Misleading test for dead

When we bought the house one room had a light switch with a 'on at dusk off after X hours feature'. Something was wrong with this (working intermittently), so I replaced it with a normal light switch.

There are four MCBs marked lights and it was not clear which was the relevant one. So I switched all of them off and tested for dead with a multimeter.

Having replaced the switch with an ordinary but old one that we had, and the switch was working correctly, I wanted to determine which was the correct MCB. So I left the switch hanging off the wall, turned on the MCBs one at a time and after each one I used the multimeter to test for live.

In every case the meter showed no voltage. I know the meter is okay as I checked it on a socket and it showed c. 240V.

When I was testing for live I was connecting to the screws that hold the wires in. It is an old light switch and these screws have a lot of patina on them (they are very dull / dirty looking). So I wonder if this patina was acting as an insulator.

Does that sound plausible?

If so then (and this has only just occurred to me) I could replace those screws with ones from the switch that I removed. Those are chromed and so should be okay.


NB I subsequently realised that I could work out which was the correct MCB, by switching the light on and turning the MCBs off one at a time.

 

Did the light work? You may not have proved dead, especially if you didn’t prove it was live first. Could be a switch live, as it might have been wired that way with the initial off after a certain period?

 

Yes.

I did say that I could have worked out which was the correct MCB by switching the light on and turning the MCBs off one at a time.

No switched live. Just a live & neutral to the switch.

 

Neutral is the name given to the conductor(wire) that completes the circuit from the load(appliance) back to the supply.
It is NOT a name for any black or blue wire.
Wires can be anything we want them to be; they do not know what colour they are.

 

You did, but wasn’t sure if you meant in the 1st place.

Anyway, which wires are you testing between to confirm live/dead? Did you test for this “no voltage” before carrying out any work? Any photos of wiring?

 

@EFLImpudence

I have no idea what you mean. I did not mention any colours. There are two wires in the switch. What do you suggest that I call them if not live & neutral?

About the old switch I said "Something was wrong with this (working intermittently)", so it could not be used to reliable determine anything.

"Just a live & neutral to the switch" - These two.

"There are four MCBs marked lights and it was not clear which was the relevant one. So I switched all of them off and tested for dead with a multimeter."

No. It's a light switch with two wires in it.

 

Perhaps look at this wiring diagram, which is what @EFLImpudence might have been on about. Perhaps you’re testing between live and switched live. Perhaps you’re concentrating on 2 wires, but have no understanding of how they operate.

 

Live and SWITCHED LIVE

The voltage between then will be ZERO when the switch is ON but both of them will be LIVE .both will be 230 VOLT above EARTH. DANGEROUS

The voltage between then will be 230 VOLTS when the switch is OFF one of them will be LIVE 230 VOLT above EARTH. DANGEROUS

The other one will have path to NEUTRAL via the lamp but it is NOT Neutral

 

Thanks for the clarification.

Are you saying that the two wires in a light switch are never L&N but always L&SL?

I can't be 100% sure but I am pretty certain that I would have left the switch in the off position except when I was testing the light.

The room is in use ATM but I will check later on.

 

Well, you wouldn't want to dead short live to neutral would you.

So the answer is yes, almost always unless it's a more esoteric arrangement, but as always never assume.

 

Yes that is what I said

 

Oh dear, to start with both line and neutral are live, in the main to isolated with a domestic installation you switch off the isolator or RCD, most MCB's in domestic are single module width and do not switch neutral, there are some RCBO's single module width which do switch the neutral but don't monitor neutral for over load.

To test for dead you test the tester normally with a special inverter unit, then test the items, and then re-test the tester. The regulations do allow with a TN supply to switch only the line, but first you need to be sure it is a TN supply not a TT supply.

Unless linked so line and neutral are switched together one should not switch neutral however this does not mean everyone follows the rule book. Also with three phase I have seen two phases only switched so the motor is always live, so seeing some thing works or does not work does not mean it is live or dead.

