Insulation Test Result Help

[Darkness]

I have a 16a radial circuit with just the one socket at the end. For some reason now it trips the RCD when I switch the mcb on.

I have a megger insulation tester which was given to me before a neighbour moved overseas and it was calibrated a month before he left so I know how to use it I think but the readings mean nothing to me.

Anyhow, I keep getting a reading between 12-16mΩ if thats how you put it. I looked at the cable t&e and it seems fine from the outside so can someone tell me what this reading says about the cable condition? Any chance there is a short between the cores?

Many thanks.

 

[Adam_151]

Assuming you mean MΩ (meggaohms) and not mΩ (milliohms)!...

Anything above 1 meg is acceptable, but the reading you got is lower than I generally like to see on a single circuit.

Which cores are you conducting the test between? and at what voltage?

After you measured 12meg and put the circuit back in the board, does it trip straight away, or does it hold for a bit... could be an intermitant fault

 

[Darkness]

Using L+N at 500v. It trips straight away when power is switched on. I never tried L+E etc.

On a TT system with a Hager 30ma RCD.

 

[ban-all-sheds]

An IR tester isn't expecting mΩ resistances, so I can't believe that it would display anything other than zero once it drops below a few '00K.

12MΩ won't create enough leakage to trip an RCD, and if the general background of the installation is that it's only 20μA away from tripping I'd expect it to do that if someone sneezed near a socket.

 

[Adam_151]

Try testing between earth and the other two cores. This should show up the fault causing the rcd to trip

 

[Taylortwocities]

You must disconnect all the conductors for the circuit being tested at the consumer unit and the circuit must have everything unplugged.

Test the IR between

Earth and Live
Earth and neutral
Live and neutral

You should see >200MΩ on all the tests. Anything more than 2MΩ is a cause for concern.

If that is all OK the we need to look at the connections in consumer unit. But dio the tests first and then we'll think about the next step.

 

[STI]

Anything more than 2MΩ is a cause for concern

Isnt it anything less than 2MOhm is cause for concern

 

[LiamPope]

L-N IR testing won't show up an RCD tripping fault.

 

[riveralt]

I think it would be better if you did the testing in the proper order - What were your results for continuity (R1 +R2)?

 

[Darkness]

These are the results I got from a quick test earlier.

L+N = 12-17MΩ
L+E = 12-17MΩ
N+E = 2.1MΩ

 

[riveralt]

These are the results I got from a quick test earlier.

L+N = 12-17MΩ
L+E = 12-17MΩ
N+E = 2.1MΩ

Are you sure there isn't another socket somewhere and something plugged in?.
Why are you doing an insulation resistance test - is it because of the RCD tripping? Because you should do the tests in order.....
Have you looked behind the socket you have or have you done a continuity test.

Describe how you are doing the insulation resistance tests!

 

[ban-all-sheds]

Using L+N at 500v.

I misinterpreted that completely.

I thought you meant you were testing (L+N combined) to earth.

I would say "why on earth..." but you might think it's a pun, so I'll say why the **** were you testing between L & N when it's an earth fault you have a problem with?

I have a megger insulation tester which was given to me

Sell it on eBay.

I know how to use it I think

Or put it away until you know where to use it.

N+E = 2.1MΩ

That's 110μA - 2 orders of magnitude away from RCD trip current....