Lighting earth problem

JonC2010 · 18 Aug 2010

Firstly, please accept my apologies if this has been covered before, I couldn't find the answer when searching.

In my flat I have two lights in the bedroom. Both are simple roses and I (well, the missus!) wanted to change them to better fittings. I bought two light fittings to replace them, a simple job or so I thought, I have done it many times before.

After fitting one of the the lights however the circuit breaker for that lighting circuit tripped as soon as the light was turned on. The new light fittings have metal parts and require earthing, unlike the roses. I have pinned the problem down to the earth wire, without it connected everything is fine, with it connected the trip goes immediately. I thought the fitting was faulty so tried the other one, exactly the same problem.

I then put a multimeter across the wires coming from the light fittings. All seems fine, in particular the earth wires (there are two in each light) do not seem to be connected to the live or neutral ones.

I then tried the other ceiling rose with exactly the same results, with both light fittings.

What's going on? Am I missing something obvious? Surely both fittings can't be faulty? And if so, wouldn't I be able to detect a short circuit with the multimeter?

Any advice would be most appreciated, as I am stuck with two unused light fittings and a missus who is certain I have broken something!

MalcofArabia · 18 Aug 2010

If the lighting was RCD protected you may have a neutral earth cross somewhere on the circuit.

While you had the earths in a plastic rose no problem, but by fitting metal fittings you may now have unbalanced the circuit with a crossed connection.

Or you may have mistaken a swich live with a neutral as they can be both the same colours ( Blue or Black). In the perfect world they should be sleeved to indicate a line cable but often are not.

JonC2010 · 18 Aug 2010

Thank you for your reply MalcofArabia

I was thinking of the earth/neutral cross as you suggest, and if so will tell the landlord that his wiring is suspect. But before I did this I wanted to be sure it wasn't me doing something silly!

The switched lives in both fittings are sleeved so I don't think I did make a mistake, but it's certainly possible I guess. But if so, then a) the new lights wouldn't have worked fine when the earth wasn't connected and b) the simple roses wouldn't have worked when I put them back. Is this correct, do you think?

Thanks again!

ericmark · 18 Aug 2010

JonC2010 said:
I then put a multimeter across the wires coming from the light fittings. All seems fine, in particular the earth wires (there are two in each light) do not seem to be connected to the live or neutral ones.

Seems you have done right thing and you should have found a fault but do remember the meter we use tests with 500 volt and 15ma = 15k ohms what range was the meter on?

Seems unlikely a neutral earth fault on supply or it would have tripped with original lamps. Yes I know you had no earth connected but the problem with neutral earth faults is the current draw and as you take power any connection between neutral and earth will trip the RCD.

Not sure if lamp is earthed does it still trip the RCD with bulb removed. Some old fluorescent lamps did have earth connection but I thought this is no longer used?

But I wonder if the tester is just not a high enough voltage to find the fault?

LiamPope · 18 Aug 2010

First things first - what tripped? RCD (will have a test button, will usually be two modules wide and there will usually be only one or two of them in the consumer unit) or MCB (individual switches one module wide, one for each circuit, no test button)? For completeness you could also have all RCBOs which would look like single width individual switches for each circuit, each with a test button. Photo would help if you're not sure.

When you say it works without the earth connected can you be more specific? When it is not connected and the light is working without tripping, do you have all the earth wires at the rose connected to each other (but of course not connected to the light fitting earth), or all the earth wires seperate and disconnected. I'm trying to determine if it's the act of connecting the earth to the light fittings that is causing the tripping, or whether it could be just the act of connecting all the earth cables together (with the light fitting being a red herring) that is causing the tripping.

Are both the bedroom lights switched together or independantly? Is there two-way switching (eg a wall switch and pull-cord by the bed?).

The answers can help determine if the fault lies with the light fittings or a (potentially very nasty) fault in your house wiring.

The answers will help determine if

JonC2010 · 18 Aug 2010

Ericmark and LiamPope - thank you for your replies...

I am not sure what the meter was on to be honest! I will check again tonight and see. I haven't tried it without the bulb in, I can do that too and report back. The light fitting is a chandelier type, for want of a better word, with 5 candle bulbs.

As far as what tripped, I don't have all the info as I sit here but will check. The consumer unit has a number of single width modules with, I think, no test buttons on the modules. Just one of these tripped and controls the lighting for that particular bedroom and one other bedroom, I believe. I will examine the unit and take pics.

When I said it worked without earth connected I meant without connecting the earth wires from the light fitting to the earth wires in the ceiling. The earth wires in the ceiling (there are two) are and have always been connected together. It's the action of including the earth from the light fitting that causes the circuit to trip.

Both bedroom lights are switched with a single wall switch.

I hope this helps, thanks again for your assistance!

electronicsuk · 18 Aug 2010

Hopefully this is obvious, but for your own safety it would be best that you don't attempt to even test these lights without an earth connection. It sounds as if there could be a L-E fault, in which case not connecting the earth would render the entire fitting live.

