Main Breaker tripping

[zog]

Don't have the details for the Micrologic 6.0A but this is (One of and most common) GF sensing schemes (Residual) used by Siemens in thier STIII trip unit.

Under normal system conditions, (without the presence of a ground fault) the vector sum
of the phase currents being monitored by the trip unit is zero. This is also true under the condition of an overcurrent phaseto- phase fault and phase unbalance condition. When a
phase-to-ground fault occurs, the vector sum of the phase currents is directly proportional to the magnitude of the ground fault. The trip unit?s microprocessor uses this vector
sum data in the execution of the ground fault protection function. The trip unit utilizes the internal breaker current sensors. No external current sensors are required.

 

[electrics]

yes it is true but, what if the loads are single loads using phase-phase voltages?

 

[crod4x4]

The breaker has shown a led light for the lsd which is the short time protection.

The system does have a neutral ct for the micrologic.
Voltages is 208/120 4wire

Have 4-8kw heaters
4-6kw heaters
2-4kw heaters

 

[zog]

yes it is true but, what if the loads are single loads using phase-phase voltages?

Then they cause a phase imbalance if not properly distributed across phases. That's the whole point, I don't know what else I can say to make it clear.

 

[zog]

The breaker has shown a led light for the lsd which is the short time protection.

The system does have a neutral ct for the micrologic.
Voltages is 208/120 4wire

Have 4-8kw heaters
4-6kw heaters
2-4kw heaters

OK then, the breaker is doing it's job. If you are getting a ST trip then you have something failing, maybe a bad heater that cycles on at night.

Have you tested the heaters?

I would ask if the breaker was tested but you said it was replaced.

 

[electrics]

i know it will cause an imbalance, but how does the relay will distinguish this imbalance from ground fault if loads are single and use phase-phase voltages?

 

[zog]

i know it will cause an imbalance, but how does the relay will distinguish this imbalance from ground fault if loads are single and use phase-phase voltages?

It won't, it will trip thinking there is a GF. I have seen this many times while investigating "Nuisance trips" at manufacturing plants where they had unbalanced single phase loads.

All moot point now that the OP posted the new info.

 

[jim dungar]

i know it will cause an imbalance, but how does the relay will distinguish this imbalance from ground fault if loads are single and use phase-phase voltages?

Single phase loads (L-N) should only be used on systems with neutral current sensors. Single phase loads (L-L) can be used on any breaker.

The GF, in the OP, uses a summation of the phase currents+the neutral current sensor. This breaker is not designed to treat phase imbalance as ground faults.