Replacing a Dimmer Switch (Careless Shock)

This is more of a telling off for my own carelessness and a reminder that LABELS ON A UNIT MIGHT NOT BE CORRECT and you can't trust them, unless you did the install....

While not a disaster, it could have been.

Our house is 15 years old. Extension is 10. We bought it 14 months ago and I've been sorting out bodged DIY projects from the previous owner since.

While placing a light switch face plate back, after measuring the backbox depth for a replacement, I got a minor shock - despite the "UPSTAIRS LIGHTS" isolator being off.

Carelessly I hadn't tested the live connection on the switch itself when I unscrewed it.
(I've triple-checked for live currents as I'm very careful, but I lapsed tonight.)

Turns out the lights are wired into the "SMOKE ALARM" ring. Nice.

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Oddly, I'm not sure why the RCD didn't flip when I got the shock...seems odd?

 

10 milli-Amp will give a very noticable shock but it needs 30 milli-Amp to trip an RCD

Sensible as it provides an indication ( light not working ) if the MCB for the smoke alarms has tripped ( or been turned ) OFF

 

Fair point on the MCB for smoke alarms I guess - it just seems a little odd (before searching online, I assumed that mains safety devices would likely need to be separated on their own circuit).

Regardless, I'll be practising the "all off" method before doing even minor work as you can never be too safe.
(for example, there could have been a loose wire behind the switch which I could have contacted with)

 

TEST

FOR

DEAD

Get a multimeter, measure AC volts from live to earth and neutral to earth before starting any work. To be super-safe, you should then test your multimeter on a known live source to check that it does read a-okay.

Nozzle

 

Smoke alarms are not, or should not be, on a ring. Only 13 amp sockets can be on a ring.

 

as far as i know, it is common that Mains smoke alarms are to be wired into a light not light into alarms, but i guess it works the same way.

 

Sometimes (in good design terms) a smoke detector circuit isn't on an RCD bank.

Logic being that you don't want the smoke detectors to switch off when a superior fault trips an RCD. Same logic with having smokes and a lighting circuit together. No lights indicates no smoke detection working.

Linked smoke detection should have battery backup, and the units tend to chirp and flash the neon / LED when mains fails.

 

Think you know that most 30ma RCD's will trip around the 15-20ma range. 30ma is the maximum allowed.

But what if a circuit is fed directly from before the main isolator in the CU, thus having no over load or residual current protection whats so ever. Yes highly unlikely, but would not surprise me if such a thing has been done before.

And line to neutral!!! Plus, if your going to use a multi meter, be sure you have you have it set up and configured correctly!

Very True, but I guess one could technically find them fed off a fused spur from a ring final circuit.

Then put them on their own RCBO.

 

Yes I did know that, maybe I should have said " but it needs 30 milli-Amp to ensure that the the RCD will operate and thus limit the duration of the shock"

During the time it takes for the RCD to trip the earth fault current could be several amps.

 

can anyone recommend a good "how to" on multimeter use in household environments? either text or you tube video? ive only used mine when working on car electrics which are pretty simple. and at the very least id like to be able to test sockets and switches before changing them

 

If you're poking multimeter probes into mains circuits be careful! Most meter probes come with 2cm or so of bare metal at their tips. Touch that inadvertently with a finger and you could live to regret it, or not live . Best to use probes insulated to within a few mm of the tip.

 

Late to the party but just a hint, you can check if they're suitable as they should be marked CATIII or CATIV, as should the meter they're connected to.