# Msha ground continuity testing



## 2dogs (Feb 7, 2014)

Anybody have experience with doing ground testing for msha? We are running into a problem when we measure the circuit ohms. We start out with a 500' spool of wire and have .7 ohms of resistance, and when we measure to the motor housing will will wind up with -.6 ohms. We think it's because we are using fluke 87 meters and they don't have enough power to power the loop. I don't know if this is the right meter for this, or if there is something else out there that guys are using. It's not happening with every motor on the plants, and doesn't matter how many different meters you use, we will sometimes wind up with negative ohms.


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## brian john (Mar 11, 2007)

2dogs said:


> Anybody have experience with doing ground testing for msha? We are running into a problem when we measure the circuit ohms. We start out with a 500' spool of wire and have .7 ohms of resistance, and when we measure to the motor housing will will wind up with -.6 ohms. We think it's because we are using fluke 87 meters and they don't have enough power to power the loop. I don't know if this is the right meter for this, or if there is something else out there that guys are using. It's not happening with every motor on the plants, and doesn't matter how many different meters you use, we will sometimes wind up with negative ohms.


I have done some MSHA and it depends what test they are asking for a 3-point or it sounds like a 2-point but in either case you have the WRONG EQUIPMENT.


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## 2dogs (Feb 7, 2014)

brian john said:


> I have done some MSHA and it depends what test they are asking for a 3-point or it sounds like a 2-point but in either case you have the WRONG EQUIPMENT.


We are not doing the three point fall of potential testing for the ground bed, we set two ground plates and call it done. This is for checking the equipment ground path from the motor frame to the ground plate at the generator. Msha states that this reading has to be below 1 ohm. We are winding up with negative readings on the flukes, which makes no sense since we start off with around .7 ohms in the 500' 12awg test wire.


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## Lone Crapshooter (Nov 8, 2008)

In the past I have done a lot of ground grid testing and what the customer speced was a 50 amp test current between the disconnected (from equipment ) ground pigtail and ground rod. 
We used a small Multi-amp motor overload tester for the current source and a DMM to measure the output voltage. Then using Ohms Law we would calculate the impedance of the ground grid. We used #2 welding lead for the test leads. One lead was about 30' long and the other lead was at least 150' long.
Probably we should have had a parallel set of leads to measure the voltage at the test points rather than at the voltage source. 

LC


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## don_resqcapt19 (Jul 18, 2010)

MSHA is not looking to test anything associated with the connection to earth. They want to verify the integrity of the EGC from its origin to the equipment it is connected to. 
I am not sure why you are getting a negative number. Is the test wire laying close to any energized conductors that could induce voltage into the test wire?
That being said a DMM is not really the correct tool for this testing. You need a tester designed for testing low resistances. I am not sure how much current you really need to make this test valid. The insulation tester I use has a low ohm function(0-99 ohms) and it uses 200 mA for that test. A true low resistance tester will use a much higher current.


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## 2dogs (Feb 7, 2014)

don_resqcapt19 said:


> MSHA is not looking to test anything associated with the connection to earth. They want to verify the integrity of the EGC from its origin to the equipment it is connected to. I am not sure why you are getting a negative number. Is the test wire laying close to any energized conductors that could induce voltage into the test wire? That being said a DMM is not really the correct tool for this testing. You need a tester designed for testing low resistances. I am not sure how much current you really need to make this test valid. The insulation tester I use has a low ohm function(0-99 ohms) and it uses 200 mA for that test. A true low resistance tester will use a much higher current.


The plant is not running when they perform this test, so there isn't any induced voltage. I agree that a dmm isn't the right tool, and we might try a Simpson analog swing needle meter. There isn't enough power from the 9v battery in the dmm to power the loop. We use our megger for distances longer than 500' because of the 6 d cell batteries in it.


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## Big John (May 23, 2010)

I've measured continuity on literal miles of telephone wire with Fluke multimeters, so I don't think the 500 feet is the cause of the problem.

When we do equipment ground continuity testing, all we've ever used is a DMM. I agree a higher current source would provide a better test, but I'm not familiar with a standard that requires it.

Just out of curiosity, what happens when you measure AC and DC voltage across that same circuit? I think you have something coupling current into the wire and it's messing up the measurement circuitry.


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## gnuuser (Jan 13, 2013)

correct a dmm is not very accurate for this job
you need an insulation tester preferably an analog one preferably one with the reflective strip so you can get an accurate view reading)
or dependant upon low values requirements a vtvm (vacuum tube volt meter).

battery operated meters can show issues with accuracy as battery charge gets low or deteriorates:laughing:


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## Big John (May 23, 2010)

Our standards for these tests are that anything greater than 0.5Ω is suspect. Not saying it doesn't exist, I've never used a megger that had resolution accurate to within 500 milliohms.


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## BillyB692 (Apr 9, 2021)

I am way late to the party on this post, but I stumbled upon it and thought I would share just in case people are still looking for information. I have done lots of MSHA ground testing. Your DMM is fine for most cases. Typically there is a lot of induced voltage and all kinds of interference that will hamper the resistance measurement in a DMM, but if you're reading below 1 Ohm net then you're good. If you have a coil resistance of 1 ohm and your reading in the negatives, then your net is 0. If the ground is good, you will have a very low resistance, especially in a plant with conduit due to how many parallel paths the ground can take. Anything above .5-1Ohm i would double check with an analog resistance meter. We have had readings upwards of 10 Ohms on a DMM that will only be .5 Ohm on an analog meter.


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## paulengr (Oct 8, 2017)

MSHA uses a 4 ohm resistor to check the ground fault relay. But for readings most mines use the NEC suggested limit of 25 ohms. The IEEE substation standard says 1 ohm or less. MSHA does not have a requirement of their own.


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## SWDweller (Dec 9, 2020)

When I was working the open pit mine in NM we had books for all of the fixed electrical equipment. We would go out and test each machine once a month. (by building) While I get the concept of wanting everything to be grounded and verified. I did not understand people (electricians) digging deep into the metal because their reading was to high at that moment. The man from MSHA would look at the books occasionally. If it were all that important I would have thought there would have been some designation, test from here to here. Some of my co workers thought that their readings had to be lower than anyone else. I guess they felt that by lower readings they were doing a better job, Personally I thought accuracy was much more important.

The good news was each year we would go to a 4 day class for grounding. I once watched a electirican drive a ground rod and connect it to the electrical panel for the shed. Then he took his Fluke 87 and read ohms from the ground rod to the metal skin of the building. O.XX was his reading and he exclaimed Wow that is a great ground. I asked him if he went to the same grounding class I did. He said ya for the last 15 years he had attended. I was convinced that he did not know the difference between continuity and grounding/bonding.


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