What MCB should I use for a mixed cable ring?

Postby Saguenay » Tue Jan 24, 2012 11:00 am

Whilst renovating a bedroom I decided that I would need to put in 3 extra double sockets to make 5 in all. After taking up the floorboards I discovered a complete 'rat's nest' of cables underneath. Some have been simply cut and left but with no power, junction boxes with spurs all over the place mixing 2.5mm and old multi stranded 3mm cables feeding sockets in the kitchen on the ground floor and some cables disappearing into the wall cavity going who knows where. The house was built in 1904 and quite a few electricians have been in doing their own thing over the years! The CU is very old with wired fuses and the fuses that serve both upper and lower rings (complete with spurs) are, as far as I can see, rated at 30 amps. Bearing in mind that a lot of the newer cables are 2.5mm could I change the 30 Amp wired fuses with modern 20 Amp MCBs for the time being subject to replacing the CU with a modern unit at a later date?
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Postby stoneyboy » Tue Jan 24, 2012 10:27 pm

Saguenay ,
A good idea to downrate the circuits but you may get tripping if say the kitchen circuit has a washing machine, toaster and kettle on at the same time.
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Postby ericmark » Wed Jan 25, 2012 1:43 am

I have come across this in the past where over the years there have been re-wires without removing the old cables.

I have found one can get some sort of idea as to what feeds what by measuring the loop impedance where there is a ring as one leaves the fuse box one sees the impedance raise the fall as one returns to the fuse box with large jumps for those sockets on spurs. Not the correct way but it gives one a quick indication and also tells one if you could convert to MCB's.

The problem with a MCB is it's not a continuous as with the fuse but has a straight line where the magnetic part takes over from the thermal part so there is a very defined point of pass or fail on the loop impedance.

Testing the ring exists for all that's line, neutral and earth is of course important and the correct method is to create a loop and then with a low ohm meter test the resistance and all sockets which are part of the ring will have the same reading. However going to all that work is rather pointless if the loop impedance will not pass anyway.

The MCB as I said has too distinct parts the thermal bit will cause it to eventually trip at the rated current but this could take hours. The magnetic part will ensure it will trip within the time allowed with a short circuit but with a short circuit the current needs to be 5, 10, or 20 times the rated value for a B, C or D type MCB. So for example a B32 MCB will need 160 amps to flow to trip it which equates to a loop impedance of 1.44 ohms. With a fuse being slightly over 1.15 ohms for a BS 1361 30 amp fuse will just mean it takes slightly longer. But with a MCB being over the 1.44 limit means it could take many times longer as it needs to heat up the thermal part instead to the magnetic bit working.

Once you have the data then you can decide the best way forward but without the data guessing is just not good enough.

For the DIY man the first consideration has to be is it worth buying or hiring the test gear? The loop tester, low and high ohm ohmmeter and the RCD tester which also records the time taken to trip will as a set cost around £750 to buy or £75 to hire. Normally min of one week to hire. So you would need to work fast to complete all work within the week.

As for the professional there is the problem in leaving work not deemed as safe. One is not permitted to leave a house uninhabitable without finding alternative accommodation but also one can't re-energise a circuit deemed as dangerous and in the world of blame that gives the electrician a real problem. To complete the tests he has to switch off the power and if he then finds a fault he can't re-energise however neither can he lock it off so once out the door you could re-energise. However once you know it's dangerous then to re-energise should some one be injured you would be libel.

It's that sucking through teeth and jobs worth comment. So if you think it will fail then to get an electrician to tell you what you already know is rather pointless.

So the next best is the socket tester with a loop tester built in. There are two popular types one by socket & see and the other by Martindale both just over £50 the SOK34 or SOK36 or EZ150 these are not as good as the loop tester used by the pros but do a reasonable job I would expect the EZ150 would likely be best for you. Try Test4less they seem to have low prices.

However at £44.16 plus vat still not cheap. By time you have bought or hired the test gear one wonders if it's really worth DIY?

The http://www.esc.org.uk/industry/industry ... ce-guides/ Number 8 does tell you some of the problems with using socket testers. Have a read then say if we can help.
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Postby Saguenay » Wed Jan 25, 2012 5:42 pm

Thanks stoneyboy and ericmark. Certainly food for thought. I have uncovered another cable (3mm) which has been severed and is still live. I managed to trace it back to the fusebox and it appears to be the other half of the ground floor ring so all the kitchen appliances and other paraphenalia on the ground floor seem to be operating on one giant spur!!!! After drawing out a circuit diagram of the rat's nest I think that I have located where the second half of the ring is. I shall do some testing to see if my thoughts are correct. Will keep you advised as to the continuing saga.
Thanks again.
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Postby Saguenay » Thu Feb 09, 2012 6:56 pm

This item is now closed for me. Had a Eureka moment when I tracked down a severed cable which was supposed to serve the ground floor ring. A bit of reconnecting and rerouting has got rid of the undesirable spurs and all is now good. Thanks once again guys.
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