main two pole 32 amp breaker tripping problem
Normal breakers (not GFCI/RCBO or AFCI) trip because you are drawing too much power.

If it sometimes trips when you turn it on, and sometimes doesn't, that means either you were using more power elsewhere on that circuit during the times it tripped, or it is right at the edge of what that circuit can handle and it drew more power than usual or was on for a longer time.  Or the breaker is defective.

A clamp on ammeter can tell you how much power is being draw on a particular circuit.  If you had an electrician out to fix this, why did they check for lose connections and then stop looking?