question

ben avatar image

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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1 Answer
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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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.

The MCB continues to nuisance trip, now at a pace of between once and twice every day.

Each time, the loads are measured between 40 and 50A through the MCB. The ambient air temperature around the enclosure is stable at about 25C.

The last time it tripped, my thermal imager showed the side of the MCB to be at roughly 60C.

The fan does not engage on this unit, although I an contemplating overriding that to see if it can be cooled down and perform better.

What should I do next?