Low N/E insulation resistance

[pig]

Hi all,
Just installed a brand new radial circuit for the upstairs sockets with Prysmian 2.5mm T+E, around 21 metres in length. When testing the insulation resistance of the circuit upon connection to the consumer unit, I seem to repeatedly get around 49 Megohms when measuring Neutral to Earth. The result seems to get lower as I lower the test voltage, and is more around 100 Megohms when testing at 1000v. I thought that it may be a dodgy bit of cable so separated the conductors at a few sockets and tested again but get the same results each time, sometimes as low as 26 Megohms. Line to earth and line to neutral are both at least 400 Megohms at each location. I tested the remainder of the 100M drum which probably still has around 80M left on it at 1000V, and get >1000 Megohms between conductors. 49 Megohms doesn't quite satisfy me for a brand new circuit. Any thoughts?

 

[derek]

Did you try splitting the circuit in half and then carrying out the test again? I assume when you IR tested that you disconnected the circuit from the board.

 

[Pete999]

Hi all,
Just installed a brand new radial circuit for the upstairs sockets with Prysmian 2.5mm T+E, around 21 metres in length. When testing the insulation resistance of the circuit upon connection to the consumer unit, I seem to repeatedly get around 49 Megohms when measuring Neutral to Earth. The result seems to get lower as I lower the test voltage, and is more around 100 Megohms when testing at 1000v. I thought that it may be a dodgy bit of cable so separated the conductors at a few sockets and tested again but get the same results each time, sometimes as low as 26 Megohms. Line to earth and line to neutral are both at least 400 Megohms at each location. I tested the remainder of the 100M drum which probably still has around 80M left on it at 1000V, and get >1000 Megohms between conductors. 49 Megohms doesn't quite satisfy me for a brand new circuit. Any thoughts?

What MFT are you using?

 

[telectrix]

could the cable have gotten a bitof damp in it due to the wet weather? or even some damp in a socket outlet.

 

[westward10]

26 meg is not a low reading.

 

[Intoelectrics]

As Derek mentioned, have you tested this whilst the circuit is connected at the consumer board, if so that might be the problem. Test again with the circuit completely isolate from the CCU.
But as westward stated 26 meg is not low.

 

[Lucien Nunes]

The fact that the bulk cable reads good and the resistance increases with voltage both point to condensation in the socket outlets, especially if they are in rooms that are not continuously heated or there is plaster drying nearby. Opening them to split the circuit may be lowering the readings as more condensation takes place around the terminals. You might be able to prove it if you can find a section with a lowish reading and warm the associated sockets front and back with a fanheater for a few minutes.

 

[pig]

Yes I tested the cable before it was even connected to the terminals in the CU. I have just retested the circuit again and have now got 67 meg. Using a Fluke 1662 MFT. Not too worried just originally 26 didn't quite satisfy me as high enough for a brand new circuit. Thanks.