Choking a Ground Conductor

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eesonny

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I recently installed a ground loop in a foundation for a building with uninsulated 2/0 awg and when I coiled cable above the concrete level I used rigid steel conduit. I was told that the steel conduit would choke the ground and it was not good practice to use steel conduit. Can someone explain how the conduit chokes the ground?

Thanks,
Sonny
 
Re: Choking a Ground Conductor

I can not explain the electrical theory part of this but I can tell you the code.

250.64(E) Enclosures for Grounding Electrode Conductors. Metal enclosures for grounding electrode conductors shall be electrically continuous from the point of attachment to cabinets or equipment to the grounding electrode and shall be securely fastened to the ground clamp or fitting. Metal enclosures that are not physically continuous from cabinet or equipment to the grounding electrode shall be made electrically continuous by bonding each end to the grounding electrode conductor. Where a raceway is used as protection for a grounding electrode conductor, the installation shall comply with the requirements of the appropriate raceway article.
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It is easer to use PVC and eliminate the bonding issue. :)

In your installation the rigid steel conduit is required to be bonded to the grounding electrode conductor at each end of the conduit.


Bob
 
Re: Choking a Ground Conductor

I would also like to read a description of this choke event that does not choke me.

I always understood, from the old school, that the metal is bonded to prevent a flash over in the event of a transient overvoltage surge.

I can't buy this choke effect for many reasons, namely; current can not be induced in an open circuit. Without opposing magnetic fields there can not be a choke effect.

I would like someone to direct me to any approved scientific paper or publication that can substantiate this choke phenomenon.

[ April 21, 2004, 08:39 AM: Message edited by: bennie ]
 
Re: Choking a Ground Conductor

An additional reason for the metal conduit to be connected to the cabinet or equipment and to the ground clamp is for mechanical rigidity of the conduit. A floppy conduit will eventually cut the conductor.

This choke effect started in a handbook by someone who pretended to be an expert, by fabricating something that is an insult to logic.
 
Re: Choking a Ground Conductor

Bennie,

I agree, the choke explanation seems to be very far fetched.

However, the choke effect does exist. Ferrite chokes are very common in mitigating EMI caused by high frequency currents. Take a look at the cable between your PC and the CRT, that fat piece is a ferrite core. (For more information go to http://www.ferrishield.com/)
 
Re: Choking a Ground Conductor

Bennie some call it "girdling" while others call it "choking". You are not inducing a current into the conduit per se, rather adding inductance.

It is caused by the magnetic permability of of the iron in steel conduit greater than 1. It acts as an magnetic amplifier. In radio design we use ferrite beads around conductors to act as RF chokes and inductors, it is the same principle transformer use.

When I get home tonight, and if you are still interested, I can give you the IBN number of a book written that fully describes the principles and math. In a nut shell, if you compare a length of 750 KCM cable installed in a conduit, and one not in conduit, the voltage drop ratio is about 14:1 to 25:1 for a 10KVA @ 8 milli-sec rise time. I will give you some real numbers tonight.
 
Re: Choking a Ground Conductor

Hey Bennie your first post was great :) , but what is the purpose of going personal on your second post?


Originally posted by bennie:
This choke effect started in a handbook by someone who pretended to be an expert, by fabricating something that is an insult to logic.
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If you have some facts put em out here. :)

Just my opinion for what ever that's worth, Bob
 
Re: Choking a Ground Conductor

Sorry Bob: I get carried away.

I now have code books back to 1940. I am missing 1942,& 1943 editions. Not one book mentions choke effect for the ground conductor to earth.

I can see some effect from concentrating the magnetic field, but in a ten foot length?

Would a transient surge care?
 
Re: Choking a Ground Conductor

Originally posted by bennie: This choke effect started in a handbook by someone who pretended to be an expert, by fabricating something that is an insult to logic.
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I don?t think so. The effect is real, though I will be quick to admit that the effect is most likely to be insignificant.

If you bond the conduit to the EGC at both ends of the conduit, then the two will operate as parallel resistances. The net resistance of the combination will be lower than the resistance of the conductor or the EGC alone. That would tend to increase the current flowing through the EGC. In a fault condition, that would be a good thing.

If you do not bond the conduit to the EGC, then the two will form both a capacitor and an inductor. There is capacitance between any two metal objects not in contact with each other. There is inductance between a wire and the conduit that surrounds it, as there is between two adjacent wires. The impact of the capacitance and inductance can only be to increase the total effective impedance of the EGC ? it cannot decrease it. Therefore, in the event of a fault, less current will flow in the EGC.

The bottom line: If you bond, it helps the EGC clear a fault. If you don?t bond, it hinders the EGC in its attempt to clear a fault.

How much of an issue is this really? Well, probably not much. The amount of reactance added to the circuit depends on many physical factors. It would be hard to calculate for all possible combinations of diameters and lengths of conductors and conduits. The easiest way to avoid having to measure or calculate the impact is simply to forbid it. I equate that prohibition to the rule that says you can?t use electronic devices during take off or landing of a commercial aircraft. How many computers and CD players would it take, before a specific type of aircraft would experience any significant interference? Here?s the Airlines? answer: ?I don?t know, and I don?t want to spend the money to find out, so let?s just forbid it.? For my part, I find that approach reasonable.
 
Re: Choking a Ground Conductor

Charlie, I think the subject is GEC rather than EGC. With a EGC ran with circuit conductors you get magnetic cancelation, but not with a single GEC
 
Re: Choking a Ground Conductor

I tried Bob, but could not generate a current flow to imitate a lightning surge.

I understand a low side lightning surge is created by capacitance coupling from the high to low side of the end user transformer. The impedance, of the secondary winding, mitigates the intensity of the current.

A minute choke effect seems immaterial.

I told my wife " when I die and get to heaven I want to ask Thomas Edison a lot of questions" she said " maybe Thomas didn't make it to heaven" I then said " then you can ask him". :D ;)

Joke only, I sincerely care for my wife of 50 years.
 
Re: Choking a Ground Conductor

Everybody is posting fast, the posts will be out of context, but we have faced this dilemma before :eek:

I mentioned the 1940's because the words are the same. I am not sure ferrite cores were in use then. I built my first crystal radio set in 1940. I could receive one station KFI Los Angeles, California. It was a clear channel 50 thousand watt AM station.
 
Re: Choking a Ground Conductor

Originally posted by dereckbc: Charlie, I think the subject is GEC rather than EGC. With a EGC ran with circuit conductors you get magnetic cancellation, but not with a single GEC
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OK. Thanks Dereck. I see that you are right. I guess its time for me to join that EDA organization (Engineers Against Dyslexia). :D :D

However, my explanation of the physical situation still applies, although we may now be talking about lightning strikes. So I?ll change the bottom line to, ?If you bond, it helps the GEC carry the lightning strike to planet Earth. If you don?t bond, it hinders the GEC in its attempt to carry the lightning strike to planet Earth.?
 
Re: Choking a Ground Conductor

I went to a class on this and they showed a video or something about the effect. They were measureing quite a lot of amps on the conduitin the circuit, no lightning just running current through the electrode grounding conductor. I don't remember the numbers exactly but they were high under certian circumstances, has anyone heard of this test?
 
Re: Choking a Ground Conductor

I would like to see the test and read the specifics of the procedure.

I may be wrong, but an extremely high current would take out the transformer and the #6 cu ground wire. The conduit would be toast also.

The AC component of a high current, and voltage transient, would be choked by the transformer winding impedance. This, I can accept as fact.

The DC Component would not be affected by the inductor choke effect.
 
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