My main breakers keep shutting off when I put on central ac.it happened all of a sudden. I had ac service man come out to check unit,said unitvis running fine

Breaker could be Bad, may be loose wires on the breaker or burnt corroded buss that the breaker plugs onto.
Loose connections generate heat.

Along with the possibilities outlined by Strat, I would be interested to know what size main circuit breaker you have and what the power requirements for the AC are... what is the voltage and running load (amperes) of the AC?
(please do have the conductor connections checked ASAP)

Ooops... Duh, I see 100 amp main breaker... though what is the AC?