Change over switch

[Satori]

I'm trying to figure out what sort of switch is needed to simultaneously bring a shower unit on line whilst isolating, say, an immersion heater; and vice versa. Don't want to overload the 60 A supply fuse.

Thanks.

 

[EFLImpudence]

Not necessary. You won't.

 

[Satori]

That's a risk I'd rather not take. Occasionally the washing machine, bathroom heater and kettle are on simultaneously. Combined with a shower unit and background consumption the total load would be more or less 60 amps.

Hence, I don't want the immersion heater and shower unit to be powered simultaneously. Is there not a fairly simple sort of change over switch available for this application other than the obvious solution of switching off the immersion heater before showering?

 

[333rocky333]

changover switches usually have two outputs but one common input, as a showers likely to be a 45 amp circuit and an imersion 20 amp, then additional larger than normal wiring to the imersion FCU or in line mcbs may be required, so a standard switch is not really suitable on its own but could be with extra work

 

[plugwash]

Changeover switches big enough to handle a shower are not cheap. http://www.tlc-direct.co.uk/Products/KMT5B37I.html . As 333rocky says you would also have to make sure the immersion was adequately protected by a fuse or MCB and that the wiring between the changeover switch and the fuse/MCB used to protect the immersion was adequate for connection to the shower circuit (easiest way would be to use the same size cable as the shower circuit)

Honestly I think you are worrying needlessly. The chances of all those appliances not just being on but actually drawing full power at once for any significant length of time is pretty small and fuses DO NOT blow immediately under small overloads.

 

[JohnW2]

That's a risk I'd rather not take. Occasionally the washing machine, bathroom heater and kettle are on simultaneously. Combined with a shower unit and background consumption the total load would be more or less 60 amps.

As others have said, I really think you are worrying unnecessarily. It takes a lot more than 60A, flowing for a very appreciable period of time to cause a 60A fuse to blow (e.g. 100A would probably take well over half an hour to blow the fuse). Countless households have all the things you mention, plus a lot more (tumble dryers, ovens, hobs, microwave, fan heaters, toasters etc. etc.) many of which may occasionally be on simultaneously, but it's very rare for such 'overloads' to result in blowing of a 60A or 80A service fuse.

Kind Regards, John

 

[Satori]

Concerned more about the 16 mm² distribution cable taking >60 amps than the blowing the supply fuse (which I'd also rather not do).

It is precisely the fact that the 60 A BS 1361 fuse requires about 100 amps (99th percentile or whatever) over an hour before blowing which makes the matter of overloading the distribution cable a risk I'd rather avoid.

Just how conservative are the ampacity figures for these cables? Will a 16mm² T&E safely carry, say, 80 amps for twenty minutes?

This Klöckner-Moeller cam switch is probably the easiest solution.

Thanks for the replies.

 

[ericmark]

I think this is what your looking for I have never used one and there seems very limited info as far as I can see it would also need a relay of contractor as I can see 1A written on it.

But really I would agree with the others I would not worry about it.

 

[JohnW2]

Concerned more about the 16 mm² distribution cable taking >60 amps than the blowing the supply fuse (which I'd also rather not do). It is precisely the fact that the 60 A BS 1361 fuse requires about 100 amps (99th percentile or whatever) over an hour before blowing which makes the matter of overloading the distribution cable a risk I'd rather avoid. Just how conservative are the ampacity figures for these cables? Will a 16mm² T&E safely carry, say, 80 amps for twenty minutes?

They are very conservative. By virtue of the the operating characteristics of MCBs and cartridge fuses, cables are deemed to be able to safely carry at least 1.45 times their rated values for 1 hour - e.g. 16mm² with installation method C is deemed to be able to carry at least 123A (85A x 1.45). However, as often discussed, here, even that figure is undoubtedly very conservative.

Kind Regards, John

 

[westie101]

Cut-out fuse will carry a 47% overload prety much indefinately.

We very rarely get called out to a blown cut-out fuse in these conditions, apart from the inconveniance the DNO will replace the fuse FOC anyway, usually within 3 hours during the day

 

[JohnW2]

Cut-out fuse will carry a 47% overload prety much indefinately. ... We very rarely get called out to a blown cut-out fuse in these conditions, apart from the inconveniance the DNO will replace the fuse FOC anyway, usually within 3 hours during the day

Indeed. However, as you will have seen, although the concern the OP initially expressed was that he might 'overload' the cut-out fuse, he is now worrying because it might not operate if he overloads it, and is therefore now worrying about the cables. I'm sure that we can all agree that he is in no different a position from most other UK householders and is worrying totally unnecessarily.

Kind Regards, John

 

[JohnW2]

I suppose we should not really be surprised, or two dismissive of, those who have such concerns - after all, it's not unreasonable for 'the man in the street' to assume that a "60A fuse" or a "cable with a CCC of 85A" mean what they appear to say. If anything, perhaps we should be surprised that a lot more people do not think about these things, 'add up loads' and then find reason for apparent concern. These are, AFAIAA, not things which are generally taught in schools - maybe they should be (along with the total load capacity of a double 13A socket!)?

Kind Regards, John

 

[Satori]

That 45% extra capacity is more than sufficient to cover the potential overload.

Thanks to each of you for the helpful and informative replies.

 

[JohnW2]

That 45% extra capacity is more than sufficient to cover the potential overload.

Indeed, and you would have even more leeway than that for half an hour.

Thanks to each of you for the helpful and informative replies.

You're welcome.

Kind Regards, John