Simpson Maxidry 452 shorting mains saftey switch?

Hi there i have a simpson maxidry 452, when running it wthout turning the heating switch on it seems to run fine but when i turn the heating switch to on it shorts the mains saftey switch in the fuse box within a few minutes. I have done an ohms test on the heater element and it seems to test ok, 33.3ohms (approx). I have fitted a new blower belt because the old one was broken, cleaned the dryer out completely from dust, etc and checked all the connections! is there something i am missing? thanks, Mark