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

 

[westward10]

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’re not diy are you?

You do not disconnect the cpcs for an IR test.

 

[cliffed]

You do not disconnect the cpcs for an IR test.

You may have to, if there is a fault on that cable

 

[pc1966]

You might disconnect the CPC after a test shows a problem to try and work out what it is.

The reason for having the CPC to the MET when testing is your IR fault could be to something that is not on that cable's CPC (another circuit, or even some structural part like a nail in to plasterboard, etc). If all CPCs are together and to the MET at test time you have a better chance of catching that sort of problem.

 

[Lucien Nunes]

0.00 is a dead short fella

I agree that when a zero reading comes up on an insulation test a short-circuit is likely, but as the circuit has been live it can't be the case here unless the earthing is O/C. An insulation tester will display 0.0 MΩ when the resistance is below 50kΩ, but at that resistance 230V will only pass 5mA of leakage, so an installation might function normally without symptoms if the ~1W of heat dissipated is not concentrated at one spot, and nothing will trip. It could be a very small load running to earth (an indicator light or tiny PSU) or a serious case of damp. A continuity test is needed distinguish between that scenario and an actual short-circuit, which is likely to be 50,000 times lower resistance.

 

[Grant1987]

You do not disconnect the cpcs for an IR test.

Yes you do when you are testing that individual circuit/cable so you’ve never disconnected an Earth when fault finding??

 

[westward10]

Yes you do when you are testing that individual circuit/cable so you’ve never disconnected an Earth when fault finding??

Cpcs are not disconnected to carry out an IR test.

 

[Grant1987]

You do not disconnect the cpcs for an IR test.

When you have multip

Cpcs are not disconnected to carry out an IR test.

I didn’t say that I wudn disconnect earths on doing normal IR test. the guy posting has multiple faults by swnd of it and is testing across the common earth and neutral bar as far as I’m aware? In that situation I disconnect the neutrals out and test and then find the circuit which is at fault. Once that’s established I will take the Earth out aswell and test individually for that cable to be definitive
[automerge]1600975081[/automerge]

Cpcs are not disconnected to carry out an IR test.

You don’t need to try lecturing me on fault finding I’ve carried out in the hundreds of fault finding especially within the area I live with old mining community with terraced housing full of damp Old accessories etc. . you won’t believe a lot of the faults I’ve found and learnt from over the years

 

[westward10]

I don't believe I am lecturing anyone.

 

[Grant1987]

I don't believe I am lecturing anyone.

Ok well just thought you was telling me I was wrong

 

[GBDamo]

Did you unplug/disconnect any devices?

Hidden spurs with flaming neons, the devils own work.