Choking of grounding electrode conductor (gec)

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d2568maaee

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SITUATION: I recently inspected a service ground istallation at a building. The installation was as follows:

2 - 3/4x10' rods @ 10' spacing with #2 bare solid GEC. Exposed section of GEC extends up to the service disconnect installed in 3/4" PVC. PVC clamped @ 2 places using metal pipe clamps.

I was told that the metal pipe clamps would choke the ground and is not good practice to use metal clamps with pvc conduit for this application. Can someone tell me if the previous statement is true and if so, provide me with some direction or corrective measure?:)

Sincerely,

Michael A. Armentor, P.E.
Illumination and Electrical Facilities Engineer
LaDOTD Bridge Design
1201 Capitol Access Road Rm 609G
Baton Rouge, LA 70802

"Life is a gift to you. The way you live your life is your gift to those who come after. Live it well, enjoy today, and do something fun. Be happy, thankful, and seek God!”:grin:
 
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roger

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Fl
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The "choke" would only happen if these metal clamps completely wrap around the conduit as in a mineralac type of strap. A typical open type of strap/clamp would not be a concern and if a mineralac strap were used a nylon screw with a non-metallic spacer could be used.

Roger
 

infinity

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Would two 1" pieces of metal around the conduit/condcutor really make any difference? I understand the choke effect but this seems to border on silly.
 

don_resqcapt19

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When a single conductor that carries AC current is run through ferrous metal that completely encircles the conductor the impedance is increased and the current flow is limited...choked. In extreme cases, there will be enough heat to melt the ferrous metal and or the conductor.
 

jeremysterling

Senior Member
Location
Austin, TX
Out of curiosity, does the 3/4" PVC terminal end have a ferrous lock-ring?:roll:

That is the question that is often posed on single conductor applications.

Another issue for the OP to deal with!:grin:
 

d2568maaee

Member
JeremySterling:

Good Catch. Thanks.


The contractor did install a 3/4 male adapter with pvc threaded locknut. They either knew what to doing or got lucky and installed the correct lock nut. I'll give them the benefit of the doubt and say they knew what they doing!!:roll: :)
 

JWCELECTRIC

Senior Member
Location
Massachusetts
SITUATION: I recently inspected a service ground istallation at a building. The installation was as follows:

2 - 3/4x10' rods @ 10' spacing with #2 bare solid GEC. Exposed section of GEC extends up to the service disconnect installed in 3/4" PVC. PVC clamped @ 2 places using metal pipe clamps.

I was told that the metal pipe clamps would choke the ground and is not good practice to use metal clamps with pvc conduit for this application. Can someone tell me if the previous statement is true and if so, provide me with some direction or corrective measure?:)

Sincerely,

Michael A. Armentor, P.E.
Illumination and Electrical Facilities Engineer
LaDOTD Bridge Design
1201 Capitol Access Road Rm 609G
Baton Rouge, LA 70802

"Life is a gift to you. The way you live your life is your gift to those who come after. Live it well, enjoy today, and do something fun. Be happy, thankful, and seek God!?:grin:

Hey Michael,

That's a Great signiture Quote. Thanks for that Gift!

-JWC
 

mark32

Senior Member
Location
Currently in NJ
JeremySterling:

Good Catch. Thanks.


The contractor did install a 3/4 male adapter with pvc threaded locknut. They either knew what to doing or got lucky and installed the correct lock nut. I'll give them the benefit of the doubt and say they knew what they doing!!:roll: :)


Is it against code to install a metal locknut on a pvc fitting?
 

roger

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Fl
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Retired Electrician
Is it against code to install a metal locknut on a pvc fitting?

No, but in this particular case a non metalic or non ferous metalic lock nut should be used.

Roger
 

Cold Fusion

Senior Member
Location
way north
No, but in this particular case a non metalic or non ferous metalic lock nut should be used.

Roger

I don't completely understand "choke effect". So I have a series of dumb questions.

Don said:
When a single conductor that carries AC current is run through ferrous metal that completely encircles the conductor the impedance is increased and the current flow is limited...choked. In extreme cases, there will be enough heat to melt the ferrous metal and or the conductor.

I got this part (maybe). The ferrous metal (conduit) encircling the conductor acts as a transformer core. So, ∑NI = 0 But there is no secondary. So, ∑NI ≠ 0
Or maybe it has to, so the current (eddy currents? circulating currents?) flows in the conduit and that is what heats up the conduit?

Next: So a locknut on a PVC conduit is enough to cause trouble. Then one might also think that the metal straps wouldn't be any different?

One more: So we don't put metal straps on, no conduit that isn't bonded at both ends, and no locknut on PVC. WE just have the grounding conductor passed through a hole in the ferrous metal box. Isn't the metal box essentially a ferrous ring around the grounding conductor? If so why doesn't it cause the same type of issues?

