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amp
November 11th, 2010, 06:49 PM
I ran across a post I printed out for reference from 2007. I was about to pitch it and after I reviewed it, I started to question just how much I understood it. Some of the graphics people sent me…just confused me a bit. So I’ll give the first part of my question a try. Just some basics first…So I understand in a “street to meter base to main panel” situation you put the neutrals and grounds for all the branch circuits on the same bar. That’s easily seen in a main panel. As far as the SEU cable coming from the street, through the meter base, and into the panel……The braided neutral (2/0) gets connected between the hots or on the neutral lug of course. Again, easily seen. So for easy thinking let’s use a receptacle in a house that’s always “hot”. The return path for that receptacle (when it’s in use) will be the neutral wire coming back from the receptacle to the main panel’s neutral/ground bar. So, why doesn’t that return current seek the ground on that neutral/ground bar and leave the house via the 8’ ground bar outside? I do understand the neutral current goes back to the pole…..but what is its exact path when everything is connected correctly? The same question applies to if a hot broke loose in a grounded metal box (and it touches the metal box)…..that ground fault will get back to the main panel’s neutral/ground bar via the ground path (the bare ground conductor). Why does it choose the ground rod that you have connected to the neutral ground bar instead of any of the neutrals on that same neutral/ground bar that everything is connected to? Is it an impedence thing….path of least resistance? Not sure. Thanks in advance.

suemarkp
November 11th, 2010, 07:43 PM
All neutral (or fault) current from the main panel back to the transformer takes all paths. The least resistance one is the neutral wire to the transformer. But some current flows in the earth, and the amount varies by resistance. If you have a metal pipe network in your neighborhood, a lot can go that way too (going up your neighbors neutral to his pole over the pole to pole grounded conductor to your transformer.

Earth is kind of a weird conductor. THink of it as a shell. The larger diameter the shell, the lower the resistance. A ground rod has a rather small shell arrund it (5/8 of an inch). That limits the current into the earth. That soil then touches adjacent soil. Once you're out a 3' diameter cylinder from a ground rod, you've added about all the resistance you're going to add. So electricity can go a long way, but it has a "high insertion loss" where it goes into and out of the earth.

A code target for ground electrode systems is 25 ohms. But they can be much higher or much lower. However, nothing is going to compare to the less than 1 ohm of the service neutral (unless it breaks!).

amp
November 13th, 2010, 07:14 PM
Thanks Suemarkp! It's making more sence for sure. So, as you said, neutral or fault current takes all paths. So is it fair to say that a little fault or neutral current could go back to my main panel.......and then go out of the house by way of the ground rod instead of the neutral neutral going to the transformer? Or is it unlikely because the "shell" you were referring to is too small with the ground rod......and lot larger or easier with the neutral going to the street? I'm probalby just rewording what you said, but i wanted to make sure I had it right. so really then, the purpose of a ground conductor in a living room's receptacle is just to get fault current back to the main panel. And then it goes out of the house via the neutral to theh pole. The ground rod and metal water pipe are extra safety measures. Right?

suemarkp
November 13th, 2010, 07:31 PM
There is one main purpose for the ground prong in a receptacle -- to provide a low resistance path back to the source. It is intended that this be used to guard metal appliances from becoming a shock hazard by connecting that metal back to the power source. This is called bonding.

All fault current is going to get back to the panel on the grounding wire. From there 99% or 95% or 80% (house specific) will get back to the transformer neutral via the neutral wire. The remaining will take a path from your ground electrode to every neighbor's ground electrode and end up back at the neutral. Your current in the earth could be going for miles and miles to get back to the transformer. The longer the distance it travels, the less current that path will have (could be milli or micro amps).

The main purpose of your ground rods is to provide a discharge path for electricity that has a high potential relative to the earth (lightning, static). This gets those voltage sources out of the distribution system and in to the earth so they don't damage your house or the distribution equipment.

The ground rod does nothing to help prevent shocks. If the utility distribution system was not grounded, your house would be safer with no ground electrodes. You need the bonding path back to the power source, but the earth does not have to be involved. But since the power company grounded it, you need to ground your side too.

amp
November 23rd, 2010, 03:37 PM
So I get the theory behind the neutral and ground for sure. Neutral current or fault current gets back to the transformer by many paths. Your examples help me out for sure. I guess when I was thinking of fault current I was considering a wire that comes loose in a receptacle box. And of course, the lowest resistance path is the ground wire. But once that gets back to the main panel it’s attached to the bonded neutral/ground bar….which then takes the fault back to the transformer via the braided neutral…say 90%, 95%, or 99% like you said.
So that leads me to my next question. If you have a setup where it’s transformer to meter base to main disconnect to sub (acting as a service panel) you need to isolate your subpanel’s neutrals and grounds. I understand the connections one would make with the SER and SEU cable. I’ll take a shot at this. Say you have objectionable current from a receptacle in your house……maybe a loose hot conductor touches the side of a metal box. That current will get back to the subpanel (to its isolated ground bar) by way of the ground conductor. If the neutral and ground were on the same bar (this is wrong, I know)….then what would happen?...Do you run the risk of that objectionable current ruining other circuits in your subpanel that contains all your house’s circuits? Is that the issue? Thanks!

suemarkp
November 23rd, 2010, 06:16 PM
Not really. I think the two main reasons for separating the neutral and grounding wires past the main disconnect are:

A combination neutral/ground carries current all the time which has the potential to make a bad connection from thermal cycling and arcs that blow out metal during faults. This bad connection can increase the voltage drop in a fault path so you could get shocked.

A neutral that comes loose can have 120V on it. If the neutral and ground are a shared conductor, then the parts bonded by that bonding jumper now become live. An example would be a 3 wire feeder to a subpanel. If the neutral to the sub comes loose at the main panel, you will have 120V on the neutral whenever any circuit on the subpanel is energized. That can make all metallic raceways and the panel enclosure on that subpanel lethal shock hazards. You will have a visual clue of this (none of the circuits will work because of the broken neutral). Complicating things would be the return path via circuits on the other leg (giving dim and bright lights from the subpanel). This still has a shock hazard since the neutral will vary between 0 to 120V to ground depending on balance. But again, you have a visual clue.

Wgoodrich
November 23rd, 2010, 07:07 PM
I want to try an explanation ground rod versus neutral at transformer in a laymen language attempt. Maybe it will help you to understand what SueMarkup is saying.

Picture in your mind a controlled easy flow of air being say your body flowing across a path. This being the current flowing out of the transformer through the panel and load. Now picture at the end of the path where the air is flowing to a hole that has a suction in it that will guide the current back to the transformer and a second path with a wind blowing causing a resistance of your easy flow of air to go down that second path being the ground rod to earth.

You would say the sucking pipe would swallow all or most of that easy flow of air being that flow of current.

The above picture being a normal usage of electricity from tranformer through an appliance doing work and a natural flow back to the transformer on the return path.

Now picture your body shot out of a cannon blasting across that path multiplied by 10,000 times in the amount of force and size, and the two choices being the small sucking tube of the neutral path back to the panel and the huge open area of earth that is blowing a wind back against you.

When flowing natural easyily you would expect you to flow into the sucking tube back to the transformer rather than buck the wind blowing against you making the resistance of your path. But during a short circuit is is more like an explosion of current flow blasting toward that neutral. That huge blast is coming so hard so fast that resisting wind would not even be noticed thus bypassing the neutral and blasting into the huge area of the land mass of earth.

While this is not a scientific explanation it should give you a better picture of the easy normal flow of current when things are operating correctly versus the explosive action that happens when you hear that big hum of a dead shorted circuit.

Then also don't forget the grounding electrode is primarily designed to obsorb that same explosive force of lightening hitting the outside of the house hopefully also being obsorbed by the massive area of earth.

Hope this does not confuse but lends a better picture of action actually happening and force involved.

Wg

amp
November 29th, 2010, 03:44 PM
Ok, I think I got it. Per the posts below..."The main purpose of your ground rod is to provide a discharge path for electricity that has a high potential relative to the earth (lightning, static)". This gets those voltage sources out of the distribution system and in to the earth so they don't damage your house or the distribution equipment. Makes sense...got it.
Next, "A neutral that comes loose can have 120V on it. If the neutral and ground are a shared conductor (don't think this is what is was asking), If you have a setup where it’s transformer to meter base to main disconnect to sub (acting as a service panel) you need to isolate your subpanel’s neutrals and grounds. Say you have objectionable current from a receptacle in your house……maybe a loose hot conductor touches the side of a metal box. That current will get back to the subpanel (to its isolated ground bar) by way of the ground conductor. If the neutral and ground were on the same bar (this is wrong, I know)….then what would happen?...Do you run the risk of that objectionable current ruining other circuits in your subpanel that contains all your house’s circuits? I guess I need to know what happens if they're not isolated. Am I missing this in a post? Just not sure how the isolation helps in the sub (acting as your main panel) vs a setup when you keep them together like transformer, to main panel.

amp
November 29th, 2010, 04:03 PM
I guess if I just simplify the previous post...since I get what the neutral and ground do....it comes down to these questions:

1. With a "transformer to meter base to Service panel" setup you keep the grounds and neutrals together in the SPanel. In the case of normal current returnng to the sp. It will go to the sp via the neutral and to the transformer. Objectionable current (say a hot wire in a receptacle box comes loose and touches the metal box). Same deal, but it gets back to the sp via the ground conductor. And since the neutrals and ground condutors are on the same bar it will either go to ground or the transformer. Right?
Why isn't the sp getting enegized with the objectionable current.


2. With a "transformer to meter base to dissconnect to sub (acting as a service panel" setup the sub has the neutrals and grounds isolated. Why do you do this? Normal current returns from a receptacle in the house via the neutral to the sub. I assume it then goes back to the dissconnect and returns to the transformer. got it. What about current coming back on the ground wire if there was a wire that came loose in a receptacle box? it goes back to the sub and then to the dissconnect right? so are you isolating them so you don't energize the subpanel and ruin circuits?

suemarkp
November 29th, 2010, 09:09 PM
1. It is. But current by itself isn't a problem, its the voltage. If there are thousands of amps, you can get a large voltage drop across a conductor (or pipe, or box) which could shock you. But as you are seeing, the code making panels think this is OK at the Service (main disconnect and meter enclosures), but NOT OK at a sub panel. I think they would like the Service to have a 4th wire (separate grounding conductor from neutral) run from the pole. But they have to fight all the power companies to do that, and the NESC is a different code book than the NEC.

