MSHA motor ground test question.

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S Harris

Member
Location
Atwood, IL
During the annual motor ground test at the local stone quarry we ran across 2 motors that have a high resistance on the EGC, reading of 2.44 ohms. if we leave our test lead on the EGC and lift it from the frame of the motor the resistance reading go down to .25 ohms, but as soon as you touch the EGC to the frame it goes up to 2.44 ohms. Does any one have an idea why this is happening. We only have 2 motors that this is happening to, last year the readings were fine. Thank you.
 

meternerd

Senior Member
Location
Athol, ID
Occupation
retired water & electric utility electrician, meter/relay tech
Without knowing your hookups, it's only a guess, but I'd say you're subject to the evil "high impedance meter" gremlin. Any number of things can affect the readings. Stray voltages, RF from some other area, induced voltage from adjacent wiring, capacitive coupling of parallel conductors....the list goes on. Get an analog meter and my bet is the weird readings will disappear. A DLRO (Digital Low Resistance Ohmmeter) is best for looking for accurate low resistance readings, but most folks don't have access to 'em. Uses a much higher test voltage/current than a standard 9V VOM.
 

PetrosA

Senior Member
If you're using the exact same setup as during last year's testing, then you might have a problem. As Don wrote, we need more information about how you hooked up and I'd also like to know what meter you used.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
A similar problem came up earlier in a different thread. Although the OP there never came back with a resolution the symptom there that strange readings stopped when the breaker to the feeder supplying the area of the (disconnected) equipment under test suggested strongly that there was current in the EGC and disconnecting the ground to test resistance put enough of a voltage across the ohmmeter that its readings were not correct.

The very first thing I would do when preparing to measure the ground resistance would be to check for voltage (and if voltage, also current with an ammeter in circuit or a jumper and a clamp type meter) across the two points where I am about to measure the resistance.
 

S Harris

Member
Location
Atwood, IL
Can you give us some details as to how you connect your meter for this testing?


Attached is an older diagram of how we perform the test. Once we disconnect the EGC from the motor frame and test the EGC the resistance goes down but when you touch the EGC back to the frame the resistance goes high.
Diagram.JPG
 

PetrosA

Senior Member
I'm not an expert in motors, but I'm going to agree with GoldDigger that these aren't just wonky readings and that maybe you have insulation breakdown on those two motors. I would check for voltage between the motor frame and EGC with the EGC disconnected from the frame as voltage leaking to ground could be throwing your meter off in resistance mode. If you do, then you have to figure out where it's coming from but that's a whole different set of tests :)
 
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