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Excellent response Mr LNThe 0.05MΩ is L > N, so a load, possibly a fan timer? Won't like insulation test volts up it.
But you say the L+N > E is 0.3MΩ which is abnormally low and quite possibly at the fault location. It might be variable, a dead short at times and a meg at others. A likely scenario is that a cable is damaged with the conductors almost touching, which actually make contact when something moves, e.g. a heating pipe expands or a floorboard rocks. The arc has left carbonised insulation which is the cause of your low IR.
This is one of the things about IR testing to find a fault. It doesn't necessarily reveal the low resistance that caused / causes the bang, but it can lead you to anything suspicious - e.g. a damp section of wiring or even the aftermath of the fault - which does aid location.
I'd be splitting the circuit up in search of any section below a few MΩ.
Apologies to SuffolkSpark who kind of suggested this as well and I poo poo'd it.
Do you really think tracking down the 0.3M ohms to earth will lead to the tripping MCB fault Lucien?
I was going to try terminating the line conductor in one of the MCB's on the RCD side of the board to see if it tripped the RCD. If it does trip the RCD then I know I am looking at a L-E fault and I will track down the 0.3Meg. However if it doesn't trip the RCD then would you agree that the 0.3Meg to earth is a red herring as the fault would then likely be L-N?
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