Insulation Resistance

[danijah]

Hello there guys, after adding an additional socket outlet in my kitchen fed from an FCU. I carried out the test sequences needed to fill out the Minor works cert using my Megger MFT 1552. The earth continuity was satisfactory but, there seems to be a breakdown between Line - Neutral when i do my Insulation resistance test. here are my results; L-E 23.0 Mohms, L-N 0.97 Mohms, and N-E 27.0 Mohms. Can anybody offer any advice on the process of elimination.

 

[paul b b]

Where are you doing the test from? you need only test the cable you have installed so make sure its not connected the the exsisting ring or you may fry things. Failing that you may have dodgy cable. Oh and neons can effect results.

 

[danijah]

I tested from the CU on the ring circuit because my house is on one ring. I disconnected all my appliances and equipment. Theres no sockects with neons on them. Maybe i should of tested the ring before i added the socket to establish if there was any faults first (all part of learning i suppose). The length of the actual spur is only 100mm max.

 

[Zeespark]

Do a continuity test on each leg of the circuit, then do an IR test on each leg, this will find the faulty leg. On top of that, test all legs, not just until you find the first leg with issues, there could be issues lurking elsewhere. It does pay to test circuits first, I would rather test first and get paid for remedial work, rather than doing the install and potentially having to do remedial work for virtually free.

AndyGuinness

 

[danijah]

Thanks for the advice, i'll do that as soon as. Yeah your bang right about testing first and saving yourself the hassle of free labour. Luckily its my own property. :Salute

 

[matt.leung]

as u are fault finding, it helps to draw the circuit out in a piece of paper, so you can see where cables run to and from. also jot down your results on the diagram...

 

[Zeespark]

There is a minor issue with that method Matt, you may know what socket comes in line with the next, but you are unlikely to know which actual T+E goes where without marking/identifying the cable inside the box. To do this, I test each leg, make up a flag with insulation tape, and mark on it what part of the run the leg belongs too. A piece of advice for this, don't use black insulation tape.

AndyGuinness

 

[kme]

970 ohms L-N suggests a (small) load still connected somewhere............bell transformer, aerial amp in the loft etc... Something of this ilk is usually the culprit in these situations.

 

[Doc Hudson]

Maybe i should of tested the ring before i added the socket to establish if there was any faults first (all part of learning i suppose).

I consider testing the original circuit as a basic essential, part of verifying the installation is satisfactory prior to doing any alterations, while you are checking earthing bonding and existing protective measures are up to scratch. Also I do a quick test on the new added cables before I connect to the existing circuit. As you say a lesson to be learned, a few minutes checking and testing can identify problems earlier rather than later.

Doc H.

 

[Evans Electric]

If the length of the spur is 100mm (which means 4" to me) just dis, it and test the existing , sounds like you're meggering something still connected as my esteemed colleagues have said.

I assume its not tripping the RCD /MCB .

With nothing connected its just a loop of wiring , in which case its faulty cable, dampness, cable spiked, nailed , screwed , pierced by plate screws , alien intervention or its one of Batty's old jobs

 

[batty]

If the length of the spur is 100mm (which means 4" to me) just dis, it and test the existing , sounds like you're meggering something still connected as my esteemed colleagues have said. I assume its not tripping the RCD /MCB .

With nothing connected its just a loop of wiring , in which case its faulty cable, dampness, cable spiked, nailed , screwed , pierced by plate screws , alien intervention or its one of Batty's old jobs

Can't be one of mine my shorts are a lot better than that.

 

[kme]

Can't be one of mine my shorts are a lot better than that.

View attachment 2277

If you say so - but I can`t say I`m convinced............................

 

[ProDave]

970 ohms L-N suggests a (small) load still connected somewhere............bell transformer, aerial amp in the loft etc... Something of this ilk is usually the culprit in these situations.

0.97M ohms is in fact 970K ohms (970 thousand ohms) so unlikely to be a bell transformer.

More likely to be a neon in a FCU somewhere or indeed a faulty cable.

Could also be dust and debris in a back box.

That leakage would result in a current fow at 230V of 0.237mA, so pretty insignificant.

 

[kme]

Well spotted mate. I`ll blame me age; think my eyesight is failing.........