Loft light not switching off

mutley74

Got a light in the loft on a simple 1way circuit with simple bayonet fitting for bulb.

For some reason light will not switch off. I tried changing the switch think but fault still persists with new switch. DAK have an idea what the fault could be?

(i have had to remove the bulb to avoid leaving the light being left on).

DVardysShadow

Did it ever work correctly?

If it did, my money is on the switch cable being crushed under a heavy piece of loft jetsam, shorting the 2 wires. This in turn indicates that either the switch cable does not have an earth wire or that the earth wire is not connected in.

SailorSam

If whoever wired it up didn't know what they were doing, you may find it's been taken from another light fitting, probably the landing, in which case when that light is switched off the loft may go off as well.

dkmax_2

DVardysShadow wrote: »

If it did, my money is on the switch cable being crushed under a heavy piece of loft jetsam, shorting the 2 wires. This in turn indicates that either the switch cable does not have an earth wire or that the earth wire is not connected in.

That could not happen in any normal 1-way arrangement. You would have to short across the switch contacts which could only happen in the switch enclosure. Shorting anywhere else within one cable (any L-N or L-E pair even if the earth is disconnected) will not result in the light coming on and moreover the fuse/MCB/RCD would trip. A two-way arrangement can be compromised as you suggest.

mutley74

DVardysShadow wrote: »

Did it ever work correctly?

If it did, my money is on the switch cable being crushed under a heavy piece of loft jetsam, shorting the 2 wires. This in turn indicates that either the switch cable does not have an earth wire or that the earth wire is not connected in.

yes it has worked well for over 20 years its in my folks house. If a cable was crushed would this not short the system? The switch only controls 1 light, not on 2 way with any other system. (there is no earth cable - only 2 cables in the switch box when i tried changing the switch)

someone did fit a handle to the wall not far from the switch recently, if the screw fitting hit a cable could this be the problem?

ormus

dkmax wrote: »

That could not happen in any normal 1-way arrangement. You would have to short across the switch contacts which could only happen in the switch enclosure. Shorting anywhere else within one cable (any L-N or L-E pair even if the earth is disconnected) will not result in the light coming on and moreover the fuse/MCB/RCD would trip. A two-way arrangement can be compromised as you suggest.

connecting the two red wires (as above) together anywhere, will turn the light on.
thats what a switch does.
wires crushed or a nail/screw will do it.


you need to read up on simple electrical circuits.

DVardysShadow

mutley74 wrote: »

yes it has worked well for over 20 years its in my folks house. If a cable was crushed would this not short the system? The switch only controls 1 light, not on 2 way with any other system. (there is no earth cable - only 2 cables in the switch box when i tried changing the switch)

someone did fit a handle to the wall not far from the switch recently, if the screw fitting hit a cable could this be the problem?

Crushing the switch cable would not short the system if there is no earth wire - it would just keep the light on permanently. It is worth looking at the handle - perhpas a screw has gone through the cable. Be very careful. If this is what has happened, the handle is probably live .

dkmax wrote: »

That could not happen in any normal 1-way arrangement. You would have to short across the switch contacts which could only happen in the switch enclosure. Shorting anywhere else within one cable (any L-N or L-E pair even if the earth is disconnected) will not result in the light coming on and moreover the fuse/MCB/RCD would trip. A two-way arrangement can be compromised as you suggest.

What ormus said above. I was very careful to say that this is the switch cable. OP has confirmed that the circuit is at least 20 years old - which means that it is less likely that there is an earth wire.

dkmax_2

ormus wrote: »

connecting the two red wires (as above) together anywhere, will turn the light on.

Agreed, but that's only if your two notionally "red" wires are contained within the *same* cable. What you have is a loop-in or junction system and not a "simple" 1-way where the T&E drop to the switch does not exist.

L

---

| / |

---

Bulb
N

---

That's a simple one way - L-N pair in and out of the switch. Shorting any of those pairs will not light the bulb. That's the way I see most lofts wired (you have free access to the cable and switch so you can wire them inline) but perhaps I am picturing the wrong scenario.

And I did miss DVS saying "switch cable" so I'm sorry about any misunderstanding.

dkmax_2

mutley74 wrote: »

someone did fit a handle to the wall not far from the switch recently, if the screw fitting hit a cable could this be the problem?

Ah, so the switch *is* in a wall - for some unknown reason I thought it was in the loft. Then this is a loop-through / junction system and the DVS scenario is likely to be correct, particularly when you have mentioned some recent fitting near the switch. Is it in a horizontal or vertical line from the switch ?

Don't remove any screws or nails without switching the circuit off. You are entitled to replace a damaged cable under Part P regs but I'd always recommend getting an electrician in. The cable may be plastered into the wall too so it might involve some redecoration unless you can site the switch elsewhere.

ormus

typical domestic light circuit.
(old colors).

if the loft light has a ceiling rose, then it will be similar to this.

same thing applies with a nail/screw, to the com and the switched live wires.

dkmax_2

ormus wrote: »

if the loft light has a ceiling rose, then it will be similar to this.

Although it doesn't matter if it is wired as ceiling rose or via a junction box, so long as there is a live and switched live drop. The OP will know this having replaced the switch already.

For sake of completion, as you know, very old lighting arrangements (circa late 60s or earlier, I would say) tend to have junction boxes, modern ones have loop-in roses - the rose just doubles up as a junction box where the looped-in/through live is passed on.

The last light of a lighting circuit even in a modern rewire may just be wired inline, in which case there will be four conductors at the switch enclosure and two at the light fitting (ignoring the protective conductor). With the inline arrangement it becomes less likely that a penetrating object will just short the live and switched live although it is not impossible.