motor still ran???
motor still ran???
(OP)
We had a ground fault indicator light come on in one of our substaions recently.After troubleshooting we found out that a 480v 3phase motor was wired wrong,OOPS!One of the phase wires was terminated to ground at the motor and the EGC was hooked up to one of the t-leads of the motor.My question is how did the motor run without any tripping or problems.





RE: motor still ran???
System grounding?
Type and size of motor?
Motor protection?
Circuit protection?
What is your "ground fault indicator light"?
How loaded is the motor?
What should have tripped?
Define "problems"?
RE: motor still ran???
RE: motor still ran???
For example, take your motor overload relay and circuit breaker for instance. In some scenarios, a ground fault can take a high resistance/impedance pathway, meaning the current flowing to ground is not terribly high. If that's the case, and the total current flow to ground is at or below the thermal overload threshold of your OL relay, there is no reason for the relay to trip. And if it's too low for the OL relay to trip, it is definitely too low for the either the thermal or magnetic trips on the circuit breaker as well.
In another scenario, even if you had one lead of the motor branch circuit going to ground and you have a corner grounded delta system to start off with, and the phase that was mis-wired was the one that referenced to ground anyway, that might look only like a single phased system to the motor starter, not a ground fault. If your motor is relatively unloaded (i.e. under 58% load), a basic NEMA style motor starter would think that is just hunky-dory.
Your scenario makes it sound as though your company needs to hire a PE to do a system coordination study to ensure that any potential problem like this will be seen and reacted to by your protection equipment. That's what coordination studies are all about. If I were you and your bosses, I would not fool around and try to be cheap about this, there appear to be some serious deficiencies in your system as a whole, i.e. a fire just looking for an excuse to happen.
RE: motor still ran???
RE: motor still ran???
RE: motor still ran???
You make no mention of any kind of ground protective circuitry or device?
Do you have a ground conductor ran out to the motor?
Dan Bentler
RE: motor still ran???
RE: motor still ran???
RE: motor still ran???
I like ungrounded systems with ground indicators. This is just another good example of why.
davidbeach, this is an illustration of the theory and application of "ground detector lights". Study up and ask around utilities and airports and you'll find a lot more applications and fans like myself.
RE: motor still ran???