Insulation Test Help

[hightower]

If I wanted to run an insulation resistance test on a particular length of cable do I need to disconnect the neutral at the DB as well as isolating the line, or will isolating the line be sufficient? Hope this is enough information, will post up more if needed though.

 

[Pete999]

Disconnected the lot Line Neutral and cpc, if you are only testing one length of cable

 

[hightower]

Thanks Pete, what if the stretch of cable is like this? There's no neutral as such in the cable as it's a live feed in/out and strappers. Say if I want to run the test on the 3-core cable at the switch, and the junction box isn't accessible without ripping up carpet and boards. Would it be ok to run the test at say the left-hand switch with only isolating the supply at the MCB? Or would I still need to disconnect the neutral/CPC for the whole circuit at the CU?

 

[Pete999]

Thanks Pete, what if the stretch of cable is like this? There's no neutral as such in the cable as it's a live feed in/out and strappers. Say if I want to run the test on the 3-core cable at the switch, and the junction box isn't accessible without ripping up carpet and boards. Would it be ok to run the test at say the left-hand switch with only isolating the supply at the MCB? Or would I still need to disconnect the neutral/CPC for the whole circuit at the CU?

I would disconnect the supply cable, Remove all the lamps, carry out an IR test L to N, L to Earth and N to earth ensuring you operate the 2 way switching to be certain of testing all of the cores, if you detect a fault then it would be a case of disconnecting at the light and switches and test each individual cable, you may have to access the JB as well.

 

[hightower]

This is what I'd propose to do, if you think this will work.

1) Isolate the line at the CU for this circuit
2) Disconnect the line at the light (as that is accessible)
3) Disconnect all wires at both switches

That way, I have a segregated circuit, and then run the following IR tests (I was thinking to start on 250v so I don't accidentally make a hash of something I may have overlooked):

1) IR test between left hand yellow and left hand blue
2) IR test between left hand yellow and left hand red
3) IR test between left hand blue and left hand red

And then repeat the same tests at the right hand switch.

That way if a short circuit fault exists where I've coloured brown for example, it should show up on the test between yellow and blue at the left hand switch, but no other tests.

Or if a short circuit fault exists where I've marked green for example, (across yellow, blue and red) it should show on all tests on the right hand switch, and even the test on the left hand switch between blue and red, but it shouldn't show on tests on the left hand switch involving the yellow and blue/red.

I hope I've explained things alright, it's kind of tricky getting things down on paper but I am quite enjoying thinking about this - I do like trying to solve problems like this.

 

[Pete999]

This is what I propose to do, if you think this will work.

1) Isolate the line at the CU for this circuit
2) Disconnect the line at the light (as that is accessible)
3) Disconnect all wires at both switches

That way, I have a segregated circuit, and then run the following IR tests (I was thinking to start on 250v so I don't accidentally make a hash of something I may have overlooked):

1) IR test between left hand yellow and left hand blue
2) IR test between left hand yellow and left hand red
3) IR test between left hand blue and left hand red

And then repeat the same tests at the right hand switch.

That way if a short circuit fault exists where I've coloured brown for example, it should show up on the test between yellow and blue at the left hand switch, but no other tests.

Or if a short circuit fault exists where I've marked green for example, (across yellow, blue and red) it should show on all tests on the right hand switch, and even the test on the left hand switch between blue and red, but it shouldn't show on tests on the left hand switch involving the yellow and blue/red.

I hope I've explained things alright, it's kind of tricky getting things down on paper but I am quite enjoying thinking about this - I do like trying to solve problems like this.

View attachment 30811

You will pick that fault up the way I mentioned, why are you so interested in the three core?

 

[hightower]

You will pick that fault up the way I mentioned, why are you so interested in the three core?

Just a fault that my dads has - I think the 3 core is the issue because when connected the MCB trips. The light is currently wired up via a PIR off an adjacent socket, so the 3 core is disconnected (except for the yellow feed in which is still connected at the junction box, but just tucked to the back of the box and left to forget about).

The PIR has been in for years now, and he's happy enough with it as it would be a right sod to replace the cable. I'm just using this scenario as a fault finding mission to try and get my head around testing routines and problem solving. I'm certain the cable is shagged, and would need replacing to wire it all back up, like I say, just wanting to try and understand the fault better in a bid to aid my learning.

(BTW, the PIR job wasn't me haha! And it certainly wasn't me who left a live wire in a bit of tape at the back of the box - would appreciate thoughts on this though since we're now on the subject)

 

[Pete999]

Just a fault that my dads has - I think the 3 core is the issue because when connected the MCB trips. The light is currently wired up via a PIR off an adjacent socket, so the 3 core is disconnected (except for the yellow feed in which is still connected at the junction box, but just tucked to the back of the box and left to forget about).

The PIR has been in for years now, and he's happy enough with it as it would be a right sod to replace the cable. I'm just using this scenario as a fault finding mission to try and get my head around testing routines and problem solving. I'm certain the cable is shagged, and would need replacing to wire it all back up, like I say, just wanting to try and understand the fault better in a bid to aid my learning.

(BTW, the PIR job wasn't me haha! And it certainly wasn't me who left a live wire in a bit of tape at the back of the box - would appreciate thoughts on this though since we're now on the subject)

Hang on you have a two lighting circuit and a PIR on the same circuit?

 

[hightower]

Hang on you have a two lighting circuit and a PIR on the same circuit?

No, the 2-way is part of the original wiring and where a fault lies. The 3-core in the diagram was disconnected at the switches, although the yellow feed-in at the left hand switch is still live but in a connector block at the back of the box. A PIR was then installed which runs off the upstairs ring (basically 3amp plug with flex running to PIR, and then up to light). The PIR was installed as a cheap fix to save chasing walls to replace the faulty cable.