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826 posts

Ultimate Geek


  # 2278196 17-Jul-2019 11:06
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Rikkitic:

 

If he's working on the light fitting as per the topic, it doesn't matter what the switches are doing. Light out = safe to work on. You don't need a multimeter for that.

 

 

Broken light also means light out :)

 

You need some sort of tool, even if there is some disagreement over what is suitable for the job. Older lighting installs often had phase connected while the swich was off, so you cannot assume it's safe. Personally, I don't assume anything is wired correctly.


Fat bottom Trump
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  # 2278204 17-Jul-2019 11:17
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You know the right circuit has been isolated because the light goes out when the circuit breaker is switched. It is unlikely that another circuit uses the same box but it doesn't matter if it does since there is no need to open it. How do you know that a fault hasn't caused your whole house to become live? Or that a bus won't run over you when you step outside? You are just reaching for things to play king of the mountain here. 

 

 





I reject your reality and substitute my own. - Adam Savage
 


 
 
 
 


98 posts

Master Geek


  # 2278207 17-Jul-2019 11:23
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Rikkitic:

 

You know the right circuit has been isolated because the light goes out when the circuit breaker is switched. It is unlikely that another circuit uses the same box but it doesn't matter if it does since there is no need to open it. How do you know that a fault hasn't caused your whole house to become live? Or that a bus won't run over you when you step outside? You are just reaching for things to play king of the mountain here. 

 

 

 

 

And if the light fitting isn't functional to begin with?

 

I'm not reaching at all - just someone with a little experience in the topic area. 


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  # 2278208 17-Jul-2019 11:25
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SirHumphreyAppleby:

 

Broken light also means light out :)

 

You need some sort of tool, even if there is some disagreement over what is suitable for the job. Older lighting installs often had phase connected while the swich was off, so you cannot assume it's safe. Personally, I don't assume anything is wired correctly.

 

 

Sorry, I was responding to the post before yours. When I have done this kind of thing in the past, I always switched whatever I was testing on and off a few times to make sure I was actually controlling the circuit. Yes, the light could suddenly burn out while being switched, but that is high on the list of improbabilities. Your meter could also break. If the circuit is turned off at the board, and the light goes out, then that fuse or circuit breaker is controlling that circuit. I think this discussion is approaching how many angels can dance on the head of a pin territory. Unlikely things can always happen so if you really want to be sure, then just switch off the mains altogether and hope lighting doesn't strike while you are replacing the fitting.

 

 





I reject your reality and substitute my own. - Adam Savage
 




1392 posts

Uber Geek


  # 2278210 17-Jul-2019 11:27
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Thanks for all of the responses here.

 

I take on board all the comments about not making assumptions, especially when dealing with live current.  In this particular case, I suspect (but am not 100% sure) that the wiring in the sleepout is very simple.  There is a simple switchboard with a mains switch and two circuit breakers.  One is for the garage (which the sleepout is part of) - I know this, because I've had to reset the circuit breaker when something plugged in in the garage went awry.  I suspect the second is for the sleepout.  There are four electrical sockets, and two light sockets.  One in the main room, and one in the bathroom.  There is only one switch for each light socket.

 

There is always the possibility that another current runs through the light circuit, but I think it is unlikely.  When I switch off the mains at the switchboard for the sleepout, all power to the building is off - every socket, every light in the garage and the sleepout is dead.  I know this because we have something plugged into every socket and light fitting, and all appliances go off.  So I think it will be reasonably safe if I switch off the mains for the garage/sleepout building.  I will, of course, take every precaution and won't proceed if it seems more complicated or confusing than I suspect.  I have no desire for a jolt...


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  # 2278213 17-Jul-2019 11:30
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TheMantis:

 

I'm not reaching at all - just someone with a little experience in the topic area. 

 

 

Are you suggesting I don't have any? This has turned into a 'I know more than you' competition, as often happens with these sorts of discussions. Good. You know more than me. You win. Happy?

 

 





I reject your reality and substitute my own. - Adam Savage
 


2894 posts

Uber Geek


  # 2278218 17-Jul-2019 11:44
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I originally replied to the statement that there wouldn't be power at the switch because that may have been true in the past with multiple switch plates either by rule or good practice but that is no longer the case.

 
 
 
 


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Ultimate Geek

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  # 2278277 17-Jul-2019 13:40
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BlueShift:You shouldn't need to turn off the main, as long as you turn off the wall switch to the light. Then, like you say, its just a couple of wires.

 

Be careful. On (very) old houses, it was very common to "feed" the fitting and just run a twin cable to the switch so there would be a live wire at the fitting at all times.

 

(Very uncommon though on houses post 1970)





Rob

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Uber Geek


  # 2278315 17-Jul-2019 14:41
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Could you do he job without turning off the mains switch is an entirely different question to should you turn off the mains switch.

 

If the OP doesn't have the correct tools or knowledge to confidently prove the circuit is dead, then the most fool proof (no slight intended) and life-safe advice should be dispensed (ie turn off the mains switch).

