Possible Soft Start Damage from 3-Phase Fault

[adamscb]

Forum,

We have an 800A feeder breaker in our plant that feeds two 200hp motors, each with a soft start. Each soft start has it's own upstream thermal-magnetic breaker. We tried starting one of the motors up one day and the feeder breaker tripped on Instantaneous, showing 4800 amps on all three phases (INST trip setpoint is 4800). From my understanding that indicates a three-phase fault. Technically if the coordination was correct, the thermal magnetic breaker for this motor should have tripped first, but I think the electronic trip unit on the feeder breaker is just too fast for the fault to trip the thermal mag. We looked at the wiring going to one of the motors and it looked pretty bad - scorch marks and such, so we think we found that culprit. We threw the thermal mag for that motor down and locked it out.

The really interesting thing - we wanted to start the other motor today, but when we immediately threw the thermal mag breaker up, it turned on the motor completely bypassing the soft start. We're pretty convinced that the SCRs are shorted in the soft start. My question is could the three-phase fault on the other branch cause damage to the soft start on this side? My thinking is it's all connected to the same feeder breaker, so would the other soft start see high amounts of fault current as well as the branch in which the three-phase fault occurred? Obviously if the breaker was down then no, but if it was up could it damaged the other soft start? Let me know what you're thoughts are on this.

 

[kwired]

Was the second motor running when the ground fault occurred? The inertia of the driven load may have contributed some energy into the fault.

Does the soft starter have bypass contacts that close after starting process is finished? If so maybe they are welded closed?

 

[Jraef]

...

The really interesting thing - we wanted to start the other motor today, but when we immediately threw the thermal mag breaker up, it turned on the motor completely bypassing the soft start. We're pretty convinced that the SCRs are shorted in the soft startv. My question is could the three-phase fault on the other branch cause damage to the soft start on this side? My thinking is it's all connected to the same feeder breaker, so would the other soft start see high amounts of fault current as well as the branch in which the three-phase fault occurred? Obviously if the breaker was down then no, but if it was up could it damaged the other soft start? Let me know what you're thoughts are on this.

In order of your issues:
1) your 400A T-M Breakers have an instantaneous Trip, many in that size range are adjustable. But the maximum range is always 10X the breaker rating, in this case 4,000A. So your 800A main set to 4800A SHOULDN'T have tripped first on Instantaneous even if the mag trips on the 400A breaker were cranked to max. You mention that your 800A breaker has an electronic trip, so likely LSI and it’s probably more likely that it tripped on your S setting, not I. Your S may be set too low or too short or both. If by chance it’s an LSIG, it may have tripped on GF.

2) Can a Fault in one circuit damage the SCRs in another? Yes. A high dV/dt (change in voltage over change in Rome) spike caused by an arcing fault can cause SCRs to “self commutate” meaning they turn on when not commanded to. Do that enough times and sooner or later they turn on at the wrong times compared to each other and that then can cause them to short on high dI/dt (change in current over change in time).

It’s also possible, depending on the make and model of the soft starter, that you have one of the cheap ones that have SCRs in only two phases, where the third pole is just a bus bar straight through. Those are particularly susceptible to failing in the way you describe because you ALREADY have one half of a complete circuit through one winding by virtue of that bus bar pole, so you only need to have one SCR short to complete the path. Post the make and model and I can tell you if that’s what you have.