reliable low impedance fault current path

[L. W. Brittian]

I sincerely hope that one of you kind electrical experts can give me a bit of help. I sincerely would like to have a professional answer by questions, please!

As the equipment grounding conductor is necessary (according to the NEC) for safety, and as it must be reliable, and as it must have a low impedance path to facilitate the operation of the ocpd I am sure that you check it out very carefully.

As a professional I am sure that before you walk away from a job that you installed that you check to insure that the OCPD will operate. And that it is likely that you measure the Z of the equipment grounding conductor before your call for a cover up inspection.

Can you please .... tell me how you insure in a measurable-quantifiable (using some type of meter) way that the conductor is reliable, that it provides the necessary low z path and that the circuit breaker/fuse or over-current protective device will operate to clear a fault?

Also can you please tell me what the maximum Z value for each of the sizes of breakers listed in the NEC that you use to insure that the equipment grounding conductor will facilitate operation of the ocpd?

As the word "low" is relative, please reply using numbers. For example for a 20 amp/single pole/MCCB/120 V ac nominal circuit the maximum Z you allow is? As the code uses the word Impedance, I am sure that you do not use some DC resistance value. If so what is your authority of reference that supports this method?

I am confidant that all city electrical inspectors are doing a great job to insure the safety of the electrical systems be thy residential single or multi family, or commercial or industrial they inspect and that they require that the all important "safety" wire be tested to insure its compliance with the NEC requirements.

On those jobs inspected by the local city electrical inspector (or an AHJ), how many have you had the inspector reject the work because the Z was too high to facilitate the operation of the ocpd?

I am sure that as a true professional, you are concerned deeply about the operational safety of the electrical systems you install and would never walk away from a system not knowing if the ocpd will operate to clear a fault current condition.

Please excuse me for asking such a simple sophomoric question.

I know that the EGC is checked according to the BS Regs., I just do not know how it is done in the States.

Lw

 

[Aleman]

What I do is use the correctly sized conductor and make good quality connections. At this point I consider it to be good.
I do always run an EGC even though you can sometimes use the raceway itself. If I measure anything it is with a regular meter, nothing
fancy but good quality. After I pull a circuit, I will measure my voltages to ground and leg to leg etc. I don't test the OCPD, unless it's a GFCI.
As far as I know there is no need to in normal circumstances. Maybe in some types of facilities or applications it might be required
to become more retentive on this type of thing.

 

[junkhound]

And that it is likely that you measure the Z of the equipment grounding conductor -- first impression was ha ha hahh......

But, yes, on some critical military systems.

Never have seen impedance measured (e.g. complex impedance, RLC) on simple commercial work; as already stated, most of that is "correctly sized conductor and make good quality connections".

Impedance is very well controlled on military aircraft and commercial airliner cables, impedance is by design and routing controlled by drawings. Bond impedance (typically bolted with hydraulically crimped lugs) is checked to 3 milliohms or 6 milliohms depending on criticality of system, many different test equipment models to do this, typically at 10 A. Some very critical nuclear sites have 70 micro-ohm dc bonding impedances required and > 120 dB 50 ohms MS202 bond attenuation at 100 MHz checked and connections sealed with non-acetic acid RTV or other sealant.

On some military systems, even check cable routing with TDR and spectrum analyzer, have never seen that done on commercial work except some security systems.

note: the above system examples totally disregard NEC as it is inadequate for any except safety reasons.

 

[junkhound]

re:

can you please tell me what the maximum Z value for each of the sizes of breakers listed in the NEC

After re-reading the OP, wondered if the OP is a psychology major in cahoots with a EE student, turning out a term paper on seeing what inane web responses they can garner ?

FWIW, OP should already know that as an example Z for 30A ocpd; Z = (low line volts expressed as complex number)/(30+trip factor); say about 3 ohms. By the time you get that much 10 AWG, your voltage drop is the line voltage so 'ya gots lots bigger fish to fry' - aka "ya done went way over 3% voltage drop'.

BTW, thought that the British BS specs were superseded by EN (or whatever EU calls them now) sometime in the late 1990s?