Testing integrity of lighting wiring after plasterboard ceiling fit

[markharro]

I was helping our joiner plasterboard some ceilings and I was asking him how you deal with stray screws that might hit a cable for lights/smoke detectors etc. He said it best to get the electrician to test the cabling before the ceilings are skimmed.

 

Problem - our electrician is so busy it may be hard for him to find time to do this before the ceilings are fininished.

 

So, is it possible to test myself without buying expensive testing kit? Is it easy? How is it done. I suspect @ProDave will have the answers!?

 

thanks

[Mattg4321]

Not very easy to test absolutely everything unless everything has been connected up (obviously without light fittings connected to wiring). 
 

Pretty time consuming to go around and test every leg/run of cable. 
 

Usual practice is to test during/after 2nd fix and deal with the problems then, as much as that can be a pain when it happens. 
 

Cables should be clipped or run at a depth in the wall or ceiling so the screws don’t reach them and can’t be damaged 
 

 

Edited November 9 by Mattg4321

[ProDave]

You will need an insulation resistance tester, if you are not doing it for a living you can get one cheap on ebay for example  https://www.ebay.co.uk/itm/256703940830

 

You need to measure insulation resistance between each of the 3 cores of each and every cable.  Yes it can be tedious. 

[markharro]

10 hours ago, ProDave said:

You will need an insulation resistance tester

@ProDave thanks very much for the link. Thats cheap enough to buy and its only really 3 ceilings in the house I am concerned about. How do you use this instrument in practise though? So lets say I have a 3 core wire for a light sticking through the ceiling - I connect each core in turn to one of the inputs on the tester ie earth or line? then what? do I need to trace the cable right back to the consumer unit (where currently all our cables is just hanging coiled to be connected ultimately? Find the correct cable and attach the correct core to the other input? If so, how is this done because the rooms are upstairs and the consumer unit downstairs!? Do I need an extension wire and connect that to one core at the consumer unit end and then run that up the stairs and into the bedroom and plug it in to the tester? Thanks again

[ProDave]

You are testing that none of the cores of the cable are shorted to another of the cores of the same cable, which is typically what will happen if there is a screw through a cable.  So start with one probe on bare earth wire of the cable and one probe on the brown, if you get good, high insulation repeat with one probe on the earth and one on the blue.  To be thorough test again with one probe on the brown and one on the blue.

 

Typically a light cable in the ceiling will go to the light switch box, as most houses now are wired "loop at switch" not directly to the consumer unit.

[markharro]

Ah thanks...so does that mean that I only need to connect at the one end of the cable run? It sounds like it from what you say? If so that's a whole lot easier than I'd imagined.

Also @ProDave can I ask what I do with these doubled cable ends on the photos.....one for smoke detector and other for lights. Just do the procedure on both cable? 

[G and J]

9 minutes ago, ProDave said:

Typically a light cable in the ceiling will go to the light switch box, as most houses now are wired "loop at switch" not directly to the consumer unit.

I’ve always run the loop round the roses, then a wire for each switch from the rose to it’s switch.  
 

Is there a particular advantage in running loop at switch?

[ProDave]

2 minutes ago, markharro said:

Ah thanks...so does that mean that I only need to connect at the one end of the cable run? It sounds like it from what you say? If so that's a whole lot easier than I'd imagined.

Also @ProDave can I ask what I do with these doubled cable ends on the photos.....one for smoke detector and other for lights. Just do the procedure on both cable? 

Yes test both cables individually

[ProDave]

1 minute ago, G and J said:

I’ve always run the loop round the roses, then a wire for each switch from the rose to it’s switch.  
 

Is there a particular advantage in running loop at switch?

Most people consider that the old way of doing it but there is no right or wrong way.

 

The disadvantage of loop at light, as that is called, is some consider it more tiresome terminating at a ceiling, and a lot of modern light fittings have inadequate provision for that many cables within the light fitting.

 

Loop at switch I find easier to terminate as you are working at a nice height.  It also provides a neutral at the switch if you want any form of smart switch, and in the case of say a string of downlights it really is the only sensible way to do it. 

[G and J]

2 minutes ago, ProDave said:

Yes test both cables individually

Goodness I’m full of questions (or something) this morning….

 

So typically a short to earth is less likely and hence not tested for, or indeed, is difficult to detect in a modern timber frame house?

