Circuit Breaker Selection

[Hawkas]

I have a situation where I have a sub-circuit control box protected by a 20A 3 pole circuit breaker. Within the control box are 1 x 10A 3 pole circuit breaker, and 1 x 6A single pole circuit breaker. The problem we are having is that the 20A circuit breaker trips at random times, however the smaller breakers in the control box do not. We have performed insulation tests on the supply cable and it passes all tests.
We started with a C curve breaker, and now have a D curve breaker but we still have the nuisance tripping.
My concern is that the 4mmsq 4 core and earth cable, only has a 2.5mmsq earth and I wondered if this was causing an earth loop impedance fault ?? the cable run is just under 100 metres.

Does anyone have an experience with this, or have any suggestions.

 

[iceworm]

First - a disclaimer - I'm an NEC guy. Leaves me a bit cold on mm^2 wire sizes. Still, the wire size is between #12 and #10, equipment bonding conductor is between #14 and #12. Circuit is 3ph, 20A, probably 400V, ~300 feet. Sounds okay toi me - unless there are startup voltage drop issues

My concern is that the 4mmsq 4 core and earth cable, only has a 2.5mmsq earth and I wondered if this was causing an earth loop impedance fault ?? the cable run is just under 100 metres.

What is an "earth loop impedance fault"? I know what "earth loop impedance" is. I even know how to measure it. But I have never heard of a CB trip element that measures the earth loop impedance and trips on, - what?, high impedance? I don't have any knowledge concerning this.

Does this 20A cb have a ground fault trip - probably called RCD maybe?

You have changed the 20A CB so it isn't likely the the issue is a bad CB. Changeing from a C curve to D curve changes the inrush portion a bit but doesn't change the long time portion of the trip. So when is this nuisance trip occuring? At startup? Random while running?

You did not mention that anyone has checked the inrush or steady-state current. Seems likely you have done this. Maybe consider a power analyzer to clearly record any inrush.

My buddy the EMT says, "If you hear hoofbeats, think horses - not zebras" If the CB is tripping, think over current. Only suggestion I have is - Measure it. However, if it has a sophisticated trip unit - then you tell us, what else does the trip unit measure and possibly trip on?

ice

Harmless flakes working together can unleash an avalanche of destruction

 

[waross]

Tell us about the loads?

Bill
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"Why not the best?"
Jimmy Carter