Another factor may be what you're calling "objectionable current". To me, this is current flowing in the normal course of business where you don't want it. A fault is not normal business. Neutral current is. So when the neutral is being used to bond equipment (either a range or dryer in the old days, or a subpanel in an outbuilding prior to 2008 ) the current in the neutral is also flowing in the panel enclosure. This didn't shock too many people for the hundred years or so of using that approach, but the code making panels deemed it unsafe. I think ranges/dryers were the bigger offenders because there is not much of a clue when the neutral degrades or breaks. In a feeder or service, you get a lot of warning signs when the neutral goes (lights going brighter/dimmer).

But again, code allows this neutral current to flow on service raceways and enclosures, but not for subpanels.

2. Current doesn't necessarily ruin circuits, it is the applied voltage. That is the point of bonding everything -- provide a low resistance path back to the source and get the serving breaker to trip.

The reason the neutral and ground are separated in subpanels is to keep normal day-to-day neutral current off the enclosure and to provide a bonding path that doesn't normally carry current (grounding conductors only carry current during faults). My previous post (# 6) explains the advantages of separating the neutral and ground.

amp
December 8th, 2010, 05:02 PM
So based on your #1 response in the last post – the main panel does get energized when a hot wire comes loose in a receptacle in your living room and that fault current goes back to the panel (not normal business) using the ground conductor. Then it goes back to the pole probably by way of the neutral since the neutrals and grounds are bonded together with the panel. I guess I figured the path of least resistance once the fault gets back to the panel (via the ground) is the braided neutral which goes out your house to the pole. And the reason I’m not getting shocked when I touch my panel is because the lower resistance path to the source (transformer) is the braided neutral going to the pole. Correct assumption? Let me know if I’m thinking about this right.

Neutral current is about the same. Code allows neutral current to flow on meter enclosures/main dissconnects as in the above paragraph, but not subpanels as you stated. I guess the whole issue with separating neutrals and grounds would be nice to do at a main disconnect if you had an extra conductor (ground) coming from the pole. Since you don't, you bond neut+ground at main dissconnects and isolate at subpanels in order to keep neutral current off the subpanel when neutral current is going back to the main. right? low chance of getting zapped by neutral current at the sub is an advantage!
And having a single ground conductor to carry only faults is good too. If a fault happens in your house and the fault has a single path from the sub (which houses all your breakers) back to the main dissconnect and ultimately back to the source/transformer...this is good.

suemarkp
December 8th, 2010, 05:40 PM
Mostly. The return current takes all paths to the pole transformer. So some current goes there via the earth and some (most actually) goes via the service neutral. The current in each path is proportional to its resistance between main panel and pole on that path compared to the other paths.

As long as there is one low resistance path to the pole, the voltage on the panel chassis during a fault will remain low. If the neutral were to break and all the current was returning via grounds instead of the neutral, the resistance of that path may be 20 to 200 ohms. 20 amps times 10 ohms is 200V, so even with a 10 ohm path all 120V is going to be dropped between the panel and ground electrode. That means any bonded metal can shock you if you're standing on the earth.

Your understanding of faults and paths to subpanels is correct.

Roger
December 9th, 2010, 10:18 PM
Hi Amp

Just thought I would drop in on this thread and post some of my drawings (stubbie) & (Bruto) & (Roger) usernames... to give a bit of visual aid for different circumstances and maybe Myself and Mark and others can use them to your understanding if you want to ask more questions.

Effective Ground Fault Path is shown below by the red dots in the first drawing. This is the intentional path installed by the utility and dwelling electrician to insure the lowest possible impedance for fault current to return to the center tap (Xo) of the serving transformer. This must be intact to get (insure) a circuit breaker to trip during a ground fault event to protect humans.

I'm not showing the insignificant amount of current that takes paths to ground (earth) or that is on bonded metal and other points. Mark explained the details in an earlier series of posts.

http://media8.dropshots.com/photos/440526/20090904/164906.jpg

In this next drawing we have a 3 wire feeder from the service equipment to a panel in a detached building under the allowances of NEC 2005 and prior. The red dots show the path that ground fault current would take if a ground fault occurred on a branch circuit served by that sub-panel. Notice we have the neutral and ground bonded because the only low impedance path back to the service equipment is the feeder neutral.

http://media8.dropshots.com/photos/440526/20091028/120224.jpg

This next drawing shows what happens if you add a ground bar and do not bond it to the neutral of the 3 wire feeder from the service equipment. Fault Current must use earth to return to the center tap of the transformer. This is a high impedance/resistance path and if a ground fault occurred there would be enough current flow thru the branch circuit breaker to cause it to open and clear the fault. Your panel metal will come to line voltage and you are going to get shocked if you touch it. If you use ohms law this becomes apparent.
Resistance of earth lets say is 20 ohms 120 volts/20 ohms = 6 amps. That is what is flowing thru the breaker due to the earths resistance. As you can see a 15 amp breaker will not trip with 6 amps flowing thru it.

http://media6.dropshots.com/photos/440526/20091105/151221.jpg

This last drawing is what happens when you run a 4 wire feeder to a sub - panel and you bond the neutral and ground. This is where you have unwanted (objectionable) current on the bonded metal and equipment ground during normal operation of your electrical system. In the case of this drawing .. system neutral current will split equally on the parallel paths provided by bonding neutral and ground on a 4 wire feeder to a panel. So if for example I have 40 amps of neutral current being returned from branch circuits after all cancellation has been factored in. It is very possible that half that current (20 amps) is flowing back to the service equipment on the equipment ground of the feeder. This is extremely dangerous if the right conditions exist and you get in contact with that unwanted current. For example the feeder neutral comes apart or is broken. Now because you bonded the equipment ground to the neutral all the neutral current is going to be using the equipment ground of the feeder to return to the service equipment and then to the transformer. And again I'm not mentioning the current that is going to earth as it is insignificant.

Hope this has been a benefit to your questions

http://media8.dropshots.com/photos/440526/20100203/124124.jpg

Wgoodrich
December 10th, 2010, 11:40 AM
Amp

One thing you keep bringing up is term objectionable current. There is a fact that may help you understand what is happening with a ground rod. If you had a house with no neutral connected back to the transformer and tried to use the ground rod only as your neutral return path source the best you would get would be a very few dim light bulbs lighting about 20 % and no 120 volt motor that would run if they had to rely on the return path of a ground rod only. Ground rods work efficiently only during high power surges like a dead short fault or lightening strike. Low power such as normal 110 volt loads won't work relying just on that ground rod as a return path.

Wg

mikoal
June 15th, 2011, 03:56 PM
Hi everyone and Mark.
First time poster and definitely a newbie. I know this is a long post, but if someone can dissect my problems that would be great!
i'm eager to learn!

Probably of all the threads this came closest to answering my questions. I think I'll have quite an extensive list and I hope I'll understand your responses. Here goes:

(I know this statement below is wrong, but would like someone to make it easier for me to understand)
I dont generally see ground wires running from load to ground. There is usually hots and neutrals in conduits, and sometimes bonding wires which are connected to the ground. Usually ground rods are connected ground wires then to busses is what I've seen.

1) Is this true? (no is my answer, but it seems like i dont see a lot of ground wires running with the other conductors, is it because it's usually taking a different physical path? IE bonding equipment?)

2) The reason why I ask because I found some electrical drawings showing 4 wires (3 hots and neutral) +##G. So I would assume that is the grounding conductor (earth ground)? Electrical drawings always have +6G or whatever size the ground needs to be. Yet looking at receptacles the ground wire is just bonded to the metal box, which I assume is connected to the neutral bus bar which is grounded. I have trouble visualizing the ground wire and where it goes.

3)
So in that case looking at a receptacle there are 3 wires, hot, neutral, ground.
Hot will be source
Ground will be bonded to the metal box which is all bonded back to the bus bar then to the rod.
a) Are all the grounded bus bars bonded? Why or why not? (I believe they are)


Neutral will carry neutral current going back to the source then will hit a point (I think its a few points, not just 1 point) where either it should choose the ground rod path or the source/transformer/neutral path. according to your posts, the least resistant path is the source/transformer/neutral path.
All neutral (or fault) current from the main panel back to the transformer takes all paths. The least resistance one is the neutral wire to the transformer. A code target for ground electrode systems is 25 ohms. But they can be much higher or much lower. However, nothing is going to compare to the less than 1 ohm of the service neutral (unless it breaks!).

b) So the neutral current will be going back to the source, but at every ground rod point, it chooses the neutral path. is that correct? That is the reason why the neutral conductor carries current and under normal condition a grounding conductor should not.

4) similar to the OP, what happens when
a) the circuit receives an overloaded current when the equipment draws too much current, the returning current will once again go to the source ? If no OLPD was installed what will be damaged? Wires, equipment, transformer, and panels?

b) If there is a short or a fault current then current will jump onto the ground conductor.
i) Does this make all the metal enclosures thats bonded a shock hazard?
ii) will the current on the ground immediately go towards the neutral conductor since the resistance is less than the ground path? If so, doesnt this damage the system?
iii) Also you said some currents will go into the ground then up the poles into other people's neutrals. So do you mean that the current doesnt really dissipate? Also if you start sending large amount of energy into the gorund will you damage their equipment?

5) What is the purpose of the grounding conductor then?


The main purpose of your ground rods is to provide a discharge path for electricity that has a high potential relative to the earth (lightning, static). This gets those voltage sources out of the distribution system and in to the earth so they don't damage your house or the distribution equipment.

a) But how? If barely any current goes down that path?


There is one main purpose for the ground prong in a receptacle -- to provide a low resistance path back to the source. It is intended that this
be used to guard metal appliances from becoming a shock hazard by connecting that metal back to the power source.
The ground rod does nothing to help prevent shocks. If the utility distribution system was not grounded.

b) you mentioned that ground prong is to gaurd metal appliances from being a shock hazard, then in the same post you mentioned that ground rod doesnt prevent shocks, can you explain?

your house would be safer with no ground electrodes. You need the bonding path back to the power source, but the earth does not have to be involved. But since the power company grounded it, you need to ground your side too.
6) I can't find the link, but i read somehwere that the systems without ground would be safer. Can anyone remember why a ground was installed and why is it safer?



