Equipment ground theory

[alwayslearning]

From readings it seems like in the old days, electricians ran a hot from the "fuse box", out to the load and back via the neutral to the neutral bus. Then in the 70's(?) someone came up with the idea of running a third wire, the equipment ground to everything in a building.

My thought is that the purpose of this third wire is to provide a low resistance route back to the neutral bus so that if a hot wire comes in contact with a conductive structure (j box, switchbox, appliance, light fixture switch casing etc etc) instead of providing a place for a person to become electrocuted, the sudden flow of large current will trip the breaker. Perhaps there is fire hazard protection too?

[Ohm1]

Good speculation!


If you break the title down, it points to what you specified in your speculation (theory): Equipment--A general term including material, fittings, devices, appliances, luminaires, apparatus, and the like used as a part of, or in connection with, an electrical installation.

Then Grounding (Not grounded) conductor--A conductor used to connect equipment or the grounded circuit of a wiring system to a grounding electrode or eletrodes.

Good work practices are essential when making sure that a grounding path is not interrupted. If the grounding path has a problem (or defect), then there could be problem with the OCPD opening quickly, and even a chance of fire, or worst.

[alwayslearning]

Thanks Ohm, I am still having trouble imagining a scenario where a lack of ground wire results in a fire or fire possibility.
I guess a hot wire could contact some route of high resistance (and resistance = heat) in the case of a broken neutral??
AL

[Wgoodrich]

Below is a copied section of the NEC that may help. Always learning you are correct the intent of the equipment groudning system [not the grounding electrode system but equipment grounding system {there is a difference in intent}] is to direct any current that comes into contact with any noncurrent carrying metallic parts that people normally touche every day from becoming energized. The equipment grounding path creates a low resistant path back to the center tap of the transformer where the power came from also tripping the breaker due to interupting ratings at the same time.

COPIED SECTION 2002 NEC;

250.2 Definitions.
Effective Ground-Fault Current Path. An intentionally constructed, permanent, low-impedance electrically conductive path designed and intended to carry current under ground-fault conditions from the point of a ground fault on a wiring system to the electrical supply source.
Ground Fault. An unintentional, electrically conducting connection between an ungrounded conductor of an electrical circuit and the normally non–current-carrying conductors, metallic enclosures, metallic raceways, metallic equipment, or earth.
Ground-Fault Current Path. An electrically conductive path from the point of a ground fault on a wiring system through normally non–current-carrying conductors, equipment, or the earth to the electrical supply source.
FPN:Examples of ground-fault current paths could consist of any combination of equipment grounding conductors, metallic raceways, metallic cable sheaths, electrical equipment, and any other electrically conductive material such as metal water and gas piping, steel framing members, stucco mesh, metal ducting, reinforcing steel, shields of communications cables, and the earth itself.


Hope this helps

Wg

[alwayslearning]

Seems like the server hiccupped there Sat pm.
Anyway, I can understand how the Equipment Ground keeps us safe, in those systems where the bare, third wire in an ordinary circuit connects to the neutral -- the circuit can be completed and the breaker trips.

BUT how about those systems where the Equipment Ground is NOT bonded to the neutral.... Does current actually pass into the 10' copper Ground Rod and cause the breaker to trip?? Hard to believe.

Thanks!

[suemarkp]

What systems have an equipment ground that is NOT bonded to the service neutral? I can't think of any. The grounding electrode system can not conduct enough current through the earth at 120V or 240V to trip a breaker, nor is it intended to. The grounding electrode system is to dissipate lightning strikes or huge voltage surges that come down the powerline.

An equipment grounding conductor that doesn't at some point connect to the neutral provides no fault current protection. If you're thinking about subpanels where the neutral and ground are separated, remember that in the service where the feeder(s) to that subpanel originate, the neutral and grounding conductors are bonded together.

[alwayslearning]

That answers it Mark, thanks.
I was thinking of subpanels. And you might have missed my thread about my detached garage with a distribution panel... we were talking about where one goes to connect the neutral and ground bus bars. So I got to thinking about why one would want them disconnected anyway. I guess I still don't have an answer for that one. Why not connect them in all cases, even if the main panel has it covered?

