Earth Fault Tripping while Starting


Thread Starter


Hi everyone!!

Here in a process plant we have been dealing with a severe problem. A 315kW, 6.6kV fed compressor motor is very frequently Tripping on Earth Fault while starting by 7SJ600 O/C relay. We have checked everything we could do. We have changed CT's, relay everything. The settings are same for another same rating motor which is running continuously without any problem. Both of its settings match to every protection including dynamic settings.

If any one could come up with any suggestion Please!!!
> What have you done to check to see if there actually is an earth fault?

We have checked IR of the motor through feeder. It's greater than 3 Gigaohms. Even we have checked the Motor Terminal box for any abnormality but found nothing.

CT's connections and tightness were checked and found OK. All the control circuit was checked to find no abnormality.

Bob Peterson

Presumably this unit has been operational for some time and this fault is something new

Has something changed recently? Like somebody did some maintenance on the motor?

Since you have replaced all of the sensing equipment, it would appear that in all likelihood you actually have a fault.

When do you get this fault? Does it happen during starting? Or after a long period of running? You really are going to have to give more information or we are just guessing at what it might be?
Nothing was changed. It has been running normal for the past 15 years. And no PM was done within 2 months since it has started tripping. As I stated earlier this motor is tripping on E/F while starting itself.

Bob Peterson

Is this some kind of reduced voltage starter or across the line starter? It's possible that there's some kind of earth fault in the starter itself.
Since you replaced the ground fault instrumentation and you have a similar motor which is not tripping, it would appear that your ground fault instrumentation is measuring something that it thinks is a ground fault. These types of instruments measure the current with a single CT (like your home GFCI) or using three CT's and summing them together to measure the residual current and tripping if it exceed a certain value. It would appear that you are having a problem of some sort on one of the motor phases. I believe that the Siemens 7SJ600 has some software called DIGSI that allows you to monitor the fault waveforms when hooked up with a PC via a serial port. I would suggest reviewing that to see if that can provide some additional information to troubleshooting this problem. If you do not have the software, I would suggest that you hire a company that is experienced in troubleshooting and making measurements on MV motors and have them make measurements that can be recorded on the motor phases (voltage & current) during a motor startup. This could give you the location of the problem and its electrical characteristics which can lead to what is causing it.

William (Bill) L. Mostia, Jr. PE
ISA Fellow, SIS-TECH Fellow,
FS Eng. (TUV Rheinland)
SIS-TECH Solutions, LP

"No trees were killed to send this message, but a large number of electrons were terribly inconvenienced." Neil deGrasse Tyson

Any information is provided on a Caveat Emptor basis.
1. Is the motor a delta connected?
If it is star connected then is it neutral grounded?

One thing you must check, that is winding resistances and inductances. If it is star connected and neutral grounded then, unbalancing in three diff phase reactance may cause a residual earth current. In that case you need to measure it and rectify.

BTW, what is your earth fault setting?
May be a large inrush current are saturating a CT and, then, a Ground false signal is detected. Eventually you can exchange CT between motors and test again.

From the 7SJ600 relay in the operating annunciation, we have observed that dynamic settings are not getting on sometimes, and hence the motor is tripping while starting.