motor still ran???

[evan63]

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.

 

[davidbeach]

We're not mind readers, you haven't presented anywhere near enough information to even begin to formulate an answer.

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"?

 

[evan63]

The motor is a 2hp,480v,3ph,1700rpm.It's fed from 480v starter with overload protection at mcc so I thought the breaker at the mcc should have tripped due to a phase to ground but it didn't,the ground fault indicator light at the substation just came on letting us know there was a ground fault somewhere on that feed.The mcc is fed by an delta-wye transformer 4160pri.-480v sec.I'm not sure what type of system ground we are using, someone said something about it being an above grounded system.I'm going to try to get more information.The ground fault indicator light is on the transformer in the substation that feeds the mcc.I'm just trying to figure out what type of ground system we have so I can understand how the motor operated.

 

[jraef]

As davidbeach indicated, it's a lot more complicated than can be determined with what you are able to tell us, but to answer your question generally, ground faults can come in all sizes and flavors, as does ground fault protection equipment. If the protection equipment you have does not specifically look for the type of ground fault you had (or can have), it will not protect for it.

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.

 

[evan63]

Thank's for your help.I'm going to talk to the electrical supervisor and plant engineer and see if they can provide some information to prove the system has proper grounding protection.I'll post any information I get and see what you think.

 

[davidbeach]

My guess, based on "ground fault indicator light is on the transformer in the substation" and no other information is that the 480V system isn't grounded. In which case there is no protection that would open for a single phase to ground fault, the whole point of an ungrounded system. The light (one of three?) comes on and indicates that there is troubleshooting to do. Motor had all three phases, one of them just had to take a bit more round about path, why would it care? But that is all just purely speculation until all the previous questions are answered along with any necessary follow up questions.

 

[ssn596]

Do you have only the ground detector lamps?
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

 

[davidbeach]

I take the "EGC" as being an Equipment Ground Conductor. So, assuming ungrounded system, three phases and a ground conductor to the motor. Two phase connected properly. One phase connected to ground and then ground connected to the third phase. 2hp motor means low currents, and all wires the same size, either 14 (heaven forbid) or 12, or even 10 if long runs not to stingy. The motor wouldn't know any different, nor would the starter. Same would happen with a high resistance grounded system depending on what the ground fault indicator light really is.

 

[evan63]

Hey guy's thank's for your information and sorry if the information I gave wasn't enough.But I did find out today that we have a high resistance grounding system.So with only one of the phases grounded the ground fault indicator light on the transformer came on allowing us to locate fault and plan downtime to correct.Thank's for the help!

 

[HCBFlash]

So the simple answer to your fairly straightforward question is; It ran because it was not significantly overloaded, and was powered by an ungrounded 3-phase system. The ground fault protection scheme in use is high-impedance grounding with indication only.

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.

 

[davidbeach]

HCBFlash, I know a bit about "ground detector lights." What wasn't given, until evan63's 30 Aug 07 18:41 post was that it was a high resistance grounding system and a single indication. So I couldn't tell whether it was a set of three lights where one goes out and the others bright in the presence of a ground fault, a single light connected across a grounding resistor, or what it might be. I've seen far too many variations in "ground detector lights" over the years to have any clear picture of exactly what is being described when just those words are used. I'm glad you were able to figure it out so clearly a week after the OP finally told us what he had.