motor grounding test

[kbclutter]

ANNUAL MOTOR TEST



We are required to do a motor electrical ground test annually.
Here is how we have been told to do it and was wondering if it was a standard.

Doing the test on each piece of equipment.
Meter type: Analog ? Digital (Analog is best)
Meter Setting: At the time of each motor test
Meter Reading: At the time of each motor test.
The length of the wire used for a lead.
The size of the wire used for a lead.
The resistance of the lead wire.
The voltage to ground reading to each motor.
The incoming voltage to each motor.
The fuse/breaker size to each motor.

Testing method of each piece of equipment.
Equipment frame to the source of power. (This could be at the MCC or it could be at the service panel.)

1. Find the voltage to ground reading.
2. Ohm reading from equipment frame to source.
3. The resistance of the lead wire.
4. Subtract line 3 from line 2. This is actual ohms.
5. Divide line 4 into line 1. This is the amps to ground.
6. Amps to ground should be 9 to 10 times greater than fuse/breaker size.

 

[rcwilson]

Motor Ground Tests??

Motor Ground Tests??

The procedure seems to be aimed at determining the ground fault currents at the motors to verify the fuses or circuit breakers will clear ground faults quickly. It seems like you are asked to measure voltage, measure DC resistance , adjust the reading for lead length and then calculate current.

Unusual test. IMHO it provides bogus data since it uses a questionable DC resistance measurement to calculate an AC fault current while ignoring any upstream system impedance and fault return path impedance.

Actual fault current will be less than the measured/calculated value due to the impedance of the total circuit: utility transformer, main service, MCC, motor leads and ground fault return path.

Also, measuring low values of ohms resistance (<5 ohms) with an analog meter will not provide accurate results. A wheatstone bridge or low resistance ohmeter (Ductor) is needed to get accurate resistance readings. But resistance measurements alone cannot be used to calculate AC ground fault currents. The AC impedance of the return path would have to be measured. While this could be done with a variac, precision VM & AM and long test leads, the results would still be questionable.

I suggest you ask what is the purpose of the test. If it is to verify fast clearing of faults, doing a visual inspection of the conduit and grounding connections would probably provide more benefit.

 

[kbclutter]

The procedure seems to be aimed at determining the ground fault currents at the motors to verify the fuses or circuit breakers will clear ground faults quickly. It seems like you are asked to measure voltage, measure DC resistance , adjust the reading for lead length and then calculate current.

Unusual test. IMHO it provides bogus data since it uses a questionable DC resistance measurement to calculate an AC fault current while ignoring any upstream system impedance and fault return path impedance.

Actual fault current will be less than the measured/calculated value due to the impedance of the total circuit: utility transformer, main service, MCC, motor leads and ground fault return path.

Also, measuring low values of ohms resistance (<5 ohms) with an analog meter will not provide accurate results. A wheatstone bridge or low resistance ohmeter (Ductor) is needed to get accurate resistance readings. But resistance measurements alone cannot be used to calculate AC ground fault currents. The AC impedance of the return path would have to be measured. While this could be done with a variac, precision VM & AM and long test leads, the results would still be questionable.

I suggest you ask what is the purpose of the test. If it is to verify fast clearing of faults, doing a visual inspection of the conduit and grounding connections would probably provide more benefit.

Bob, Thanks for responding! The objective is to make sure that employees will not come in contact with an entergized motor case or even conduit.
Unusual test. I would agree. We do get some bogus data. After doing the math we get negative numbers at times.
All I want to do is make sure that employees will not be able to come in contact with 267v if they touch the equipment. The breaker will trip before this can happen.
We are regulated under (MSHA) Mine Safety & Health Administration. They want to know if the equipment has a good ground. They say less then 1 ohm is required. Less then 1 ohm is ok on some things but the voltage and breaker size could make that to not be a good rule of thumb. They also made the comment. It should be 9 to 10 times higher the the breaker or fuse.
They also said to use a Megger to get this reading. Maybe I am wrong but I do not use a megger for that. I check for insulation problems.

So Bob, how do we know if the breaker will trip in a ground fault? How do we know that this will happen? Is their an exceptable standard or a system that is commonly used today by others?