DIY Doctor

Adding Sockets to a Studio Area

Postby synthguy » Fri Dec 27, 2019 3:49 pm

Hi,
I want to add a bunch of extra sockets to my studio.
I've put up some trunking around the room .
I have an existing double socket that I plan to use as the feed. The socket I on a ring main.
Cant I just add a bunch of additional sockets form the socket and then return it back to the same socket thus completing the ring ?

[original socket]--- {daisy chained sockets] --- [original socket]
or can I only create a spur off this socket
[original socket] ----[Fuse 13A] --- [daisy chain socket]

Thanks
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Postby Mr White » Fri Dec 27, 2019 9:50 pm

It is your choice, but If it were myself, I would add the new sockets to the ring.
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Postby ericmark » Sat Dec 28, 2019 10:33 am

[quote="Mr White"]It is your choice, but If it were myself, I would add the new sockets to the ring.[/quote]
I would agree, but I am an electrician so have the testing tools, 1.37Ω is the loop impedance limit for a B32 MCB it was 1.44Ω but a 5% margin for error has been added.

In the old days of fuses, if the ring was two long then it would possibly take 0.5 seconds instead of 0.4 seconds to rupture fuse with a short circuit, but when we went to the MCB if has two bits, magnetic and thermal, the magnetic bit will trip in around 0.01 seconds but to do that a set amount of current must run through it, with a B type 5 times rated current, if the current is just under that point then the thermal part of trip takes over, so instead of 0.01 seconds it can take minutes to trip. It still protects the cable, but too slow to help any one who makes contact.

So we measure the prospective fault current or the earth loop impedance this can be measured with a low ohm ohmmeter, using 250 mA or a loop impedance meter, latter is easy, plug in press button and read both ohms and amps by flick of a switch, with ring centre socket, with radial end socket, if not sure just measure a few.

The problem is the £170 price tag on the tester, I could measure the R1 + R2 but that is a lot more complex, and involves removing the cover from the consumer unit, some thing I am not happy telling a DIY person to do.

As an electrician I have measured it many times, so looking at size of house and if one, two or more ring circuits, I will have a good idea of likely figures using the loop impedance meter. So I know if likely on the edge, or if likely no problem, so with latter I may take a chance and extend ring then test.

But I have to make out a minor works certificate so need the readings to enter on it.

Now a 13A fuse needs around 2.7Ω to pass, unlikely to exceed that, so with no meters fitting a FCU then a row of sockets, it is more likely to pass.

Of course today all new sockets must be RCD protected, again a RCD FCU is easy way where rest of house does not have it, but where a RCD is fitted, then the risk of a MCB not tripping fast enough is a lot less, RCD must trip in 40 mS again a problem as can't measure 40 mS on a stop watch, so need a proper tester, but I have not tested the 14 RCD's in this house, other than push button, my son has my tester, and keeps forgetting to bring it back. And I am not really worried, found RCD's that are slow, but still far faster than a fuse.

So as with any job, you select a method, (the method statement) then do a risk assessment, so ring final has the testing problem, and a fused spur has possible volt drop and over load problem.

In theory a ring final is limited to 106 meters, this is because of volt drop, with load taken at 20 amp central and 12 amp even spread, so uses 26 amp for calculation not 32 amp. However with an incoming impedance of 0.35Ω at centre of ring it will be 0.94Ω to keep within the 5% volt drop, so allowed 1.37Ω for the tripping of the MCB so volt drop rather than the tripping current of MCB should be limiting factor, however when I was a lad never remember working out volt drop, and it was less then as was set to 4%, so 88 meters was the limit.

To work out volt drop the tabulated mV/A/M has to be corrected and the formula uses square roots etc, although dab hand with slide rule, I was not going to do that calculation in a hurry, OK today I have a java script program to work it all out, so easy, but when I started we used "Rule of Thumb" even that has changed name to historical something.

Maybe guessamate is better name? And to be far 95% of time we were OK. So how new is original, how big is the house, how many rings has it got, all have a bearing on which method to use.

So it is yet another "just" job. To be fair likely no problem, but don't think I should say yes go ahead, when you may have a mansion. Even electricians get it wrong, I found a lovely extension to a 4 mm radial using 2.5 mm cable, where clearly the guy doing the job thought it was a ring, but it was a radial.
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