[markharro]

Brilliant thanks Dave. I will snap that tester up. One last question...how do I know if a reading is "good". Does the screen show literally "good" or "bad" as the result or does it give a numerical figure (if so am I looking for a figure above or below X to be "good" etc. Thanks

[G and J]

1 minute ago, ProDave said:

Most people consider that the old way of doing it but there is no right or wrong way.

 

The disadvantage of loop at light, as that is called, is some consider it more tiresome terminating at a ceiling, and a lot of modern light fittings have inadequate provision for that many cables within the light fitting.

 

Loop at switch I find easier to terminate as you are working at a nice height.  It also provides a neutral at the switch if you want any form of smart switch, and in the case of say a string of downlights it really is the only sensible way to do it. 

And the only downside is that you need a deeper backbox on the light switch, which with timber frame is dead easy.  
 

I like that and will use that.  Another buildhub membership info nugget banked.  Again, thank you.

[ProDave]

18 minutes ago, markharro said:

Brilliant thanks Dave. I will snap that tester up. One last question...how do I know if a reading is "good". Does the screen show literally "good" or "bad" as the result or does it give a numerical figure (if so am I looking for a figure above or below X to be "good" etc. Thanks

You will get a reading in megaohms when set to insulation testing, and you are hoping for a very high reading, even infinity is good.  anything less than 1 megaohm wants investigating, but typically a screw through a cable will read 0

 

If you get a low reading, look first at the other end of the cable in case the cores are touch each other.

[ProDave]

17 minutes ago, G and J said:

And the only downside is that you need a deeper backbox on the light switch, which with timber frame is dead easy.  
 

I like that and will use that.  Another buildhub membership info nugget banked.  Again, thank you.

If you are doing a service void of 25mm and 12mm plasterboard, that is a perfect match to a 35mm back box.

[markharro]

On 10/11/2024 at 10:24, ProDave said:

anything less than 1 megaohm wants investigating

@ProDave Ive got the Robin tester now. You will see the instructions that came with it. I read this as saying that if the display reads "1" this indicates at least 200 Megaohms? I have tested one cable now and it reads 1 so that must be good. My question is the instructions dont say what the display reads if there is a problem! Will it read 0 or something else? thanks again

[G and J]

If you hold the two probes onto something, say maybe a piece of damp wood - which clearly won’t be highly electrically insulative, what does the display say on test?

 

If a perfectly insulated cable gives a ‘1’ then so perhaps might a duff test lead.  I would assume that shorting the probes won’t damage the device, (but I can’t guarantee it as I’ve never used one of these things so I’ll let others comment on that), but if I’m using a normal resistance meter the first thing I do is hold the probes together and ensure that the display reacts accordingly.  Testing for an absence of a reading is always a bit dubious.

[ProDave]

It's okay to short the probes.  The damp piece of wood is a good test to give some confidence that it is working properly.

[G and J]

4 minutes ago, ProDave said:

It's okay to short the probes.  The damp piece of wood is a good test to give some confidence that it is working properly.

Presumably this tester squirts a high voltage across the probes.  Is that ac or dc and is it tiny enough so it won’t act as a defibrillator?

 

Or should you shout ‘Clear’ just before each test?

[ProDave]

3 minutes ago, G and J said:

Presumably this tester squirts a high voltage across the probes.  Is that ac or dc and is it tiny enough so it won’t act as a defibrillator?

 

Or should you shout ‘Clear’ just before each test?

It outputs up to 500V or 1000V depending what you have selected, but only a low current is available.  It will certainly smart if you touch the probes.  Try not to.

 

[G and J]

10 minutes ago, ProDave said:

It outputs up to 500V or 1000V depending what you have selected, but only a low current is available.  It will certainly smart if you touch the probes.  Try not to.

 

As a kid I dismantled a flashgun.  The most obvious way to remove the pcb was to grab this cylinder looking thing (the biggest component on the pcb), my finger and thumb at each end of what I now know to be a capacitor.  Ouch.  Couldn’t instantly let go and had to fling out my arm to dislodge the pesky thing.  My hand hurt for days.  
 

Presumably this tester is a little more gentle?  Of course I’m now tempted to buy one just to play with it….

[ProDave]

DC is nasty as it is hard to let go.  But these things are designed to only output a low current, so if you did touch it the voltage would drop a lot lower.

 

It's volts that jolts, it's mills (milliamps) that kills.