Power circuit design

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Hi guys,

Ive opened up a can of worms on a job and need a bit of advice although i pretty much think i know the answer just after confirmation to my nightmare scenario.

I am in the process of partially rewiring a house which will be notified to building control.

on initial testing the r1 +r2 on all the sockets was fine, the insulation resistance was fine so i thought i should be in a good position to change the board with little disruption as the lights were all earthed etc.

2 wires were going into the circuit breaker for the power circuit so presumed it was either a ring circuit or 2 radial circuits. Cable size is 2.5mm

There are 11 sockets on the circuit so worst case 2 radials wouldnt be an issue.

I have lifted up a couple of floor boards and found a junction box with 2.5mm cable coming out of it. There is the 2.5mm from the consumer and 3 cables coming out which are going to the sockets. this is the same on the other 2.5 coming off the mcb. So in effect i have 2 "spider ciruits" both going into junction boxs then going off to the sockets.

Im concerned that i now have a spur off a spur off a spur etc and also possible cable underrating. If it was 2 radials i was going to just split them up

I really dont want to have to rewire the whole house but i now cant trust the design as there could be junction boxes anywhere?

Any suggestions other than a complete rewire of the power circuits as i know these spiders are not best practise on a lighting circuit never mind a power circuit.
 
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Have you done any tests to determine if the circuit is actually a ring (albiet with a mess of spurs) or two independent runs from the CU?

The first thing to do in a situation like this is to start mapping out the situation. Usually mapping will involve a combination of testing (either with a tone tracer or shorting one end and measuring for the short to find the other) and inspection. Is there a crawlspace? if so crawl round it (carefully) and start tracing cables. If not look for floorboards that look like they have been lifted in the past as these very likely cover cable routes.

Once you have a map of the situation you can start working out the best way to bring it into compliance. It is perfectly ok for radial circuits to branch so a "spider" configuration on a radial is not nessacerally a problem.
 
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Ok thanks for the quick reply. I am going to go back to the drawing board and test with crossed neutral and lives for a ring although im pretty sure after seeing the jb thats its not. Does the jb not count as a spur? if so i thought it would need to be down rated with an fcu ?
 
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Ive been researching this a bit more and as far as I am aware this is whats known as a branch circuit? and my only real concern is the cable rating so if i put a 20amp mcb on this circuit i should be ok?
 
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Yes, apart from being subject to any adverse installation conditions, then a 20A mcb should be fine (or even 25A if you can obtain one)
If you suspect there are more junction boxes then I really would want to be checking the connections in them though.
 
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I am in the process of partially rewiring a house which will be notified to building control.
By what process?
on initial testing the r1 +r2 on all the sockets was fine, the insulation resistance was fine so i thought i should be in a good position to change the board with little disruption as the lights were all earthed etc.
Okay, but no Ze or Zs readings?
2 wires were going into the circuit breaker for the power circuit so presumed it was either a ring circuit or 2 radial circuits. Cable size is 2.5mm
Presumed? did you not test for end to end and r1+r2? What rating was the circuit?

I have lifted up a couple of floor boards and found a junction box with 2.5mm cable coming out of it. There is the 2.5mm from the consumer and 3 cables coming out which are going to the sockets. this is the same on the other 2.5 coming off the mcb. So in effect i have 2 "spider ciruits" both going into junction boxs then going off to the sockets.
Are you sure? Could they not be one spurred socket of each junction box and the RFC continues? But if you have four cables at the JB, sounds dodgy!
Im concerned that i now have a spur off a spur off a spur etc and also possible cable underrating. If it was 2 radials i was going to just split them up
Well the rating of the protective devices will help, you need to test the circuit out as a ring final, end to end continuity and r1+r2 of the live/earth loop.
.
I really dont want to have to rewire the whole house but i now cant trust the design as there could be junction boxes anywhere?
You never can, that is why investigation and testing is required.
Any suggestions other than a complete rewire of the power circuits as i know these spiders are not best practise on a lighting circuit never mind a power circuit.
Firstly, do the correct test to prove continuity and r1+r2 of a ring final.
 
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