We want two ring mains in the kitchen, one for the readily sockets serving moveable appliances -toaster, kettle etc; and another serving fixed appliances - washer, drier etc. I want the fixed applianced circuit to have a 4 pole 40 amp switch, to serve both sides of the ring, so that in an emergency ,such as we forget to take the cat out of the washing machine, we can isolate all fixed appliances by flicking one switch. Is such a switch available, ideally one that has an appearance in keeping with a chrome Crabtree socket?
Alternatively, could we legally run a single 6mm t+e cable from the fuse box to a dp switch in the kitchen and run a ring main serving the fixed appliances from that? If so, could I take both rings from that point?
I'll check with Building control re installing it legally. The cooker and oven will be gas.
The latter would seem the most sensible solution - switching seperate ends of a ring seems somewhat pointless. As long as the cable capacity is suitable, this would be the simplest approach - DP switches are somewhat easier to find...
thought that'd be illegal, but obviously it isn't, and I don't understand why. Doesn't that mean that in a small house, I can theortically disconnect one side of each ring thereby converting each ring to a perfectly acceptable radial circuit? And why isn't it considered a spur off a spur of a spur off a spur? It seems to avoid one safety factor that I thought was a main point of a ring - if there is an undetected bad connection in the earth or neutral somewhere along the way, there is a second path that will still afford protection.
=2E......... THe din solution sounds clever, but a bit complex in that it has got to be passively reliable for 20 years or so. And if someone else has to deal with it in years to come, simple to understand. So I think I'll go with the 6mm feed to the kitchen ring.
Radial socket circuits have always been allowed but British electricians tend to have an obsession with rings. There are two standard radial circuits: the 20 A and the 32 A.
The 32 A circuit is wired in 4 mm^2 cable (minimum) and can serve up to
75 m^2 floor area. It can have spurs in 2.5 mm^2 subject to the same rules as for the standard ring circuit.
The 20 A circuit uses 2.5 mm^2 cable (minimum) and can serve up to 50 m^2 floor area. This circuit can fork and branch as you wish, in a similar manner to a lighting circuit.
As with the ring, the cable sizes mentioned above are subject to design checks for voltage drop, earth fault loop impedance and the various cable derating factors. All circuits must be adequately rated for the load likely to be connected and the design should ensure that small overloads of long duration are unlikely to occur ("long duration" here means several hours or more).
The original raison d'être of the ring was simply economy in the use of copper. No special earthing provisions are required for ordinary socket circuits unless the protective conductor current is expected to exceed
10 mA. Above that figure the high integrity earthing requirements [section 543.7] come into effect and the ring circuit (with separate earth connections at all sockets and in the CU) is one of the available solutions.
That's a non-standard circuit and will need to be documented in your design records. Access to the ends of the ring will be needed for continuity testing and the location of this point should be documented.
You are aware of the requirement for 30 mA RCD protection for all socket circuits now, and for T&E cables behind plaster?
Well no, because a standard ring will typically be wired with 2.5mm^2 cable and protected by a 32A MCB. If you split it into two radials, you would need to reduce the MCB rating for each of them to 20A.
It is a spur of a spur etc. Hence the cable has to be able to carry the full design load for the circuit. So typically 4mm^2 cable would be used for a 32A ring. Note however the reduction in the maximum allowable floor area served with a radial as Andy mentioned.
It is an area where ring circuits perform better than radials (and on balance, they tend to out perform radials with most real world faults IMHO). However radials are perfectly acceptable, and in some cases preferable for reasons of simplicity or topology.
Contactor based solutions ought to be reliable, and are quite commonly used in industry. Its basically a posh version of using a relay to switch a bigger current load than your switch can handle alone. As Dave H mentioned, the contactor could be in the consumer unit if you have enough spare space (they often need about three modules space).
A 4mm radial for the group switched appliances would probably be simpler. And then a normal ring circuit for all the other sockets.
Understood thanks... Presumably, to make it simpler still, by using a single radial only for all the outlets in the kitchen. It'll be 6 mm t+e since I already have a coil. It'd first serve the five sockets for non-fixed appliances, then continue via a 40amp dp switch w/neon to the 4 fixed appliance sockets. Is that ok?
The main difficulty with 6mm^2 is it can be a pig to work with in the confines of a normal socket box. You may also find difficulty getting 2 x wires in the terminals on some brands of socket.
You could use it as a "backbone" and spur from it in thinner cable for the sockets etc, that way the terminations on the 6mm^2 can be in more suitable junction boxes.
I think that's ok. It's a gas cooker, gas tumble drier, so the biggest drain is the washing machine.
Good point. I actually realised that in the middle of the night! Spurs would work, but that'd mean a lot of extra boxes scattered about the wall. So I'll get some 4mm cable. instead.
I came across this calculator for cable size & voltage drop which looks good.
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