Low L–N insulation readings on ring final – normal?

[jcaan]

Hi all,

I’m testing a domestic ring final with my megger MFT and have come across some weird insulation resistance readings. Here’s what I’ve done and observed:

Setup:

Disconnected the circuit completely from the CU/MCB.

All sockets and other loads switched off.

Tested all six conductors (L, N, E for both ends of the ring at thw CU).

Results :

L–E → very high, >999 MΩ

N–E → very high, >999 MΩ

L–N → very low, ~0.06 MΩ at the CU, even with all ring wires disconnected.

Tests at the sockets (whilst leaving CU in and out disconnected):

With the socket switch ON → L–N low (~0.06 MΩ)

With the socket switch OFF → L–N high (>999 MΩ)

All sockets work perfectly and there are no faults in normal operation.

It looks like the low L–N readings happen at the CU. High L–N readings only occur when socket switches are off (only at the sockets test)

My question:
Is this a genuine insulation fault I should be worried about, or is it just an artifact of the ring circuit?

Would appreciate advice from electricians who’ve seen this behavior before.

 

[EFLImpudence]

Separate the conductors at mid-point of circuit to see on which side the low reading is.
Then keep dividing until you find the cause.

High L–N readings only occur when socket switches are off (only at the sockets test)

Well of course. You are disconnecting the circuit by switching off.




Another socket somewhere with something plugged in.

 

[Harry Bloomfield]

It looks like the low L–N readings happen at the CU. High L–N readings only occur when socket switches are off (only at the sockets test)

Do any of the sockets, include a neon indicator, or perhaps RCD outlets? Test again, but find out which socket, when turned on, causes the IR to fall.

No, the IR should not reduce, if all of the sockets are just normal sockets. If they are all just normal sockets, then one, or more is damaged/faulty, perhaps burned.

 

[Murdochcat]

I’d split the ring to narrow down the fault , that’s assuming there is a fault, not just a load or rcd outlet you’re not aware of.

Hidden accessories are a pita

 

[endecotp]

With the socket switch ON → L–N low (~0.06 MΩ)

With the socket switch OFF → L–N high (>999 MΩ)

Edit: hang on, this is a *plug-in* tester, right? Well of course you'll only see the fault when the switch is on.

Is this a genuine insulation fault I should be worried about, or is it just an artifact of the ring circuit?

It's either a genuine fault, or you're doing something wrong. It's not "an artefact of the ring circuit".

Edit: any USB sockets?

 

[jcaan]

Edit: hang on, this is a *plug-in* tester, right? Well of course you'll only see the fault when the switch is on.




It's either a genuine fault, or you're doing something wrong. It's not "an artefact of the ring circuit".

Edit: any USB sockets?

Good point I'm using kewtech R2 test connector to comment my megger too.

 

[jcaan]

Yeaah guys im gonna split the ring and see what's going on. Its a new build apartment and has its own CU in the apartment

 

[Murdochcat]

New build that hasn’t been lived in ?

 

[jcaan]

New build that hasn’t been lived in ?

No it has been lived in. Everything works fine. I was just testing my MFT initially. Until I came up with this reading.

 

[pete01]

Was just going to add, any USB sockets? I hate them

 

[jcaan]

Was just going to add, any USB sockets? I hate them

No USB, no neon indicators or anything... just standard sockets.

 

[pete01]

No USB, no neon indicators or anything... just standard sockets.

I think as suggested above split it in half and test each side.

 

[Murdochcat]

No it has been lived in. Everything works fine. I was just testing my MFT initially. Until I came up with this reading.

Are you doing an EICR?

 

[jcaan]

Are you doing an EICR?

No I just had an MFT and I was just having a quick practice of how to use it. Then stumbled across this 'issue'.

 

[Murdochcat]

No I just had an MFT and I was just having a quick practice of how to use it. Then stumbled across this 'issue'.

I think you are looking for a hidden accessory or maybe a RCD socket/ FCU