We have a a CaC2 manufacturing plant with an electric arc furnace. Incoming is 22kV which steps down to 11kV by an out door 22/11kV transformer, then goes to an old OCB, then to the indoor furnace transformer and finally to the electrodes. The furnace trandformer supplies 60-80 volts at 3000-5000 amps. We frequently faces transformer breakdown, especially during rainy season. We couldn't really make out what is happening each time, but we thought of various reasons such as those mentioned in the topic subject. Consultation with a number of electrical engineers could not yield a definitive reason nor a reliable preventive measure. One last and final proposal is to install an outdoor VCB before the 22/11kV transformer. Ofcourse, it is going to cost a lot and hence this request for advise. We have lightning arreastors in the system. If anybody can write a few lines, it is highly appreciated.