More than two paths to ground

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dicklaxt

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We all know that grounding networks made up of rods,conductors and grids in a closed loop through out a facility proper with taps to different pieces of equipment is an effective way to ground equipment and it always has a path to ground if the main ground loop conductor is broken or severed in any way.The integrity of the system remains in tact.

This brings up a question involving the grounds of a service panel and subpanel.Lets assume we have a driven rod at the service panel/meter loop that is all tied to the main panel groundbus which in itself is bonded to the main panel neutral bus.We now extend the two hots(120/240), a neutral and a ground to a subpanel.The neutral bus at the sub panel is isolated from the ground bus at the sub panel. Lets only talk about connections to the ground bus from device ground terminals via the branch ckt ground wires at the sub panel.This ground bus as stated previously remains isolated from the neutral bus.Now going back to the opening paragraph and the effectiveness of a ground loop that provides more than one path to ground,,,,,,,,,can this same basic logic be applied here?In other words if a second ground wire were added to the subpanel ground bus, exited the subpanel by its lonesome and routed(direct buried) remote from ground wire in the sub panel feeder conduit all the way back to the ground rod at the main panel and used as a loop ground wire and used to ground additional equipment along the way, thus giving all device's two paths to ground.If this can be done, can additional rods be added to the remote ground wire to maintain low impedance on the ground system as the sub panel is 700 feet removed.

Done safely and all within Code parameters of course.

I think I covered it all,holler if something is amiss.

dick
 
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Dick, welcome to the forum! :)

If I understand your scenario, yes, additional EGC pathways are permitted, as long as the one sized for, and run with the feeder is maintained.

What's important is that you do not create a second, isolated EGC/electrode system that is not bonded to the building EGC/electrode system.
 
I'm not sure, but it sounds as though you thinking the ground rod / earth plays a part in clearing a fault, is this the gist of your post?

Roger
 
Welcome to the Forum.

The buried EGC will be a high impedance path because it is seperated from the ungrounded conductors, it needs to be routed with the ungrounded conductors to maintain a low impedance path.

The fault current is attempting to return to the transformer that is providing the fault current, not ground or the ground rods. Some small portion of current may travel through the earth, building steel, water pipes, or other conductive material to get back to the transformer. The majority of current will use the EGC routed with the ungrounded conductors.

If the single effective EGC is broken, insufficient fault current may flow to trip the upstream OCPD.

At 700 feet I expect the only way to acomplish what you want will be to use aluminum conduit ( first EGC ) & an insulated wire for the second EGC.

You might want to consider OCPDs with ground fault protection.
 
I was afraid that fault current was a stumbling block,,okay then what about this scenario, extend sub panel ground as described but do not close the loop back to the main panel ground.

dick
 
Let's clarify what we are trying to accomplish, are you trying to make sure there is a low impedance fault clearing path or are you looking for auxiliary grounding for say static discharge, equipotential, or some other reason?

I'm confused as to the reason for all the GE references

Roger
 
Roger,A positive two paths to ground system,I guess an easy way would be two additional rods tied together but independat of the system ground.

dick
 
dicklaxt said:
Roger,A positive two paths to ground system,I guess an easy way would be two additional rods tied together but independat of the system ground.

dick
Click to expand...

LarryFine said:
Dick, welcome to the forum! :)

If I understand your scenario, yes, additional EGC pathways are permitted, as long as the one sized for, and run with the feeder is maintained.

What's important is that you do not create a second, isolated EGC/electrode system that is not bonded to the building EGC/electrode system.
Click to expand...

We are still unsure what you want to accomplish.
Please read this: http://forums.mikeholt.com/showthread.php?t=95495
 
As described in the OP it is an extension of the system ground and runs in parallel with it but as a direct buried conductor and not in the conduit with the feeders and the other ground .The intent was two create two paths to ground but it has been stated this would end up being a high impedance so I guess the problem goes away and the system ground is or will be it.BTW ground fault mains at the main panel were always in the picture.

dick
 
I understand your intent now. If you have metallic conduit it will serve better than the buried loop conductor you are considering. It is always in close proximity with the ungrounded (phase) conductors & thus will have less impedance.

Steve
 
Many electricians today will run an extra equipment grounding conductor in the conduit (even though not required by the NEC) to provide a back up EGC in case the conduit is broken at a coupling or sort. I feel in a properly installed conduit system this should not occur in the first place, and in most cases the conduit will provide a much lower impedance then even a larger EGC can provide.
Extra ground rods do not provide any fault current protection and might even invite a lightning strike farther into the buildings electrical system to get to this and other ground rods placed away from the service.

Only proper design which includes proper installation will provide for a proper fault current path back to source.

Also you mention that the main service is GFP protected, keep in mind that if you have any SDS (transformers) a ground fault will not trip a GFP ahead of the SDS because the GFP will only see line to line current from a ground fault after a SDS, another GFP would have to be placed on the secondary side of the SDS for farther GFP protection.
 
Thanks guys I'm on board with you all now,I realize extra rods do not help fault clearing I was merely trying to introduce a safety ground by extending a system gound ,,thats "Bad News Bears" I have come to realize by you answers and further thought on my part and at the same time making a reduntant path.Thanks again for the eye opener.Sometimes the trees get in the way when you are trying to see the forest.

dick
 
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