easy test for duff part of electric circuit

[Tom]

Hi All,

Before anyone says look out, its ok the power is off for the part of the circuit I need to test... And, naturally, I only want to test it whilst its off.

Part of a lighting circuit has been shown (I assume) to be shorting - a qualified electrician has disconnected the part that is causing the problem. But they were unable to tell where the fault actually is located.

With the problem part now safely isolated and off, what equipment can I use to locate the fault by a process of elimination? ie. removing switches or disconnecting sections of the circuit until the bad part is pin pointed ?

I have a multi-volt/amp/resistance meter and I assume that would that show me when a direct short is occurring? I suppose everything in circuit would need to be disconnected ie. light bulbs taken out just in case the 2 way switch is actually in the on position.

Any advice ?

Tom

[Tabby]

Assumptions tend to be wrong in these sort of situations.

maybe ?title=3DMultimeterNT

[John Rumm]

Usually fairly easy to find with a bit of educated guesswork and some trial and error.

Your resistance meter will probably be the most useful...

You may have got some clues from what went wrong in the first place:

Shorts can occur Live to Neutral, Live to Earth, and Earth to Neutral,

LN shorts will trip a MCB or blow a fuse, but won't trip a RCD[1] LE shorts will trip a MCB or blow a fuse, and will trip a RCD NE shorts will not trip a MCB or fuse but will trip a RCD

[1] You possibly don't have a RCD on your lighting circuit unless you have a whole house RCD, a TT earthing system, or a very recently rewired place.

Very old installs may not have an earth on the lighting circuits at all.

A couple of obvious checks:

A fuse/MCB trip that happens only when a particular switch is turned on will often be caused by damage to its switch wire (typically a LE short caused by a nail etc). Have you nailed or screwed into any walls lately?

Changed any bulbs, light fittings, lamp shades recently? If so check the wiring in the rose affected since its easy to pull a wire out of a terminal.

Tests

Assuming the fault is there all the time regardless of switch position, and you can't think of any recent changes that coincided with the manifestation of the fault, then you will need to do some tests.

The first job is to identify the type of short: with the light switches all off (and bulbs out on two way circuits if unsure), measure the resistance between L, N & E at a light switch or ceiling rose, see if you can see if there is a short (i.e. very low resistance). Ideally you ought to see an open circuit between each wire - however chances are you will see a short between L & N or L & E.

Assuming there is a short, now you need to find it. Chances are your lighting circuit starts at the CU and run from one ceiling rose to the next in a long chain. This is the "loop in" system. Identifiable by having three cables (at least) to each ceiling rose.

roughly where each light position is in the chain will help a little. Pick a position roughly in the middle and take the cover off the rose. Make a note of the cable which is the switch wire - it will be the one with one wire to the live terminal and the other to one side of the bulb. Before you disconnect anything, mark the neutral wire with something so that you know its the switched live (if you are lucky the electrician who installed it put some sleeving or tape on it for you!) Disconnect the "in" and "out" cables and repeat your resistance check between each of their wires. Chances are you will see a short only on one cable. You have now eliminated half the circuit! Repeat this activity about half way along the remaining segment with the fault in it. After a few trips round this loop you will be down to one section of cable with a fault - or will have spotted a loose wire in a rose etc.

Once you know where it is, and assuming its not a fault in a rose, you can either replace or repair the problem cable.

[Mike Clarke]

John Rumm has already given an excellent outline of the things you could do but I'd add just one more. You know the circuit has been disconnected, but even so, the first thing you should do is to physically check that it really is "dead" by checking for voltage between line and earth, and neutral and earth with your meter set for measuring voltage before attempting to make any resistance measurements. Mistakes in identifying cables can happen, especially if groping around in a dimly lit loft or under floorboards. It's also good practice to check your voltmeter beforehand by checking for voltage on a known live circuit.

[george [dicegeorge]]

there's an orange screwdriver which lights up when its near to electricity, safer than those neon things you have to touch to wires

First I'd draw a diagram of all the switches and lights, with red and black and blue and brown for those colours.

[g]

[newshound]

Excellent advice, also the "Volt sticks" which don't actually have a screwdriver blade, just a translucent plastic cone at the end. About a tenner from Maplins, half that price from Ebay, CPC, and similar sources. This one is even cheaper

measuring tapes, you need to have several of them otherwise you can never find one when you need it.

Same rule applies as with neon screwdrivers, check that they are working against a known live before trusting that something is really dead.

Very good for finding a break within a flex or cable. In my experience less likely than a neon to give a "false positive" because of capacitative coupling.

[Frank Erskine]

The same as Stanley knives...

[newshound]

Good point, but I seem to be saturated with Stanley knives, whereas I still buy a couple of steel tapes every time Tesco has them for £1

[The Medway Handyman]

>>> Like measuring tapes, you need to have several of them otherwise you

And pencils...

[Tim Lamb]

Fixing nails and screws through cables have been mentioned but incandescent lamps mounted upright can short L/N during failure.

Is the fault still there; now you have removed the lamps?

regards

>

[Dave Plowman (News)]

The majority can be done by examination. With lighting, it is most likely to be a fitting. Especially where a ceiling rose with loop in loop out has been replaced with a fitting which doesn't offer this. Then think of any work which has been done around the house - could a nail etc have pierced a cable? Have switches etc been removed for decoration?

A short doesn't just develop. It has been caused in some way.

[D.M.Chapman]

Call me paranoid, but I always retest a meter after it's told me that something is indeed dead.

[A.Lee]

I had an 'experience' when trying to trace a fault in a lighting circuit. No visible damage to lamps/holders/pendants. Disconnected each, one by one, to see if it tripped. Narrowed down to 3 lights at the far end of the house. Took them all apart, still couldnt find a fault. Went into loft, traced the cables back, could still not see any damage. Had to disconnect each in turn. Traced the line to earth fault to 3 cables behind a top hat style steel cable covering. Bit the bullet, and made a mess with the plasterwork, and pulled off the covering, and the fault was plain to see, when fixing the covering, the sparky had slightly pinched one of the cables near to a nail. It had took 13 years since the extension was built for the wire to push through the insulation to touch the earth - and that was the fault, the nail was putting pressure on the cable, and had squeezed out the live cable. Something I hadnt seen before, but has made me more aware of this when affixing cables. Alan.

[newshound]

Creep.

That's the scientific name for the process, rather than a description of the sparky (or you!)

As you say, a useful lesson.

[John Weston]

I've seen that in a socket. A "professional" had left the wire ends long, as they should be, and pushed them into the back-box when installing faceplate. Unfortunately, the Line wire was straight out of the terminal, bending only where it touched the metal back-box. It must have been 10 years later when the insulation had suffered creep, resulting in almighty bang, which took out my network... Now I see why back boxes are earthed. I trust my DIY repair will last longer :-)