Not really. I think the two main reasons for separating the neutral and grounding wires past the main disconnect are:

A combination neutral/ground carries current all the time which has the potential to make a bad connection from thermal cycling and arcs that blow out metal during faults. This bad connection can increase the voltage drop in a fault path so you could get shocked.
QUOTE]
7) I thought they were always seperated? Unless you were epxlaining what would happen if they werent? If there are examples of times where its not seperated, why?


[QUOTE=suemarkp;113667]So when the neutral is being used to bond equipment (either a range or dryer in the old days, or a subpanel in an outbuilding prior to 2008 ) the current in the neutral is also flowing in the panel enclosure. This didn't shock too many people for the hundred years or so of using that approach, but the code making panels deemed it unsafe.

8) Neutrals are bonded to ground, and grounding conductors are used to bond equipment?


But again, code allows this neutral current to flow on service raceways and enclosures, but not for subpanels.

9) What do you mean neutral current doesnt flow in subpanels? wouldn't the current have to return to the SP then to the source?

THANKS A LOT!

mikoal
June 17th, 2011, 07:06 AM
Was my post too long? Or does anyone and/or Mark knows the answer to the Qs?

suemarkp
June 17th, 2011, 01:43 PM
Didn't remember seeing the post....

First, it is important to differentiate between bonding and grounding. It is confusing that the main service is grounded, so all bonding wires get you back to a ground. But the word bonding or grounding should be used to convey the main purpose of a conductor. What the code calls equipment grounding conductors should really be called equipment bonding conductors. Most equipment does not need to be connected to the earth. But if it is metal, it needs a low resistance connection to that metal back to the service common return point (grounded conductor).

1) So all circuits should have a bonding conductor. The code calls it "equipment grounding conductor" and common use is just "ground wire". A metallic raceway is permitted to be the equipment grounding conductor, so the green/bare grounding wires may not always be present. Most people run them anyway, as a conduit can come apart (especially EMT with setscrew fittings).

2) The ground wire you see is really a bonding wire. If using a metallic raceway, running a wire to the box is sufficient. This wire is sized per 250.122 which is based on the size of the circuit breaker protecting the wires.

3a) Not sure of your terms here. "Grounded" in NEC use means the white wire. It at one point, connects to earth. The equipment grounding (bonding) wires also connect to the white wire at only one point (main service or transformer). So "grounded" busses are connected to the panel chassis at one one place. In all down stream panels, the grounded bus will be insulated from the panel chassis. The grounding busses are always directly connected to the panel chassis. These are usually earthed at one place, but code permits ground additional electrodes to be placed anywhere to grounding conductors (but they aren't going to do much of anything).

3b) Current doesn't "choose", it takes all paths. So as it returns to the source if there are two paths (ground electrode to dirt to electrode to transformer, and a neutral wire) it will take both paths. But most current will take the neutral wire path. At subpanels and receptacle outlets, the bare grounding wire doesn't normally carry current. Should there be a fault and now it does, that current will ride every grounding wire, conduit, or other equipment that has an eventual connection back to the circuit source. The current on those paths will vary based on its resistance. The wire or conduit feeding that circuit should have the lowest resistance back to the circuit source, so most current will go that way.

4a) Yes, if a piece of equipment draws too much current, the terminations can overheat, and wire insulation can melt. Overheated terminations tend to have higher resistance, so the problem will occur sooner next time. Overheating a receptacle can make the springs weak so they don't hold the plug tightly and that connection gets a higher resistance making it hotter.

4b) A fault can happen multiples ways -- hot to hot, hot to neutral, hot to bonded metal. Only in the hot to bonded metal case will the metal object become electrified. As long as it is bonded via a grounding conductor sized the proper size, it should not be a shock hazard. If it is doing its job, the breaker should quickly blow turning off the circuit because the grounding wire is bonded to the neutral back at the beginning of the power distribution. Earth (dirt) most likely won't be involved in this situation. The system should be designed for a short term high current surge (the AIC rating of the panels is what this is for, and circuit breakers and wire sizes limit the max fault current available -- you just need to be sure you don't have a system that can fault 30K amps through a 10K AIC system).

The current going up the pole to the utility transformer happens during normal conditions, not just faults. That is because there is not a separate grounding wire form the utility. The neutral at the utility pole and the main service disconnect are connected to ground electrodes by design. So if there is any current flowing towards the utility neutral, there will be some in the earth too. All current returns to its source, it doesn't magically dissipate into the earth. But it takes the earth as one of its paths in the utility distribution. The resistance of the earth is rather high compared to wires, so you don't really put much current through the earth. If you did, you'd probably see steam coming out near the entry and exit points in the dirt.

5) The purpose of the equipment grounding conductor is to provide a low resistance path back to the grounded side of the power source. The purpose of a ground electrode conductor is to connect the grounded side of the power source to the earth.

5a) Ground rods will allow a lot of current of the voltage is high (e.g. tens of thousands of volts from a high tension utility line or millions of volts from a lightning strike. Assume your ground electrode has a resistance between 10 and 100 ohms. Use ohms law: I = E/R. At 120V and 10 ohms, all you can get is 12 amps into the earth. This won't trip a circuit breaker. A 100KV, you can get 10,000A into the earth. That can melt many conductor sizes.

5b) the ground prong is for bonding. It allows current to go directly back to the source during a fault which trips the circuit breaker. If all you did was tie that ground prong into the dirt and nothing else, the current would still go back to the source but only through the dirt. The resistance would be too high to trip the circuit breaker and the faulted appliance would continue to fault until you remove its power source. The device would also be a shock hazard because the 120V would be dropped in the dirt (that's where all the resistance is) so the faulted device chassis would be 120V to the earth 3' or more away from the ground rod.

6) Grounding is done because of lightning strikes to the utility system and static electricity build up. Because they've connected their system to the earth, and worse, they connected the secondary of your serving transformer to the earth, it has potential to the earth from each ungrounded conductor.

7) The grounded (neutral) and grounding (earthing/bonding) conductors are one and the same ONLY at the utility and their drop to your building. At the main disconnect, that is the last place grounding (green/bare) wires and grounded (white) wires touch.

8) Usually. Grounding conductors are used to bond equipment. Some conductor needs to be grounded to earth on most systems. This is usually the neutral. But some systems don't have a neutral (e.g. corner grounded delta). But whatever point is chosen to be earthed will be the white or grey wire in the system.

9) Neutral current doesn't flow in subpanel ENCLOSURES and RACEWAYS. Code now requires all subpanels to have their neutral isolated from the subpanel chassis. Neutral current should only be flowing on the neutral wire (which must also be white insulated). Only when the current gets back to the main disconnect panel do neutral and ground become one.

mikoal
June 20th, 2011, 08:35 AM
First I'd like to thank you mark for taking the time to answer my silly questions
I've got some followup questions for you.

1+2)
i) What would be the definition of a metal raceway, an electrical conduit?
ii) Also why can this replace a grounding conductor? In the raceway case all metal raceways will be bonded. In the case with the grounding conductor, what is the conductor doing or attaching to?

3a)

i)ok.....I was thinking of mulitple points with grounding busses, so.....multiple ground rods at different locations 1) transformer 2) main..... 3) another point down the line.
Is this allowed? if so in what situations? Won't this cause ground loops?

ii)So you are saying that the neutral is only connected at one point (main/trans) and not further down to other ground busses?

iii)Also if there were multiple grounding busses at different locations, should these busses be connected together? So lets say the Main has grounding busses and bonds to panel 1,2,3. Panel 4 has its own grounding bus and bonds to panel 5,6,7. Does panel 4 bond/connect to panel 3's chassis? Or am I way off base here?

3b) Just to clarify, the source point of 2 paths. You are referring to the grounding bus which the neutral is conected to? At this point the current goes to ground rod, into earth, into transformer OR continue to neutral back to transformer?

4a) ok

4b)
i) The hot the neutral fault, would that be similar to an overload?
ii) What would a hot-hot and hot-neutral fault be like?
iii) assuming no OLPD was installed, then what would happen? Would the fault current go on the grounding conductor, then at the source most will go to the neutral and also creating a shock hazard on the metal cases?
iv) If there is always current travelling in the earth, why doesnt anyone get shocked? Same potential?



5) The purpose of the equipment grounding conductor is to provide a low resistance path back to the grounded side of the power source.

What do you mean grounded side of power source?
You say it provides a low resistant path, but the neutral path is relatively lower resistant.....?

5a) So from what it sounds like the purpose of the ground is for HV disasters. Otherwise

-during normal operation it doesnt do much. No current flows here or suppose to flow here, but since currents take all direction, i guess some will go in this direction.
-during faults, it doesnt sound like it really does much either....maybe offer protection and tripping the breaker? but this could be done wihtout ground and with just the neutral cant it? Cant we use the neutral as the grounding and grounded conductor if the utility didnt ground their system?
Actually even if there side is grounded, why can't we still use the neutral to bond to the chassis and metal enclosures?

But Either way the equipment still gets damaged, or breakers get trip. Since most current goes to the neutral, with grounding conductors or with grounded conductors.

-during high voltage lightning strikes. True a lot of current goes through the ground. but wont a lot MORE go to the neutral and destroy everyhting? Since relaitvely speaking the resistance is lower on the neutral path.

5b)so the bigger the R, the lower the I, and the bigger the R the greater the V drop, therefore causing hazard?

6) If theres no ground lightnight will kill your system right? But even if there was, I still have my same reasoning that majority of current will always go to neutral and harm the system. so i still dont see why the utility decided to use grounds.

7) Sorry i meant to say other than the main disc point.
So if they were to touch at points further down the line what would happen?

8 ) In the case of a 2 wire circuit the bonding wire would be grounded conductor or the grounding conductor?

9) Ok but neutral current flows in the subpanel.......and nuetral current doesnt flow in enclosures and raceways (only for grounding conductor path)

And the reason why neutrla current flows are the main encloure and service raceway is because the neutral is bonded to the grounding bus?

suemarkp
June 20th, 2011, 03:44 PM
Conduits are one form of raceway. Metal gutters or troughs can be raceways. Even some cables with metal skins are permitted to use that skin as a grounding conductor (but not all). Likewise, flexible metal conduit can only be used for a grounding conductor is it is listed for such use and there are certain amp limits.

