Splitting a ring?

[Paul5152]

While having some building work done, a 2.5mm T&E cable was found in an inconvenient place (coming from loft to floor level) . It's on a 20A MCB circuit, which I had assumed was a radial circuit

On investigating to work out which sockets would be disconnected if that wire was removed, I found that *none* would be. Looking inside the consumer unit there are two 2.5mm wires going to the MCB

So, it looks like it's a 2.5mm ring main with a 20A MCB. Which wasn't what I expected at all. (it looks like there are another 2 circuits the same in the CU, and a 32A ring with the same size cable, for the kitchen)

(the house was completely rewired a couple of years ago, before we bought it, so I'm not sure why it was done this way - maybe the 20A MCBs were on a special at Aldi that week... )

Now, I believe it'll be fine to remove the inconvenient wire (and retest earth continuity/impedance) and convert the ring to a radial, but am I missing something?

 

[Jurassicspark]

On investigating to work out which sockets would be disconnected if that wire was removed, I found that *none* would be. Looking inside the consumer unit there are two 2.5mm wires going to the MCB

Ok.

So, it looks like it's a 2.5mm ring main with a 20A MCB. Which wasn't what I expected at all. (it looks like there are another 2 circuits the same in the CU, and a 32A ring with the same size cable, for the kitchen)

It might not be a ring, it could be a spur supplying something else.

(the house was completely rewired a couple of years ago, before we bought it, so I'm not sure why it was done this way - maybe the 20A MCBs were on a special at Aldi that week... )

It could have been a ring before but the OPD was reduced due to a high Zs or an open circuit in the ring.

Now, I believe it'll be fine to remove the inconvenient wire (and retest earth continuity/impedance) and convert the ring to a radial, but am I missing something?

Check continuity of all the cores from the two cables. If it actually is a ring you'll find continuity. If it isn't a ring then the other cable leaving the consumer unit will be supplying something else. See where it goes.

 

[blup]

It could be a split ring but maybe in the first place two radials from one breaker. Havve you identified any sockets working off either of them.

 

[Mottie]

Calm down @HERTS P&D. Not that sort of ring.

 

[Murdochcat]

It could be a radial circuit with 2 cables at the MCB

 

[ericmark]

I have in the past found mistakes, either a ring final broken, or a 4mm² radial that someone thought was a ring final and added 2.5 mm² in error, and the simple way to make safe is to replace the B32 with a B20 MCB.

If the B20 is tripping, then you need to look further, but often a B20 is enough anyway.

Theory, is that the circuit is short enough, so if a 20 amp load is put at the end (radial) or centre (ring final) and the remaining is even spaced, that the volt drop will not exceed 6%. The equates to 106 meters of cable with a ring final, and 32 meters of cable with a 20 amp radial, so splitting a ring can result in over the permitted volt drop, but this is unlikely to cause a problem, as long as a short circuit will trip the MCB on the magnetic part, it is OK.

The problem is the meter used to test this is a bit expensive. This one I think cost around £85, there are cheap testers,

but they will only test a radial, their pass mark is too high for a ring final, and the one shown cost £44.34 so is it worth getting a tester that expensive that will not work with a ring final?

And to be frank, as long as the circuit is RCD protected, it is not that important anyway.