What is this single pole switch to empty ceiling box blows breaker when switched on
Something is wrong with the wiring to the ceiling fan or with the connections to the switch.

Possibly in between the last succesful test and the first unsuccessful use of that switch something has damaged the wiring (e.g. a nail into the wall? strain on the wires?)

I would isolate the circuit at the breaker box, test for voltage at the switch and then test the wires attached to the fan switch for unexpected continuity between live &amp; neutral, live &amp; earth. I'd then detach the switch from all wiring and test for continuity across it's contacts with the switch in each state.