Testing motor grounding in a gravel pit.

[Uncle Bill]

What I'm wodering is how other people around the country do ground testing in gravel pits. What type of equipment / method do you use? More specifically, what type of meter do you use to measure between the reference ground and the motor frames?

Here's how I do it: first, I test the rod(s) at the incoming service using the 3 point fall of potential method. Once this rod has been checked and proves to be an acceptable ground, (it typically measures less than 1 Ohm) this ground rod then becomes my reference ground. Next, I use the same ground testing instrument wired for the "2 point bonding" test and proceed to check the resistance (bond) between all the motor frames back to the reference ground. To do this I take a long length of #12 and run it out from my test instrument (which is still connected on one side to the reference ground rod) and clip it to the motor frame being tested. Now I take my reading and then subtract my lead resistance to determine the true resistance back to the reference ground rod. After subtracting the lead resistance a well grounded / bonded motor will measure less than .1 Ohms.
That, anyway, is how I do the test.


I would be very interested to learn what other methods or equipment are used for this type of testing.

Thanks!
Uncle Bill (my first post, by the way...)

 

[ActionDave]

We have done it using a long piece of wire from the motor starters up to each motor and a megger to test the EGC. Then a clamp on ground tester on ground rods and record the results.

 

[Uncle Bill]

Thanks for your reply ActionDave.
I have a question: you?re using a megger to test the EGC. So you must have it set to read Ohms, not megohms, right? I just want to be clear on it. I?ve heard of using an older analog Simpson meter to test the grounding because it?s less likely to be effected by stray currents than a digital meter would be.
Anybody know if there is a meter on the market designed for simple 2 point testing? A meter that won?t be effected by stray current? Something small / handheld? (Not big and heavy like I'm used to...)

 

[ActionDave]

No we used megohms. It could have been at 500V. I don't remember.

Full disclosure the shop I work for is not any kind of special testing savvy outfit. There was a gravel pit guy that we had worked on his motors. He asked us if we would do some testing of his grounds. Conversation between him and my boss went something like-

What kind of testing?

I need somebody to test my grounds annually. How would you do that?

I dunno. Megger I suppose. Here's how one works.

Sounds good. Do that.

Next week me and another electrician are off to the gravel pit with a megger and a five hundred foot roll of MTW with alligator clips on each end to test EGC. Did it for three years and then the place sold.

 

[electric_cal]

We do a great deal of ground testing at sand and gravel operations. They are covered under the Mine Safety and Health Administration. MSHA.
We follow their guidelines in regards to test readings. Below 25 ohms for the main earth electrode, and 1 ohm or below on direct testing of motors and power equipment. We use an AEMC 6470-B Digital Ground Resistance Tester.

This tester also comes in handi when we are testing IT sites, Communication sites, and new construction for other contractors.

 

[Uncle Bill]

ActionDave, I appreciate your candor. Thanks.

Electric Cal, I'm gonna take a look at the AEMC unit you mentioned. There's also a meter out there by Extech Instruments.
I will often spend a day doing this sort of testing, and a lighter, faster unit would sure be nice.

 

[Greg1707]

unfamiliar with this topic

unfamiliar with this topic

Can someone please elaborate about this topic? I am not familiar with these issues. I assume you are testing the resistance to ground on the GEC? Is the about reducing the risk of lightning strikes?

 

[electric_cal]

Can someone please elaborate about this topic? I am not familiar with these issues. I assume you are testing the resistance to ground on the GEC? Is the about reducing the risk of lightning strikes?

Facilities and properties covered under MSHA are required to perform ground testing once every year. The testing is to verify that the required grounding is in place and that it tests within the safety guidelines published by MSHA. The testing is not in regards to possible lightning strikes, but to help prevent electrocution to employees.

The main earth electrodes must test under the 25 ohm rule. There must be a direct conductor from the electrodes to the various MCCs or electrical panels that tests at 1 ohm or below. The equipment and motors must also test at 1 ohm or below to their respective MCCs or electrical panels.

When a new piece of equipment or motor is added to the facility or plant, it must also be tested to verify that direct grounding exists. This also includes motors that are removed for repair and then placed back into service.

One issue that you will run into if you begin this type of work, is that if you work more then 5 full days per year at MSHA sites, you will need to become MSHA certified. This requires that you have 24 hours of safety training from an MSHA certified instructor at first, and then 8 hours of safety training each year after that to keep your certification.

All of the above can be found at the MSHA website. Good luck!