Testing after switchboard upgrade

[ElectricalApprentice]

Hey guys,

Sorry for the long intro to the question.

I upgraded a domestic switchboard recently, including adding RCD protection to power and lighting circuits. During the upgrade I carried an IR test on each sub circuit, with one power circuit giving a failed 0Mohm result. I suspected that an appliance was plugged in, however at this time, after an extensive and time consuming search with the homeowner, nothing was found. I momentarily livened the circuit and the RCD did not trip, further indicating an appliance. The home owner then remembered an old pump being plugged into a socket under the house which had been boarded over and this turned out to be the cause of the low reading.

This got me thinking, am I responsible for the sub circuits when re-livening them after an upgrade? The homeowner may never have remembered that pump and it may have taken hours to find it.

My understanding is that we are only responsible for the work we complete, which in my case was only the protective devices however does this apply to existing work downstream?

Thanks.

[DougP]

An appliance that isn't faulty being plugged in shouldn't give you a failed reading, because you only test from each of A & N to earth. So presumably you were checking between A & N? That isn't a required test.

It's probably debatable how much testing should be done when replacing a switchboard, but IMO as a minimum, you should be doing an earth continuity test for all circuits (because you have re-terminated all the earths presumably), as well as testing polarity of final subcircuits and testing the RCDs as required for a new installation.

The IR test is probably the only one that isn't required, as you didn't install any subcircuit wiring.

[ElectricalApprentice]

Hi Doug,

The test was between A+N TO E. The result was slightly above 0ohm so probably just not low enough to trip the RCD.

Thanks for you answer, I have always carried out all the testes within section 8, hence the IR test.

[AlecK]

1 megohm on 230V gives 0.23 mA .
The 10k ohm allowed for eg MIMS elements gives 23 mA - usually but not always enough to trip a 30 mA RCD
Conversely; 30 mA (RCD must trip) comes at 7667 ohms.

so just how low was the reading?
slightly above 0 ohm (as stated) should certainly result in RCD tripping
Slightly above 0 Megohm could be a wide range, depending on the scale / range of the meter being used

Point being, we should always find "the" cause of a bad test result, not just assume we've found it.
Or at least find out exactly how bad it is.

As for responsibility; ESR 13 makes us responsible for everything we work on; and also to ensure that what we do doesn't reduce the safety of any other part of the installation

That said; it's highly unlikely anything you did at swbd could have adversely affected IR of the pump circuit;
However an overall IR test of entire installation would be warranted, to ensure no cross-overs; and that would have given a low test that would then need to be tracked down at least to find which subcircuit - just to prove it wasn't something you'd done.