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ben avatar image
ben asked

Breaker trip curve on Autotransfomer 100A

On my fairly new autotransformer 100A, I have experienced the breaker opening a couple of times this week under high load.

The input circuit breaker is being fed by a Quattro 15kVA connected to a Venus monitor. That monitor shows a continuous load of roughly 13kW prior to the trip.

I have additional motorized appliances with nominal loads of 1500W. My suspicion is that one of those motors is drawing additional locked rotor current, and the total is causing the autotransformer's input breaker to trip.

However, even using a conservative 6x for locked rotor current, I would expect only 38A more (1500W * 6A / 240V), and only for a few milliseconds.

I'm wondering if I should be looking at replacing this circuit breaker (perhaps it is not operating to specification), or if this is expected behavior, or even if we know what the trip curve is for this particular model.


Autotransformer
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Daniël Boekel (Victron Energy Staff) avatar image
Daniël Boekel (Victron Energy Staff) answered ·

Hi @ben,

Please check if the breaker is getting hot, check connections, etc to try and find the cause.

if you pull 60A continuously and add 'roughly' another 38A, you're getting close to the rating.

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ben avatar image ben ♦ commented ·

Indeed, 60A + 38A is very close. But the 38A would only be for a few milliseconds, and I would expect the breaker to handle that.

The breaker in the Autotransformer 100A is a Chana 100A MCB with a shunt release. Chana's standard offering in that line allows for a Type C or Type D trip curve, both of which would allow even 1.5x current (150A, or in this case 90A of transient) for upwards of ten seconds.

I suspect the breaker is operating out of specification, but I can't be sure. It may well be getting too hot. I'll come back here with more findings, the next time I'm able to recreate the problem.

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