Quite - my very point ... and, as I said, assuming that was done, the fact that N&E are connected upstream of the CU (at cutout, generator or wherever), as mentioned by TTC, is not, in itself, relevant to IR testing.Well, we hope he disconnected the L & N busbars from the incoming supply by opening the main switch.
On another point in relation to testing with MCBs connected, can we be sure that no modern MCBs contain any 'electronics' which could affect IR measurements (or even make them susceptible to damage from IR testing)?
Kind Regards, John.