Low Insulation Resistance readings N-E

[TheCodboy]

Looking for some guidance here. I'm currently shadowing a guy and we're testing a new extension on a school. 3 phase TNCS supply from the original busbar. Feeds a section board and from the section board it feeds 4 3 phase DB's. The DBs have one feed in and then split to a separate power and lighting DB. (Schneider). R1+R2 tests all fine on feed to the section board, then sub mains. IR between L/E & L/N all good, N/E 0.01 at the DB's. Obviously TNCS is linked so i understand the low IR values there, but even with the TP&N isolators off and the incoming earth removed from the earth bar, testing from the other side of the TP&N isolator, im still getting 0.01 N/E. Where is it still linked out? As I say I'm shadowing a guy as its been a while since I've done much testing, he was confident to energize the DB's, which he did, and everything is fine. I know its probably something very simple but its driving me mad why I cant figure it out. I'm getting a lot of "thats fine dont worry" but i like to understand for myself. Hope someone can point me in the right direction. Thanks in advance

 

[davesparks]

Are you testing through the RCBO's in the DB?

 

[TheCodboy]

Are you testing through the RCBO's in the DB?

No, testing from the top of the rcbo's. We're also getting the same result on the MCB circuits. Its on every DB too. Im obviously missing something simple.

 

[davesparks]

No, testing from the top of the rcbo's. We're also getting the same result on the MCB circuits. Its on every DB too. Im obviously missing something simple.

Unless those RCBO's are switching the neutral then you will still be testing through the electronics between the neutral and earth within the RCBO.

 

[TheCodboy]

Unless those RCBO's are switching the neutral then you will still be testing through the electronics between the neutral and earth within the RCBO.

That would make sense as I'm not sure if they are switching the neutral, but why am I getting the same on the non RCD/RCBO circuits in the lighting DB?

 

[davesparks]

That would make sense as I'm not sure if they are switching the neutral, but why am I getting the same on the non RCD/RCBO circuits in the lighting DB?

How exactly are you carrying out the tests?

If you are testing with the circuits connected to the neutral bar then it will be the same thing as the RCBO's are also connected to the neutral bar.

 

[TheCodboy]

How exactly are you carrying out the tests?

If you are testing with the circuits connected to the neutral bar then it will be the same thing as the RCBO's are also connected to the neutral bar.

Yeah I'm testing the same. The 2 DB' are fed from the same feed. So if I understand you correctly,the 2 neutrals (lighting & power) are essentially the same neutral and the rcbos are probably causing this issue.