IR testing - Checking my logic

[catdog1121]

Did a check on a ring circuit, due to board having to remain live I did it at a socket.. Was an RCBO, had checked RCD side and all tested ok, when I did IR live and neutral to cpc I got 0Mohms, when I did resistance test got a very low reading 0. something.

Now I knew that there was something going on cause the rcbo was fine and it turned on which it would not if it was effectively a dead short.

So my theory and I just wanted to check this with others to make sure I am correct, supply is TNCS, 3 phase, passes through a 3 pole mcb to a distribution board that has only 3 pole isolator, rcbo will most likely have been single pole only, so my thinking is neutral is not isolated at all even when you turn off the rcbo and there is a path right back to the supply fuse where obviously earth and neutral become one. So doing Neutral to cpc I got a lot resistance reading and because loads were plugged in also got a low reading from live to cpc...

Because of this ended up putting it as a limitation, but is my theory correct? Obviously if I had disconnected the rcbo at the board then I could have done IR testing however the board was a bit of a mess and with it being live and not the easiest place to work on I thought it safer to do from a socket but obviously found the limitation that I could do end to end and R1+R2 but could not do IR. I guess this is also true of a board that has been isolated live and neutral the IR test will effectively be a global IR unless you disconnect the neutral. (Obviously IR was done at 250v and the megger does not actually let you ramp up anyway if reading is low.)

 

[loz2754]

but is my theory correct? Obviously if I had disconnected the rcbo at the board then I could have done IR testing

Yes, most likely

 

[timhoward]

I guess this is also true of a board that has been isolated live and neutral the IR test will effectively be a global IR unless you disconnect the neutral

Yes, and forgetting a board had SP devices is exactly how I finished off a faulty fridge freezer once....
As you say you are effectively doing a L-E test of the circuit, and a N-E test of all circuits.

In your scenario I'd have looked to see if I could safely get the RCBO N fly lead out or the N conductors out of the RCBO.
EDIT being 101% sure I had the right one as I have no ambitions to become a N conductor.

 

[catdog1121]

Yes, and forgetting a board had SP devices is exactly how I finished off a faulty fridge freezer once....
As you say you are effectively doing a L-E test of the circuit, and a N-E test of all circuits.

In your scenario I'd have looked to see if I could safely get the RCBO N fly lead out or the N conductors out of the RCBO.

Yeah it was at the top of the board in a small cupboard and was a bit of a mess of cables. I was only doing an EICR on the one circuit and given where it was just wasn't worth the risk.. I just put it down as an operational limitation.

Glad it wasnt just me that was a bit dim.... I have definitely changed with amendment 2 to only using 250v for IR unless I know I have just installed the circuit and there is nothing connected..