Air Conditioning - AC Tripping Breaker in Chatham, NJ

Our technician arrived onsite for a no air conditioning issue. The customer mentioned the breaker for the 1st floor system kept tripping but was staying on now.  He checked the power coming into contactor and was not getting any. He removed the disconnect and cover panel and checked for the incoming power and did have it there. The tech found both fuses blown in the disconnect box. our tech replaced both fuses put cover plate back on inside contactor and when putting disconnect back in, saw a spark then found breaker had tripped.