IR connundrum

[Mark.W]

Hey Guys

I've got a bit of a head scratcher that I was hoping someone could help with.

I have a ring main feeding 7 points. When testing IR btw L&N I get 967Mohm but when testing N-E & L-E I get 455Mohm. I split the ring and tested as 2 radials and the results suddenly come back all clear at >1000Mohm L-N L-E and N-E??? When I reconnect the ring it's back down to 455Mohm??

I've checked all connections at the sockets and all good.

Any ideas??

Cheers
Mark

 

[Mark.W]

Just a thought but it could be the damp! I'm on a site in Poole and it's very moist where I am.

 

[Pete999]

Are you disconnecting ALL the circuit conductors from the CO Ls, Ns and cpcs??

 

[Pete999]

Just a thought but it could be the damp! I'm on a site in Poole and it's very moist where I am.

Well it would be damp in a poll wouldn't it? sorry my attempt at humour

 

[HandySparks]

Is ring continuity OK?

 

[Mark.W]

Yeah end to ends are good, as is r1+r2

R1 - 0.35
Rn - 0.39
R2 - 0.64
R1+R2 - 0.27

 

[HandySparks]

Yeah end to ends are good, as is r1+r2

R1 - 0.35
Rn - 0.39
R2 - 0.64
R1+R2 - 0.27

OK. Obviously, missing ring continuity could affect IR results when the ring is split.

BTW, I guess you mean r1, rn and r2.

 

[Mark.W]

Yeah it's auto correct lol

Start of
A new line
Auto capitalises
The first letter

 

[HandySparks]

Actually, looking at the figures again.
If you have about 500M IR for a complete ring, then splitting into two halves would give about 1000M for each half if the 'fault' is evenly distributed.

I don't really see a problem, especially as the figures are so high.

 

[tony mc]

Keep the test on for at least a minute and see how the reading change i would say that the reading would slowly go up slowly if it is a damp problem but I wouldnt lose any sleep over the readings posted!

 

[Pat H]

I'd agree the values look ok. The L-N us passing through two conductor insulations. L-E and N-E are only passing through the line conductor insulators as the CPC is bare.
A little moisture in the cable and you will see lower values.
Your tested proved the values where consistent even when split.

 

[westward10]

I don't see a problem here.

 

[telectrix]

the only problem as i see it is OP considering that 455Mohms is worthy of a second look. many MFTs only read up to 299Meg.

 

[Pete999]

the only problem as i see it is OP considering that 455Mohms is worthy of a second look. many MFTs only read up to 299Meg.

probably a strange thought, but any chance the OP could be misreading the MFT and stating OHMS instead.

 

[westward10]

the only problem as i see it is OP considering that 455Mohms is worthy of a second look. many MFTs only read up to 299Meg.

Mmm, second look then look somewhere else.

 

[PEG]

Although i admire the OP's fastidious nature,i don't see a great issue.

If he ever does an EICR on a 3 storey victorian semi,with all sockets connected to one circuit by one means or another...his head may explode

 

[Lucien Nunes]

Fixed that for you:

I have a ring main feeding 7 points. When testing IR btw L&N I get an excellent high reading and when testing N-E & L-E I get another excellent high reading. I split the ring and tested as 2 radials and the results suddenly come back as excellent, even higher readings as you would expect, L-N L-E and N-E. When I reconnect the ring it's back to its original excellent high reading.

 

[Mark.W]

Thanks for all the replies guys. I agree the results are good but I wasn't worried about them I was just curious as to why I was getting a lower reading when testing to earth. I was hoping to see them all around the same value.

 

[Deleted member 72666]

Just a thought but it could be the damp! I'm on a site in Poole and it's very moist where I am.

Pools usually are!

 

[Risteard]

Are you disconnecting ALL the circuit conductors from the CO Ls, Ns and cpcs??

Since the 17th Edition came out it has been required to have the cpcs connected to the Earthing arrangement and not disconnected.

That said, I really wouldn't be worrying about 455 million Ohms!

 

[Andy_G]

Th

Hey Guys

I've got a bit of a head scratcher that I was hoping someone could help with.

I have a ring main feeding 7 points. When testing IR btw L&N I get 967Mohm but when testing N-E & L-E I get 455Mohm. I split the ring and tested as 2 radials and the results suddenly come back all clear at >1000Mohm L-N L-E and N-E??? When I reconnect the ring it's back down to 455Mohm??

I've checked all connections at the sockets and all good.

Any ideas??

Cheers
Mark

this is the kind of 'conundrum' I like to get on jobs, I.e the type you can just come back to think about when you are bored with nothing to do for an hour..455 Meg ..I want some of those conundrums in 2017

 

[Andy_G]

I t

Thanks for all the replies guys. I agree the results are good but I wasn't worried about them I was just curious as to why I was getting a lower reading when testing to earth. I was hoping to see them all around the same value.

otally understand why you wanted to know the actual reason as it is always helpful to know why things happen as opposed to just knowing that they happen,electrical theory is fascinating and the more you learn,the more you realise how little you know...hence why most avoid asking 'why'....

 

[1Justin]

Hey Guys

I've got a bit of a head scratcher that I was hoping someone could help with.

I have a ring main feeding 7 points. When testing IR btw L&N I get 967Mohm but when testing N-E & L-E I get 455Mohm. I split the ring and tested as 2 radials and the results suddenly come back all clear at >1000Mohm L-N L-E and N-E??? When I reconnect the ring it's back down to 455Mohm??

I've checked all connections at the sockets and all good.

Any ideas??

Cheers
Mark

Disregarding, for the moment that with figures that high you shouldn't hesitate to move on. However it still can make sense.
The structure of T&E cable means there is about twice as much insulation between L-N as between LE or NE. So any leakage will be expected to be larger in the latter case. When you split the ring, assuming you broke it near the middle, each half L-E + N-E would be approximately doubled. Hence all 1000Mohm.
Wait till you get a damp installation 1000 times worse than you have there, and have to make a judgement..