Amps rating of switches

[hard-work]

I was in a neighbours place last week. I notice the Bosch induction hob had a 20A switch isolator adjacent on the wall. This has been fitted for about 5 years. I never said anything, but looked up Bosch induction hobs and they come to approx 7.2 kW maximum which is approx 31 A. It is a one bedroomed place with two people. The hob, I would assume, has never been full on, that is all rings being on max. I know about the diversity factor and that cables can be downsized because the probability of a cooking appliance being fully on in all its functions is very, very rare, hence why cables can be downsized.

1. Does this apply to switches?
2. What is the max' amps a 20A switch isolator can take? I know they have overload.

I have not said anything to them, as I am gaining information first before I approach them with some firm advice.

 

[ericmark]

The isolator/switch should be rated no smaller than the over load device. So a cable rated 22 amp supplied from a 20A MCB feeding a 20A switch which in turn feeds a 32A cooker is OK, as it will fail safe.

However if on a 32A MCB theory is cable and isolator should also be at least 32A, however we regularly see 63A RCD's fed from a 100A fuse feeding 3 x 32A MCB's which in theory is wrong, and it is unlikely there will be a problem using a 20A switch/isolator for a 32A supply even if not correct.

So may be better to either say nothing, or ask them what size trip feeds the cooker before saying anything.

 

[seasickstevie]

... upBosch induction hobs and they come to approx 7.2 watts maximum

I guess we can take that as 7.2kw (?)

 

[JohnW2]

The isolator/switch should be rated no smaller than the over load device. So a cable rated 22 amp supplied from a 20A MCB feeding a 20A switch which in turn feeds a 32A cooker is OK, as it will fail safe.

I'm not at all sure that is theoretically correct. The cable should, indeed, be "rated no smaller than the overload device", but it is OK to consider diversity for that (essentially because what matters to the cable is the average current over a significant period of time).

However, a switch might be called upon to break the maximum possible instantaneous load, which means that application of diversity is not appropriate. If it were a 20A MCB, that would allow 31A to flow for quite a time - so I would have thought that the switch should really be rated to be able to safely break that current.

Kind Regards, John

 

[EFLImpudence]

How do you know it is a 20A switch?

20A

32A

What is the OPD rating?

7.2kW @ 240V = 30A (28.8 @ 230V) which after diversity is 16A.
Not likely, but it could have a 20A OPD.

 

[JohnW2]

7.2kW @ 240V = 30A (28.8 @ 230V) which after diversity is 16A.

Indeed - but, as I have just written, I don't think it appropriate to apply diversity when considering a switch - since that switch might be called upon to break the maximum possible instantaneous current.

Kind Regards, John

 

[EFLImpudence]

Yes, I agree. I was just saying that it could have a 20A OPD on the circuit. It could be 2.5mm².

 

[JohnW2]

Yes, I agree. I was just saying that it could have a 20A OPD on the circuit. It could be 2.5mm².

Agreed and agreed - but a 20A switch (if, as you say, that's what it is) would nevertheless be technically a bit iffy.

Kind Regards, John

 

[hard-work]

I guess we can take that as 7.2kw (?)

Yep.

 

[hard-work]

I will find out maybe tomorrow. I know the CU has an RCD and a dedicated mcb. For arguments sake, say the mcb is 32A with the cable say 6mm all the way (or say the cable can take 7.2 kW) I read here that the diversity factor dictates it is fine and the 20A isolator is fine. It is the very same as this one, MK, that is how I know it is 20A.

 

[hard-work]

Indeed - but, as I have just written, I don't think it appropriate to apply diversity when considering a switch - since that switch might be called upon to break the maximum possible instantaneous current.

Kind Regards, John

These MK 20A switches do not break unless someone throws the switch.

 

[JohnD]

So if it's a 32A MCB and a 32A load, it should of course be the 32A switch.

Quite normal on a cooker circuit. For some reason the wrong switch may have been fitted, perhaps by somebody who didn't know better.

 

[hard-work]

I would agree that 32A minimum switch should be used. But is this safe? Will the contacts of the 20A switch melt if the hob is run at maximum, the whole 7.2kW. I have a gut feeling the cable is OK and the mcb is OK, it just the switch which is in question.

 

[JohnD]

maybe they would. The maker is under no obligation to sell a product whose performance exceeds its rating.

You sometimes see old switches where the contacts are burned or the springs have softened, though I think this can be due to dirt or tarnish. Long ago you used to find platinum points in some switchgear, and gold in certain telephone switches. I haven't heard of either lately.

 

[JohnW2]

I would agree that 32A minimum switch should be used. But is this safe? Will the contacts of the 20A switch melt if the hob is run at maximum, the whole 7.2kW.

In reality, almost certainly not. However, if, for some reason, you tried switching that switch off whilst the hob was running at that maximum load, you could get arcing aross the cointacts, leading to the switch contacts welding together (i.e. 'permanently on').

Kind Regards, John