 

Yes, you can (and I have) replaced light fittings only turning the circuit off at the switch or C.B. However, without a multi-meter and the knowledge to confidently use it you are staking your life on 1) there not being a secondary fault such as a welded switch contact or water in a switch or 2) a parallel path because someone previously wired something incorrectly , 3) the circuit being wired to an old or incorrect standard such as neutral switching.

 

Should you be teaching electrical noob's best trade practice or a shortcut that makes a number of assumptions and could hurt them?

 

I have encountered circuits that were neutral switched, so the light switch controlling the bulb satisfactorily does not mean the circuit is safe to touch. Also even in a properly wired circuit, contamination in the switch such as water, ants or metal oxides from arcing can flow insufficient current to illuminate the bulb, but pass plenty of current to shock you and knock you off the ladder.  Floating/ghost voltages can also be induced by parallel circuit - commonly these can't be felt through the thick skin of your hands, but will give a small shock when touching the sensitive skin under your forearm, or on your neck and face. Whilst these are not dangerous, they can scare the heck out of the uninitiated, causing them to recoil and fall off the ladder.

 

If you turn off the mains switch, yes you can cause inconvenience in the rest of the house, but you eliminate a host of latent hazards.


255 posts

Ultimate Geek


  # 2278597 17-Jul-2019 20:42
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Slightly off topic, but I recently changed a 2 pin plug on my vacuum  clearer to a 3 pin plug. The old plug only had 2 lugs but the new plug had 3.

 

All I did was  connect the 2 cables from the cleaner to the 2 top lugs on the 3 pin new plug.

 

Any problems with this?




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Uber Geek


  # 2278601 17-Jul-2019 20:58
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All done. Bought a comparable fitting from mitre 10. Switched off the mains. Disconnected the old fitting. It had three wires - black, green and red. A quick search confirmed these were the old colors, but verified which was which. Slotted the wires into the new fitting, screwed them down. Good as gold!



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Uber Geek


  # 2278667 18-Jul-2019 08:47
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So the next question - fixing the light in the bathroom has thrown fresh light (pun intended) on how dim the single light fitting is in the main room of the sleepout, even with the brightest LED bulb available.  Is it possible to just swap out the single light fitting (ceiling mounted) with something like this with three lights instead of the single light?  Or is the wiring different for the two types of fitting?


928 posts

Ultimate Geek


  # 2278693 18-Jul-2019 09:16
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BlueShift:

 

If you're the home owner, and you're replacing like with like, you don't need a sparkie.

 

You shouldn't need to turn off the main, as long as you turn off the wall switch to the light. Then, like you say, its just a couple of wires.

 

I've replaced most of the light fittings in my home (incandescent downlights to LEDs), and a few switch plates as well.

 

 

Bad idea.

 

You KNOW the phase is dangerous, however the Neutral can be just as dangerous (if not more so because of complacency).

 

Just turn the mains switch off to be sure


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  # 2278699 18-Jul-2019 09:29
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Lizard1977:

 

So the next question - fixing the light in the bathroom has thrown fresh light (pun intended) on how dim the single light fitting is in the main room of the sleepout, even with the brightest LED bulb available.  Is it possible to just swap out the single light fitting (ceiling mounted) with something like this with three lights instead of the single light?  Or is the wiring different for the two types of fitting?

 

 

This shouldn't be a problem though I have seen some fittings like this that can be a little confusing. The lights are wired in parallel internally and there will be the usual two or three connection points for the power. It can be installed just like a single fitting. Where it can be confusing to non-experts is when all the connections are made on a single terminal block, with different wires running to different places. The connection points are usually clearly marked but you have to double-check that you are connecting the right wires to the right places,





I reject your reality and substitute my own. - Adam Savage
 


928 posts

Ultimate Geek


  # 2278877 18-Jul-2019 12:29
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Rikkitic:

 

SirHumphreyAppleby:

 

Broken light also means light out :)

 

You need some sort of tool, even if there is some disagreement over what is suitable for the job. Older lighting installs often had phase connected while the swich was off, so you cannot assume it's safe. Personally, I don't assume anything is wired correctly.

 

 

Sorry, I was responding to the post before yours. When I have done this kind of thing in the past, I always switched whatever I was testing on and off a few times to make sure I was actually controlling the circuit. Yes, the light could suddenly burn out while being switched, but that is high on the list of improbabilities. Your meter could also break. If the circuit is turned off at the board, and the light goes out, then that fuse or circuit breaker is controlling that circuit. I think this discussion is approaching how many angels can dance on the head of a pin territory. Unlikely things can always happen so if you really want to be sure, then just switch off the mains altogether and hope lighting doesn't strike while you are replacing the fitting.

 

 

 

 

The "Correct" way to test with a multimeter is

 

Verify, test, verify .

 

So you verify the meter works correctly, do the test and verify it still works correctly.

 

Yes, the circuit breaker may control the phase, but it does NOT control the neutral, and if the neutral has been illegally wired so it is a back feed for another circuit it can be lethal , and yes I have come across this. NEVER trust the idiot who was there before you. And yes, I am a registered electrician.

 

My advice for home owners is to turn off the mains switch, and then STILL test between phase and earth and phase and neutral before and after.


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