Insulation Resistance

[Jamieblade1]

Hi guys,

I've recently just completed a rewire of a small 2 bed bungalow consisting of 4 circuits. I have tested the kitchen ring and it has tested fine, the same with the lighting circuit. On testing the heating circuit and general ring main, the insulation resistance between L-N on both circuits is 0.3 Mohms. Between L-E and N-E they test fine at over 200Mohms. I have looked for the obvious i.e nails through cable in capping etc twists and kinks in cable. The cables move freely in the capping when pulled. The 1.5mm supplying the fused spur in loft for the heating is no longer than 6 meters in length. The general sockets however, I have split the ring down to find the leg which is causing the low reading and it seems to be the supply from the consumer unit to the first socket. But as I've said there are no nails, plaster, clips or anything on the cables. Really frustrating now, especially as the bungalow has just been skimmed!

Any chance it could be faulty cable?

Any help will be greatly appreciated.

Cheers

 

[Plonker 3]

Have you got all loads disconnected including any neon indicators on FCU?

 

[Deleted member 9648]

If your 'low' reading is L-N there will be a load connected somewhere......hopefully 500v up it's jacksie hasn't trashed it.
Usual missed culprits are heating controls,neons and tv amps. 0.3 megs is also typical of a surge protected lead.

 

[Silly Sausage]

If your 'low' reading is L-N there will be a load connected somewhere......hopefully 500v up it's jacksie hasn't trashed it.
Usual missed culprits are heating controls,neons and tv amps.

He says he's just rewired, so I assume he knows that there isn't a load connected!

OP, did you IR your cables before the plastering?

 

[drzsta]

Heating is likely to be the Neon on the FCU.

With the socket circuit, disconnect both ends of the leg you think is the issue, if it is the leg that goes from c.u. to first socket (like you think) and still has poor reading - no other option other than to replace

 

[Deleted member 9648]

He says he's just rewired, so I assume he knows that there isn't a load connected!

OP, did you IR your cables before the plastering?

Regardless...odds are if it's ok to earth it'll be a load. A real fault will almost always be to earth as the earth wire is between the live and neutral making a L-N fault without earth as well highly unlikely.

 

[Jamieblade1]

There are no loads connected and FCU's are non-neon. Basically just a 1.5mm up to a FCU with about a 6 meter length for heating, that's it, no combi fitted as yet. The property is completely empty.

 

[Octopus]

Just been skimmed - damp??????

 

[Jamieblade1]

The walls in contact with the cables mentioned are completely dry, they were plastered first before the rest of the bungalow last week.

 

[Deleted member 9648]

Look again for something connected.....if it was damp it would give a low reading to earth as well.

 

[drzsta]

I would completely disconnect from C.U. and accessories and test again.

 

[Jamieblade1]

100% nothing connected whatsoever, the heating goes straight from CU to FCU with no load (no combi fitted) and the first leg from the CU to the first socket also reading the same L-N, 0.3mohms

 

[Octopus]

Are you saying that you have a 2.5mm ring with a 1.5mm spur from the mcb/cu for the boiler?

 

[Plonker 3]

Have you disconnected the cables from the DB then carried out the IR test?

 

[Deleted member 9648]

So there's nothing connected and nothing wrong.....but you are getting 0.3 megs. Doesnt really stack up does it?

 

[Jamieblade1]

No the heating is directly from CU on its own circuit. The leg in question is the ring main to the rest of the bungalow. From the CU to the first socket, it's a reading of 0.3mohms L-N. From the first socket to the rest of the bungalow, it tests completely fine.

 

[Plonker 3]

Hi guys,

I've recently just completed a rewire of a small 2 bed bungalow consisting of 4 circuits. I have tested the kitchen ring and it has tested fine, the same with the lighting circuit. On testing the heating circuit and general ring main, the insulation resistance between L-N on both circuits is 0.3 Mohms. Between L-E and N-E they test fine at over 200Mohms. I have looked for the obvious i.e nails through cable in capping etc twists and kinks in cable. The cables move freely in the capping when pulled. The 1.5mm supplying the fused spur in loft for the heating is no longer than 6 meters in length. The general sockets however, I have split the ring down to find the leg which is causing the low reading and it seems to be the supply from the consumer unit to the first socket. But as I've said there are no nails, plaster, clips or anything on the cables. Really frustrating now, especially as the bungalow has just been skimmed!

Any chance it could be faulty cable?

Any help will be greatly appreciated.

Cheers

How have you done this of the job has been plastered?

 

[Jamieblade1]

Because the cables move freely in the capping, I have also managed to pull the leg out from CU to first socket and there's is no physical damage at all.

 

[Plonker 3]

So when you pulled the leg out why didn't you change it? Or even carry out a IR test on it?

 

[Jamieblade1]

Yeah I will when I go back tomorrow, I'd just had enough after trying to get my head round why both cables would show the exact same fault L-N when they're not even from the same drum and different size.