25kVA Transformer Inrush Current

The transformer is a static device and is built to withstand its peak inrush current, so it shouldn't be a problem. The contactor rating should be checked to see if it is built for this type of duty.

Taking six attempts to get the breaker to stay closed is about right since the actual inrush current depends upon where in the cycle the breaker is closed. Closing it +/- 15 degrees from the peak will result in the lowest inrush current, that happens twice per cycle or 60/360 or 1/6th of the time so your experience is statistically correct. Maybe you can program your DCS to close the contactor during the window surrounding the peaks thereby eliminating the problem.

What is the primary voltage. You have a 16 amps breaker and 32 amp inrush. I would have thought the 16 would have been able to take that amount of current.

I agree with TonyS, you most likely have the wrong type of circuit breaker for the application.

if you are only reading 32A inrush on a transformer of that size, that just means you have a cheap meter that is way way too slow to be able to see the REQAL inrush current. As RamConsult said, the peak varies by when in the sine wave the transformer is energized, but the worst case scenario can result in 20X the normal primary current if there is any residual magnetism (rare, but possible). Usually the peaks are in the range of 10-15X. Your circuit breaker instantaneous (magnetic) trips must be able to hold that inrush for 2-3 cycles of power without tripping. That's what the C nd D curves do for you.

If you have a 16A Molded Case Circuit Breaker, it should have adjustable magnetic trips on it, you will need to adjust them up to where they hold in.

As TonyS told, type B is only for non linear load. For transformers, it should be type C. (For welding machines it should be type D). Please check the type of MCB, provided by the contractor. The actual peak inrush current may be much higher than the 32A especially in the initial part of a cycle. The inrush value within the initial few cycles, (say within 20 cycles) is approximately 250% of the 16 A. If the breaker do not have sufficient capacity to withstand this inrush, it definitely trips.

there is a correction: in the above quote, type B is for linear load( not for non linear).