testing of equipment ground circuit

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gws

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What is the ideal testing equipment and procedure to test the resistance of the equipment grounding circuit, from equipment back to the electrical service or to sub-panels located throughout an manufacturing plant. The plant is a damp location due to it's process being to cut and shape stone (a lot of wet saws). It is not practicle to run a piece of THHN conductor from the main electrical servcie to the equipment in order to have a loop. Therefore testing the ohms and reducing the total amount by the amount of ohms the testing piece of THHN added to the testing loop.

Has anyone been in this situation before?

Thanks
 

charlie b

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Sorry, but I do not understand the question. I generally don?t think of equipment grounding in the context of a ?circuit,? and I generally have no interest whatsoever in the resistance of the associated wires. From the main service panel to each downstream panel, you will have an Equipment Grounding Conductor. There is a process for selecting its size, not for selecting one that has a given resistance value. From any branch circuit panel to any piece of equipment, you have another EGC, and it is also sized by the same process. As long as you have the right sizes for all the EGCs, and as long as you have confidence that they are tightly screwed into their respective termination points, why would there be a need to measure any resistance values? :confused:
 

petersonra

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Location
Northern illinois
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engineer
I don't think you can test the EGC "circuit" with any degree of reliability. Most times jboxes and conduits are indirectly or directly connected to the floor and to the frame of the building, and to other equipment such as piping, making those things some part of the EGC "circuit", even though they can't be specifically used for that purpose.

Whatever measurement you made would include the parallel resistance of these things that cannot be considered the EGC or part of it.

I suspect the best you can do is a visual inspection.
 

gar

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Location
Ann Arbor, Michigan
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EE
100721-1130 EST

gws:

I do not understand your question either.

If your question ultimately is related to people getting shocks, then some form of GFCI might be important.

How could you measure the resistance of an EGC? If it is part of a single circuit from the main panel to the point where you want to make the measurement, and no other loads or connections exist to the EGC or neutral, then with no load measure the voltage between neutral and the EGC. I will assume the result is very close to 0.0 millivolts.

I am assuming you are talking fairly long runs, maybe 200 to 1000 ft. Suppose copper wire, size is #6, and we are at a 1000 ft destination, then the 20 deg C resistance is about 0.395 ohms. 10 amperes thru this is a voltage drop of 3.95 V.

Put your volt meter between neutral and EGC, and from a hot wire to EGC supply 10 A and you should read about 4 V. This is using the neutral as a long test lead. Because the neutral and EGC are very close together the 1 turn loop formed has little cross sectional area and therefore the voltage induced in this loop from the current flow is probably negligible.

Next you can switch the current to the neutral and measure the neutral resistance with the EGC being the long test lead.

Now suppose there is a 30 A breaker supplying this circuit. It will be worse if the breaker is larger. A 30 A breaker may take a very long time to trip at 30 A or may never trip. 30 A thru my above example would produce about 12 V on the EGC relative to the surrounding wet earth. Entirely possible this could be lethal to a person soaked in conductive water.

Not that you eliminate EGCs, but much better protection will be provided by the addition of sensitive GFCIs.

What happens to the equipment chassis potential if all hots, neutral, and EGC are the same wire size and material, and a dead short occurs between a hot and the EGC? The equipment chassis voltage relative to earth will rise to about 1/2 of the hot supply voltage until the breaker trips.

.
 

gws

Member
question reworded

question reworded

The question is to accurately test the integrity of the grounding conductor. You can not visually inspect the connections and assume there is nothing is wrong anywhere else. It is required to be tested and documented each year and must have a 25 ohm or less amount of resistance.
 

hurk27

Senior Member
Such testing is required on a yearly bases for MSHA installations. The results will include the parallel paths.
A loop impedance tester can be used for the testing.

Reading the article on loop detectors I saw this:
Picture a ground-fault loop as follows: A fault occurs, current travels through the grounding conductor back to the service, then down the ground connector to the ground rod or grid, and into the soil. Parallel paths then exist through the soil and the grounded neutral conductor back to the supply transformer;

Ok maybe at a voltage above 4160?
 

brian john

Senior Member
Location
Leesburg, VA
The question is to accurately test the integrity of the grounding conductor. You can not visually inspect the connections and assume there is nothing is wrong anywhere else. It is required to be tested and documented each year and must have a 25 ohm or less amount of resistance.

An Earth Ground resistance tester has a two point test which according to manufactures I have talked to this can be utilized for point to point readings, HIGHLY INACCURATE based on by field test.

You can also use a ductor/micro-ohm meter/low resistance ohm meter, but long leads are expensive and I have never seen leads longer than 30, feet each limiting you too 60 feet.

What type of plant is this?
Is this a metal fabricated facility, with EMT or rigid, metallic water piping, duct work? Damn near impossible for the readings to be above .5 ohms (and that is high) based on all the parallel paths, assuming you do not have IG circuits and all EGC's are properly installed.
 

mcclary's electrical

Senior Member
Location
VA
and note during the msha testing required in most rock quarrys, they only verify that there is a ground path. They do not verify, what path , or how many paths. The only verify it's there.
 

don_resqcapt19

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Location
Illinois
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retired electrician
I have done installations at some MSHA facilities but have never been involved in the required ground path testing so I am not sure exactly what they use.
I do know that they use a "ground check" conductor in a lot of their flexible cables for portable equipment. In some cases this conductor is connected to a ground check device that will open the circuit on the loss of the equipment grounding path.
 

dbuckley

Senior Member
What I think you are talking about is a "prospective short circuit current" tester, which is a mandatory tool for electricians in many juristictions, in order that you can prove that under short conditions a breaker will open in the permitted time.

They are quite cool - you plug it in to the receptical, press the button, and it tells you what the current would be under short circuit conditions.

Try calling Fluke and/or Amprobe.
 

gws

Member
The plant processes stone materials. It is one story old building and is always wet due to the wet saws. All the circuits are THHN in metal conduit. All the machinery tested is hard wired and almost all is 3 phase. When talking to Fluke and Amprobe in the past, I do not get much help. I have used their earth ground test unit that is a clamp-on style. The down side to this is it can, at times, give a false low reading due to it reading a loop. It induces a voltage on the conductor being test and then measures the current flow. If the grounding conductors goes back to the servcie and the service as proper grounding the test is accurate. You are getting a true measure of the resistance in the grounding conductor. If however, you have a ground loop at the equipment, the induced voltage may make a loop at the equipment and the high amp reading may not be due to a good ground circuit but rather a ground loop. You may have a low resistance reading yet not a low resistance grounding circuit.

If you try to use one of Fluke's larger ground testing equipment you most likely with be using the stake system for out door use.

Thus, my problem.
 
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