Current on ground wire

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just the cowboy

just the cowboy

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newburgh,ny
Hey guys

I went to check a compressor that tripped a breaker last week, did normal checks megged motor all seemed ok.
Reset breaker and started compressor started no problems, checked current a=188 b= 190 c= 194 current around all three was .3 amps maybe. Looked good, I checked the current on the ground wire coming into the disconnect to the compressor and had 3.8 amps on the ground wire. This got me because I have never seen that much on the ground wire, checked both wires that were under the bonding lug one had 3 amps one had .8 amps showing me current was flowing on conduit also I guess.

Shut off compressor and current went away, I thought great motor problem. Fired up a second compressor and had same thing only more current on that ground. Fired up third compressor more current on ground. Now I know I don't have three bad compressors.

They are the only thing on this 1500a panel. We have two services feeding the building both are 13.2kv/480v wye/wye. The compressors are 100 hp. 150 hp. and 200 hp. I see 11-18 amps on what I think is the bonding jumper from neutral to ground in the switchgear and it don't have any 277 loads. This is a new building just built.

Can this much current just be induced I have never seen that much current on ground wire in 40 years.

Thoughts
 
Stray current from grounded conductor of control circuit? If 24 volt control circuit that 3.8 amps is only 91.2 VA.

If all current carrying conductors are in close proximity you shouldn't get much for induction effects.

Capacitive coupling? maybe a little more likely.

Is this direct line connected motors or VFD driven motors?
 
5.3A is the imbalance in the current between the three current readings.

Depending on the scale used to measure this could account for the unexpected current if the transformer is a wye.
 
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SG-1 said:
5.3A is the imbalance in the current between the three current readings.

Depending on the scale used to measure this could account for the unexpected current if the transformer is a wye.
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If no line to neutral loads there is no unbalance current to flow on a neutral conductor. For a three phase motor load with no connection to a wye point, if unequal impedance through each winding or unbalanced supply voltage then it has no choice but to balance itself across the three supply leads.

If someone did connect midpoint of wye connected motor windings to the EGC, then it would carry any line to neutral imbalance current.
 
kwired said:
If no line to neutral loads there is no unbalance current to flow on a neutral conductor. For a three phase motor load with no connection to a wye point, if unequal impedance through each winding or unbalanced supply voltage then it has no choice but to balance itself across the three supply leads.

If someone did connect midpoint of wye connected motor windings to the EGC, then it would carry any line to neutral imbalance current.
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That's what I am thinking, since all three are all behaving the same way. Someone may have decided they needed to be grounded. He did not say if this just started or was just noticed.
 
SG-1 said:
That's what I am thinking, since all three are all behaving the same way. Someone may have decided they needed to be grounded. He did not say if this just started or was just noticed.
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If they are VFD driven motors I would think if you grounded the wye point of the motors - the VFD would likely trip on ground fault condition. Never seen it happen or tried to see if it would, but from other experiences it don't take much fault current at all for the drive to trip on ground fault. Most those experiences were for under 20 HP though. A 100 + HP drive may not reject such fault current of only 3.8 amps like OP was measuring?
 
some answers

some answers

Only three phase power input was changed, no wires in compressor were changed.

The ground bus is connecter to building ground grid.

One is off of a VFD the other two are reduced voltage wye/delta.
 
Screw compressor

Screw compressor

It was suggested from another site that it might be the result of the screw compressors and moving oil. Waiting to hear back from the manufacture, I did talk to the service rep and he said when they see it they just drive a ground rod at the compressor.

I still don't like to see current on the ground wire, I'm old school ground wire is for faults only.

Thoughts

Cowboy
 
just the cowboy said:
It was suggested from another site that it might be the result of the screw compressors and moving oil. Waiting to hear back from the manufacture, I did talk to the service rep and he said when they see it they just drive a ground rod at the compressor.

I still don't like to see current on the ground wire, I'm old school ground wire is for faults only.

Thoughts

Cowboy
Click to expand...

I've seen and heard static caused from diesel fuel flowing in cheap dispenser hoses, but never put an amp meter on them or the EG. I would think it would be DC but as I said, never checked.
 
just the cowboy said:
It was suggested from another site that it might be the result of the screw compressors and moving oil. Waiting to hear back from the manufacture, I did talk to the service rep and he said when they see it they just drive a ground rod at the compressor.

I still don't like to see current on the ground wire, I'm old school ground wire is for faults only.

Thoughts

Cowboy
Click to expand...

I’ve been around screw compressors most of my professional life and have never heard of current induced by oil flow.
 
Thanks

Thanks

retirede said:
I’ve been around screw compressors most of my professional life and have never heard of current induced by oil flow.
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I have never seen that much current on a ground wire in forty years. It was a thought I needed to follow. Compressor tech said to just drive a ground rod next to the machine but to me this would just mask the problem.

Cowboy
 
just the cowboy said:
I have never seen that much current on a ground wire in forty years. It was a thought I needed to follow. Compressor tech said to just drive a ground rod next to the machine but to me this would just mask the problem.

Cowboy
Click to expand...

This current is static discharge? Ground rod near the unit doesn't eliminate the current - just provides an additional path for it to flow. If the EGC is lower resistance path a majority of the current still flows on it.
 
kwired said:
This current is static discharge? Ground rod near the unit doesn't eliminate the current - just provides an additional path for it to flow. If the EGC is lower resistance path a majority of the current still flows on it.
Click to expand...

I have no idea what this current is, was just grabbing at straws. If it is 11 amps of discharge WOW.

I just know I never saw this much current on a ground wire without a problem somewhere. I don't like it and won't give up till I find out why.
 
kwired said:
This current is static discharge? Ground rod near the unit doesn't eliminate the current - just provides an additional path for it to flow. If the EGC is lower resistance path a majority of the current still flows on it.
Click to expand...

I don’t think it’s static charge that’s causing this amount of current continuously.
And I agree - a local ground rod will not eliminate the current on the egc no matter what the cause.
 
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