The grounding conductor is required to bond all metal boxes, raceways, enclosures, ground pins, etc. If you run metal conduit from box to box and connect it well to boxes and enclosures, then it is just as good or better than a wire (it has much more area and current handling ability that a wire inside it). However, it has been assembled and could come apart (through settling of a building, damage from getting hit or snagged, expansion/contraction, corrosion, etc). The most robust design has both a wire and a metal raceway.

Ground electrodes are required at the Service Disconnect. However, if structural steel is one of your electrodes, you'll most likely be connecting metal conduit to that structure at many locations. You are also permitted to have additional ground electrodes, as long as they are all bonded together with a #6 or larger copper wire. The NEC doesn't care about ground loops. In their mind, the ground never carries current except during fault conditions, so the more paths to earth or the power return point, the better.

Yes, the neutral connects once to the grounding system via the Main Bonding Jumper. A new system can start at a transformer, but if it is just panels to panels to circuits, the ground and neutral are only supposed to touch at one point at the beginning.

The ground busses at each panel MUST be connected together via an equipment grounding conductor (wire sized per 250.122 or a non flex metal raceway). You may not use the earth as a bonding conductor, because its resistance is too high. This is easy to do -- you just run the bonding conductor with the power wires feeding the panel.

"At this point the current goes to ground rod, into earth, into transformer OR continue to neutral back to transformer?" No, it goes both ways. Assuming you're at the main service panel, that service is connected to the earth. The neutral at the utility transformer is also connected to the earth. So current returning from the transformer power source via the neutral will take both paths. However, 99% will follow the neutral wire to the transformer because its resistance is so much lower than the parallel path in earth.

Hot-Hot and Hot-Neutral faults carry the current through the normal path. It is just excessive and will burn up the equipment if the breaker doesn't blow. But the breaker will blow if the fault is bad enough. A small leak would just draw excessive current, but maybe not enough to pop the breaker. Eventually it will. The grounding wire is not used in this case. However, if the breaker never tripped, things would eventually melt. If a hot wire touched the metal structure/enclosure of the equipment, that metal would become energized too. It would be a shock hazard until the breaker pops. Grounding it will not help if you have the breaker failing to trip as a premise. However, current will follow all wires touching in the fault (so maybe half would follow the grounding wire and half would follow the neutral).

Yes, there is always current flowing in the earth. The reason you don't get shock from this is that there isn't enough flowing to cause a shock voltage between your feet. Even if you put in two rods 100' apart and run a long wire from each to a volt meter, you'll most likely only measure 10 to 20 volts. Now if there was a 10KV wire touching the ground, there would be a dangerous area at a certain distance from that wire. There could be something like 10KV at the wire/earth point, 9KV 1' away, 5KV 2' away, 2KV 3' away, 500V 4' away, 100V 5' away, and so on. If you had one foot 4' from the wire and the other at 5' from the wire, you'd have a 400V shock between your feet. At 10' away, there is probably no more hazard.

The strange thing with the earth is it has a high "insertion loss". That is, at a rod where voltage is applied to the earth (either current going in or coming out), it has a moderate resistance (5 to 200 ohms or so). If you think of concentric circles of earth going away from that rod, those circles keep getting larger in diameter. As the diameter increases, the resistance goes down. Once you're 10' or so from a rod, you have a huge volume of earth to carry your current. So current can go extremely long distances at little loss. 99% of the loss happens where the power goes into the earth and where it comes out.

The equipment grounding conductor is a parallel wire to the grounded conductor. It just doesn't normally carry current. It must go to the same place as the source grounded conductor if it has any hope of returning errant current back to the source. The only time current should be on this wire is if a Hot wire comes loose and faults to this bonded metal. I used the term grounded conductor because not all power systems have neutrals. The neutral or grounded conductor is not supposed to be connected to a metal frame/enclosure of equipment. That's why the ground wire is there -- provides a path very similar to that of the neutral/grounded conductor should a hot wire come loose or some fault occurs shorting the hot to the frame.

If the utility didn't ground (earth) their system, we would probably have no need to ground (earth) our system either. In that case, bonding would not be required because the earth is no longer connected to the power source and touching something with one hand wouldn't shock you.

Yes, the earth ground doesn't do much at voltages under 1000V (maybe even 5 KV). The reason we don't use the neutral to bond equipment is if the neutral gets broken, then the chassis will come up to the supply voltage and be a shock hazard (the voltage won't be dropped through the load to near zero because the return path is broken). If you've ever worked on a live circuit, say to disconnect the neutral in a panel for a light circuit and think the neutral won't shock you, it will if the light is switched on. The light will limit the current through you, but if its a 1 amp light and it only takes 30mA to kill you, that isn't a lot of help.

Most equipment shouldn't get damaged if protected by a breaker the size the manufacturer specifies. The problem is the other way around -- damaged or failing equipment causes excessive current to flow and the breaker stops the situation from melting equipment and putting shocking voltages on metal enclosures/frames.

One of the drawbacks of connecting one side of the power system to earth is that it allows lighting voltage to follow that path. However, at the high voltages of lightning, many strange things can happen. Things you thought were insulators become conductors. All bets are really off in a lightning strike. I think the reason for earthing the enclosures and a conductor of the system is to try and limit the voltage to other parts. This keeps the insulation from being damaged at the expense of a current surge melting things. Melted wire is obvious, voltage damaged insulation is not.

5b) yes.

If grounded and grounding touch further down the line than the main disconnect, then nothing bad happens during normal conditions. Wire connections can degrade when current flows because they heat up and cool down. So having those wire touch potentially will degrade the grounding connections. You can also get a mild spark when you disconnect a wire with current flowing on it. So it isn't a huge thing to have the wires touch, but the code says the equipment ground is not to carry current during normal function. Possible degradation is the only thing I can think of for why this is.

In a 2 wire circuit, you have an ungrounded (black) and grounded (white) conductor. The bonding wire would be a third wire, either green or bare, and run with the other two. When someone says "two wire circuit" they aren't covering the bonding wire, which is implied to be there. Kind of like when you buy house cable (NM or Romex cable). It will say 12-2, as it has two #12 insulated wires, but it also has a 3rd bare wire. But 12-3 has 3 insulated wires and a 4th bare one....

The reason current flows on Service raceways and enclosures is because the neutral and ground are the same conductor (there is no separate bonding wire from the utility to the service) and both the Service panel and Utility transformer neutrals are connected to the earth and to their enclosures.

mikoal
June 22nd, 2011, 07:27 AM
1+2) ok, so metal raceways are grounded by bonding them to the ground. In the case where the metal raceway doesnt exist, the grounded conductor is used and connects to???........

3a
i) is there scenarios where it is recommended to have multple ground points (bonded together)? Also you mentioned that NEC doenst care about ground loops, but it doesnt mean then dont exist though right? If they did exist, what problems can it cause?

ii) What happens if neutral connets at mulitple points?
iii)OK

3b) OK

4b)
i+ii)OK
iii)
"However, current will follow all wires touching in the fault (so maybe half would follow the grounding wire and half would follow the neutral)."

Why half? Wouldnt most go to the neutral?

iv)In the 10KV case, where does the current go to? There is the voltage drop from 0' to the 10' range, so then there must be current flow. What will be the direction and where does it go? In my mind I would have thought dissipate, but now I'm thinking going up the grounding conductor to the neutrla of another system. If so......then wouldnt this 1) damage the system, 2) the shock hazard should last from the point of the 10kV to the grounding conductor since that is the current travel path?

5a)
i) Where do these losses go? OR what does it transform into? Heat?
So it still isnt' a lower resistant than the neutral path.
Also, with losses, doesnt that mean there is a greater resistance? IE:Insertion loss?

ii)For the bonded neutral answer..... if the supply side was not grounded, then if the neutral was broken, there will be no shock hazard right? Acutally if there wasn't a break in the neutral, shouldnt there be a shock hazard anyways since neutral current will be flowing through the neutral and will energize whatever it is bonded with?

iii) Right, but the point I was trying to make iwth the breaker/damaged equipment, is that either way it will happen with or without grounded and grounding conductors since most current goes to the neutral anyways.

iv) Hmmm...maybe to simplify things for me a summary of the two scenarios would be good

In the case of lighting strike, what would happen if the system had
1) Ground rods and all is bonded
2) Ground rod but the rest isnt' bonded(neutral no connected to it)
3) no grounds

I would think in all 3 cases everything will be damage. I still dont see why having grounds are beneficial in HV situations since current travel all paths and neutral has lowest R, it will go there most.

In the case of regular fault, what would happen if the system had
1) Ground rods and all is bonded
2) Ground rod but the rest isnt' bonded (neutral no connected to it)
3) no grounds

6) Is essentially the same question as above
"If theres no ground lightnight will kill your system right? But even if there was, I still have my same reasoning that majority of current will always go to neutral and harm the system. so i still dont see why the utility decided to use grounds."

7) degradation..... but aside from that, you can have multi grounds, and multi grounding to grounded connections.

8 ) dont know why I asked this? yes grounding conductor is used to bond

9) ok

suemarkp
June 22nd, 2011, 09:42 PM
1+2) Are you asking about grounding or bonding? Metal things and receptacle "ground" pins must be bonded (even though code says grounded). In a PVC raceway, you are still required to have this grounding/bonding conductor because it is needed at the end to connect to equipment. The green or bare equipment groundING conductor does this. It eventually goes back to the main bonding jumper where the equipment grounds, grounded conductor (usually the neutral), and the ground electrode conductor all come together.

3) The NEC requires all spliced grounding wires in a box to be interconnected. So if 3 or 4 separate circuits all route to a common device or junction box, they must all be connected. So this is a NEC mandated ground loop. The NEC typically has one ground electrode location per building/structure at it main disconnect point. No more are required.

If you're asking what happens if neutral and ground connect at more than one point, then those conductors become parallel. This means the grounding wire will be sharing current with the neutral in that paralleled section.

4b) If during a fault both the neutral and grounding conductors are involved, the current in each would be about the same because the wires are both similarly sized and both terminate at the same location. In circuits 30A and under, the ground and neutral are the same size. Above 30A, the grounds don't grow as quick as the neutral conductor. But the size of grounding wires is sized to ensure low resistance at huge faults and not melt. There's no issue of heat damaging insulation (as there is with neutrals) because the grounding wire isn't required to be insulated.

