Iv'e recently been doing the electrics for a new bathroam have made sure everything is on rcbo's. The main problem i have is that the old electrics are in a very bad condition and the lighting circuit for the bathroom area was showing up a neutral to earth fault so after looking in the attic for the fault i found it to be a stray piece of cable and after testing again i find that the fault has cleared and my tester is reading >299 on an insulation resistance test, this is until however i screw in an edison screw bulb into the bathroom light at which point it goes down to 0.45 megaohms. Is there any reason for this as i really don't need the lighting circuit to keep going off.