pulling grounding conductors

Status
Not open for further replies.
Just for my information, if it was a parallel run of 3 conduits, there would actually be three 3/0s in parallel for the EGC. For the calculations that Don did, wouldn't the other two 3/0s contribute to the grounding path?

In other words, at 12000 amps, a 3/0 is good for 50 feet? But what about the other parallel 3/0s? Shouldn't they be considered to carry some of the fault?
 
don_resqcapt19 said:
Bob,
Doing a quick run with the GEMI software (from the steel tube institute) it shows that with a 12,000 amp fault and 600 kcmil circuit conductors a 3" EMT is a suitable EGC for about 325'
Click to expand...

Does this calculation take into account the building steel in parallel with the EMT? In Iwire's installation, there would be hundreds of parallel paths for ground fault current to get back to the source.
 
crossman said:
Just for my information, if it was a parallel run of 3 conduits, there would actually be three 3/0s in parallel for the EGC. For the calculations that Don did, wouldn't the other two 3/0s contribute to the grounding path?

In other words, at 12000 amps, a 3/0 is good for 50 feet? But what about the other parallel 3/0s? Shouldn't they be considered to carry some of the fault?
Click to expand...

Each one has to be full size as per 310.4/250.122. If a fault occured in 1 conduit the path to source is one 3/0. (in a PVC install for instance)
 
Crossman,
Does this calculation take into account the building steel in parallel with the EMT? In Iwire's installation, there would be hundreds of parallel paths for ground fault current to get back to the source.
Click to expand...
No it does not. With high fault currents very little current flows on those remote paths as a result of the inductive reactance. As you separate the circuit conductors and the fault return path the inductive reactance goes up. There is at least one case study showing a single properly sized EGC in a 4' wide non-conductive cable tray limited the current enough to put the breaker into the short time trip range. The was a result of about 30" of separation between the circuit conductors and the EGC. If the EGC would have been run with the circuit conductors, the current would have been in the instantaneous range. With remote paths like a building steel and no other path, you may been in the long time trip range or even in the normal operating range of the OCPD.
Don
 
Take a look at the following from this IAEIarticle.
It can easily be demonstrated by testing that only an internal bonding conductor can divert a sizable amount of fault current from a metal raceway (approximately a 50-50 division). None of the other possible paths (building steel or external bonding) will have any appreciable effect on the direction of current flow.
Click to expand...
 
Let's look at some instances:

1200 amp feeder to equipment, 3 parallel metal raceways as EGC

1. Ground fault can occur on the load side of the equipment

2. Ground fault can occur in the equipment

3. Ground fault can occur inside one of the feeder conduits

In the first two, I think we will all agree that all three conduits will serve as a fault path. After all, won't the ungrounded conductors in each conduit supply current to the fault?

Number 3 could be different as mentioned in the IAEI article. But assuming no wires burn in half, wouldn't there be conductors in each of the pipes contributing to the fault? Not as much because of the increaed length of wire to the equipment and then back to the fault, but still contributing? And the EGCs in those pipes would have the lower impedance just like the faulted pipe?

It can easily be demonstrated by testing that only an internal bonding conductor can divert a sizable amount of fault current from a metal raceway (approximately a 50-50 division). None of the other possible paths (building steel or external bonding) will have any appreciable effect on the direction of current flow.
Click to expand...

I heard something about this before, but never set down and thought about it. I am definitely interested. I'll do some research and can anyone post some more links to the actual studies? Any thoughts or theory to show me what is happening with the impedances of a grounding path that is widely seperated from the phase conductors?
 
Thinking:

The building steel and other multiple "outside" grounding paths will be sharing the fault current, so the overall current flow in any one of these paths is going to be fairly small. And therefore the magnetic field around any of these paths is going to be fairly small. And therefore, the inductive reactance of any of these paths is going to be relatively small.

So here is my uneducated guess as to what is happening. The reason there isn't enough fault current to trip the breaker isn't because of the isolated ground fault paths, it is due to the inductive reactance of the phase conductor inside the metal pipe. Without having the fault current in close proximity to the phase conductor, the current in the phase wire will cause a substantial magnetic field interacting with the pipe, which causes inductive reactance in the phase wire, reducing the fault current. Now, if the return path for the fault current were right next to the phase wire, the two magnetic fields would cancel, and the inductive reactance would be minimal, allowing a high current flow.

So again, it isn't that the multiple ground paths have a high reactance, it is the phase wire/conduit that has the high reactance.

Thoughts?
 
nakulak said:
-the whole fact that the dirt may or may not do it is part of why its dangerous to not have a separate utility grounding conductor
-the stray current due to poor utility neutrals causing current to flow through water mains and such is also another reason that the utility should have a grounding conductor, taking the current off all these unwanted things and putting it directly back to source
Click to expand...

I'm with ya 100%. The GEC installed at customer premises doesn't do much for the customer. It'd be great for the utility to do the bond and run an EGC.
 
I don't think that's all that outlandish, Peter. I think Ryan's even gearing up some proposals to make that a legal option in the next cycle.

As far as pulling a supplemental EGC goes, I look at it this way: A cable assembly is not assembled in 10' pieces in the field. It's continuous between boxes.

EMT is a different animal, it's far easier to compromise the integrity of the EGC between boxes, IMO.
 
georgestolz said:
I don't think that's all that outlandish, Peter. I think Ryan's even gearing up some proposals to make that a legal option in the next cycle.
Click to expand...

Maybe so, but a move away from the MGN would be a massive change for the utilities. It may fall under the NEC as an option, but don't poco's have the freedom to ignore it?

Just like the ever elusive AFCI that works as advertised, I'll believe this change when I see it.

I don't think a separate EGC from the poco x-former is a bad thing by any means, but I'm just highly skeptical that it will every become a reality in our US utility system that is entirely MGN.
 
George,
What does that have to do with the location of the main bonding jumper?
Click to expand...
It would elimnate many grounding electrode points that are now part of the utility MGN system...the ones at each service. That being said the NEC would have no control what so ever over the installation of an EGC from the utility to the service equipment. That is outside the scope of the NEC. You would have to get that change made in the NESC and the utilties to agree with it as the NESC is not a legally adopted code in many areas.
Don
 
maybe it won't happen in a million years.

then again, maybe once the word gets around, and the news starts covering every death from "stray voltage", the ambulance chasing lawyers will get involved.

after the lawsuits start amounting to something, that's when we will see what shakes out.

I give it about 20 yrs.
 
Fine....I didn't want to do this...but you all leave me no choice.

What's the real reason that we run EGC's in metal raceway systems like EMT?

Hold on to your seats: Because copper.org says that we should.

There, that settles it. No more debate and discussion. Just keep running those EGC's so you can make all those commodity traders happy.
 
Status
Not open for further replies.
Top