These faults are uncommon. The typical fault is a hot wire to enclosure. In this case, the grounding conductor gets the most current because the neutral still has the load in series with it, whereas the metal chassis is nothing but metal and the equipment ground conductor.

The 10KV case is a power line. So the current from that line touching the earth needs to return to the power source via its neutral. That neutral is connected to earth at almost every utility pole and every house. So the current will flow in all directions going up all those ground rods to find its way back to the utility neutral (some current, perhaps just micro amps, goes up EVERY house and power pole on that utility system). Current is supposed to return via the neutral. The only thing "wrong" here is that it is going through the earth instead of transformers on all the power poles. So no damage will occur. The shock hazard ends once enough earth is spreading the current wide enough that the voltage between your feet is small. This exact distance depends on the earth conductivity and the applied voltage (and how far apart your feet are). But it will be around 10'.

The reason there is no shock hazard past this point is because the voltage drop in the earth from that point outward is too small. Pretend the earth was made of solid copper instead of dirt. If a wire fell onto this copper earth and some other point on that copper earth was the return line (another grounded power pole), you would not be shocked because there would be no voltage drop in this sold earth of copper. It is the same reason a bird on a wire doesn't get shocked -- that few inches of copper between its feet has so little resistance that the voltage between its feet is very very small. The voltage relative to the earth is very high, but the bird isn't touching that.

5a) what losses?
5a ii) No shock hazard if the neutral is broken on an ungrounded system. Not following the next part -- why would be bond an ungrounded system?
5a iii) Not following the question.

5a iv:
1 & 2) I don't think bonding makes much of a difference here unless we're talking about where the lightning hits. It if hits the utility wires and comes down the service lines to your house, the ground rod is all that matters. If the lightning hits a metal box or light pole, if is has a bonding/grounding conductor then that provides (in theory) a lower resistance path to earth via the service ground rod. If it has no bonding conductor, the lightning may take some weird path through the air or surrounding structure to reach the earth, or could follow the supply wires back to a place where it can easily arc to a ground.
3) With no ground, then the lightning has no easy way to get to the earth. It is going to jump all over the place potentially damaging insulation and burning holes in various things.

Grounding doesn't solve all issues with lightning. No one has tamed it, and all bets are off with lightning strikes. But it seems to help with damage prevention in the utility distribution system, especially if it can dissipate over a lot of ground rods over a lot of area. It started out ungrounded, but it was determined long ago that grounding the utility system made things last longer and created less hidden damage.

In the HV situation, lets say a tree takes out one of the high voltage lines at the street feeding your house. The high voltage line on the tree is a hazard and putting an electric gradient into the earth. Your service neutral is not really involved with this because it far away from where the tree fell. However, because the voltage is so high, the current going into the earth can be significant as it returns to the utility via all the grounds at each power pole. If there is a circuit breaker on the utility line, it will trip stopping the power line from creating that hazardous voltage gradient all around the power line. The voltage on that line needs to be substantial in order to flow 100 amps or more through the earth. But power lines are high voltage, so this trip mechanism will work. If it was just a 120V line falling to earth, you probably would never get 15A flowing (the smallest breakers used are usually 15A or larger). So a 120V line will stay live on the earth until someone shuts it off. The only saving grace is that the hazardous radius around a 120V line is about 1.5'. These types of lower voltage faults are the ones that shock people and animals around faulted street light poles that people failed to bond.

6) Lightning is not sourced from the power system. It seeks the earth. The only reason it would care about a neutral would be if it provided a lower resistance path to earth than something else. A metal utility transformer whose case is connected to the neutral and that neutral is in turn connected to the earth will hopefully allow the lightning strike to flow around the transformer can and into the earth. If that metal can was not grounded, the lightning may just go through it seeking the wooden pole on its way into the earth. That would damage it internally, and it would fail sooner (or maybe immediately).

I can't really tell you the pros/cons of why the utility system is grounded. Fact is, it is grounded whether you want it grounded or not and that is not going to change. That causes us to have to bond things since the dirt is a possible return path for power.

mikoal
June 30th, 2011, 09:43 AM
Sorry for the late reply but was on site the past few days.



1+2) I guess this question originated when you said that a bonded metal raceway can replace a grounding conductor. I'm just trying to see how.
In the case of bonded metal raceways, grounding conductor is connected to the raceways providing faults to travel that path, and it seems to me it just does that. Bond to metal raceways and enclosures.
Whereas if a metal raceway did not exist, that would mean a grounding conductor would be required. Since there are no metal raceways and enclosures to protect. What is this grounding conductor doing? What is its purpose? Providing ground at the equipment end? If so....what is providing ground on the equipment end in the above scenario (metal raceways)

3a)
i) I was asking what happens when there are multiple ground rods in different parts of the circuit. Do these ground points need to be all bonded together?
ii) if neutral connets at mulitple points, this will be very bad, similar to using the grounded as the grounding?

4b)
The reason why I thought it would damage the systems on the other poles is. I'm think they are working in fine balance, and out of no where your system faults creates an imbalance, shoots a bunch of current into their system, making it unbalanced and supplying it with too much current. But I guess since this whole system is all interconnected, even when mine is unbalance, it makes the entire system a net imbalance thus the "additional" current isn't actually additional, is that correct?

Ok let's break this 10KV into ground into a few parts.
If you are hanging on a hot wire in the air, you are safe since there is no path for current to complete right? But once u land, you have a path to ground and you will be shocked?
In the 10KV case, the ground is the conductor.....what is the path of current if you were to stand in the 0 to 10' range?
Also why would the distance of your feet matter. Does hanging on the hot wires with 2 hands matter?

I think the bird analogy makes sense, due to low V drop and low R, there is no harm.
Ok now I'm having another mind block.......generally when looking at a curcuit we calculate the load or the R for current flow.
for example in a cable that has 10 ohms resistance at 50Volts drop, the total current is 5 amps throughout the wire.
Now....just because to 0-1" portion has 0 ohms, and 2"-3" portion of the wire have 9 ohms etc........doesnt mean that 0-1" portion has less current flow right?
the 5A still will go trhough the entire wire.
If thats the case, why does it matter how far a birds legs are apart? Since a high current is flowing at all parts of the cable....

5a) the loss as in the voltage drop in the earth. Does it heat up the soil?
Also does the earth generally have a high R or not? You compared it to copper (low R), then you said 10' dissipation (seems high?)
What is insertion loss?

5a ii) Why would there be no shock hazard if the neutral was broken on an ungrounded system? Is it because no path for the current to travel?
The second part refers to using the neutral to bond for a return path for the faults (when hot touches the enclosures). But I'm guessing that is pointless since there is no ground, and if there is no ground, there is no point in bonding since there is no path to shock us.

5a iii) I dont remember.......

5a iv: + 6)
For the lightniing striking a metal enclosure, wouldnt it still have more current going to the path of least resistance? IE the neutral path to source? Thus damaging the system?

"Lightning is not sourced from the power system. It seeks the earth. The only reason it would care about a neutral would be if it provided a lower resistance path to earth than something else."
But it does.....doesnt it?

7) So having multi grounds (grounded to grounding points) can cause degredation and a parallel path for current to travel?

suemarkp
June 30th, 2011, 07:29 PM
1/2) Yes, the equipment grounding conductor is to ground (really bond) your utilization equipment at the end (it also bonds metal wiring methods if they exist). This is done via wire, metal raceway, or a combination (if you transition from metal to plastic, then you must attach a bonding wire in the last metal box and run that wire in the plastic wiring method until you either change to metal again or terminate it). The final place this goes is the ground pin on a receptacle or the green or bare wire in a piece of equipment.

3) Circuits don't usually have ground rods, just services or feeders at detached structures. You can add as many electrodes as you want to those which in theory decreases the overall grounding system impedance. Sometimes, a manufacturer will specify that a machine have a ground rod. I don't know why, but they do. These would be an auxiliary rod and are only required to be connected via the size equipment grounding conductor of the circuit it is on.

Yes, connecting the neutral and ground at multiple points is considered "bad". It puts "objectionable current" on the grounding wire at the places where it is paralleled with the neutral.

4b) If two totally separate circuits share a common conductor, there is generally no harm unless the total current overheats a wire. The power company and lightning are two separate systems. Extra current riding on the grounded neutral from lightning hitting a power pole doesn't really affect the utility circuit unless the total current overheats the wire. It would be the same as connecting the - side of your 12V car battery to your neutral and the + on a separate wire to a remote location. At the remote location, you pick up the neutral to get your - return path. This power being used by the 12V circuit does not affect the power company circuit other than having more amps in the neutral wire than expected. But that current has no reason flow up transformers or into other houses. It just rides the neutral from where the battery attaches to where the load attaches.

In the 10KV powerline on the ground case, the current is going into the earth where the bare wire end is touching the earth. It then flows in all directions to every grounded pole or other location with a neutral grounded panel with ground electrodes. Just assume the current goes in all directions seeking all those ground rods. The reason you can be shocked near the power line is because of the resistance of the earth. The earth is a poor conductor. A #2 "wire" made out of dirt would have thousands of ohms of resistance. A #0000 earth "wire" would have less resistance because it is fatter. Since the earth is so huge, the further you get from the wire the more earth can be used as conductor so the resistance begins to fall. If the earth was made of copper, you would not get shocked no matter how far apart your feet are because there is virtually no voltage dropped in a piece of copper that huge (hundreds of amps * .0001 ohm is not much voltage). But there is a lot of voltage dropped in the earth, especially closest to where a conductor or electrode is inserted into (or laying on) the earth. At 120V, most of the voltage is dropped within 3'. They call this "step potential" because most people can step 2.5 to 3'. Put both feet on the down wire, and you don't get shocked (its a good conductor). Put one foot on the wire and another 2' away on the dirt and now you may have 70% of the applied voltage across both feet. If you stand 10' away from the downed wire and put the other foot 13' away, you probably won't get shocked because 95% of the voltage has been dropped in the first 3 to 5 feet of earth where the power line is touching.

