I worked in a project in an old and small hydro power station where we replaced existing MOCBs with VCBs. In the second day from installation there was fire in the VCB when the breaker tripped. Everybody guessed it was manufacturing fault and there was air leekage to the VCB. A spare VCB was used. But unfortunately it also caught fire. Later it was discovered that the excitation system of the 500KW generator was faulty; the field breaker wouldn't trip when the main breaker tripped. The generator is rated at 6.3 KV but is stepped up through a transformer to 11KV. The VCB is at 11KV at which synchronization is done. Can the blame be placed on faulty excitation for the fire in the breaker? I thought VCBs should be designed to withstand worst case condition of generator terminal voltage, which occurs when the generator is unloaded and maximum excitation current that the exciter can supply is supplied. The blame can't be put on the system side, because other two similar units on the same 11KV BUS were functioning normally. Thanks.