I am replacing a pendant light together with the light switch. I've totally disconnected them from the mains wiring -- I basically have two sets of three wires sticking out of the wall and out of the ceiling.
When I turn the power back on at the consumer unit, and use a mains tester (screwdriver type) on the three wires sticking out of the ceiling, all three are shown as LIVE.
This strikes me as very odd.
Is this necessarily indicative of a serious wiring problem somewhere?
Thanks Alexei
Neon type testers can give misleading results. To be certain if a wire is live, connect one side of a (mains) light bulb to a known neutral and probe the unknown wires with the other bulb connection.
Bob
On 2 Oct 2005 09:35:50 -0700, "Alexei" scrawled:
That's because the neon screwdriver method of testing is absolutely useless. It goes in the usefulness file along with cheap stud and pipe detectors.
Nah, quite normal. It'll probably tell you your elbows live but fail to tell you a live cable is live.
Serious lack of professionalism tbh. Get a proper tester on the job.
This would explain it, thanks. Out of interest, under what circumstances could a neon-type tester give misleading results?
I'll do the lightbulb test some other day, when there's sufficient daylight.
On 2 Oct 2005 10:29:33 -0700,it is alleged that "Alexei" spake thusly in uk.d-i-y:
If one or more of the cables that are disconnected is running parallel to a live cable. If it's not actually a neon screwdriver but one of the 'continuity testing and mains testing' Hypersensitive LED ones I noticed B&Q have on clearance, the value of 'running parallel to' changes to 'within spitting distance of'.
On 2 Oct 2005 10:29:33 -0700, "Alexei" scrawled:
Whenever you use it.
If you're planning on doing any more electrical work it would be worthwhile investing in a reasonable multimeter. You can pick up something respectable from around £20.
Here a full account of what I've been trying to do.
I am installing a new pendant light. To this end, I've disconnected the existing (working) light, and wired the new light, connecting red to brown, black to blue and the third wire to both green/yellow and the mounting bracket (the pendant is mostly metal).
This seemed to work perfectly. I've left the light on for about 15 minutes, after which the switch at the consumer unit tripped. I've checked the bulbs, suspecting one might have blown, but all appeared OK. I've turned the trip switch back on, but the light wouldn't work.
I suspected the dimmer switch may have been to blame. I disconnected it to take a look and it looked dead (evidence of smoke).
I replaced the switch with a 450W-rated dimmer (the load is 5x60W, the old dimmer was rated for 400W), wiring black to C and red to L1. The new one died exactly the same death the moment I tried to turn it on.
At this point I've removed both the light and the switch to see if I've managed to screw something up, but everything looks OK as far as wiring goes: all the wires are connected where I intended them to be connected, there is no exposed wiring, etc.
Messing about with the tester was just a quick sanity check; I didn't realise they are unreliable. Stuart's comment about lack of professionalism is a fair one; however, it is worth bearing in mind that this is a DIY forum and I felt perfectly up to this relatively straightforward job.
I am probably going to get a qualified electrician to sort this out. In the meantime I would be most interested to hear what steps you would take if you were trying to figure this out.
Thanks Alexei
On 2 Oct 2005 11:17:36 -0700,it is alleged that "Alexei" spake thusly in uk.d-i-y:
In your original post, you stated you had "two sets of three wires sticking out of the wall and out of the ceiling."
Just to clarify, did you mean 'one set of 3 at the switch, and one set of 3 at the lighting point' ?
If so, it sounds like you're connecting things up right, (although the black at the switch should technically be sleeved red or brown).
If the fault is occurring with this apparently perfectly normal wiring, you have 2 choices.
1) Call an electrician (guaranteed but pricy).2) Try swapping parts, obtain a 'normal' lampholder and 60w lamp, and a light switch of the 99p variety, and see if the fault recurs when you swap them in. If it doesn't, the fault was in one of the 2 things you just eliminated.
It sounds to me like one of 3 possibilities.
A) The new dimmer was faulty. (unlikely as a second also died the same way)
B) The new light fixture is faulty (possibly caused during manufacture)
C) The junction box (which must be present somewhere, probably above the ceiling) has an intermittent short circuit, this should be considered when trying 'turn it on and see' methods, not trying to induce paranoia, but a slow smouldering fire under the floor is A Bad Thing.
A neon tester can pick up induced voltages in wires which run parallel to live wires, and which are *not* actually live. Far better to use a voltmeter - or a multi-meter set to an appropriate scale for measuring 240v AC.
It's possible that your dimmer switches don't like the type of load you're giving them even though, in theory, they have more than enough capacity. have you checked what happens when you use a bog standard on-off switch rather than a dimmer? It's easier to get it working with an ordinary switch first, and then worry about dimming, later.
On 2 Oct 2005 11:17:36 -0700, "Alexei" scrawled:
Well, evidently it's not straight forward enough. ;)
Sounds like you've pulled down the old pendant, blatantly ignored the loop in and switch wire connections and wired the light across the live and neutral, in parallel with the dimmer switch, causing the light to work and the switch to explode.
When you wired it up did the switch work and the light dim up and down as it should?
I would get my test meter out so we're stuffed at this point as you haven't got one!
What would you be looking for in a multimeter for this type of work?
I use one of those for non-mains work:
Alexei
Yes, everything worked as it should. The lights turned on and off and dimmed up and down as expected.
Alexei
On 2 Oct 2005 12:00:34 -0700, "Alexei" scrawled:
No, me neither.
I've used AlphaTek for some good cheap meters in the past. Robin also do an autoranging rugged DMM for £20 odd (I think, I could be making that bit about Robin up).
As a side note, it's a pity Maplin can't spell 'professional' (as in proffessional multimeter, down on the left).
Is it fair to say that this type of tester can give false positive but not false negatives (i.e. if a wire is live, a working tester will always show it as live)? This is purely hypothetically of course, I am not going to rely on one from now on. :-)
No, I've not checked that yet, but am intending to. The reason I've not tried this earlier is that I wasn't expecting the second dimmer to explode in an apparently working setup. Otherwise the second switch would have been an ordinary one.
Alexei
Chip,
Thanks for the detailed reply.
That's right.
Swapping parts sounds like a good plan. I've got all the parts I need, and will probably have another stab at this before calling an electrician.
As suggested, I'll try a normal switch.
I have the old pendant (basically a lightbulb-on-a-flex type thing), which I can try. FWIW, I've used an ohmmeter to measure the resistance across the wires on the new fixture, and there's no evidence of any short-circuits (by no means a definitive test).
That's a scary thought. I take comfort in knowing that everything worked fine until I started messing around with it, and the room got completely rewired about 4 months ago.
Alexei
On 2 Oct 2005 12:13:47 -0700, "Alexei" scrawled:
I wouldn't rely on a neon tester to give a true anything. I think the term "working tester" when used in the sense of a neon screwdriver is stretching the truth somewhat.
On 2 Oct 2005 12:07:39 -0700, "Alexei" scrawled:
Hmmm, were any other lights on the circuit switched on at the same time? Did any of them display strange behaviour whilst the testing of this one was going on?
On 2 Oct 2005 12:25:13 -0700,it is alleged that "Alexei" spake thusly in uk.d-i-y:
[snip]
Hmm, being fairly new, I wonder if a connection could be loose and arcing, which arcs can then spread to earth or neutral tripping the breaker?
The multimeter advice you have got from other respondents is certainly a good idea too.
Since I am treating this as a learning experience, I would appreciate if you could outline initial tests that you would do with your meter if you were in this situation.
Thanks Alexei
There are no other lights on that circuit.
Alexei
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