LV Motor ground faults and time delays

[Dvhez]

Hello!

I'm trying to coordinate ground fault protections for several low voltage motors (2, 5 and 11 kW) that are protected by a C441 Motor Insight (Eaton). About the pickup current, I'm considering 3% of Full Load Current of the motor but for the instantaneous region I'm having trouble as the delay settings vary from 1-60 seconds and the upstream protection has an earth fault time delay of 300 ms (Incoming of the Motor Control Center - NGH800 Eaton), which is the maximum setting.

What I'm doing wrong? I'm considering both protections directly from ETAP's library and re-checked those delay ranges from both of their catalogues.

 

[dpc]

These are definite-time functions with a pickup setting and time delay setting. To coordinate, the downstream protection must be faster than the upstream device, so it should have a shorter time delay. The motor protection needs a much shorter time delay than the 1.0 sec shown. This device has nothing downstream of it that it needs to coordinate with - it can be set with no time delay or a very minimal time delay of a cycle or two if nuisance tripping occurs on motor starting.

 

[Dvhez]

Thank you dpc!. That's what I'm trying to achieve but by design, the minimum delay time that I can set on the C441 is 1 second. At first I thought that ETAP had it wrong but the catalogue and user manual also says that the time delay setting goes from 1 second to 60 seconds.

 

[LionelHutz]

Is the power source resistance grounded?

My opinion is that trying to detect a low current ground fault on a solidly grounded system is pointless because any insulation failure results in a phase to ground short.

I expect that specific C441 protection feature is for resistance grounded systems.

 

[Dvhez]

The upstream transformer that energizes this system is solidly grounded. Please can you clarify more why it would not make sense to detect low intensity ground faults?
Thanks!

 

[RRaghunath]

For any high current faults (be it phase or ground fault), the upstream MCCB or Fuses are expected to operate / isolate the motor feeder.
The ground fault protection in this relay is only for low current faults that may not require to be coordinated with the switchboard incomer protection. This also explains why Alarm-No-Trip option is provided with this function.
The time delay is a necessity as the ground fault protection is using phase CTs and may maloperate during motor starting.
I haven't used this relay but this is my understanding from what I find in the manual.

R Raghunath

 

[LionelHutz]

Please can you clarify more why it would not make sense to detect low intensity ground faults?

Because low intensity ground faults simply don't happen on solidly grounded systems. An insulation failure immediately results in a phase to ground short.

 

[dpc]

Interesting. If you really need to coordinate for a ground fault at the motor (or feeder), you will need to replace that relay or add a specific ground fault relay designed for use on a solidly-grounded system. Some type of flux summation CT and ground relay could be added.

 

[wcaseyharman]

I am assuming this is a typical MCC where the C441 overload protection is on the starter and short circuit protection is a set fuses or a a circuit breaker in the bucket.

In this case beware of setting a ground element on a solidly grounded system. The starter is likely not rated to interrupt full short circuit ground fault current, and if you set a fast ground element you’ll overduty the starter and it will fail under ground fault conditions. I wonder if Eaton doesn’t give you the option of an instantaneous ground to prevent this from happening.

I ran into this doing arc flash mitigation on 4160 MCCs. I almost overdutied those starters doing arc flash mitigation by speeding up the GE relays.

 

[electricpete]

> The upstream transformer that energizes this system is solidly grounded.

Just to double check, you're saying the transformer low side winding is grounded wye?