SETTING GROUND FAULT PROTECTION FOR MOTOR EQUIPMENT

engineerintraining12

Senior Member
Location
Saint Louis
Hope everyone is doing well. I had a question about setting the ground fault protection for a 500 hp motor that is being protected by a 1000 amp breaker at 480V. The breaker has LSIG setting capabilities and I wanted to coordinate the ground fault setting on this 1000 amp breaker with the 4000 amp main breaker that also has ground fault protection. I just wanted to know, should I be careful not to set the ground fault too low on the 1000 amp breaker that feeds the 500 hp motor? I want to coordinate it with the main, but am afraid that I will be dealing with nuisance tripping if I set the ground fault protection too low.
 

topgone

Senior Member
If you expect a ground fault to occur on the motor side, then it is wise to set your GF setting to be very low at the motor end, much lower than those GF setting upstream of your motor protection!
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
My understanding of this kind of protection is kind of limited but as I see it you are looking for a ground fault current that's not enough to trip the breaker, so I am guessing you should set it somewhere along the lines of less than what the rating of the breaker is.

If for instance you said the ground fall protection on the motor breaker at 500 amps, it is almost certain that it will trip before an upstream breaker would trip.
 

engineerintraining12

Senior Member
Location
Saint Louis
This breaker has 3 ground fault pickup settings. 480 amps, 720 amps, and 1200 amps. (Technically the sensor is rated for 1200 amps, but I have set the long time to 1000 amps.) I put my ground fault pickup setting for this breaker at 720 amps to coordinate with the 4000 amp main that has a 1200 amp setting for its ground fault pickup.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
I don't work with motors of that size, but absent a VFD I wouldn't expect much ground current from the motor without an actual fault.

With a VFD you can see large capacitive coupling current spikes at each pwm transition.

So on a system with two 480V VFD fed motors of about 50hp each, we needed to set our residual current relay system above 5A to avoid nuisance tripping.

I seriously doubt you would have problems at the 480A setting.

Jon
 

ron

Senior Member
It is often a rule of thumb, but if you don't know any better and don't have another breaker to coordinate with downstream, ~20% of the handle rating is a fair GFP setting, so the next setting near 200A you have available, 480A is fine.
 

Xptpcrewx

Senior Member
Location
Las Vegas, Nevada, USA
Occupation
Power System Engineer
Hope everyone is doing well. I had a question about setting the ground fault protection for a 500 hp motor that is being protected by a 1000 amp breaker at 480V. The breaker has LSIG setting capabilities and I wanted to coordinate the ground fault setting on this 1000 amp breaker with the 4000 amp main breaker that also has ground fault protection. I just wanted to know, should I be careful not to set the ground fault too low on the 1000 amp breaker that feeds the 500 hp motor? I want to coordinate it with the main, but am afraid that I will be dealing with nuisance tripping if I set the ground fault protection too low.
As you are probably aware, there are two components to coordination - pickup and time delay. You want to set these with reference to the main breaker and motor starting characteristic. Obviously more information is needed based on the TCCs involved but there are no rules of thumb that you could apply since coordination is evaluated as a whole for both main and motor feeder. If you want to PM me I could probably model it for you.


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