Adding new light switch to existing circuit

[guy999]

Hi
Had a chap in (65 years plus) to swap a ceiling light for new down lights, add an additional switch for the lights and re plaster ceiling, in utility room.
He added downlights, wired to old ceiling rose, switch worked no problems.

Later added new switch, think he wired this direct from old ceiling rose, didnt put in bulbs and plastered.
I later added the bulbs, turned on, when tried to turn off blew main fuse board. This happened a couple of times.
So asked someone to look at, bare in mind i dont know electrics plus i'm colour blind!

However read wiki here and he sounds right!

Says switches are 1 way not 2 way, cable for new switch is twin+earth, should have 3 cables and earth which should be connected to existing switch.

Before i get him back in, anyway he could have added switch correctly using 2 core plus earth... not sure what he will say.

Thanks

 

[JohnW2]

Says switches are 1 way not 2 way, cable for new switch is twin+earth, should have 3 cables and earth which should be connected to existing switch. ... Before i get him back in, anyway he could have added switch correctly using 2 core plus earth... not sure what he will say.

Yep, that's right. To have two switches controlling one light in the normal way requires that the switches both be 2-way ones, and that the cable between them is 3-core (and earth).

So, the person who did it had no idea of what he was doing. However, it gets worse, since using the wrong switches and cable would not, in itself, "blow your main fuse board" - so he's presumably done something else wrong as well.

If it were me, I wouldn't trust that person to 'put things right' - you really need someone who has much more of an idea of what they are doing!

Kind Regards, John

 

[guy999]

Thanks, thought so. Chap came well recommended plus with age trustworthy!
Ceiling has been boarded, plastered and new cables away. So thinking of caping new switch and not using...but is it safe?

Trying to second guess his reply, yo how he did this.
And no, wont be using again.

 

[Doggit]

The odd thing, is that the lights came on, but the fuse blew when you turned them off; this suggests that the wiring is okay, but the bulbs may be at fault. How long were they on for before you switched them off. It's possible that if he's used the earth as the third cable, then there may be an earth connection somewhere that's causing feeding the live back down to earth when you switch the lights off.

I'd be inclined to talk to the first installer, and see if he can find the fault, but at some stage, warn him that if it needs redoing, then you expect a refund of his charges. It could be something quite simple, and we all have off days.

 

[JohnW2]

The odd thing, is that the lights came on, but the fuse blew when you turned them off ....

That is indeed odd, particularly if they are 1-way switches (see below).

It's possible that if he's used the earth as the third cable ....

It's certainly been done! However, although there may have been some misunderstanding, the second person appears to have said that they were 1-way switches, in which case the first guy would not really have had anything he could do with a third core (whether insulated or not!).

Kind Regards, John

 

[bernardgreen]

Two way lights that trip the MCB when the light is turned OFF may have been wired with Live and Neutral looped at the switches

When a switch is operated to turn the lamp ON there is no current being broken so no arc between the contacts in the switch.
When a switch is operated to turn the lamp OFF there is an arc created by the breaking current. If this arc continues long enough for the moving contact to reach the other contact the arc is now across the Live and Neutral and will continue as a plasma short circuit between Live and Neutral until the MCB trips.

Older switches with large gaps between contacts did not sustain the arc because the gap was two wide and this economical way of wiring lights was trouble free (*) but modern switches with only a 3 mm gap between contacts will sustain the arc.

(*) it is not a sensible way to wire two way because the two contacts in the lamp holder can be both Live when the lamp is OFF

 

[securespark]

Aren't you switching neutrals there?

 

[securespark]

but is it safe?

Probably not.

The best thing to do is take some pictures (or get someone to take pictures for you) of the junction box - old ceiling rose you say (probably sited near where the surface-mounted fitting used to be) and the wiring inside the new and existing switches.

Please turn off the electricity before exposing anything (Oooh Matron!)

 

[JohnW2]

Two way lights that trip the MCB when the light is turned OFF may have been wired with Live and Neutral looped at the switches

Theoretically true - but, as I wrote in the previous message, it appears that one of the problems is that the OP only has 1-way switches, in which case what you postulate would obviously not be possible.

Kind Regards, John

 

[guy999]

Thanks for your replies, need to have a good read through to throughly understand.
But yes only used one way switches not 2 way.
Wasn't sure if he was trying to do some old fashioned way, now found out he's actually 75 not 65 years old! My house is around 25 years old.
Waiting for him to come back.

 

[guy999]

Also they lights were only on for few seconds before turning off, when they blew/tripped the fuse board when turned off.