Only the green/yellow wire has just one use, and with plumbers even seen that wire used as line. Far too common on immersion heater thermostats to find a green/yellow is line. And although tradition in this country black is neutral, in Germany at one point black was line, their three phase colours were brown, black, black when ours were red, yellow, blue we both changed to brown, black, grey with blue as neutral, however this has resulted in colours meaning nothing. We did get twin and earth red, red, and bare copper for lights, but unless one is going to order a whole role of cable to get brown, brown, and bare copper is rather hard to find, so most over sleeve, and sleeves can fall off.

Personally I like the neon screwdriver, not to test for dead, but to alert if some error like a borrowed neutral is found. Yes today we have the clamp on ammeter, but mine starts at 0.1 amp, and 30 mA can kill, so 100 mA is really too much.

Even turning off the whole consumer unit does not mean safe to work, did it in this house, turned off whole CU in the flat (Ex garage) walked up stairs to find all still working, further investigation found a fuse box sandwiched between old ceiling and new ceiling with a hole in new ceiling just big enough to get hand in and turn off isolator. Could likely have removed a fuse, but not sure if I would have been able to replace it. Which also meant that very little in house was RCD protected.

Tradition in the UK was to run power ceiling rose to ceiling rose and only take switched and un-switched line to switch plus since 1966 the earth, however with the MR16 spot lights there has been a move to do like rest of Europe and wire switch to switch and a wire switch to lamp, also the need for two way switching and smart switches needing a neutral has resulted in many electricians running triple and earth to light switch to allow for future changes.

So today there is no standard, no standard wiring and no standard colours, at least that you can trust. And the number of borrowed neutrals due to people splitting the lighting supply to upper and lower floors I have lost count, I have even caught an electrician who should have known better when he found landing lights two way was causing a borrowed neutral, instead of getting rid of borrowed neutral simply put the two MCB's on the same RCD, and did not even seem to realise what he had done was wrong.

Yes I do use one of these to lock of the RCBO which only locks off the line, however I know my home is on a TN supply, and I am aware there may be voltage on the neutral so I work with care, which includes a neon screwdriver, I have had power return when working on an installation and also a nasty belt where another electrician was using an insulation tester on the same system, good job foreman was a good runner, had I caught him I may have been sacked.

Other electrician could not work out the odd insulation resistance readings he was getting!

 

Strictly speaking you implied it, hence my question.

I have checked and I do get zero when the switch is on & 230V when the switch is off.

So previously, either I was testing with the switch on and did not notice or I was not making contact. Thanks for sorting out my confusion.


One supplemental question. I have often seen people here tell a poster to test for dead but I have no recollection of anyone saying that the test for dead must be done in a particular configuration, e.g. light switch off as here.

Have I just missed that?

Of course not.

But live must connect to neutral somewhere to complete the circuit.

Looking at the third part of the diagram here

that connection is made at the bulb and the switch opens / closes a gap in the live wire.

I don't see any physical reason that there could not be, say, two live wires going to the bulb with live & neutral going to the switch.

If there a technical reason that lights are arranged in this way or is it just convention?

 

Live--------Switch-----Lamp-------Neutral

 

No

No, one is connected one side of the load and one to the other side.

If there were two live (correct sense) wires going to the switch, the lamp would not light, and
if a live and neutral (correct sense) went to the switch then, when turned on, a short circuit would be created and, without a fuse/MCB, the switch and wires would melt.
Loads have a resistance which is what stops that happening.

The fundamental flaw in your thinking is that the wires are determined by colour and are Live and Neutral before you connect them.
Their purpose and definition is determined by where you connect them.

What way? The circuit is merely a conductor split by the supply, lamp and switch positions.

A circuit:



You could insert a switch in the Line or Neutral, but it must be placed in the line.

If you disconnected the Neutral wire from the supply then it would become a Line - albeit at reduced Voltage.