I think this really answered your later questions but maybe not 4B. The current flowing doesn't matter for shock. It is current and the resistance of what it is flowing through. If 10A are going through a conductor with .1 ohm, there has to be be 1V across that conductor. If 10A is going through a 100 ohm conductor, there has to be 1000V across that resistance. Current flow is based on end to end resistance. In the power line case, lets say you have 10KV applied to the earth, and there is 200 ohms of resistance to the nearest grounded pole. Another pole in the opposite direction could be 201 ohms. Another pole a mile away may be 205 ohms. However, the earth is not a linear resistor (10' of earth doesn't have 10X the resistance of 1' of earth). So if there is roughly a 200 ohm load across the 10KV power line, then 50 amps will be flowing out of that power line in directions that are too complex to model (just assume there is 50A in all directions from the fallen line).

Since the earth is a non-linear resistor, lets say the total resistance from dropped line (point A) to a point on the earth at distance X is at follows:
point B, x=1', resistance = 100 ohms
point C, x=2', resistance = 150 ohms
point D, x=3', resistance = 180 ohms
point E, x=4', resistance = 192 ohms
point F, x=5', resistance = 198 ohms
point G, x=6', resistance = 199 ohms

So if you're standing with a foot at point B and one at point E, the circuit resistance between your feet is 192 - 100 = 92 ohms. With 50A going through that earth, there is 4600V across your feet.

If you stand with a foot at point F and one at point G, the circuit resistance between your feet is 199 - 198 = 1 ohms. With 50A going through that earth, there is 50V across your feet. Shouldn't be a shock hazard unless you're wet.

5a) Yes, any resistance with current flowing through it is going to heat up. So the earth is heated with all these "stray currents" flowing through it. I answered the earth resistance above.

Insertion loss can be explained two ways. In electronics, there is loss at a connector. You could have a really long low loss wire, but if the connector has 1 ohm of resistance at each end, there is no way to get less than 2 ohms of total resistance. It will be 2 + .000X when you add the low loss cable. If you can deal with 3 ohms of total loss, there's no point in buying cable with less than 1 ohms of loss (2 ohms for connectors, 1 ohm for cable). But a cable with more then 1 ohm of loss won't work (overall resistance too high).

The earth is the same way. A ground rod has low resistance. But stick two in the earth a few feet apart and you'll get 100's of ohms of resistance. If you space those rods a mile apart, the resistance will only be a bit higher (say 101 ohms instead of 100 ohms when close). This relates to surface area of the ground electrode and how much soil it can touch. But once the electricity reaches the soil, the entire surface of a soil particle conducts to the next soil particle. The farther you get from the rod, the more circumference there is in a "shell of dirt" to conduct to the next shell further out.

5a ii). Yes, in an ungrounded system, you need to touch both ends of the broken circuit in order to get shocked.

5a IV / 6) Because metal enclosures are grounded, that enclosure should have the lowest resistance path for a lightning strike. If the neutral is grounded, that is another good path, but the lightning has to pass through the metal box to reach that neutral. That has to be a higher resistance path than the metal box (it has to jump through the air gap from box to neutral lug). That is, except at lightning voltages many strange things can happen. What we think of as an insulator can become a conductor at those voltages (even air). So blanket statements when it comes to lightning can be dangerous. But an earth grounded metal box is a good protection barrier for things inside when it comes to a lightning strike.

7) multi grounds are fine as long as it is ground to ground and not neutral-ground connections. It lowers the effective ground path resistance. The only time it can be an issue is when some appliance manufacturer is sending current on the grounding wire (which they aren't supposed to do) instead of the neutral. Ground currents can confuse the circuit leaking/bypassing through the bonding pathway. Some radio equipment does need to connect to the earth, so the grounding/bonding wire provides a pathway to get there. That can make it problematic to keep other circuit currents off the bonding/grounding pathway. Computer interfaces also used to connect their cable shields to the signal "ground". When these interfaces cables were long and connected to computers in different buildings (or even the same building but different transformers), then a lot of current could flow on those shield wires causing all kinds of problems (including going down the ground wire in the circuit potentially putting many amps on raceways). This has been eliminated with fiber-optic and unshielded balanced interfaces.

mikoal
July 4th, 2011, 12:21 PM
1/2) Is this to prevent the equipment from becoming a shock hazard? And trip the breaker if there were any faults. Or is there another reason?

3a)
i) What about having multi ground rods at like sub panels. Would this pose a problem (for both bonded and un bonded with the original (service ground point))?

4b)
I can't seem to picture what you are describing. You are saying if there was a power source with hot and neutral connected to a load.....then a battery. (not sure where to connect this), it would then create additional current and may over heat the conductors. You said theres no reason to go up other Tx......this may be the case for the lightniing or fault current on the neutral (which i would assume not only damage the wires but also damage the Tx you are at due to high currents rushing in, and going on the neutral conductor path to the Tx since its less R.
However if the lightning or fault lands on the grounding conductor, the wouldnt it travel up to other Tx's and other houses, thus damaging their Tx and equpment?

Does it work like that? You are saying that dirt is poor conductor but due to the area, it is considered as a good conductor since the earth is so large right? Then...air is a bad conductor, then using all of the atmosphere, would that make it a good conductor? Or Am i missing something here. Or is it because there are bits and pieces of conductible elements in the earth and they are being all used to lessen the R?
As for current flows, it travels through you because you are acting as a parallel path for the current is that correct? I was at first thinking of a single wire (you the human) sticking out of the ground, and no current would flow since no path. But the legs act like a wire connecting 2 points. So this would apply to birds on a power line too, so why wouldnt the birds get shocked by standing on the hot? Also they would get shocked "more" if they sat on Hot and Neutral because why?

"So if you're standing with a foot at point B and one at point E, the circuit resistance between your feet is 192 - 100 = 92 ohms. With 50A going through that earth, there is 4600V across your feet.

If you stand with a foot at point F and one at point G, the circuit resistance between your feet is 199 - 198 = 1 ohms. With 50A going through that earth, there is 50V across your feet. Shouldn't be a shock hazard unless you're wet.
"
Ok as for the ranges for it being dangerous......I read that it is the current that kills you not the voltage. So V=IR, and if the voltage is high enough, there will be a lot of current going through earht, so why does it matter where you stand if the current still is going to pass through you?
And also would the quoted item apply to birds on the transmission lines if the trans line were similar to the earht's Resistance (non linear , etc, etc)

5a) Earth's resistance is pretty interesting

5a ii). ok

5a IV / 6) "Because metal enclosures are grounded, that enclosure should have the lowest resistance path for a lightning strike. If the neutral is grounded, that is another good path, but the lightning has to pass through the metal box to reach that neutral. That has to be a higher resistance path than the metal box (it has to jump through the air gap from box to neutral lug). That is, except at lightning voltages many strange things can happen. What we think of as an insulator can become a conductor at those voltages (even air). So blanket statements when it comes to lightning can be dangerous. But an earth grounded metal box is a good protection barrier for things inside when it comes to a lightning strike."

Yes it is higher R to jump from box to N at that point, but picture the path of the electricity, it hits the box goes on the box, then on the wire that is bonded to the box (gronding conductor), flows all the way to the service point where the neutral is bonded, then shouldnt it go to the Tx rather then into the earht........or at least more to the Tx due to lower R than the path to earth? thus damaging the Tx and equipment?

7) "multi grounds are fine as long as it is ground to ground and not neutral-ground connections. It lowers the effective ground path resistance. The only time it can be an issue is when some appliance manufacturer is sending current on the grounding wire (which they aren't supposed to do) instead of the neutral. Ground currents can confuse the circuit leaking/bypassing through the bonding pathway. Some radio equipment does need to connect to the earth, so the grounding/bonding wire provides a pathway to get there. That can make it problematic to keep other circuit currents off the bonding/grounding pathway. Computer interfaces also used to connect their cable shields to the signal "ground". When these interfaces cables were long and connected to computers in different buildings (or even the same building but different transformers), then a lot of current could flow on those shield wires causing all kinds of problems (including going down the ground wire in the circuit potentially putting many amps on raceways). This has been eliminated with fiber-optic and unshielded balanced interfaces."

Back to my gnd loop question, having multi ground pts would mean different potential difference and causing current, and you said this isnt an issue for the NEC? No issues can arise from this?

suemarkp
July 4th, 2011, 01:41 PM
1/2) That is pretty much it -- prevent metal objects from becoming shock hazards. If you look at appliances that are mostly plastic, many times they won't have a grounding pin on their power cord. It doesn't do any good, so they don't use that type of plug. There are a few minor reasons to have this grounding wire connect to the earth at some point, but that is only for a few types of equipment that generate static electricity or pickup inductive or capacitively coupled voltages on its enclosures.

3a) For subpanels fed with a separate grounding conductor, having rods at them won't hurt. It doesn't really help much, but there's nothing wrong with doing it. When detached building subpanels were allowed to use a combination neutral/ground (like a Service does), that can cause neutral currents in the earth. This is really not any different than your neighbors house other than Services tend to have larger wires than small feeders to detached buildings. The smaller feeder may have a larger voltage drop across it thereby putting a bit more current into the earth than a house.

4b) I'm trying to show that current only seeks the other end of its source. Lightning originates in the sky and seeks the earth. There is no grounding conductor in the utility system -- only grounded and ungrounded. If lightning hits the grounded conductor, that conductor is connected to earth at many locations. The majority of the current is going to follow the wire to the nearest ground electrode. Less current will go further on the grounded wire to other electrodes further away. But the current is seeking the earth. The transformer winding doesn't get you there, only the grounded neutral. So lightning hitting the grounded conductor should not be traveling up the transformer winding. Same thing I was trying to indicate with my battery example. The battery current will only flow on the one wire common to the two systems, the other wires of the other circuits that don't make a circuit with the power source don't get any current from that power source.

If lightning hits one of the ungrounded conductors, there are usually lightning arrestors at each transformer. These are voltage activated short circuit devices. At a certain voltage, they will conduct between the ungrounded and grounded conductors shunting the current to earth. This trigger voltage has to be above the operating voltage though. You have the same type of device in a surge suppressor -- any spike over 200V will be shorted to the grounding and neutral conductors in a 120V suppressor.

"Poor" conductor is relative. If you make the earth out of styrofoam, it still won't conduct well no matter how large it is. Same with the air. Earth is an OK conductor, especially if damp with minerals in it. It is mostly just a surface area issue for the earth -- use a conductor with enough surface area (like a 4' x 4' metal plate) and you can get the earth resistance down to an ohm or 2. A ground rod is a poor connector to the earth.

