Low insulation resistance and tripping circuit breaker

[shug1693]

I've just done an insulation resistance test between line and earth and got 0.0 Mohms for most circuits. Obviously this is an F1, but what I don't understand is why the CB doesn't trip. Is the answer that the resolution on my tester isn't high enough? So I could for example have 0.049Mohms and this resistance is high enough to prevent a significant earth fault loop path?
Thanks for any thoughts...

 

[JBW175]

What circuits did you get the low readings on? which ones passed the test?

 

[pc1966]

The best practice guide would code that as C2 for:

• Insulation resistance of less than 1 MΩ between live conductors connected together and Earth

I could be the impedance is below the meter's resolution, as 50k would only dissipate around 1W so might not be apparent as a fault to the occupier (but could grow to be a fire hazard).

What about N to E resistance?

Or did you test L+N to E? If so you could have a load present and a N-E short which may not show up (definitely not if no RCD, sometimes as spurious trip if RCD).

 

[DPG]

Did you unplug/disconnect any devices?

 

[Dartlec]

I've just done an insulation resistance test between line and earth and got 0.0 Mohms for most circuits. Obviously this is an F1, but what I don't understand is why the CB doesn't trip. Is the answer that the resolution on my tester isn't high enough? So I could for example have 0.049Mohms and this resistance is high enough to prevent a significant earth fault loop path?
Thanks for any thoughts...

Was it the same at 250V and 500V? If it's happening on several circuits only at 500V then could there be surge protection somewhere?

 

[Grant1987]

I've just done an insulation resistance test between line and earth and got 0.0 Mohms for most circuits. Obviously this is an F1, but what I don't understand is why the CB doesn't trip. Is the answer that the resolution on my tester isn't high enough? So I could for example have 0.049Mohms and this resistance is high enough to prevent a significant earth fault loop path?
Thanks for any thoughts...

You need to disconnect the earths and neutrals out of the common bar and then test individually swnds like you have tested across the common bar. Surely you done this?
[automerge]1600810308[/automerge]

You need to disconnect the earths and neutrals out of the common bar and then test individually swnds like you have tested across the common bar. Surely you done this?

You’re not diy are you?

 

[Dustydazzler]

0.00 is a dead short fella

 

[Grant1987]

0.00 is a dead short fella

?‍ Exactly he needs to get his thinking cap on

 

[shug1693]

What circuits did you get the low readings on? which ones passed the test?

Cooker and water heater passed, upstairs and downstairs power and lighting failed.
[automerge]1600837183[/automerge]

The best practice guide would code that as C2 for:

• Insulation resistance of less than 1 MΩ between live conductors connected together and Earth

I could be the impedance is below the meter's resolution, as 50k would only dissipate around 1W so might not be apparent as a fault to the occupier (but could grow to be a fire hazard).

What about N to E resistance?

Or did you test L+N to E? If so you could have a load present and a N-E short which may not show up (definitely not if no RCD, sometimes as spurious trip if RCD).

The best practice guide would code that as C2 for:

• Insulation resistance of less than 1 MΩ between live conductors connected together and Earth

I could be the impedance is below the meter's resolution, as 50k would only dissipate around 1W so might not be apparent as a fault to the occupier (but could grow to be a fire hazard).

This is what I was getting at, thanks.
[automerge]1600837514[/automerge]

What about N to E resistance?

Or did you test L+N to E? If so you could have a load present and a N-E short which may not show up (definitely not if no RCD, sometimes as spurious trip if RCD).

L to E were the problems, L to N and N to E were fine.

 

[pc1966]

Any evidence of rodent damage?

I have seen a circuit trip the MCB then reset OK, but IR test showed high tens of k ohm so clearly not good. Was the result of mice in a home.

I guess if they eat the N insulation it might still have enough of a gap to clear 500V when tested later, but if chewing live and they go bang! it leaves some carbonised mouse around the place