Changing light switch - how to confirm live/dead circuit

[jonnyw]

Firstly as you'll note by this thread, I'm not an electrician, but am cautious/sensible enough to post the question, so should hopefully(!) be up to the job.

I have a damaged (but still working) 1 way light switch that needs replacing. I've done this before, though to do this previously I've simply turned off ALL the power at the breaker, tested the light didn't work, replaced the switch, turned everything back on, tested again.

However, I now believe there is a safer way

1. Test the circuit at current switch is live (to confirm test equipment works)
2. Remove consumer unit fuse for lighting (to isolate)
3. Test the circuit at current switch is now dead
4. Test the circuit at another switch on a different circuit (to confirm test equipment is still working)
   
5. Change light switch
6. Turn everything back on and test it works

Does this sound like the correct way to do it?

I believe the screwdrivers with lights on are deemed to be inaccurate/potentially dangerous. If so my question is how do I safely use a multimeter to test the switches are live /dead at the various stages?

For 4, could I instead just touch the two multimeter probes to ensure a reaction?

 

[PrenticeBoyofDerry]

The above is about right for safe isolation.
To test for voltage the safe method is to use a two pole/probe tester.
This can be a multi-meter or voltage indicator.
Most voltage indicator, have a compatible proving unit that can test that the tester is functioning correctly.

But if you are using a mutli-meter the process would be to prove it works on known live supply, then test for isolation with the meter, then retest the meter on known live supply again.

Generally when isolation has been proofed, you would either lock the circuit off or remove the fuse. Then place a notice at the board, stating that the circuit has been safely isolated. But there are other methods that can be used to prevent accidental or intention powering up of the circuit.
I am of the opinion, that if the fuse board contains identical (pull out) fuse holders, it would be easy for someone to re-energise the circuit, by simply pulling one out and sticking it in the circuit your working on.

http://www.ett-ni.org/media/Safe Isolation Procedure Download.pdf
https://www.ifixit.com/Guide/How+To+Use+A+Multimeter/25632

 

[ban-all-sheds]

1 & 4 involve removing a switch from the wall while it is live, not something to be advised if there are safer alternatives.

The "official" way is to use a battery-powered proving unit, but you might not be able to justify buying one of those.

IMO a safer way would be to use a socket opener or test adapter

If so my question is how do I safely use a multimeter to test the switches are live /dead at the various stages?

Set to an AC voltage range ≥ 250V, and taking great care not to touch the ends of the probes, or to slip and create a short-circuit with one.

Ideally you should replace the ones which came with your multimeter with shielded ones.

For 4, could I instead just touch the two multimeter probes to ensure a reaction?

No - with it set to voltage touching them will do nothing.

Makes me worry that you don't really know what is going on.

What did you plan to measure between at the switch?

 

[ericmark]

What you are really asking is for a risk assessment.
What a risk assessment does is get to to consider the risks and write them down, in doing that it makes you think about it. So I will attempt to go through the risks and then you can see how one needs to use a common sense approach, remember it is not what I say being correct, but that it make you think.
1) You want the light switch dead before you remove it so the bulb going on and off with what every device you select to isolate with, is rather a good start.
2) To isolate you should disconnect all live wire, both the line and neutral, however that means main isolator, not a fuse or MCB, so it depends on the system, for earthing, but many will only switch off the line, which means you can still use a table lamp to see what you are doing, but also means if you touch neutral the RCD will trip.
3) For any two wire tester to work it needs two connections, so if there is an error like line and neutral swapped going into the consumer unit, which is unlikely, but if you did have that fault the two wire tester will show dead when in fact it is live. The other problem is borrowed neutrals, where this happens a neutral wire can become live as you disconnect but show as ground potential when connected. The neon screwdriver will show up these faults.
4) On the reverse of that where items are bonded the and a neutral and live are swapped, then a neon screwdriver can show neutral as being live and worse line as being dead. Again unlikely.

So as we start to list one realises for 100% safety only way is main isolator, however you are 99.9% safe just using the bulb to show live or not, and with added protection from using a neon screwdriver, and working as if the switch is live even though switched off, then we add more 9's behind the decimal point. So I would use your first method.

I have only once worked on a site where we used proving units for 230 volt to earth supplies, I was impressed when I went on site with all the safety devices. However I found the tester

had a flaw, The tester may only show lights at 500 volt, and could have a fault where the lights don't work at 200 volt, and the proving unit only tests with 500 volt, so you could have 200 volt on the circuit and it still show as dead even with a proving unit. There are now proving units which step up and down the volts. But the point is where working in domestic the likely hood that after proving dead by simply switching the light off and one and off again using a MCB of it becoming live, specially with a neon screwdriver as back-up are very slim.

