During transformer back charging 11000/433V , 2MVA ,6.75% , I powered the transformer through a 240A MCCB on the 433V windings by applying 433V from another strong source. I noticed that the first 2 or 3 attempts (charge-trip-reset-charge-trip -reset-charge took about 10s in total ) to back charge caused the MCCB to trip but after that was able to sustain. Can anybody explain why it is able to sustain after repeated attempts ? Has it got anything to do with flux build up? If its due to flux , how much time (roughly) does a transformer core require to be totally removed of flux after the voltage is removed ?