Re: Grid switches

[Andrew Gabriel]

Can anyone think of any IEE objections to having the following arrangement?

> The circuit is a 32A B/RCBO ring main. I want to have a three way grid > switch, each way fed from the ring main. Each module is capable of 20A and > leads to a single unswitched 13A socket. There are no fuses in the system, > except in the plug tops. The feeds are to appliances, each likely to have > 2kW heating elements. > > Basically, overcurrent protection for the 20A switch is provided by the plug > top fuse, whilst short circuit is provided by the RCBO. DP Isolation is > provided by the unswitched socket. Is this allowed?

Yes.

You might want to worry about the loading and poor spread round the ring, depending what the appliances are and what else the ring feeds.

[Owain]

"Christian McArdle" wrote | Can anyone think of any IEE objections to having the following arrangement? | The circuit is a 32A B/RCBO ring main. I want to have a three way grid | switch, each way fed from the ring main. Each module is capable of 20A and | leads to a single unswitched 13A socket. There are no fuses in the system, | except in the plug tops. The feeds are to appliances, each likely to have | 2kW heating elements. | Basically, overcurrent protection for the 20A switch is provided by the plug | top fuse, whilst short circuit is provided by the RCBO. DP Isolation is | provided by the unswitched socket. Is this allowed?

The problem I can foresee, but may not in reality be problem, is that this is in effect a very high point load on the ring, and may affect current distribution around the ring ie one leg more heavily loaded than the other.

Also if these appliances now have 2kw elements, are they likely to be upgraded at a later stage to higher rated appliances? as there isn't much spare capacity on the ring unless you can show diversity. Having other sockets on the ring, to cover the 'not more unfused spurs than sockets directly on the ring' rule, also adversely affects capacity.

Owain

[Christian McArdle]

The ring main (yet to be installed) will only supply kitchen sockets and the appliances previously detailed.

Well, I've been umming and ahing and maybe I'll run the lot off a separate

32A 6mm radial. It's only one extra cable (of which I have a drum handy) and it is the easiest cable run in the world. The consumer unit has 4 spare ways (after my previously current plans) and is mounted in an adjacent cupboard. The other alternative is to keep with the single ring circuit, but run 6mm throughout instead of 2.5mm. Unlikely to get issues with unbalanced rings then. (Or I will, but it won't matter one bit).

I've found the grid switch system I'm looking at does DP switches as well as SP, so I'll probably use those too.

Christian.

[Christian McArdle]

Well, I think I've finally decided not to put these appliances on the ring main (32A RCBO). I'll run a separate 6mm clipped direct 32A MCB radial for them. I can live without the RCD protection on them. I'll probably now ditch the planned 32A cooker circuit and run the cooker off the now hardly used kitchen ring. The cooker is a gas range and only needs a 13A supply for the secondary electric oven (prob 2kW and rarely used) and ignition. Apart from that it will now only run a kettle and microwave and various other low duty cycle items.

So now, I have in the kitchen: B32A MCB 6mm radial to 3 individual unswitched 13A sockets with separate 20A grid switching. (To power washing machine, dishwasher and tumble dryer, probably run simulataneously on a regular basis).

B32A/30mA RCBO 2.5mm ring to sockets and second oven. (main oven is gas).

B16A MCB to fridge freezer.

B32A/30mA RCBO 6mm to outside lighting, electrics and shed.

B6A MCB lighting.

Christian.