Choking of grounding electrode conductor (gec)

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Roger -
Thanks you have helped a lot.

I did a search of the NEC and IEEE 142 (Green Book). "choke effect" does not show up.

Also checked the green book for "grounding electrode conductor". Two items showed up - neither had anyting to do with this subject.

So all I have is 250.64.E, and NEC Handbook Exhibit 250.28.

If anyone has any references or hints of references, I'm interested.

Tom -
I'm interested in the lightning discharge impedance issues as well, but I'll likely research this one first.

cf
 
Well lightning is in the range of 100 khz some say but the sound we hear is a lower range by the air blast and its around the audio freq of no more than 20khz which we humans can hear .

I do lots of reading about it its kinda a hobbie of mind lightning one thing for sure when it hits a ground point it doesnt matter if its metal or pvc its gone !

Lock nut is not going to matter i can see the separated phases heating up a lock nut but not a grounding electrode being a problem thur a locknut at that low a freq unless the current flow in that ground is very high which i dont think is going to be a issue .

I never heard of a choke term used in this matter a choke to me is a coil of turns of wire use to filter or used as a reactance or reactor to ac in a power supply on the ac side of a dc rectifier or used to filter or help with a freq drive to keep the return ac smooth blocks the chopping up of the ac cycle by the drive keeps the input ac power ripple smooth .

Its not the freq of lightning its the power of it that would be heating up that locknut .
 
Here is some data pulled from Soares Book of Grounding, showing the approximate impedance of several sizes of GECs through conduit, both unbonded, and bonded at each end.

The impedance pretty much doubles if run through an unbonded ferrous conduit.
 
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For what is it worth..this is the same issue as covered in 300.20 and in the Canadian Electrical code, their rule on this issue does not apply to circuits that carry less than 200 amps.
 
Volta said:
Here is some data pulled from Soares Book of Grounding, showing the approximate impedance of several sizes of GECs through conduit, both unbonded, and bonded at each end.

The impedance pretty much doubles if run through an unbonded ferrous conduit.
Click to expand...


Maybe be it's just me but after looking at that chart it seems like a few metal straps would have no significant impact on the impedance.
 
infinity said:
Maybe be it's just me but after looking at that chart it seems like a few metal straps would have no significant impact on the impedance.
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Rob,
I don't think that chart is really talking about the AC impedance. It looks like they are just looking at the reduction in resistance that occurs when you parallel the raceway with the conductor.
 
don_resqcapt19 said:
Rob,
I don't think that chart is really talking about the AC impedance. It looks like they are just looking at the reduction in resistance that occurs when you parallel the raceway with the conductor.
Click to expand...

Don, you might be right. I was just looking at the raw numbers and forming an opinion. Maybe someone could explain how this would affect AC impedance.
 
Its my understanding that the choke efect is all about the rate of change of current, delta I over delta T, dI/dT.

The function of a choke is to reduce the rate of change time, this is why we use chokes with SCR dimmers to stop buzzing. They change the risetime of the chopped sinewave from "near instant" to a "few hundred microseconds" and thats enough to teach a singing lamp or humming transformer the tune.

A single wire going through a single locknut doesn't have much impedence, and because of this, the choke effect is almost meaningless with a 60Hz sine wave. But... but put a lighting strike in the system, and you have a very fast dI/dT. The choke (the locknut) will slow down the risetime, and so with this lighting strike you can end up with thousands of volts across half an inch of solid cable. The lighting arcs around the choke, and bad things happen.
 
don_resqcapt19 said:
Rob,
I don't think that chart is really talking about the AC impedance. It looks like they are just looking at the reduction in resistance that occurs when you parallel the raceway with the conductor.
Click to expand...

I mistyped the heading for the right-hand columns, it is not supposed to say "DC Impedance of Conductor", just Impedance of Conductor. I'll try to repair that.

I think they are talking about true impedance. Earlier in the book they say "... The last two columns of that table show how the impedance of the conductor is approximately doubled when the conduit is not properly installed as required in 250.64(E)."
 
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