Neutral to Earth resistance question

[nmg196]

Hi,

I'm trying to diagnose a problem whereby my lighting circuit breaker trips when I turn on a particular light after having changed the light fitting. I'm finding it very difficult to diagnose as two problems have occurred at the same time (the switch has physically stopped operating properly but is showing no signs of having melted etc).

But before I can work out what's going on:

1. Is it right that neutral to earth should be as low as 5 ohms?

2. Is the lighting circuit likely to be on the RCD in the consumer unit, or is the RCD only for sockets? Ie, if something was leaking to earth, shouldn't the RCD have tripped rather than the circuit breaker.

Thanks,

Nick...

 

[Lectrician]

Typically, your lights will be protected by the MCB only.

You have most likely damaged the light switch by making it switch on to a direct short - causing the internal switch blades to weld shut or pitt. You need a new switch.

You first need to correctly wire the light fitting otherwise you will damage the new switch.

N and E are near to the same potential, and depending on your earthing system will be connected together at some point. Either at the meter position or the substation for TN-C-S and TN-S systems.

 

[nmg196]

But I can't see any problem with the wiring on the light fitting. So I'm a little stumped as to what caused the short in the first place. I've now isolated that switch (and lights) and the mains comes back on OK.

But as far as I can tell, the wall lights are wired up correctly with no shorts beween live>neutral or live>earth.

A fault on the switch couldn't have caused the short as far as I know as there are only live wires in the switch due to it being a 2-way setup.

What should the resistance be of two 40 watt light bulbs in parallel? I measure it to be about 54 ohms but I don't know how to convert that to amps in the AC world.

 

[Lectrician]

Describe exactly what cables you have at each light fitting, and what cables you have at the switch.

You must have miss-wired something, drilled a cable, caught one with a screw or something......

 

[nmg196]

I had hit a cable with a drill. But I dug a big hole out in the wall to inspect it and none of the internal wires had any insulation damage. I also wrapped the cable in insulating tape just in case, so I'm fairly sure this isn't the problem. I'm gussing that the only thing that could cause a circuit breaker trip is a live-earth short or a live-neutral short. However these seem to be infinite resistance (higher than 2Mohms measurable on my meter) when the bulbs are taken out, so I don't understand what could have originally caused the circuit breaker to trip.

Well I must admit I'm finding the switch wiring confusing. However I have NOT changed any switch wiring - only the wall light fittings.

I'll try and upload some pictures somewhere if I haven't sorted it in the next hour or so.

 

[Spark123]

What sort of meter are you using? Is it a 500v Insulation Resistance meter? Multimeters may not pick up a fault the same as an IR tester will. N-E of 5ohms? Same again, what did you use to measure this with?

You say you have replaced the fitting? Is there more than one cable to the light? Have you done the age old classic of assuming all blacks are neutrals and reds are lives? One of the blacks is possibly a switched live. If so you have put a dead short across the switch and probably welded the terminals shut. The viable option will be to change the switch. Before switching back on, read //www.diynot.com/wiki/electrics:blackswlive
Is it possible to replace the length of cable with the damaged insulation on it? I'm not a fan of wrapping stuff in insulation tape.

 

[nmg196]

I was using a standard multimeter - which I agree doesn't test insulation properly.

PICS BELOW

The wall switch has two circuits. One is for the wall lights (which I'm having the problem with). The other side is for the two-way ceiling light switch which I think works fine.

I have NOT changed any of the wiring in the switch.

There are two wall lights. The cable damage occured at the first one which therefore has two cables. I've dug the cable out of the wall and I'm 99% sure that the black wire which is exposed has no insulation damage. Only the white outer insulation has been 'nicked'. Even if it had been breached, I wouldn't have thought that a neutral-earth short would cause the circuit breaker to blow.

Is it even theoretically possible, looking at how my light switch is wired, that a fault on the switch could have caused a short somehow? I can't see how it could as it only contains either live or switched live wires.

As you can see, the wall light wiring is very simple and I don't see how I could have got it wrong with only blacks and reds to deal with.

 

[Lectrician]

Is it still blowing the trip with the wall lights removed?

Are you sure that 1mm lighting cable at the wall light is not damaged?

 

[nmg196]

No.. it doesn't trip with wall lights removed.

This is doing my head in! - If I disconnect the yellow wire with red band (bottom right) then why is there still 230 volts at the wall light wires?! Surely this would be the switched live going to the wall lights? Strangely, though, it's barely enough to make a neon screwdriver glow, but if I stick the same neon driver on the red wires, it glows brightly. Could it be inducting a bit of voltage from nearby wires?

Just about to connect a bog standard ceiling rose to the wall light to check I'm not going mad.

 

[nmg196]

Ok... not sure why the neon was glowing as a bulb does not come on if you connect it to those wires. Now that there's a load across the wires, the neon doesn't glow anyway!

 

[nmg196]

OK I've sussed it. I dug the wall out a bit more and found that the incoming cable actually bends off sharply to one side. That puts it directly behind the rawl plug! Behind the hole where the rawl plug used to be, I can see a small hole in the cable!

So I must have used a slightly longer screw that than was used for the last light fitting and gone through the cable on the live side

Somehow I'm going to have to try and get the wire out to repair it.

Thanks for your help guys.

 

[Steve]

QUIT WITH THE NEON SCREWDRIVER! It is NOT a reliable piece of test equipment and not to be used to prove dead or live.

 

[nmg196]

What are the alternatives? A multimeter doesn't seem any good either as it can show 230V when there is no power on the wire (too sensitive).

 

[Steve]

If a multimeter shows 230 volts when it isnt there, its battery is flat.

Seriously, a neon screwdriver, think about it. It tests for live by using your finger as an earth path. It will light up on an induced voltage, which could be anything from 100-240 volts! No decent electrician will use a neon screwdriver.

 

[nmg196]

A high voltage could exist on a disconnected cable which simply runs in parallel with a live cable for a long run. In this case, it will have a voltage applied to the cable through inductance. A volt meter as a very high resistance (to read correctly, it would have to be infinite, but obviously this is not possible) and will therefore detect voltages on cables where you could never actually draw any current from it. A volt meter will give you a reading of thousands of volts for example, from something which has a bit of static electricity in it. So just because your volt meter reads 10,000 volts, it does not mean it's dangerous to touch. It's prone to giving false positives.

As recently as this afternoon, I have seen 230V on a cable in which all wires were disconnected at both ends. Yet, put a neon on it, or touch it with your finger, and nothing happens. The battery is brand new out of the packet this morning.

I'm not an electrician (although I have studied Electronics to degree level) but would say that although I agree that a neon is not ideal, neither is a multimeter.

I would say that an ideal device, would be something which is earthed, but also has a bit of a load on it (a few hundred K ohms) to discharge any static or inducted voltage. A multimeters resistance is much too high to get rid of any inducted voltage.