Ground Continuity Checks

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mull982

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We have recently been asked to perform ground continuity checks on all of the motors here at our plant. I know the correct way of performing these checks is to string out a spare wire from the MCC to the motor and attach the one end of the spare wire to the motor frame. You can then go back to the MCC and check for continuity between the other end of this spare wire, and the ground bus thus proving the continuity of the EGC to the motor as the return path.

Somone recently came up with the idea of rather than using this extra spare wire, to simply lift the existing EGC wire off of the motor and then check continuity between the end of the EGC and the motor frame. His idea was that you could prove the continuity of the EGC by using the building structure to pass the test current back to the MCC and then back through to the end of the EGC. This would obviously require that the motor and everything in the building was bonded.

Has anyone ever heard of this method or know if it is a correct method to use?
 
This will prove the mechanical path and may be desired as well. But your specifically proving/qualifying the EGC and your way works fine, you can even use a phase conductor.
 
tryinghard said:
This will prove the mechanical path and may be desired as well. But your specifically proving/qualifying the EGC and your way works fine, you can even use a phase conductor.
Click to expand...

So you are saying that the method of lifting the EGC wire to check for continuity is a good method?
 
mull982 said:
So you are saying that the method of lifting the EGC wire to check for continuity is a good method?
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The motor frame is connected to the grounding electrode system? If so I think if there was continuity, then all would be ok and this would be an acceptable test. If there was not continuity then is the break in the EGC or the GES somewhere, or in the connection from the motor frame to the GES?

I guess, most likely the break would be in the EGC or the connection from the motor frame to the GES.

I think it's an acceptable test.:)
 
What precisely are you looking to detect? What is the common failure mode of the EGC?

The test that you describe would only provide continuity between the EGC and the nearest point that it was bonded to the building steel, but would not prove a low impedance path back to the main bonding point of the electrical system.

-Jon
 
I agree with Winnie. Testing by using a spare wire connected to the motor frame will not ensure EGC continuity if the building frame provides a parallel path. The EGC is the most important path.
 
I was thinking about this some more. If you lift the EGC at _both_ ends and determine that you do _not_ have continuity, and then re-connect the EGC at one end and find that you've restored continuity, then you will have proven both the continuity of the EGC _and_ that your measurement of continuity goes all the way back to the other end.

If there are splices in the EGC which are bonded to the building steel, then the test would only confirm continuity to the splice.

Of course, EGCs are usually parallel with the building steel, and the building steel is usually the lower impedance path.

-Jon
 
winnie said:
What precisely are you looking to detect? What is the common failure mode of the EGC?

The test that you describe would only provide continuity between the EGC and the nearest point that it was bonded to the building steel, but would not prove a low impedance path back to the main bonding point of the electrical system.

-Jon
Click to expand...

Can you explain what you meant by this?
 
mull982 said:
We have recently been asked to perform ground continuity checks on all of the motors here at our plant. I know the correct way of performing these checks is to string out a spare wire from the MCC to the motor and attach the one end of the spare wire to the motor frame. You can then go back to the MCC and check for continuity between the other end of this spare wire, and the ground bus thus proving the continuity of the EGC to the motor as the return path.

Somone recently came up with the idea of rather than using this extra spare wire, to simply lift the existing EGC wire off of the motor and then check continuity between the end of the EGC and the motor frame. His idea was that you could prove the continuity of the EGC by using the building structure to pass the test current back to the MCC and then back through to the end of the EGC. This would obviously require that the motor and everything in the building was bonded.

Has anyone ever heard of this method or know if it is a correct method to use?
Click to expand...

If you are looking to see if your "grounding path" is good you can use the method that you described. If you want to know if your "EGC wire" is good you need to lift both ends of the EGC wire and connect one end to your spare wire. It would be easy to do both methods at the same time.
 
Say you have a building with conduit all over the place, with junction boxes bolted to building steel. Additionally you have a ground wire in the conduit. This ground wire is bonded to several junction boxes along the way to the load you are considering.

The test that you describe would not verify that the EGC is continuous all the way back to the main bonding jumper in the service. Instead it would verify that the EGC was continuous back to one of the boxes where it was bonded to the building steel.

Now if you could both verify that the EGC is insulated from building steel everywhere except at the ends, and you could verify the continuity via building steel with one end connected, that would give you _more_ information about that EGC.

-Jon
 
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