LiamPope · 18 Aug 2010

Hmmm sounds like an MCB tripping. My worry was that you could have a live-earth fault somewhere on that circuit which had been 'fixed' by breaking the earth at the ceiling rose in question, and that by connecting them back together in the new light fitting, the fault is showing itself. If you had a fault like that 'fixed' how I described, you would have the earth cable in the circuit downstream of that rose live. You could then have fitted a metal light fitting to another 'affected' room on the circuit, earthed it, then switched the circuit back on and the whole light would be sitting their live and lethal. However, if the ceiling earths were always connected you cant have this problem in your installation, so fingers are pointing back to the light fittings being faulty with a live-earth fault! As Electronicsuk said that could make the whole fitting live if you disconnect the earths, so whatever you do do not be tempted to temporarily 'fix' the problem by leaving earth disconnected! But two faulty new light fittings is unlikely

My other option was 2-way switching with earth being used as switched live, but this doesn't seem to be the case either. I'm now out of ideas - good job I'm not a spark

JonC2010 · 18 Aug 2010

electronicsuk

Thank you for this, it is a sensible warning and I am being very careful indeed. Although not by any means an electrician I have spent many many years dealing with wiring issues and have learnt to be very careful!

Luckily none of the light fittings in the flat are earthed, being very simple (cheap!) fittings - with the exception of the two I just bought.

LiamPope · 18 Aug 2010

Hi again,

In case you didn't catch my edit above, and didn't fully understand electronicsuk's point - it wasn't just general 'work safe' advice.

From the info you've now given in answer to my questions, you could have a live to earth fault in the new metal light fitting. If you do not connect the earth the whole metal light fitting could be live when you turn the circuit on (and wont trip the MCB). In other words, when you tried it out without the earth connected and found it to work without tripping the MCB, everything metal on that light fitting could have been live waiting to potentially kill you had you touched it!

You would have thought your multimeter would have revealed the fault showing continuity between live and earth in the light, but it's possible the fault is just so that it needs the whole mains voltage to show itself, so your multimeter test failed to see the fault.

electronicsuk · 18 Aug 2010

Unfortunately, short of performing an insulation resistance test on the suspect fittings, I can't really see how you're going to get any further with this fault. I guess in the absence of an IR tester, you could wire one of the fittings to a plug fused at 3A and see what happens. However, that could mean potentially switching onto a L-E fault, which is far from an ideal means of testing - especially as the 3A fuse and (probably) 32A MCB are likely to let through significantly more current than the MCB that protects your lighting circuit.

sparkwright · 18 Aug 2010

Have you pinched some wires against some of the metal work of the new fittings? Look for damage to the insulation.

Send photographs if possible.

JonC2010 · 19 Aug 2010

Thanks to everyone who has replied.

Unfortunately I haven't had a chance to investigate further yet, but as soon as I can I will, and send some pics too.

Just to clarify, the standard simple roses are back on the two bedroom lights now so I am hopefully not at any risk. When I was testing the new fittings without the earth connected I was aware that the fitting was therefore potentially live and very dangerous and went nowhere near the thing! I just wanted to see if it would turn on and off without tripping the MCB.

I too am perplexed as to why the multimeter didn't show a connection between the light fitting's earth and live wires. I was fully expecting this to be the fault that was tripping the MCB. It does seem to point back to a fault in the earth wiring to the bedroom ceiling lights. Is there anything I can easily do to check whether a Neutral-Earth cross exists in the ceiling wiring? What would be the cause of this?

I have definitely not pinched any wires against the metal parts of the light fitting, I have inspected carefully and there are two light fittings exhibiting the same problem.

davelx · 19 Aug 2010

I don't see how connecting the earth to the light, or not, could make any difference if there was a fault elsewhere in the circuit. If the metalwork of the light is isolated from L & N then connecting the earth conductor to it makes no change to the circuit topology whatsoever.

There must be something wrong with the light fitting, or with the way it is attached to the rose, that is causing a connection from the metalwork of the light to something other than the earth.

LiamPope · 19 Aug 2010

As unlikely as it seems, everything is pointing to two faulty light fittings! Perhaps they came from a faulty batch? I'd try returning them and see if replacements work.

It's easy enough for a multimeter to fail to see an insulation resistance problem as on its ohms setting it only challenges the cable/device with 5V dc, wheras your mains hits it with 230V ac. Having said that, a short circuit of low enough impedence to trip the MCB, you would have thought you'd pick it up even on a multimeter

It doesn't seem you can have a fault in your wiring, as it only occurs when these light fittings are connected. It can't be a neutral-earth fault as that could only trip an RCD, not an MCB, hence the questions to determine what device is tripping.

So all points to the lights. Lacking a proper insulation resistance tester to test them, I'd be assuming they are faulty and, as above, returning them for replacements.

JonC2010 said:
Thanks to everyone who has replied.

P.S. There's an ego-massaging thanks button for that!

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Lighting earth problem

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