 

[PrenticeBoyofDerry]

What you are really asking is for a risk assessment.

I think they are really asking, how to perform the safe isolation procedures, not how to risk access.

1) You want the light switch dead before you remove it so the bulb going on and off with what every device you select to isolate with, is rather a good start.

Not if the lamp blows/goes faulty or the switch goes at the same time.

Rest of post, has ether already been covered or not required for the task of replacing a standard switch plate.

 

[jonnyw]

Thanks for everyone's responses.

I think essentially I was posting for two things. A sanity check on my proposed method (which seems to be approved of but not flawless*), and also details of how to use a multimeter to test a circuit is live, and then prove it is dead.

* I realise removing a live switch can be dangerous, thus the reason I don't want to be probing round with no idea of what I'm doing!

I suspect that all things being equal, the best/safest way for ME to get this done may be my original method via complete mains isolation. Unfortunately this switch does need replacing today.

No - with it set to voltage touching them will do nothing.

Makes me worry that you don't really know what is going on.

What did you plan to measure between at the switch?

I guess I was referring to using the resistance setting on the multi-meter - but you're right, I absolutely don't know how to use a multimeter to test if a circuit is live or dead, which is my primary reason for posting here.

Frustratingly a lot of the information you find on the net from known sites is either incomplete, or misses potentially critical steps.

E.g. both the B&Q and Dummies sites say to check that the circuit is dead, but give no indication of how. Neither site mentions the live - dead - live tests (as per the procedure I mentioned above).
http://www.dummies.com/home-garden/home-improvement/lighting/how-to-replace-a-light-switch/
http://www.diy.com/help-ideas/how-to-replace-a-light-switch/CC_npcart_400249.art

Are there any good beginner books/recommended reading for the amateur to carry out standard tasks (e.g. replacing a socket/switch/light fitting)?

 

[PrenticeBoyofDerry]

You can start by looking in Wiki, lots of useful info there and it is FOC.
https://www.diynot.com/wiki/Electrics:index

Then there is this guide with regards to safe isolation

http://www.electricalsafetyfirst.org.uk/mediafile/100117573/Best-Practice-Guide-2.pdf

With regards to basic electrical books, I would be unsure what to recommend, as I do this a profession and have no need for DIY books.

But for the price, this seems to have consistently good reviews
https://www.amazon.co.uk/Wiring-Lig...coding=UTF8&psc=1&refRID=A23BF9PWB70QQJ2JVRAH

 

[ban-all-sheds]

Bear in mind this:

Most of the book will be fine, and relevant, but it will be behind the times on things like RCD protection, Building Regulations,....

 

[John D v2.0]

The main thing that hasn't been spelled out explicitly yet is to do your voltage testing first between live and neutral, and live and earth. It should be 240 ish in both cases in the absence of a fault. And zero with the mcb or fuse out. Then secondly test neutral to earth, that should be close to zero all the time unless you're using some high current device somewhere or there's a fault.

 

[PrenticeBoyofDerry]

The main thing that hasn't been spelled out explicitly yet is to do your voltage testing first between live and neutral, and live and earth. It should be 240 ish in both cases in the absence of a fault. And zero with the mcb or fuse out. Then secondly test neutral to earth, that should be close to zero all the time unless you're using some high current device somewhere or there's a fault.

And to add to that, if you are testing at lighting switch plate, there is a good chance there will be no neutral present and a chance that the earth does not exist or has no continuity. That is why dead tests are important.

 

[deadshort]

However, I now believe there is a safer way

The safest way would be to call an electrician or switch of the supply at the mains

DS

 

[flameport]

switch of the supply at the mains

Until the main switch is defective and only switches the neutral.

Yes, I have seen that happen.

 

[deadshort]

Lucky white heather sir ?

DS

 

[JohnW2]

Until the main switch is defective and only switches the neutral. Yes, I have seen that happen.

Very little 'never' happens!

Operating the main switch still seems to be the 'safest' way to try to ensure that a particular circuit is dead - although, as you point out, there is, even then, still a need to confirm (by testing) that the circuit is dead.

Kind Regards, John

 

[deadshort]

I will now switch off the circuit mcb before switching off the mains switch, wearing my marigolds and green wellies !

Flamport, don't waste your money on lottery tickets !

DS