Domestic: 20A DP switches and ring final circuits??

[8yf13l0]

Hi all .

Seeing alot of cases where I have seen 20A DP switches in a kitchen (for example) each feeding a radial 2.5t&e to each white goods appilance or similar, and the 20A DP switch primarys are fed from the 32A kitchen ring final circuits - Is this accepteble to BS7671?

I'm guessing if it's only serving a 1 gang s/o or 13A fused spur it would not be at risk of overload at the switch?

Please advise.

Many Regards

Jamie

 

[Dave OCD]

Someone please correct me if I am wrong:-

The 20A rating of a FCU is for the contacts within the switch and not the incoming terminals?

Actually you mean a 20A DP switch as the rating of a FCU is usually 13 amps ?

 

[Dave OCD]

I agree the terminal should be able to take load as they are the same size as sockets and spurs and anyway the terminal will never run at 32amps as the ring circuit is distributed in 2 directions

Are you sure about that, I can't see how the current would differ appreciably throughout the circuit ?

 

[Paul.M]

I would have thought so Paul.

Yes I know, just wanted to "ask" instead of "inform" via a sarcastic format (I'm in that mood tonight). Very good point Dave btw.

Any switch rating is the amount of Amps it can take under normal usage without the contacts over heating/meting/burning via conducting electricity from the main supply to the load.

eg. A 6A light switch on a 10A MCB with a load of 8A.

Q. What do people think will happen in time?

 

[DPG]

Yes I know, just wanted to "ask" instead of "inform" via a sarcastic format (I'm in that mood tonight). Very good point Dave btw.

Any switch rating is the amount of Amps it can take under normal usage without the contacts over heating/meting/burning via conducting electricity from the main supply to the load.

eg. A 6A light switch on a 10A MCB with a load of 8A.

Q. What do people think will happen in time?

Being picky here, but you are saying the load on the 10A MCB is 8A. So that's OK. The 6A light switch will be OK, assuming the load it is switching is less than 10A. But I do get what you were intending to ask Daz

 

[Paul.M]

Being picky here, but you are saying the load on the 10A MCB is 8A. So that's OK. The 6A light switch will be OK, assuming the load it is switching is less than 10A. But I do get what you were intending to ask Daz

An 8A load protected by a 10A MCB is absolutely fine. The problem is with an 8A load conducting via a 6A rated switch! The contacts within the switch are over loaded and will quickly get warm then melt or fuse.

The OP's question about the 20A rating of a FCU, the 20A bit is for the load side and not the incoming terminals of the FCU.

 

[DPG]

An 8A load protected by a 10A MCB is absolutely fine. The problem is with an 8A load conducting via a 6A rated switch! The contacts within the switch are over loaded and will quickly get warm then melt or fuse.

The OP's question about the 20A rating of a FCU, the 20A bit is for the load side and not the incoming terminals of the FCU.

Ha ha, I was being picky with the way the question was worded. Not to worry. Daz

 

[telectrix]

Yes I know, just wanted to "ask" instead of "inform" via a sarcastic format (I'm in that mood tonight). Very good point Dave btw.

Any switch rating is the amount of Amps it can take under normal usage without the contacts over heating/meting/burning via conducting electricity from the main supply to the load.

eg. A 6A light switch on a 10A MCB with a load of 8A.

Q. What do people think will happen in time?

the light will go out.

 

[DPG]

But the switch will start emitting light instead Daz

 

[Paul.M]

Ha ha, I was being picky with the way the question was worded. Not to worry. Daz

It's been fun

the light will go out.

unless the contacts are fused?? (had that problem once on a dimmer switch were the winding's just fused together)

But the switch will start emitting light instead Daz

That is a safety feature, glowing switch so you can see where to turn the lights on before walking into a dark room and stub your toe on the coffee table leg! :rofl: done this, my little toe hurt for 3 days!

 

[Lucien Nunes]

OK, so we have a 20A DP control switch with its supply side as a point in an RFC protected at 32A. The load side connects to some accessory, either a single socket outlet or FCU, that will limit the load to 13A by way of the BS1362 fuse. We declare it as a fixed load so the 20A switch is within its thermal (continuous load) rating.

But what happens with a short circuit on the unfused spur? The 20A switch has to survive the fault being cleared by a 32A device without disintegrating, instead of a 20A device. Any accessory intended for use on an RFC will have been designed for that duty, but has the 20A switch? Check with manufacturers? Splitting hairs? Take it one stage further - given suitable cable would it be acceptable to control a 1A fixed load on a 40A circuit, using a bog standard 5AX light switch?

Discuss...

 

[Dave OCD]

OK, so we have a 20A DP control switch with its supply side as a point in an RFC protected at 32A. The load side connects to some accessory, either a single socket outlet or FCU, that will limit the load to 13A by way of the BS1362 fuse. We declare it as a fixed load so the 20A switch is within its thermal (continuous load) rating.

But what happens with a short circuit on the unfused spur? The 20A switch has to survive the fault being cleared by a 32A device without disintegrating, instead of a 20A device. Any accessory intended for use on an RFC will have been designed for that duty, but has the 20A switch? Check with manufacturers? Splitting hairs? Take it one stage further - given suitable cable would it be acceptable to control a 1A fixed load on a 40A circuit, using a bog standard 5AX light switch?

Discuss...

No because you'd never get the incoming 40a rated cable into the switch terminal.

 

[Dave OCD]

the light will go out.

Or stay on when the contacts have fused together.:smilewinkgrin:

 

[Lucien Nunes]

you'd never get the incoming 40a rated cable into the switch terminal.

I chose 40A for this very reason - Bare 2H2.5 MI will do 40A and will fit any decent light switch terminal. Anyway, does the cable need to be rated for 40A inside a panel and carrying a fixed load of 1A?

 

[Dave OCD]

Well let's say a 1mm cable was connected to the outgoing side of this switch feeding a 1 amp load, if there was a short in this section of the circuit the 40a MCB would still disconnect before any other damage was sustained ?

 

[rustynails]

I've done this sort of setup before but always put the fuse module before the dp switch