fanuc make ac servo spindle drive

I am having problem with my spindle drive repeatedly i/p fuses are being blown & transistor modules are also blowing I have checked the possibilities but plz suggest me about the possibilities which can also be checked.....

if insulation is taken into consideration what things should be kept in mind & how can it be rectified & one thing more please tell me what things should be kept in mind while replacing a transistor module if the drive is of fanuc make?