I've never seen this happen

peoplehouse

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Join Date
Jan 2009
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California
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I need some help understanding why this happened. We have an MCC that was blowing a fuse on the primary side of a transformer (480v.x120v.) The root cause was a mechanical bind on a conveyor, the OL device and fuse protection for this motor starter did not trip or open. Also, the circuit breaker for the 120v. control did not trip, instead one of the primary side (480 single phase)blew due to the OL device being destroyed. Replaced OL and the primary fuse blew again. I started checking for mechanical issues.
This was a bear to troubleshoot, it looked like a ground fault/dead short everywhere I put my meter (0.4 ohms). I isolated the transformer and pulled each 120v. control conductor one by one from the terminal block until the apparent short to ground changed from .4 to 135 ohms. then traced the problem to the OL on the motor starter.

Hopefully someone can explain this, Monday I will verify motor size and OL are correct.
Thank you for any insite to this
Jeff
 
peoplehouse said:
I need some help understanding why this happened. We have an MCC that was blowing a fuse on the primary side of a transformer (480v.x120v.) The root cause was a mechanical bind on a conveyor, the OL device and fuse protection for this motor starter did not trip or open. Also, the circuit breaker for the 120v. control did not trip, instead one of the primary side (480 single phase)blew due to the OL device being destroyed. Replaced OL and the primary fuse blew again. I started checking for mechanical issues.
This was a bear to troubleshoot, it looked like a ground fault/dead short everywhere I put my meter (0.4 ohms). I isolated the transformer and pulled each 120v. control conductor one by one from the terminal block until the apparent short to ground changed from .4 to 135 ohms. then traced the problem to the OL on the motor starter.

Hopefully someone can explain this, Monday I will verify motor size and OL are correct.
Thank you for any insite to this
Jeff
Click to expand...

Breakers are slow to trip compared to fuses. You would have to get the curves for the two devices and compare them. With a dead short, it is quite possible that the fuse would open before the Breaker tripped. Depending on the KVA and Impedance of the Transformer, and the size of the Fuse and Breaker.

Stu....
 
Thanks Stu, The weird thing is when I replaced the OL on the motor starter, the same thing happened when I re energized. When I isoloated the control conductors and manually pressed in the contactor, the 480 power side protecting the the motor starter fuses blew, motor windings and contactor checked good and thats when I looked for mechanical failure. I think at this point my question is how could such high current destroy motor starter overloads as soon as the contactor sucks in? Oh...this has been in service for 10 yrs.
Thanks again
Jeff
 
Are you blowing the fuses on your control power? If so you probably need to look at your contactor coil. It sounds like it might be bad.
 
Bad OL devices

Years ago I had a Heater short out to the ratchet body on the old GE contactor. 480 on the 120 controls will cause you some not so easy to find problems.
Try to explain that the heater in the OL failed and because they had an ungrounded secondary on the tranformer, they lost the control power. Not an easy task but the parts showed it was the cause.
 
I would guess that you have other problems than the 480 volt motor starter overload. Apparently the overload is being fed with more current than it can pass, so burns out. Look downstream of the overload at ALL conductors: starter wiring, starter terminals (make sure thaey are not shorted to ground), conductors in MCC wireways, motor power conductors, motor flexible conduit, motor peckerhead wiring, motor field windings.

Is your 480 volt power system fed by a Wye-Delta or Delta-Delta transformer? If your 480 volt system has no hard ground point, then you may have a ground fault somewhere else (which will not show up on a Delta system until the Second Fault happens - in your motor wiring).
 
does it "smell funny"? ...

sometimes the Bakelite (black plastic) that these things are made out of will get hot enough to "carbonize" and turn into CHARCOAL - which can conduct electricity and cause weird "shorting out" problems ...

once you get used to the "carbonized" smell it's unmistakable ... sort of a sharp pungent acidic smell - not so much like "burning" - but more like a very hot piece of plastic that's being "scorched" ... the plastic will sometimes take on a rough and "bubbled" appearance - and quite often you can scratch particles of it away from its surface with your fingernail ... it looks sort of "chalky" - but black in color ...

usual causes of this are an electrical potential across the plastic - and some type of moisture which conducts a trickle of current over a period of time ... condensation is a common culprit - especially where oil or grease is also present ...
 
If you have the overload completely isolated, there should be no conductivity to ground at all. If there is, it is defective.

You may be dealing with a grounded neutral on the 120V control power and, if the overload is not completely isolated, you may be reading that.
 
Thank you and good call outs. The 120v control is a parallel circuit that also feeds e-stops, lamps, P/B's etc. so the more I think about it some of low resistance readings may have been from one of those devices. Lancie to your point I think you nailed it, I need to go through the entire circuit for that motor, it's starting to sound like a loose connection causing excessive current.
Thanks again, everything is back up and running but I will post if I find the root cause.
 

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