MK 32amp DP switch

[JohnW2]

More generally, there's the question as to the extent to which one has to design on the basis of what inappropriate changes someone may effect in the future.

Generally John, I'd agree, however I'm sure many electricians will not take the switch off and look at the rating on the back, especially if the cable is all rated to 32A as well.

Probably true, but if they didn't 'take the switch off an look at the back', they couldn't really be sure of the CSA of the cable that was feeding it, either, could they?

However, the greater issue is probably the other part of what I wrote, to which you haven't responded. Do I take it that you would not be happy to consider the after-diversity load when deciding whether a '20A rated' switch was acceptable?

Kind Regards, John

 

[AndyPRK]

I can understand diversity being taken into consideration with an MCB selection, as the worst that will happen is it will fail safe and trip the mcb.

However if the cooker is operated at full load (which is possible at xmas) then the switch will be operated outside its design limits ?

 

[JohnW2]

I can understand diversity being taken into consideration with an MCB selection, as the worst that will happen is it will fail safe and trip the mcb. However if the cooker is operated at full load (which is possible at xmas) then the switch will be operated outside its design limits ?

That's true, but it's equally true of the cable. We (well, at least some people!) happily use diversity when selecting cable size, despite the fact that, for example, 100A is way 'outside the design limits' of 4mm² cable (clipped direct) - if one could find a 23/24 kW cooker to use at Xmas!

Kind Regards, John

 

[AndyPRK]

That's true, but it's equally true of the cable. We (well, at least some people!) happily use diversity when selecting cable size

But the cable will be protected by the MCB in that case

 

[JohnW2]

That's true, but it's equally true of the cable. We (well, at least some people!) happily use diversity when selecting cable size

But the cable will be protected by the MCB in that case

It would, but you would still be accepting in your design that the cable could be considerably 'overloaded' for a period of time before the MCB operated - is that really all that different from the situation with the switch.

I suppose it's down to the way ratings are expressed. Implicit in the design CCCs we use is the fact that a cable 'rated at', say 20A can safely carry a current of 29A for 1 hour, or higher currents for shorter periods, before an (appropriate) protective device operated. If switches were 'rated' in the same way (maybe they are?), a '20A-rated' switch would be safe with currents appreciably greater than 20A.

Kind Regards, John

 

[AndyPRK]

your using the argument a 20A switch with a 20A MCB. I can understand that.

I thought you were saying a 20A switch with a 32A MCB

 

[ban-all-sheds]

It would, but you would still be accepting in your design that the cable could be considerably 'overloaded' for a period of time before the MCB operated - is that really all that different from the situation with the switch.

No, it is not.

Just as with the cable, once you have used a diversity guideline to establish your Ib, you use a cable and a device such that Ib ≤ In ≤ Iz.

So it is the same as the situation with the cable. If In is to be 32A then the rating of the switch has to be ≥ 32A, even if Ib is only 20A. Or 20mA.

 

[JohnW2]

I thought you were saying a 20A switch with a 32A MCB

I was.

Kind Regards, John

 

[JohnW2]

So it is the same as the situation with the cable. If In is to be 32A then the rating of the switch has to be ≥ 32A, even if Ib is only 20A. Or 20mA.

Yes, I suppose that's true. [for the record, Ib had nothing to do with my argument]

Kind Regards, John

 

[ban-all-sheds]

Which argument?

The one where you repeatedly referenced the 20A Ib of the OP's oven, or the one where you referenced using the concept of diversity to determine Ib?

 

[JohnW2]

Which argument? The one where you repeatedly referenced the 20A Ib of the OP's oven, or the one where you referenced using the concept of diversity to determine Ib?

The latter.

If the OP's oven represents a true (without diversity) maximum load of 20A, and if we assume that the oven is the only load on the (switched) circuit, and that an oven cannot create an overload, then I see no problem with a 20A-rated switch.

I only started discussing (with some flawed arguments) the diversity issue when it was suggested that, although the 20A switch would be OK with the OP's current oven, someone might subsequently come along and replace it with one which represented a load (without diversity) >20A.

Kind Regards, John

 

[ldoodle]

Why do you want all them FCU's for ?

I'm not a big fan of central grid switches, but would think it would look better than all those fcu's ? ??

Just to clarify. the FCUs are already there. I am not choosing to put them in.

Where they are, they back on to the under stair cupboard so the wiring is accessible, visible and traceable, and changeable.

 

[ban-all-sheds]

Ideally, you could get a vertical one with socket.

There is one brand which does that. Seen them on ebay. One of the cheaper brands in white.

Can't find it now! I think it was a volex or similar

Contactum?




Although IMO that b***** great big red rocker is more obtrusive than the MK neon...

 

[seasickstevie]

I'm assuming all 45a switches have to be red now for some reason. All manufacturers seem to have red cooker switches now.
In fact thinking about it, they have for decades.

 

[ban-all-sheds]