I want to put up a light switch for the loft, I was hoping to use a switch with a neon light in (such as those used for water heaters), so it will warn if the loft light is left on.
The connection unit I bought has a supply, live and neutral and a load, live and neutral, I've only got the switch wire running to it.
Is there any way of wiring it up so it will act as a light switch, turning the loft light on and off and at the same time lighting the neon? I had a look at the other postings and I'm still unsure whether this is possible.
The switch needs to have a neutral run to it, otherwise it won't work. Connect the incoming live to the supply live and the loft light to the load live. Connect the neon between the load live and neutral. The neutral does not really need to be switched, but the switch is probably designed so that it is easier to connect the neon to the load neutral than to the supply neutral.
Get a L+N supply from a nearby ring or lighting circuit to the "Supply" L+N terminals. Connect the light fitting to the "Load" L+N terminals. This is the correct method.
If the wiring is already in place for a light switch, with 2 circuit wires and an earth, then connect one of the circuit cables to the "Supply" L terminal (try the red one if they have different colours). Connect the other to the "Load" L terminal. Connect the earth to either one of the N terminals. If the light stays on all the time, then reverse the 'L' connections. This is the bad noncompliant method, and, if the light circuit is RCD protected, could preload your RCD with leakage current, although probably not enough to trip the RCD on its own.
If you were in Scotland it would be simple. You would take a live feed from the radial lighting circuit to the supply side of the switch. You would then take cable from the load side of the switch to the bulb. This has the added advantage that when the switch is off, the light socket is *completely* isolated, so it is much less likely that you would electrocute yourself while changing a bulb in a dark loft. The disadvantage is that you have two lots of twin+earth running to the switch, rather than one switch cable.
However, I have no idea whether this is acceptable under the English wiring regs, and I don't have my OSG to hand. Does anyone else know?
If the wire to the switch has an earth core, check that it's connected to earth at the loft end, connect the switch wires across the supply & load Lives in the switch and the earth core to the N Load terminal. The current through the neon is negligible and quite safe to run through earth.
There is another way if you really don't want to run any more wires... Connect a neon across the switch (connect switch wires between L in and L Load, link N load to L in) This is then a 'loft light is NOT on' indicator...!
It the existing switch wire is a twin and earth cable that is in a plastic sheath it might be quite easy to replace this with a three core and earth cable. - I am assuming that the cable has a short run to your loft!
You can probably buy a few metres of 3 core cable from a shed.
In your "bad noncompliant" method, on first reading I thought you were talking about using the earth conductor in the cable to the switch as a Neutral, which I would have thought was a Very Bad Idea.
On a closer reading I see that you are saying to use the actual earth conductor, STILL CONNECTED TO EARTH AT THE LIGHT FITTING, as a "neutral" feed to the neon, which I suppose would work but, as you say, it's adding a bit of earth leakage that might encourage an RCD to trip. Would be OK though on a non-RCDed lighting circuit (such as with a split consumer unit - or as in my case two separate consumer units).
In the one case where I did this (actually for a bathroom fan switch where I added a neon) I used 3 core+earth to bring in the additional neutral in addition to live and switched live. Of course the colour coding was then all wrong and confusing so ISTR I had red for live and probably yellow (with a red bit of tape) for switched live, and of course blue for neutral. Probably highly illegal but it works for me.
Actually, your method was entirely compliant and the correct solution for your problem, provided you used black markings (preferably sleeving, rather than insulation tape) on the blue wire and red markings on the yellow to indicate that they are not additional phase conductors.
Wow, everyone thanks for the feedback, all very useful. I did think about adding in another cable, it's just annoying cause the switch has already been nicely plastered in, still it's my own stupid fault.
Would the "using the earth" method be at all dangerous? The lighting isn't on the RCD, so that wouldn't be a problem, but I don't want it to not be earthed. Would it comply with the regulations?
We all know that feeling. There ought to be a German word for it (from the same stable as "schadenfreude") - the feeling of frustration when you want to be angry about a cockup but there's totally clearly no-one to blame but yourself... ;-)
"At all" - barely, but in-the-limit yes; compliant - not really, but not so it matters hugely (though I wouldn't ever try to get away with it if done for hire-or-reward, in an Electricity-At-Work situation, etc. etc.) What you're doing is providing a return path for the tiny current through the neon along the earth wire, rather than through the circuit's neutral. Because of the construction of the neon, the current really is tiny; if you were being paranoid, you might put a 240V-rated high-ohmage resistor (half a meg or so) in series with the neon - as a second circuit element to reduce the possible current flowing if the neon or wires leading to it ever shorted out. (Neon indicators don't, however, have "failing short" as a known/reasonably-anticipatable failure mode...)
I can't think of a single-point failure which would make this arrangement positively dangerous: even if the neon or surrounding wires went short-circuit, the resulting live-to-earth fault would pop the MCB. You'd need such a fault *and* a break in the earth conductor to make the "earth" wire, and touchable things on the same side of the break, live and capable of supplying dangerous currents. And putting high resistance/impedance circuit elements from L to E is no more than what mains filters on appliances regularly do (though these filters are designed to fail safe and constructed with some care). You could even argue that the neon in the switch would be acting as an earth integrity monitor (i.e. if the neon fails to light when the switch is turned on, it indicates that the "earth" isn't one anymore - assuming the neon still works, that is ;-).
Nevertheless, using the earth conductor as a circuit return is not good practice, so this isn't truly Regs-compliant (but it's better than borrowing a neutral from some other circuit which happens to be running close by!) Final judgment on whether to run it this way has to be yours; maybe you could salve your conscience by telling yourself you'll run
3core+E in place of the existing T&E next time you redecorate ;-)
I think the neon assembly in the switch will already have a limiting resistor built in. From what I remember from my very distant school physics neon tubes have a high impedance up to their breakdown voltage (approx. 90 volts?) but the impedance becomes very low when once they start to conduct. It wouldn't hurt to add an extra resistor though, providing it doesn't make the neon too dim.
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ISTR the old Bridges power drills circa 1960 before the days of double insulation had this as a feature. They were advertised as having a "magic eye" which claimed to indicate that the drill was earthed.
Something in the same mould as "buyer's remorse", which I occasionally get (but only briefly) after buying tools that I know I will only use once every two years and probably should have hired.
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