Amps rating of switches

[hard-work]

Thanks. So, safety panic now receded.

Got this from the web:
Hob power:
- Front right: 2.2 kW
- Front left: 1.8 kW
- Back right: 1.8 kW
- Back left: 1.4 kW

So, total amps is, 31.3 A.

Two rings full will be 17.39 A, below the 20A switch rating.
Three on then it is case of do not throw the switch.

 

[hard-work]

The diversity factor calculation determines that smaller cables can be fitted. It was not determined by those on this thread if the diversity factor takes into account switches. Anyone have anything firm on that?

 

[DetlefSchmitz]

I'd be inclined to just change the switch and be done.

 

[hard-work]

I agree and if it was me I would change it. The guy who installed may have known what he was doing. I also do not want to create fear in them and just gather info on what can and cannot be done. I did go over this morning and spoke about it in a rather roundabout way, not wanting to scare them. The hob and oven have their own circuits with their own 32A mcbs, with both under an RCD. The hob consumes 31A max, so the 32A mcb is just there. He did say they used "heavy cables" all the way to the the two appliances, so that is 4mm minimum, which is OK to take the loadings.

As I have written, it is the 20A isolator that is the alarm point, with this not appearing to be such a scare as I initially thought. Also, the two isolators are above the worktop and are identical. The one supplying the oven is fine. So, if one is changed the two will need to be, to keep the looks in the smart kitchen.

 

[sparkwright]

I'd be inclined to just change the switch and be done.

Same here.

Have encountered exact same situation before, a 20A dp switch used with an unspecified electric hob, and the switch was burnt out.

In fact, this occured in two identical flats, and both were burnt out.

 

[JohnD]

to keep the looks in the smart kitchen.

for maximum smartness, you can order them engraved "hob" and "oven"

I also have "boiler" and "fridge freezer" and "extractor hood"

 

[JohnW2]

The diversity factor calculation determines that smaller cables can be fitted. It was not determined by those on this thread if the diversity factor takes into account switches. Anyone have anything firm on that?

As I've said (and as I would have thought would be obvious to anyone who understands the concepts), the concept of diversity is not appropriate to switches. Diversity estimates the maximum average current over an appreciable period of time, which is what matters in terms of heating of cables etc. A switch has to be able to safely break the maximum possible instantaneous current at the moment of switch-off - so the average current over a period of time is irrelevant, hence application of diversity inappropriate.

Kind Regards, John

 

[hard-work]

for maximum smartness, you can order them engraved "hob" and "oven"

I also have "boiler" and "fridge freezer" and "extractor hood"

Where do you get them pre-engraved?
I recall a unit made by MEM about 15 years ago. It was isolator switches with the words fridge, etc imprinted, that were smart, about the size of an mcb and went on a rail in smart box, sort of like a CU just for the kitchen. You just chose the isolator you wanted. It never appeared to catch on. A shame really as one of these could be fitted in cupboards avoiding switches over worktops. I suppose you could use grid switches but the wires are far too tight inside those for comfort.

 

[hard-work]

As I've said (and as I would have thought would be obvious to anyone who understands the concepts), the concept of diversity is not appropriate to switches.

Thanks, it did get it. That was what was giving me concern.

 

[JohnD]

Where do you get them pre-engraved?

I think some of the common ones are ordered from MK, but there are quite a few places with engraving machines. You can often pick them up on ebay from excess stock or uncollected orders for about a fiver.
https://www.ebay.co.uk/itm/MK-SURPL...ARANCE-BARGAINS/272807620052?var=571948071357

I do have a label-maker, which looks fine, but in a kitchen where cleaning sprays are used, and soup or fat might get splashed around, they can get shabby.

For inelegant work, I use white vinyl sticky tape and print the label with a Stabilo write-4-all pen. also for identifying switches, fuses, cables etc while you're working.

I found one of my self-made labels in a theatre at least 20 years after I'd applied it, still fine.

 

[bernardgreen]

A switch has to be able to safely break the maximum possible instantaneous current at the moment of switch-off

It also has to be able to take a large back EMF ( voltage spike ) when it is switching off an inductive load. The arc this creates can erode the surface area of contacts in the switch which over time reduces the current the switch can carry without contacts heating..

A switch should be able to carry a fault current for the length of time it takes for the over current protection device ( fuse or MCB ) to operate. The switch should survive this without it's contacts welding themselves together into the closed state.

 

[JohnW2]

Where do you get them pre-engraved?

A bit off-topic, but grid switches are available off-the-shelf with a wide range of things engraved on them - such as these (MK ones available from TLC) ...

MK K4896BR 20 Amp DP Grid Switch - Boiler
MK K4896CH 20 Amp DP Grid Switch - Cooker Hood
MK K4896CM 20 Amp DP Grid Switch - Coffee Machine
MK K4896DW 20 Amp DP Grid Switch - Dishwasher
MK K4896FF 20 Amp DP Grid Switch - Fridge Freezer
MK K4896FG 20 Amp DP Grid Switch - Fridge
MK K4896FN 20 Amp DP Grid Switch - Fan
MK K4896FZ 20 Amp DP Grid Switch - Freezer
MK K4896HB 20 Amp DP Grid Switch - Hob
MK K4896HR 20 Amp DP Grid Switch - Heater
MK K4896IH 20 Amp DP Grid Switch - Immersion Heater
MK K4896MW 20 Amp DP Grid Switch - Microwave
MK K4896N 20 Amp DP Grid Switch + Neon
MK K4896NIH 20 Amp DP Grid Switch - Immersion Heater + Neon
MK K4896NPH 20 Amp DP Grid Switch - Plinth Heater + Neon
MK K4896OV 20 Amp DP Grid Switch - Oven
MK K4896PH 20 Amp DP Grid Switch - Plinth Heater
MK K4896TD 20 Amp DP Grid Switch - Tumble Dryer
MK K4896WC 20 Amp DP Grid Switch - Wine Cooler
MK K4896WD 20 Amp DP Grid Switch - Waste Disposal
MK K4896WDA 20 Amp DP Grid Switch - Warming Drawer
MK K4896WDR 20 Amp DP Grid Switch - Washer Dryer
MK K4896WL 20 Amp DP Grid Switch - Worktop Lighting
MK K4896WM 20 Amp DP Grid Switch - Washing Machine

Kind Regards, John

 

[JohnW2]

A switch should be able to carry a fault current for the length of time it takes for the over current protection device ( fuse or MCB ) to operate. The switch should survive this without it's contacts welding themselves together into the closed state.

It should, but it's ability to do that has got nothing to do with the 'rating' of the switch (6A, 10A, 20A, 32A or whatever). Any switch (whether 6A or 50A 'rated') has to be able to carry the prospective fault current for the duration it could flow, which is dictated by the supply voltage, the Zs of the circuit and of the rating/characteristics of the OPD.

Kind Regards, John

 

[hard-work]

What was at the back of my mind was that if isolator switches, say a 20A, have say a 100% overload for the contacts for a certain time before problems arise. If that was the case then a 20V could take a sustained 40A for a given time without problems ( a sort of built in diversity). I know all contacts in isolators can take more than what is stamped on the back.

 

[bernardgreen]

Dead switch due to the most common error that happens when lights are replaced