Apologies I know theres been a few threads about this and IR tests, but I'm still somewhat confused. The way I understood how an RCD works is it measures the current flowing through the Line and the current flowing through the neutral on the return. If there is an imbalance of more than 0.03 amps (30mA RCD) due to a fault to earth then the mechanism recognises this and trips the RCD. Some of the other threads say that as long as the IR test is over the 2Mega ohm (regs limit) then when you fit an rcd it will be ok because thats way over the limit before an RCD trips but I just wanted to clarify that if the RCD works on a difference of currents then surely you could have over 99/299 Mega ohm on L-E and N-E and that still wouldn't guarantee fitting an RCD wouldn't trip the circuit because it would only take around 8000ohms (0.008 mega ohm) which wouldn't show on most IR testers? With that said what do you experienced sparks do prior to fitting an RCD on unprotected circuits? I've read about current leakage clamps, are these worth looking into more or is there other ways i've not thought about, or is it a case of just fit the RCD and if it trips start hunting but usually it is ok? Still new having been let out the classroom into the real world a couple of months ago so constantly questioning different things i'm coming across.