3 legged ring main

[a-z electrics]

Some advice please.. I am going to replace a 1361 fusebox, only 5 circuits, with a new consumer unit. However I have noticed that the ring 30A circuit is still energised after I pulled the circuit fuse. Further investigation revealed another 16A cct is connected and only once this fuse is pulled does the circuit go dead. So obviously it must be connected also somewhere. This strikes me as dangerous, not in current carrying capacity (the wires are old colour, stranded, about 3-4mm2 i would say) but to isolate properly.
I did a figure of 8 continuity test on one pair wires and then another pair and it is continuous, so there is a ring. Which pair is the real ring though is unclear, maybe the pair with the highest readings?
If I can't discover where the "extra" leg has been joined, what do you suggest I do? Join all three into the one circuit? If I left one wire out it would still be live..

 

[a-z electrics]

Sorry it should have said second cct is 15A, not 16

 

[Leesparkykent]

Deleted...just read OP properly.

 

[davesparks]

. This strikes me as dangerous, not in current carrying capacity (the wires are old colour, stranded, about 3-4mm2 i would say) but to isolate properly.
.

You might want to think about that a bit more, there is a ccc issue there, along with what will happen under fault conditions.

The cable is most likely to be 7/029 so far closer to 2.5 than 4

You need to find where the interconnection between the circuits is and seperate them. Having a ring fed at 3 points will have an effect on the current sharing around the ring and you will not be able to carry out meaningful testing.

As for finding it, the normal method of fault finding any circuit should find it. Split the ring, test each side, split again and repeat until you find it.
If there is any labelling on the old board it might give a few clues.
A 5 circuit board is often made up of cooker (30A) ring (30A) immersion (15A) and 2x lights (5A)
So if that's how yours is arranged you could, without further evidence, start with a theory that the 15A circuit should be the immersion heater.

 

[telectrix]

still in a good mood dave? did the landy start without a push for a change?

 

[a-z electrics]

As for finding it, the normal method of fault finding any circuit should find it. Split the ring, test each side, split again and repeat until you find it.
If there is any labelling on the old board it might give a few clues.
A 5 circuit board is often made up of cooker (30A) ring (30A) immersion (15A) and 2x lights (5A)
So if that's how yours is arranged you could, without further evidence, start with a theory that the 15A circuit should be the immersion heater.

Thanks for your response..
Well I think the 15A cct was labeled Garage skt and the fusebox is in the garage, so maybe not too bad to sort.. Hope so, because the rooms are full of stuff. I'm back there Tuesday.
Concerning the CCC, surely any load on the cct is shared by 3x3mm (or whatever size it is) so what is the concern there? After all, there's more copper with 3 wires than with 2, or am I not understanding correctly?

 

[davesparks]

I think you've missed something in the basic theory of a ring final! The current sharing around the ring would normally ensure that no cable is overloaded. but with a third connection at an unknown location will affect that current sharing. You also cannot confirm that the ring is in fact arranged as a ring with the normal sequence of tests.

For reference the current rating for 7/029 twin from the 13th edition is 20A

 

[a-z electrics]

I thought a figure of 8 test conformed whether a pair of wires are connected as a ring.. Is this not so?

 

[davesparks]

I thought a figure of 8 test conformed whether a pair of wires are connected as a ring.. Is this not so?

Yes it does, but you can't apply it to a ring with 3 ends can you!

 

[a-z electrics]

I don't see why not.. If there's 3 pairs of wires one can do 3 tests. If they each show continuity in a figure of eight test, then in my mind there is a ring with an extra leg attached. Ie two rings with a common leg. It isnt separate or it wouldn't be live until all three are disconnected, as in this case. But I'm just a novice, what do I know? I'll double check with Elecsa tm. I am quite open to be overruled.. I do agree though it shouldn't be like that. My question is, is it safe to connect all three into one fuse? It's definitely unsafe to be in two..

 

[telectrix]

it's safe for a temp. measure, but you still need to be concerned about ccc amongst other things. time to remove some sockets and do some splitting.

a guess would be the 15A feeding the garage and a cross connection from there to the RFC by some divvy that thought it would increase the load capacity tom the garage. always start with alterations in these cases.

 

[Dave OCD]

I think this could be just a simple mix up with the original installation. All you should have to do is connect one cable at a time and see which of the other two ends is live [disconnected from any terminals] , that should clarify which two are the ring and the other one will be the 16 amp radial.

 

[a-z electrics]

I think this could be just a simple mix up with the original installation. All you should have to do is connect one cable at a time and see which of the other two ends is live [disconnected from any terminals] , that should clarify which two are the ring and the other one will be the 16 amp radial.

I'll try that Dave, that would suggest the 15A cable is one leg of the ring, then by deduction the two cables terminated at the ring fuse must be the other leg and the "spur".
however I did two figure of eight tests ie cables A and B (the pair in one fuse) and A and C and they both buzzed out as connected, therefore suggesting they must all be interconnected.. I think! I didn't get round to testing the third option, B and C. I'll do that Tuesday.
Thanks anyway.. It's an interesting situation and one I haven't encountered before.

 

[davesparks]

however I did two figure of eight tests ie cables A and B (the pair in one fuse) and A and C and they both buzzed out as connected, therefore suggesting they must all be interconnected.. .

Why didn't you just bell them out 'end to end' rather than gagging about with figure 8 tests?

I thought from your OP that you had already proved that all three were connected!

 

[Dave OCD]

I'll try that Dave, that would suggest the 15A cable is one leg of the ring, then by deduction the two cables terminated at the ring fuse must be the other leg and the "spur".
however I did two figure of eight tests ie cables A and B (the pair in one fuse) and A and C and they both buzzed out as connected, therefore suggesting they must all be interconnected.. I think! I didn't get round to testing the third option, B and C. I'll do that Tuesday.
Thanks anyway.. It's an interesting situation and one I haven't encountered before.

It might be as simple as that- make sure you disconnect all plugged in equipment first too otherwise you'll probably get continuity across all 3 regardless via the common neutrals ?