Grounding a seperately derived system.

[wireman74]

Just had an engineer tell us that we could not run a ground conductor through a pvc conduit that was mounted with 3 Minerallac straps. Said it would choke the ground down. But he didn't say anything about where the conductor is running through the case of the transformer or the locknut holding the male adaptor. Anyone know why that would be a problem? I understand if its ran in a metal conduit it would have to have grounding bushings on both ends, but I have never heard of this being a problem.

 

[rambojoe]

is there a reason you are using a raceway? is it for protection?

 

[charlie b]

What "ground" are you talking about? If it is the Grounding Electrode Conductor, it will never be called upon to carry so much current that the risk of "choking" the current flow would become an issue. If it is an Equipment Grounding Conductor, it would carry a high current during a fault. If the material of the raceway interacts with the magnetic field created by the fault current, it could restrict the flow of fault current, and adversely impact the ability of the upstream breaker to detect the fault and trip. That is what "choking" means in this context. I believe that metal straps that support a PVC conduit could not have a significant impact on the fault current. But I have not done the math necessary to prove it. I suspect that your engineer has not done that math either.

Two questions:
1. Do the Minerallac straps go completely around the conduit (as opposed to just supporting it from below)?
2. Are the straps made of aluminum or some other metal that cannot be magnetized?

A "no" answer to #1 or a "yes" answer to #2 would be enough to call this a non-problem

 

[ActionDave]

Just had an engineer tell us that we could not run a ground conductor through a pvc conduit that was mounted with 3 Minerallac straps. Said it would choke the ground down. But he didn't say anything about where the conductor is running through the case of the transformer or the locknut holding the male adaptor. Anyone know why that would be a problem? I understand if its ran in a metal conduit it would have to have grounding bushings on both ends, but I have never heard of this being a problem.

I agree with you.

 

[kwired]

What "ground" are you talking about? If it is the Grounding Electrode Conductor, it will never be called upon to carry so much current that the risk of "choking" the current flow would become an issue. If it is an Equipment Grounding Conductor, it would carry a high current during a fault. If the material of the raceway interacts with the magnetic field created by the fault current, it could restrict the flow of fault current, and adversely impact the ability of the upstream breaker to detect the fault and trip. That is what "choking" means in this context. I believe that metal straps that support a PVC conduit could not have a significant impact on the fault current. But I have not done the math necessary to prove it. I suspect that your engineer has not done that math either.

Two questions:
1. Do the Minerallac straps go completely around the conduit (as opposed to just supporting it from below)?
2. Are the straps made of aluminum or some other metal that cannot be magnetized?

A "no" answer to #1 or a "yes" answer to #2 would be enough to call this a non-problem

My guess is he is talking about this or similar type of strap:



Non ferrous material would be a plus for this issue.

I do doubt that a couple straps are going to be that much of an impact for this situation however. And yes there is some of the same effect just passing through the wall of the transformer cabinet.

 

[wireman74]

It was the GEC from the transformer to ground. It was the above picture of the minerallac strap.

 

[charlie b]

Then the engineer is not thinking clearly (it happens to us all from time to time :roll: ). The GEC does not carry current, not even during a fault. So there is no concern over choking the current flow.

 

[electrofelon]

Then the engineer is not thinking clearly (it happens to us all from time to time :roll: ). The GEC does not carry current, not even during a fault. So there is no concern over choking the current flow.

Yet there is a code requirement about GECs being bonded on each end to metal raceways. Not sure the code writers agree with you.

 

[roger]

I have run into this and the simple solution (to keep from arguing) was to use nylon screws and not close the minerallac strap all the way.

Roger

 

[kwired]

Yet there is a code requirement about GECs being bonded on each end to metal raceways. Not sure the code writers agree with you.

GEC on a service might have a greater chance of carrying current compared to that of a SDS. Either case could see transient currents though current caused by a lightning surge might be the biggest threat of any significant "choking".

I have run into this and the simple solution (to keep from arguing) was to use nylon screws and not close the minerallac strap all the way.

Roger

What about stainless or other non ferrous screws even though the rest of the clamp is ferrous? I'd think it would be pretty effective.

My primary choice in this situation very well may be those non metallic "click straps" anyway.

 

[GoldDigger]

Then the engineer is not thinking clearly (it happens to us all from time to time :roll: ). The GEC does not carry current, not even during a fault. So there is no concern over choking the current flow.

Charlie, you are forgetting lightning induced currents in the GEC. These will have high frequency components that are particularly susceptible to choke effects. The EGC is, for whatever reason, presumed not to be carrying lightning associated currents.
If it were only 60Hz currents, even fault currents, the effect would be small.

Sent from my XT1585 using Tapatalk

 

[roger]

What about stainless or other non ferrous screws even though the rest of the clamp is ferrous? I'd think it would be pretty effective.

Sure except you might have to prove to the inspector that the screw is in fact stainless. The fact is a piece of string would work.

Roger

 

[kwired]

Sure except you might have to prove to the inspector that the screw is in fact stainless. The fact is a piece of string would work.

Roger

Take a magnet to the screw. If it doesn't stick it is not ferrous.

If inspector can't comprehend that, beat him with the magnet, he had no business rejecting it in the first place if he can't figure this out.

 

[david]

Just had an engineer tell us that we could not

Take a magnet to the screw. If it doesn't stick it is not ferrous.

If inspector can't comprehend that, beat him with the magnet, he had no business rejecting it in the first place if he can't figure this out.

Don't beat the inspector for something the engineer specifies

 

[kwired]

Don't beat the inspector for something the engineer specifies

Not beating him for that, beating him for his own ignorance. If he knows the reason to reject the clamp, then you tell him why the modification makes it no longer fit that reason, what else can you do other than call him bull headed.

 

[tom baker]

A good source for information on the impedance choke effect is the IEEE Green Book. It states failure to bond metallic conduit at each end will result in a 97% reduction in the current in a lighting event for the GEC. However the book (to the best of my recollection) or the NEC does not mention metallic straps.
Its not a code violation to use metal straps.

If the strap type is not called out in the specs or has language preventing metal, then the inspector has no basis for this.

 

[jaggedben]

A good source for information on the impedance choke effect is the IEEE Green Book. It states failure to bond metallic conduit at each end will result in a 97% reduction in the current in a lighting event for the GEC. ...

Is this a bad thing? Help me understand.

 

[kwired]

Is this a bad thing? Help me understand.

For a grounding electrode conductor - it may mean more current finds other paths instead of through the GEC.

Direct hit situations, it probably doesn't really matter though, you are going to have some damage.

 

[kwired]

A good source for information on the impedance choke effect is the IEEE Green Book. It states failure to bond metallic conduit at each end will result in a 97% reduction in the current in a lighting event for the GEC. However the book (to the best of my recollection) or the NEC does not mention metallic straps.
Its not a code violation to use metal straps.

If the strap type is not called out in the specs or has language preventing metal, then the inspector has no basis for this.

There will be different choke effect on several feet of metal conduit enclosing that conductor than there is on the occasional strap.

A basic one hole or two hole strap doesn't completely encircle the conductor like a saddle type clamp does either, but then if you used a non ferrous bolt on a saddle type clamp you still aren't completely encircling with ferrous metal as long as the clamp doesn't completely close when tightened.