insulation resistance

snowplough · Mar 2, 2011

Hi guys,

When doing an insulstion resistance test why do you have to leave all circuit breakers closed if your just testing one circuit say,is it because you are also checking that there is no leakage current between any conductors say a light circuit and a shower circuit ,if so why cant you just test between line and neutral at the main isolation switch in the c.u on the two screws with the main switch open. Im wondering why in the O.S.G and G.N.3
it shows one of the test leads being put on the screw on one of the C.B s.

Many Thanks

LOCKET

IQ Electrical · Mar 2, 2011

On a 'global' IR test (whole installation) all devices should be closed, lamps and sensitive components removed from test.

On a split/individual circuit test, only the circuit in question should be tested, on the outgoing side of the protective device, all other protective devices open.

Check the small print in Figure 2.3a page 39 Guidance Note 3, it reads 'main switch off circuit breakers off'.

nickblake · Mar 2, 2011

As IQ has said and has mentioned sensitive components , i have see guys doing the global test from the bus bar ,main switch off RCD's and MCB's on , i wouldnt recomend doing it this way as there is a strong possibility that you will damage the electronics in the RCD

snowplough · Mar 2, 2011

Many Thanks IQ,

At college we usually shown to do all the circuite at the c.u as i described with the leads at the main switch which i assumes covers all the individual circuits any way?

Im just trying to remember when you do an I R on A split board with RCD protecting one half , if you test at the main isolating switch as i do will it keep nuisance tripping the rcd on c.u protecting the other C.B . In GN3 tells you to disconnect rcds etc , but i cant seem to remember open the switch on the rcd to disconnect it from the c.u or even the rcd tripping when doing the IR on allthe c.u as mentioned above.

Regards

lOCKET

Adam W · Mar 2, 2011

nickblake said:
As IQ has said and has mentioned sensitive components , i have see guys doing the global test from the bus bar ,main switch off RCD's and MCB's on , i wouldnt recomend doing it this way as there is a strong possibility that you will damage the electronics in the RCD

Is there? I queried that when I was at college because I was also concerned about that, but the teacher seemed to think there was no danger of damaging the rcd at all (not that that says a lot seeing as he'd been out of the trade for a few years).

snowplough · Mar 3, 2011

If you put one of your test leads on say the line busbar just after the main switch cosumer side , and say the other test lead on the neutral busbar will this cause the rcd to trip even if there isnt any electrical connection between the line and neutral anywhere in the circuit. i no fcds will pick up on an imbalace between current flowing through the neutral and line.

Regards

Locket