Current travels through you for two reasons -- you're in parallel with some current path, and that parallel path has enough voltage across it to put a voltage across your feet. The amount of current that flows through you is proportional to that applied voltage. So if you are standing on a thick metal rail, the resistance between your feet is nearly zero. No matter how many amps travel through that rail, you don't get shocked because it just isn't imparting any resistance between your feet to cause a significant applied voltage. This is the same as a bird on a wire. Now change that metal rail to some material that has a resistance of 1 ohm per foot and your feet are 3 feet apart standing on that special rail. With a rail resistance of 1 ohm per foot, you'll have 3 ohms between your feet. The voltage imparted across your feet is proportional to the current in the rail (V = I*R). If there is 1 amp flowing in the rail, the voltage across that 3 ohm section of rail has to be 3*1 = 3V which is not enough to shock you. If there is 100 amps in that rail, there is 3*100 = 300V across that rail which will shock you (and the amps that flow through you will be well under 1 amp -- whatever ohms law says for 300V/resistance of your skin).

So it isn't the current flowing in the wire that kills you, it is the current flowing through YOU. That current depends on applied voltage. Somewhere between 50V and 300V is enough to flow sufficient amps to kill you -- depends on skin resistance, male -vs- female, plus not all people are the same.

That is what makes the earth dangerous. The voltage drop within a few feet of where the current enters the earth is high enough to create lethal voltage (and the voltage generated depends on how many amps are flowing through that resistance). Once you're past a certain distance, the current has spread out wide enough that little voltage is generated between two pieces of earth. That is why they put "equipotential grids" around swimming pool decks and inside farm buildings. This is a 1' x 1' metal mesh that keep the earth surface a relatively similar voltage level. A voltage applied to the earth (not close to the edge of the mesh) will distribute out the earth surface voltage of a large area and it won't be a problem.

5a IV) If lightning hits a metal box on your house, it seeks the earth. Most of the current will flow back to your Service on the grounding wire until it hits the Main Bonding Jumper. That connects your grounding conductor to the grounded conductor. So now it travels along the grounded conductor until it hits your ground electrode and most of the current goes there. Some will continue along the grounded conductor back to the utility pole where it will flow down to the electrode at that pole too (and further to other poles along the grounded conductor to their ground electrodes). It has no reason to seek the transformer -- that transformer is not part of the lightning source.

Because of the characteristics of lightning (it is typically a sharp voltage spike -- not a continuous voltage), a voltage spike a high frequency signal. These high frequency signals get attenuated in wires because of Skin Effect (solid -vs- stranded wires and the size of the stranding), length, and even sharp bends. So yes, that long utility neutral has a fairly low DC resistance, but its impedance (which is resistance to AC signals and higher frequency signals) is getting high to spike type voltages. So that first ground electrode will discharge most of the lightning strike.

7) Please define ground loop and multi ground points. Are you talking grounding to grounded connections, or multiple connections of the grounding conductor to itself or earth?

mikoal
July 4th, 2011, 03:36 PM
1/2) Ok

3a) Can you bond these grounds together? Would that make a difference?

4b) So current will only seek the lowest resistance (current goes all path, but majority goes to low R) when its going to the other end of its source. Meaning because lightning's path is suppose to be gruond, that it doesnt matter if the neutral path to the Tx is lower R (assuming the current is on the neutral), because it naturally wants to go to ground. So these lightning arrestors make a path to ground? What happens if the arrestor opened the circuit, what would happen then?

"Current travels through you for two reasons -- you're in parallel with some current path, and that parallel path has enough voltage across it to put a voltage across your feet. The amount of current that flows through you is proportional to that applied voltage. So if you are standing on a thick metal rail, the resistance between your feet is nearly zero. No matter how many amps travel through that rail, you don't get shocked because it just isn't imparting any resistance between your feet to cause a significant applied voltage. "

I did a quick parallel circuit calculation:
1) Somehow there is a 20V source being applied to the tracks
2) there is a 5 ohm load at the end
3) You are 5 ohm
4) Tracks are .01 ohms

So what I did was:
1) Parallel R calculation 1/5+1/0.1 = 1/ Rparalell, Rparallel= 0.00998
2) Total R= 5+0.00998
3) Itotal= 20/(5+0.00998) = 3.992032A
4) I2/I1=R1/R2. R1=5 ohms (you), R2=0.01 ohms (track)
5) I1=0.00797A (through you), I2=3.98A (through tracks)

So at such small voltage there is some current that goes through you. Also with human body's R ranging from a few thousand to few hundred thousand would play a factor too.
But I wonder, if .15A can kill you, how much V do you need?

R of dry skin = 495000 ohms

"This is the same as a bird on a wire. Now change that metal rail to some material that has a resistance of 1 ohm per foot and your feet are 3 feet apart standing on that special rail. With a rail resistance of 1 ohm per foot, you'll have 3 ohms between your feet. The voltage imparted across your feet is proportional to the current in the rail (V = I*R). If there is 1 amp flowing in the rail, the voltage across that 3 ohm section of rail has to be 3*1 = 3V which is not enough to shock you. If there is 100 amps in that rail, there is 3*100 = 300V across that rail which will shock you (and the amps that flow through you will be well under 1 amp -- whatever ohms law says for 300V/resistance of your skin)."

How do you define "enough to shock"?
I know .15A is dangerous to you heart, but I thought Volt didnt matter, like you can have thousands of V applied but if the I is 0.00000000001 youll be safe?

Actuially I just did a quick calc with 495000 ohms with 20v and 10000v on a rail way track with a 5 ohm load at the end. Both cases there isn't enough current to kill you.
10kV = .01 A, and 20V was like 10 to -7 Amps.


5a IV) So, current does have a direction and does "seek" a location? People always related current to water flowing in pipe. I'm thinking if a large dam of water were to get pushed into your pipe, the water would go to the path of least R, and not necessaily seek a certain destination.

"Because of the characteristics of lightning (it is typically a sharp voltage spike -- not a continuous voltage), a voltage spike a high frequency signal. These high frequency signals get attenuated in wires because of Skin Effect (solid -vs- stranded wires and the size of the stranding), length, and even sharp bends. So yes, that long utility neutral has a fairly low DC resistance, but its impedance (which is resistance to AC signals and higher frequency signals) is getting high to spike type voltages. So that first ground electrode will discharge most of the lightning strike."

So.....would that mean a normal 120/208V system finds the neutral to the source high R then? since its AC signal?

7)
Multiground points: multiple connections of the grounding conductor to itself or earth

Ground Loop: a current in a conductor connecting two points that are supposed to be at the same potential, often ground, but are actually at different potentials. (bonding grounding conductors)

suemarkp
July 4th, 2011, 04:15 PM
3a) All equipment grounds are to be bonded together, so I"m not sure if I know what you're asking. They meet at the bus where the main bonding jumper is -- that is where neutrals get separated from grounds. All neutral needs to connect back to the main service neutral bus. All grounding wires need to connect back to the main grounding bus.

4b) No, current seeks all paths. But the current in that path is proportional to the resistance/impedance in that path compared to the other paths. The transformer will never have a lower resistance path to earth. When lightning strikes a neutral, most of the current will follow that wire in the direction to the nearest ground rod. Could be your house, could be the utility pole. Some will definitely go to the utility neutral because that have so many grounded poles. But the further it goes the less and less current goes down those poles to the earth.

I think your rail circuit is different than what I described. I was putting both your feet on the same rail. If there are two rails, and you have one foot on each, you are in parallel with the load and have the full system voltage across your feet. The load makes no difference in this case nor does the overall current, just the 20V across the rails is what get applied to your feet.

I think you're way off on your values for skin resistance. Something more like 10,000 ohms is the most you'd want to use. See this article: http://en.wikipedia.org/wiki/Electric_shock

There is no one answer to what voltage will kill you. It depends on skin resistance and applied voltage. At high voltage, your resistance breaker down to a lower value. The ignition coil on a car can make 10000V. Plenty to overcome skin resistance. But that coil can't produce enough amps to kill you. If you had a coil with more primary current, or a different ratio so more current can come out of it yet still have enough voltage to allow lethal current to flow, then you can be killed. Or we just use 120V and pour salt water all over you.

5a IV) The water and dam analogy only goes so far. You should really think of it as a closed loop system (e.g. a pump with an intake and outlet with pipes going to the body of water). Water can leak out of a dam. Electricity doesn't necessarily leak out of its system. Tying one side of an electrical circuit to earth does create a lot more potential loss points though.

No, a 208/120V AC signal does not see it as a high impedance. That is a 60Hz signal. Skin effect get worse at higher frequencies (such as KHz, MHz). Lightning is equivalent to a pulse in the MHz region. It is the same reason that you don't wire a GPS or satellite system like a breaker panel -- the GPS and satellite system operate in the GHz range and there are many more issues to deal with at those high frequencies that are not a problem in a power panel.

7) Grounding the grounding conductor to earth or itself at multiple points doesn't hurt anything as far as the NEC is concerned. It isn't going to help much either unless that extra ground rod happens to be closer to the lightning than the main electrode. The equipment ground is not normally a current carrying conductor, so you can't have difference of potential (if I=0, then V=0 across any R). So the NEC doesn't care about "ground loops", and the more grounding interconnections you have the better the system will work and the quicker the breaker should trip (this is grounding wire to grounding wire, not grounding wire to electrode).

Ground loops are a problem with fussy equipment or equipment using grounded shields or unbalanced connections between two distant systems (generally on different power transformers).

mikoal
July 5th, 2011, 06:55 AM
3a) For some reason I was thinking you can have a main grounding bonded to the grounded at the service, then further down the lines have new ground rods that will ground/bond the metal raceways, enclosures and equipment etc. without having to connect/bond to the main ground at the service since i thought the faults will go into the gruond, but I just remembered that it does not provide a path to the source and will not trip breakers, so it will need to bond with the neutral, which would make it dangerous to have multiple gruonding to grounded points, therefore shuld be bonded back at the main service point, is that correct?

