How can I check that a neutral cable is in fact neutral? If I can measure current then clearly is isn't neutral but measuring nothing could mean that it isn't connected? I am fault finding my CH system and I need to confirm whether a black wire in the junction box in the airing cupboard is neutral.
Kevin
Don't panic if you see a *voltage* on it with a DVM, it's almost certainly caused by inductive pickup from wires that it passes alongside. If it is a voltage which can actually deliver current (check by shorting to earth with a suitable light bulb) then you may have a problem.
The trouble with unidentified wires around a heating system is they might be connected to something electronic, so you don't want to zap them with a megger (which would otherwise be a good test).
Neutrals have current through them too.
Measure the voltage between it and a known good neutral. Should be 0V (give or take a few mV)
Owain
And when the thermostat switches it becomes live.
Err, a neutral can never become live unless there's a fault somewhere.
A conductor with neutral colour insulation can quite legitimately be live (230VAC RMS).
What he means is that the wire he's identifies as neutral becomes the same potential as the live when the theromstat closes. The fault is that the wire is not connected to the neutral of the circuit.
I didn't say it couldn't. I said a can never become live unless there's a fault.
but even that isn't necessarily true. The neutral wire might have been deliberately disconnected to help with circuit tracing.
A neutral is by definition a live conductor. It /may/ (should) be near earth potential normally unless there is a fault.
It is not unusual to have the neutral switched (by a thermostat or such) inside equipment, but it should never be single pole switched in fixed wiring.
There should be the same current in the Neutral as the Live, if not there is a fault. That is a basic law of electronics.
I suspect you mean Voltage.
The neutral may not be at zero volts for a number of reasons, including faults.
Do you know if your house has a PME Earth? ( there should be a notice by the meter). If it has, with the power off, compare the resistance between a pipe and the neutral with a pipe and the earth. The cable runs should be the same so the resistance should be the same.
I think you'll see from the range of answers given that it ain't easy! It's not too difficult to establish that a wire is connected *either* to the supply's neutral *or* to earth, but distinguishing one from the other is harder.
Can you not find the other end of said cable, and see what it's connected to. That's probably the best way.
If you've got a sensitive AC voltmeter, you may get a clue by measuring the voltage between your wire and a known good earth. Start by measuring the earth to neutral voltage in a nearby 13A socket. [In my case, it's about 130mV - but this will depend on your installation]. Then measure the voltage between your mystery wire and this same earth point. If it's the same as you measured before, it's probably a neutral. If it's zero, it's probably an earth.
I rather think you're splitting hairs.
If one is fault locating or tracing circuits, it's not unreasonable to assu me that the person carrying out the work has a proper level of competency, and will be fully aware that this may be the case. If not they shouldn't be carrying out this type of work.
I have my suspicions about the competency of the OP, talking about measurin g current when it was fairly obvious he meant voltage.
I'm guessing from the question that the OP will have, at best, a multimeter and not more professional test kit. So he won't be able to measure normal circuit resistance. In any case you don't know what the cable runs might be, nor can you be sure if there is electrical continuity in the pipework, or where it might be bonded to earth.
The question about PME is good, though (provided OP knows what PME is).
With the power off, he could test for continuity with a known neutral, e.g. at a socket. If there is continuity, it probably is a neutral. If it is a switched live, e.g. from a thermostat, it would (probably) not be connected to the known neutral. Where a switched live is connected to a terminal it should really have red sleeve on it, or at least a bit of red tape as a warning.
There are wiring diagrams for CH junction boxes on the web; sorry I don't have any links but try Honeywell for a start.
I rather think that's what I said.
I am well aware that the neutral may be switched within equipment, but that's something of a red herring. The OP mentioned a junction box in his airing cupboard.
Note also that the *real* experts on this NG are studiously avoiding posting. It *could* be a can of worms. Personally, I think it is worth trying to help people in trouble even if we worry that they may not be fully competent. Perhaps all these posts should carry a "health warning": anything you do is at your own risk if you don't know the "basics". Also you may have missed out essential information.
It's not difficult to prove if a cable is a circuit neutral at a junction b ox, but that also requires one to prove that it's not a de-energised switch wire. It's not quite so easy to prove that without knowing exactly how the wiring has been configured.
Personally, I think it is worth trying to
We must agree to disagree on that topic. Interestingly, the OP has made no further comment since his original.
Determining a neutral or a de-energised switch wire requires two very simpl e proving tests before and after the event, ideally using a test lamp[1], b ut a multimeter will work just as well.
[1] I favour the test lamp, it indicates - or doesn't - immediately in your field of view, whilst one is concentrating on where the prods are directed . A meter has its uses, but usually requires a bit of manual dexterity/jugg ling and eye movement between prods and a decent view of the readout. Cheap digital meters are also prone to spurious readings, much less so with qual ity test equipment.
I cannot think of a standard wiring CH setup that does not have a neutral in the wiring centre (all motorised zone valves need a neutral).
Standard set ups are here
If the OP gives a little more detail - including the problems with the CH, maybe a photo and also says what setup he has we could get an answer to his problems.
I was not avoiding the post - it's the sort of post I enjoy. My first thought was that the OP is assuming that a black cable is a neutral (as pointed out by others).
IIRC, he had a working system, took it apart to tidy up the wiring, or whatever, thought he knew what was what but found it didn't work when re-assembled. So he's now trying to figure out which wire is for which function!
I think it's the same person who was asking about S-Plan wiring. Various people pointed him at the schematics, and I gave him a verbal description of how it is supposed to work.
If he can't sort it out from that, he needs to employ someone who can!
As we all know, the electrics for many (most?) CH systems have been installed by non-electricians, using whatever wire was to hand - so you can deduce very little from the colour of a conductor.
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