Adding a socket: finding the ring route

[SimonH2]

You can map the order, and approximate cable lengths, of socket round a ring like this. You need a means of putting a few amps through a circuit, and a sensitive voltmeter. For the former, I've used a 12V battery (actually one of those portable power supply/boost starter units) and an old headlight bulb - this will give you something like 4 to 5A.

With the power off (preferably at the main switch), drop the ends of the ring out of the board - if you use the earth then you'll get slightly higher voltage drops, but in my board the sparky put both earth tails in one bit of sleeving so I used the L or N instead.

So you form a circuit from the battery, through the bulb, round the ring (ie connect to the E wire of one of the ends in the board), and back to the battery (ie connect the E wire of the other cable to the battery). Also connect the N to one side of the battery - we'll be using this as a reference.
With the diagram you drew in advance, go round all the sockets and measure the DC voltage between N and E - any modestly reasonable digital multimeter will do - and write down the voltage next to each socket. You'll find there's a progressive change in voltage as you go round the ring, so you can determine the exact order of sockets round the ring. If you measure the current, you can calculate the cable length (you can lookup the resistance of cables in various places), but without the current you can get the relative lengths - longer cables, more voltage change.

When I did this at home, I was "surprised" by the route taken !

 

[PrenticeBoyofDerry]

I see. Had I noticed this post earlier, I would probably have asked the same question as EFLI, particularly given that this is a DIY forum.

And because it is considered a DIY forum makes what difference? There are plenty of topics on here that are beyond the scope of DIY! As I have already pointed out to the OP it seem they do not have the scope of knowledge to perform tests or actually ID a ring final circuit.

 

[ban-all-sheds]

I'm assuming that John's point is that as this is a DIY forum you should not be advising people to work on live conductors.

 

[ericmark]

There are two basic methods. One using a low ohm meter the other using a loop impedance meter, you may not loop impedance not earth loop impedance you need to test line neutral.

Step one is make a list of all sockets and MCB feed then you can add the extra info. The closer to the mid point the lower the loop impedance you can't really say which leg but you can work out approx distance from consumer unit. The second is with the low impedance tester you disconnect the wires at consumer unit and connect them in a huge loop then test line - neutral all on the ring will have same reading all spurs will have higher reading.

Between the two you can work it all out. I have not detailed exactly the method as I will guess you have not got a low ohm meter able to measure 0.01 ohms so it's all academic.

You can use the low ohm meter to do the loop impedance test as well it takes time and personally I have a set of test leads made up to speed up the process.

If all you have is a multi-meter give up now. OK I will admit using batteries and a low resistor measuring the volt drop across the resistor or other Heath Robinson set ups you could build yourself a low ohm ohmmeter you need at least 200 mA to flow it is possible. However it would take some doing.

The important bit is the records until you have all readings you will not know what is going on. It would be an interesting project to build the test gear and use it.

 

[flameport]

What advantage is that over the continuity test while dead?

The live test is generally much quicker, particularly in properties you have little or no knowledge of.

For the dead continuity test, it is necessary to unplug/disconnect every appliance on the circuit to avoid misleading results, which often means every room in the house, including all of those things plugged in behind immovable furniture, heavy kitchen appliances which will rip the flooring when moved, connections from the back of a socket to outside sheds, lights and similar.

It also avoids opening the consumer unit to disconnect wires - something which may be rather difficult and likely to cause problems if the interior is the typical mess stuffed full of wires in a random order.

 

[EFLImpudence]

I am somewhat puzzled by all that.

The method was suggested as a way of confirming (or not) the ring at a socket.

 

[SimonH2]

... a low ohm meter able to measure 0.01 ohms ...

As an aside, do you tend to take several readings at each socket ?
The reason I ask is that a while ago I found a fault in the flat - IIRC an unusually high loop resistance on the neutral. I found that the reading at each socket was inconsistent - pulling the plug out and re-inserting would change the value, as would switching the switch off and back on again.
While there was enough difference between sockets relative to the variation, and I had a good idea of the route (there was only one sensible route and I know the sparky who wired the place wasn't into wasting cable) - it might have made mapping out some rings a bit tricky.

Leaving that one, a method that doesn't tell you which leg has some limitations. In our house for example, there's one ring that does upstairs and downstairs - and I'd have never guessed the route ever ! Two bedrooms, each with two sockets, and in each room the sockets are on different legs of the ring

 

[ericmark]

I agree often the 200 mA required for a low ohm meter is not enough to get accurate enough readings likely your battery method is better. With the loop impedance meter you are dependent on a steady supply press button twice you never get same reading as to if that's due to test resistor temperature in meter or varying supply hard to tell.

But since people have told him how to measure he has not come back.

I remember doing my C&G2391 test and rejecting a set of sockets because the variation between sockets was too high considering how close they were to each other. It seems the builder of the test rig had included resistors to make it emulate a house.

Thank bubachuba - good suggestion. Unfortunately some of my rooms are inaccessible as they are banked up with furniture as we decorate other rooms. Have to go with the spur.

So really no point if you can't access sockets then simple you can't test.