4b)

"No, current seeks all paths. But the current in that path is proportional to the resistance/impedance in that path compared to the other paths.The transformer will never have a lower resistance path to earth. When lightning strikes a neutral, most of the current will follow that wire in the direction to the nearest ground rod. Could be your house, could be the utility pole. Some will definitely go to the utility neutral because that have so many grounded poles. But the further it goes the less and less current goes down those poles to the earth."


I thought that is what I said with "So current will only seek the lowest resistance (current goes all path, but majority goes to low R) when its going to the other end of its source."

Perhaps I worded it wrong, but what I'm saying there is, current seeks all path, but the lower the R, the more current, given that it is going back to the source. Because what I thought is, if you were a person driving a current "car" and you hit the point where the grounded and grounding bonded together, you can see the path to the Tx being XX ohms and the path to earth being XX ohms. I thought that the current or majority of it will go to the lowest R..........I think we've decided that the path to the Tx is relatively less ohms than earth?

But i think one crucial part I've missed is that current does have a destination, because lightning and faults will travel different paths given that they are the same Voltage and was faulted on the grounding conductor (fault would seek the Tx as source's other end, Lightning would seek ground as the source's other end.).

So initially when I thought the path to neutral was less R i think that meant that the path back to the source (tx) is less by going on the neutrla to the source instead of going to the earth, up the rod, into someone else's Tx.

The rail circuit with the calcs is with the human standing on just one of the rails with a load at the end.... I placed a load there just because I'm used to seeing something powered, the load isnt in parallel with the human.
I thought the load would matter since I wanted to get total R, then get total I, then get the V drop across your feet, then calc the current through you and the rail. Itotal= I1 + I2. And I2= Ihuman+Irail at that point.
Hmmm sorry about the skin R, i found that on a random site.

In regards to the voltage question, so if I applied a 100kV voltage drop across you, with .000001A through you....what would happen? My question is asking what would happen in high voltage with insignificant current, how would that affect a human body.

5a IV) So for a lightning strike, would that mean a large large pump with a large water source is all of a sudden introduced to an existing closed loop system? That is How I see it. Then in that case why wouldn't it use the existing closed loop path ie: back to the Tx source instead of seeking ground?

So a quick sumamry, how does one normally wire a GPS/Satellite, or how is it different than a typical breaker panel?

7) If more grounds can potentially mean more efficient trips.....then why dont we apply this to most sysmtes? or is it because it would be insignificant most of times?

Yea I thought g-loops affected controls and computer circuits etc. Then that means in those cases, there are multiple grounds?

suemarkp
July 5th, 2011, 09:21 PM
3a) Yes. Only one connection between grounded and grounding. You can put as many rods on the grounding wire or ground electrode conductor as you want.

4b) first half - you got it.

Your rail circuit isn't very realistic because you're assuming the human only has a resistance of 5 ohms. If they had wet salty feet on that rail, then they could feel the shock. Not sure if .01 ohms is realistic resistance for 3' of rail. If your legs were hundreds of feet long, then .01 may be realistic.

Applying 100KV across you is enough voltage to lower your resistance substantially. Much current will flow if the source can supply it. But if you're saying the source can only supply .000001A (1 uA), then you won't even notice it or know you're being shocked. If the source could supply 100mA, then you'd most likely be dead.

5a IV) Because electricity isn't water. You need a precise water/electricity example to make that analogy work. Pipes leak water if poorly connected. A broken wire flows nothing, but there is potential (pressure) on the end of the wire just waiting to flow current it that wire meets with the opposite pole of the power supply. In the plumbing world, air has low resistance. In electrical, air is basically leak proofing applied around all of the pipes (wires).

In high frequency electronics, wires are kept short and straight or gradual bends. I'm no high frequency electronics designer, so I don't know all the details. Striplines and transmission lines (co-ax cables) are used to move high frequency signals longer distances.

7) The reason a house has one or two ground rods instead of 100 is cost. Certainly, 100 is better than 2, but 100 will cost much more to install. And it may not do much to improve things. The NEC says one ground rod if its resistance is 20 ohms or less. If above that (or you don't feel like measuring its resistance), you need 2 rods, and they don't care what the resultant resistance is. In the desert, you may never get below 100 ohms no matter how many rods you have. In rainy Seattle, one rod may be plenty.

If people really care about the ground electrode resistance, it will have to be listed in the specifications for the job. Things like TV, radio, and cell phone towers usually have significant ground electrode design and resistance requirements (and voltage surge suppression requirements). That is because the antenna cabling (lightning attractor) is required to be grounded and it also connects directly to expensive electronics.

In computer circuits, the major problem with interconnect cable ground loops is when each system is powered from a different transformer and especially when those transformers are in different buildings. The NEC requires each transformer to have a main bonding jumper (neutral to earth/ground connection). So there will be multiple ground points when different transformers are involved. If these transformers are in the same building and the building is made from structural steel, these transformers will be grounded to the steel. The resistance from one steel member to another should be fairly low, so the ground loop resistance between transformers is rather low. But if the building steel is separated, or the computers are in different buildings, then the earth is the only connection between the transformers until you plug in a grounded cable between these two grounded computer systems. That will cause a major ground loop.

mikoal
July 6th, 2011, 07:53 AM
3a) ok

4b)
Yes I agree with the Ohms not being too realistic. I kind of just picked a small number and a relatively larger one, and used 495000 ohms later for the human.
I haven't done the math but ratio wise 495000 to 0.01 is a factor of 10 less than human:10,000ohms and rail: 0.01/300 feet. Its still no realistic I guess.


I guess my example with 100kV and with 1uA, then the person must have a lot of R (some rubber protection equipment).

Ok here is another stupid mental block.....what exactly is the supply, current or voltage? I think you can have a current or voltage source....

I got a bit turned around because you mentioned "apply 100KV" then also "Source can only supply 1uA". Does this mean source has like .1 VA as power, and it got stepped up to 100KV, and is only able to supply 1uA? Assuming no losses. In that case less than 1uA would pass through the person since th rail in parallel is much less R.

What do you mean by Voltage breaking down or lower the R?

5a IV) OK

7) Would this type of ground loop (2 tx in 2 seperate buildings bonded), only affect sensitive electronics or would it affect power as well?
It affects sensitive electronics because of the introduction of current into what? The nuetral?
This shouldnt trigger breaker trips, but instead disturb analog signals or any sensitive signals is that correct?

Now the g-loops occur because of different potentials, how does these 2 ground points have different potential?

suemarkp
July 6th, 2011, 03:02 PM
Some power sources are inherently current limited. So yes, if your 100 KV source is a 1VA transformer, it will only be able to produce 10uA of current.

There are both current and voltage sources. Most things we see are voltage sources (e.g. a generator, a battery, the power utility system). Most equipment wants a supply voltage in a very small or somewhat small range (e.g. 110-125V, 90-250V, 220-240V).

"Voltage breaking down the R" means that the resistance can change based on the supply voltage. For skin, the wikipedia article I linked to earlier indicates that skin resistance drops to around 500 ohms when the supply voltage is around 450V or higher. This is called dielectric breakdown. The same thing happens with a lot of materials. That is why lightning can shoot through a window and wall where there were other perfectly good conductors near by. At very high voltages, many things become conductors that you thought were insulators.

7) Most power systems don't care about ground loops. Normally, equipment is only connected to one power transformer. When you have two computers, each separated by some distance, they most likely are fed from different transformers. If you connect those two computers together with a grounding wire, current will flow on that wire. Most things don't care about that, but data cables and interfaces probably don't like that.

I'm not sure why there is potential between two grounded transformer grounds, but there always seems to be. Perhaps it is related to the small voltage drop at each transformer in its grounded conductor and ground electrode system and the rather high resistance of the earth compared to a wire.

mikoal
July 7th, 2011, 06:38 AM
4b)
When will one use a current source where current is constant with varying voltages? Seems kind of impractical??

In that case, even if you have something of hi R, if the V is high enough it can lower the R to cause lethal/fatal current.


7) Why would these computers be tied together if they both have seperate sources?

If no Tx was there and we connect 2 rods XX miles away, will there be a pot difference?

suemarkp
July 7th, 2011, 02:15 PM
Current sources tend to be used inside an appliance. There are also bench top power supplies that can run in constant current mode. But obviously, they would have limits as to what max voltage the current source can supply. Likewise, voltage sources have a maximum current they can supply.

7) The computer interfaces are tied together, not the computer power systems. But some interfaces are grounded so then those two separate systems power system grounds get connected by the interface cable.

If you go stick to rods in the earth, you will measure a potential difference between those rods. Distance doesn't matter (can be a few feet or hundreds of miles). The exact voltage measured will depend on the current gradient flowing through the earth near those rods and the soil resistance.

mikoal
July 7th, 2011, 02:38 PM
So it acts like a type of control? Would this Current source be for more current sensitive items?

7) When you say cpu interface, what do you mean? Like data transmission cables? If so, would wireless avoid this?

Without power source there is still pot diff in 2 points?
If so......isnt that like free energy?

suemarkp
July 7th, 2011, 03:23 PM
I'm not sure when you'd want a current source. In electronics classes, many times a current source is drawn. This was always problematic for me since I had more practical knowledge and didn't know of current sources, only voltage sources. But some things want constant current, so you power them with a current source. I just don't have any examples.

Yes, data transmission cables. The classic one that caused problems was RS-232. This cable had a ground wire plug the connector shell was grounded. RS232 could go hundreds of feet. It was commonly used to put a remote terminal for a computer in office whereas the computer was in a cold data center.

USB, which has replaced RS232, I believe uses differential signals but also has a length limit of 15'. Ethernet cables can go up to 300', but they also use differential signals and have no ground. The only possible issue with ethernet cables would be if you used shielded cabling. The cable shield usually connects to the computer chassis, so it would be possible to get a ground loop between two computers using shielded ethernet cables (or a computer and a remote network switch).

It is not really free energy. There is current flowing in the earth, and the earth has resistance. Therefore, there will be a voltage generated at the ends of this energized "resistor". There is not much current you can take from this, but what you do get could be considered free. This current has already been paid for as line losses from a metered house or commercial building.

mikoal
July 8th, 2011, 07:14 AM
Thanks Mark!
I'm always interested in learning and wanted to find a mentor or someone to throw ideas off of. If you have the time and availability is it alright with you if I came to you for future questions?

suemarkp
July 8th, 2011, 08:13 AM
Sure. May just want to post a question and anyone can answer. This thread is getting long.