9v shown between live and neutral/earth on mulit-meter

[David WE Roberts]

Hi,

I am working my way round the house room by room, and have just started on the sun lounge.

I have an arrangement of two sockets side by side in a fancy double wall box. One socket is a single 13 amp 3 pin, the other is a single round pin 3 pin (like the old 5 amp IIRC).

The wiring is grey sheathed T&E 1mm or 1.5mm AFAICS and wired as a ring i.e. two cables both wired into the socket.

The extension was built in the sixties or seventies (we think) and this wiring will have been done then. The wiring in the rest of the house does not use this scheme.

As far as I can tell this is not current(no pun intended)ly in use but when I put my cheapo digital multimeter on the A/C setting it shows U/L (overload) very briefly then settles down to showing about 9V between the live pin and the neutral pin or the earth pin; earth and neutral are at the same level. I assume that this is acceptable.

I am assuming that there is no connection to anything live, and wondering what this may have been used for in the past.

I was considering replacing the wall box to allow me to fit a twin socket, but looking at this the easiest thing to do would be to add another single socket to replace the round pin socket. Probably as a spur depending on the amount of free cable. Is there any benefit in doing this over using a distribution board/extension lead?

Bit of a rambling post, but there you go.

Cheers

Dave R

[Steve]

A digital voltmeter is probably the worst way of measuring these voltages as they (digital meters) have such high input impedance any coupled voltage is read. An moving coil type of meter would be better if you can get hold of one to test the circuit. Failing that, a resistor say 10k ohm or thereabouts connected across the digital meter terminals would shunt the meter and give more meaningful measurements.

Steve

[John Rumm]

Its is probably telling you that the other end of the wires is not connected to anything or at least switched off. Your meter is reading the voltage on the wire induced by its proximity to other live wires. The difficulty with a DVM here is it places so little load on the wire that it does not discharge it easily.

You would need to check the T&E cable size carefully first to make sure it really is suitable.

[chrisj.doran]

Round-pin sockets are commonly used for lighting (table or standard lamp) switched from elsewhere. Does it become live when you turn on the room light? Is there an extra wire inside the light switch? Or a mystery switch somewhere that appears to do nothing?

You could try tracing the cable routes with a detector, though the neigbouring 13A socket will probably confuse things. The presence of two cables to the mystery socket suggests there is/was a second one somewhere.

Chris

[David WE Roberts]

******** Good point - (following on from another post) I have one light switch which doesn't seem to do anything. I will check this and test the socket with the light switch on. If this is a switched circuit then I will have to work out where it comes from then decide if I can safely disconnect it or if I label it up and/or use it. ********

I'm not sure I explained myself correctly. I was considering removing the current cabling to the round pin socket, and connecting a second 13 Amp single socket to the one next to it.

To clarify - I have an extended double wall box which will take two single sockets (and is too wide for a double socket). One single socket is switched 13 Amp. The other single socket is round pin (5 amp?)

I propose to replace the round pin socket with another single 13 Amp socket and either link it into the ring or spur it off the other 13 Amp socket depending on available cable length. Spur is the most likely. The

13 Amp ring is standard 2.5mm T&E. I do not plan to reuse the 1mm/1.5mm cable currently connected to the round pin socket.

There are three sets of sockets in the extension. Two have a 13 Amp and a round pin pair; the third also has a telephone socket.

Cheers

Dave R

[Andy Champ]

I'm not sure, but I think the 'phone people may get antsy about that.

ISTR having an office with 3-sided poles to get wires from the false ceiling to a usable level; the installers said that the 'phone people didn't like their signals too close to the mains, and that was why the three sides were phone, mains, data. OTOH, that was a long time ago,

*and* was a private exchange...

Andy

[Andy Champ]

Ooh! In the thread "Part P is not going away ?" Rumble just said "In fact the signal cables may well be part of a fixed installation and if they were, they would be subject to Regulation 528 (segregation, separation, etc)" so perhaps I am onto something!

Andy

[The Natural Philosopher]

It has ALWAYS been part of electric regs to segregate low voltage and signal cables from mains, in seperate trunking (areas?) and if not, by distance.

[David Hansen]

On Mon, 22 Oct 2007 02:54:15 +0100 someone who may be The Natural Philosopher wrote this:-

All the cables inside a house or commercial building are likely to be low voltage, or extra-low voltage and when discussing the wiring regulations people who are familiar with them tend to stick to the correct terms.

Segregation in the regulations is largely considered in terms of safety. There is no restriction on having low voltage and extra-low voltage cables in the same enclosure, provided the extra low voltage cables are insulated to the higher voltage, including barriers at boxes, or separated by earthed metal. They can even be in the same cable, with the appropriate precautions. There are some restrictions on emergency lighting and fire alarm circuits, but these are to enable them to continue working for as long as possible if other cables have caught fire.

The regulations do draw attention to other standards on preventing interference between such circuits.

[John]

This is quite possible if your house uses a TN-S or IT supply where the earth is remote from your neutral incomer. There will probably be other properties on the same transformer and you could be reading their volt drop in the neutral.

John

[David WE Roberts]

Spot on - finally got round to testing it and it is switched from the mystery switch from a pair of switches on the wall.

I now have this urge to buy some round pin plugs and fit them to lamps just so I can use it. I may however restrain myself (or be restrained by my style consultant).

I was a little confused because the switch was connected to a completely different cable (3 + E) which I usually associate with multiple switches for one lighting circuit.

Now if I only knew what the disconnected 1.0mm T&E floating in the back of the box used to do........

[Dave Plowman (News)]

Are there two table light circuits and multiple sockets? If so the extra wire could be the second circuit - allows you to group things after installation. It's exactly what I did when wiring this place.