[suemarkp]

The reason that ground and neutral are separated in subpanels is twofold. In case 1 (a subpanel in the same building) the ground and neutral current could evenly split between the neutral and pipes. This would happen if you had a water heater or pump connected to the subpanel. It could also happen with a gas furnace. Since the pipes are required to be bonded to the service and each branch circuit grounded to the device, you have a parallel neutral. Current in water pipes can cause accelerated corrosion of the pipes. If you break any of these pipes, you could get a spark as the current is disrupted. In the gas pipe, that would be bad.

In case 2 (separate structure) you most likely will have a voltage drop in the neutral so you'd have a parallel current path between the neutral and earth because the separate structure requires a grounding electrode. The current flow in the earth will be quite low, since the voltage drop in the neutral should be 5V or less and the earth resistance is high, but it is still there. Note that this case is still allowed, but is on its way out. Note also this this case is not allowed if there is a metal pipe or other conductor between the buildings, as this causes the same problem as case 1. This case 2 is also how each individual house is fed, so there will always be some current flow between your ground rod and your neighbor's ground rod. As distance between houses increases, the current decreases. It also happens between your house and the power transformer.

Separating the ground and neutral in case 2 helps keeps the earth at the same potential in all areas. This also keeps current from continuously flowing between your grounding electrodes which is a good thing.

[Wgoodrich]

The ground rod [aka grounding electrode] and the groudnign electrode conductor is only allowed to be installed in a structure where the power enters the structure. This grounding electrode does little to nothing in serving equipment grounding system inside the house.

The purpose of a grounding electrode [aka ground rod] is to divert a huge surge of electricity such as a lightening strike into the ground before it has a chance to enter the structure. This is as suemarkup is saying equal potential with earth. This is only desired at the point of entry of the power to a structure.

That equipment grounding system [green or bare] wire installed on the load side of the main service rated panel is intended to provide a low impedence electrical path where a short to noncurrent carrying metal inside the house will direct that current down the bare or green wire instead of out to that person touching that noncurrent carrying metallic part such as the metal case of an appliance. This dead fault current surge is produce from the serving transformer. Thus this dead fault return path of that bare or green equipment grounding wire directs that current directly to that main service rated panel remaining on this green or bare equipment grounding system [not the neutral while on the load side of the main service rated panel not even in sub panels.] to that point inside the main service rated panel where this structural equipment grounding system is then joined with the neutral bar inside that main service rated panel. Then that surge of current is carried by the service neutral conductor back to the center tap of the transformer where the power came from in the first place.

The ground rod while connected to the same thing when you are talking low power such as 240 volt will do little to complete a path. If you totally disconnected the neutral service conductor from the meter to your main service rated panel and tried to run your house off that ground rod 240 volts then you might at best get a few light bulbs to light dimmly. No motor would even run off that ground rod serving as a neutral path to earth.

Now when huge power such as lightening is striking that house this ground rod will then work to divert that huge surge of current to earth hopefully keeping it out of the house. If that lightening is huge enough it will even melt down that ground rod and melt down anything in the house it wants to, even to the point of pulling nails through your top plate out the sides of your studs and welding to the romex cable inside your walls, blow up TV screens like a bomb and much more. All depending on how much power that lightening strike is producing.

Lightening strikes many times you never even noticed and was obsorbed by your ground rod or other grounding electrodes never even having that power enter your house. These are common light duty lightenin strikes found in most thunderstorms. The bigger stikes the grounding electrode can still limit damage and possibly save your home. The biggest lightening strikes will do what ever they want even passing through the air using the particles in the air as a conductor. Hope you don't experience one of those biggy strikes.

HOpe this sheds some light

Wg

[alwayslearning]

Interesting indeed.
Hey we had a house burn here a few yrs ago...apparently lightning entered through the feed wires to the house. The owners unsuccessfully sued the power company for providing this conduit into the house for the lightning surge.