The popularity of surge protected socket outlets (and extension leads) still seems to be growing, I’m all for not having my PC blown up when there’s a dodgy neutral connection on the pole outside my house, but it doesn’t come without it’s hassles. Well for us carrying out periodic testing it doesn’t anyway.
It would be nice to put the meter across the installation (having carried out the relevant isolations and load removals of course!) and get >299Megohms, oh joy, but this is the real world, and we end up with a reading way below that stated in Table 61, and the realisation we are just going to have to test each circuit one by one.
I’d say most engineers will take the precaution of connecting the live conductors together and testing to earth, in accordance with Regulation 612.3.3, this is perfectly acceptable.
But what about when we still get a low reading? (usually around 0.3Mohms)
A simple check is to carry out the insulation resistance test at 250V, and see if the value markedly improves. If so, there’s a good chance there are surge protected socket device (SPD) and /or extension leads on the system.
Is this acceptable practice?
Yes, in the Notes below Table 61 of BS7671, it clearly states the following:
Where surge protected devices (SPD) or other equipment likely to influence the verification test, or be damaged, such equipment shall be disconnected before carrying out the insulation resistance test. Where it is not reasonably practicable to disconnect such equipment (e.g. fixed socket-outlet incorporating an SPD), the test voltage for the particular circuit may be reduced to 250V d.c., but the insulation resistance shall have a value of at least 1Mohm.
Why does this work? Well simply, the SPD is set to detect high voltages, your test instrument is pouring 500V into the system, so naturally the SPD comes into play. At 250V, the voltage is what the SPD expects. Some socket outlets, or spur units contain a removable filter, which will allow the normal testing of the circuit.