UK Insulation resistance whole board.

[Bobble23]

Hi all. Changing a CU in an ex-council flat, pme system, no RCD's, and initially tried to IR test whole board L/N joined to earth, got a reading of 0.28 Ohms.

Then did each circuit individually (2 X RFC, 2 X lights, 1 X cooker), L/N joined to the circuit's CPC, and got between 0.25 and 0.30 Ohms on each one.

Any ideas what could be causing the low readings?

Thanks.

 

[telectrix]

always get that with PME. it's because the N bar is tied to earth at the service head with bonding to services contributing.

 

[Lucien Nunes]

With the main switch OFF? When you tested the circuits individually, did you remove the N's from the bar? If not, each test was still a global test, and an N-E fault on one circuit will show up on all.

 

[buzzlightyear]

Then did each circuit individually (2 X RFC, 2 X lights, 1 X cooker), L/N joined to the circuit's CPC, and got between 0.25 and 0.30 Ohms on each one

did you disconnect from the board or tails in the board .

 

[newfutile]

How are you reading ohms on an IR test ? Is it not MegaOhms ?

 

[pc1966]

Assuming it has a DP main switch which was off, most likely some circuit has a N-E fault and as they are common on the N bar you see the global test fail.

Need to pull each N out in turn to find it.

If it were L-E a MCB would have found it dam quick!

 

[pc1966]

Then did each circuit individually (2 X RFC, 2 X lights, 1 X cooker), L/N joined to the circuit's CPC, and got between 0.25 and 0.30 Ohms on each one.

Assuming the individual N was out during each test that is odd. Having a N-E fault on every circuit is a bit more than coincidence and more like an installation with some serious flaws!

 

[newfutile]

if the 0.3 ohms reading per circuit is correct a global 0.06 ohms would be a global reading drawing 3.833 KW .either measurement errors , SPDs or item still in circuit (possibly wired incorrectly ) is there an RCD present?

 

[Bobble23]

if the 0.3 ohms reading per circuit is correct a global 0.06 ohms would be a global reading drawing 3.833 KW .either measurement errors , SPDs or item still in circuit (possibly wired incorrectly ) is there an RCD present?

 

[Bobble23]

Sorry, my mistake - I meant MegaOhms, not Ohms.

No RCDs, no SPDs and an empty property so I couldn't even find anything plugged in. I did the individual tests removing L, N and CPC from the board, so that's why I can't understand why they all had pretty much the same low reading. Wiring is singles in conduit, but can't see how that can make a difference.

It seems there is a problem but I can't even think what that problem might be!