What brings this up is I recall an inspector telling an electric crew they could not use a cgb to route a grounding conductor to the isnde of a ferrous box. The electric crew had used a cgb because the area is subject to 40kt wind driven snow/sleet/rain and they wanted to seal the opening around the conductor. The grounding conductor was a green, insulated 1/0. He said it was a problem with "Choke effect".

I couldn't (can't) see the difference between the cgb and the steel box.

I obviously don't understand the physics.

cf
 

iwire

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Location
Massachusetts
Next: So a locknut on a PVC conduit is enough to cause trouble. Then one might also think that the metal straps wouldn't be any different?

I have seen metal straps on PVC cause a problem but it was not a GEC or EGC.

I know some folks that where remodeling an active TV station and it required running a temp 1600 amp feeder. Someone decided to run four PVC conduits on a strut rack and then made them up as isolated phase (A in one pipe, B in another pipe etc.)

The TV station called them back in when they kept smelling melting PVC, it was the strut straps melting into the PVC. That was a tough repair.
 

roger

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Location
Fl
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Retired Electrician
I couldn't (can't) see the difference between the cgb and the steel box.

I obviously don't understand the physics.

cf

CF, if a ferrous metallic fitting is in contact with the enclosure that the GEC is bonded to there would not be a "Choke" in that the box and the fitting are one, the choke would occur if the ferrous ring (ie locknut or encircling strap) were isolated from the enclosure or GEC.

Roger
 

Cold Fusion

Senior Member
Location
way north
No, but in this particular case a non metalic or non ferous metalic lock nut should be used.

Roger

CF, if a ferrous metallic fitting is in contact with the enclosure that the GEC is bonded to there would not be a "Choke" in that the box and the fitting are one, the choke would occur if the ferrous ring (ie locknut or encircling strap) were isolated from the enclosure or GEC.

Roger

Confusion mounting:
So if the locknut in post 445 were in metalic contact with the enclosure, it would not be a problem?

Take a look at the attached sketches. Here's what I have been told (Inspectors, code classes, some even from an MH grounding class)

Case 1: Nobody likes this one. Grounding conductor has high impedance.

Case 2: Everybody likes this one. RSC is in parallel with grounding conductor. Impedance is low.

I'm okay with these two, although I have never figured out the physics.

Case 3: This is from an inspector that later did happen to become a state AHJ. This is not a grounding conductor, but phase conductors. He wanted the top of the stub bonded to the cabletray. This was to keep the stub from increasing the impedance during a fault - "choke effect." he said. This one looks to be the same as a locknut on the PVC or a CGB.

:-?:-?

I'm listening

cf
 

roger

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Location
Fl
Occupation
Retired Electrician
Confusion mounting:
So if the locknut in post 445 were in metalic contact with the enclosure, it would not be a problem?

Take a look at the attached sketches. Here's what I have been told (Inspectors, code classes, some even from an MH grounding class)

Case 1: Nobody likes this one. Grounding conductor has high impedance.

Case 2: Everybody likes this one. RSC is in parallel with grounding conductor. Impedance is low.

I'm okay with these two, although I have never figured out the physics.

Case 3: This is from an inspector that later did happen to become a state AHJ. This is not a grounding conductor, but phase conductors. He wanted the top of the stub bonded to the cabletray. This was to keep the stub from increasing the impedance during a fault - "choke effect." he said. This one looks to be the same as a locknut on the PVC or a CGB.

:-?:-?

I'm listening

cf

In the first diagram #2 is correct, in the second diagram, with all due respect, the inspector turned AHJ is lost.

Roger
 

Cold Fusion

Senior Member
Location
way north
In the first diagram #2 is correct, in the second diagram, with all due respect, the inspector turned AHJ is lost. ...
Absolutely with due respect. He was a good guy.

I am pretty sure I understand the issue in Bob's post 14. The isolated phases would act the same if they were run into an enclosure with out opening the steel between the penetrations - inductive heating.

Tell me some more about:
"So if the locknut in post 445 were in metalic contact with the enclosure, it would not be a problem?"

And how that would relate to having a cgb as the exit fitting for a grounding conductor?

And how come penetrating the box with no fitting is not a problem?

cf
 

roger

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Fl
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Retired Electrician
Tell me some more about:
"So if the locknut in post 445 were in metalic contact with the enclosure, it would not be a problem?"
Since the GEC is going to bond to this enclosure a locknut that is in contact with this enclosure essentially becomes part of the enclosure

And how that would relate to having a cgb as the exit fitting for a grounding conductor?
Same as above.

And how come penetrating the box with no fitting is not a problem?
Same as above again.

The problem arises when a isolated ferrous ring is put around a current carrying conductor.

Roger
 
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