A low IR test with a megger tester doesn't indicate a 'short' it indicates an insulation or earth leakage fault. Just incidentally if it's an RCD tripping fault why are you testing between the L+N instead of linking the L+N together and testing to earth?It's not a shared n. Been working for the past year they said. IR test 0mohms between L and N.
Could damp cause it to trip after 10 secs? Why dies it happen after that long. Surly with a short it would trip straight away?