Induction hob.

[Bogart]

Am about to install an induction hob and can either use single phase or three phase. Will use singles in conduit. Maximun load is 11Kw so either single phase in 6mm or 3phase using 2.5mm. If I use 3 phase I will get a normal cooker isolation switch to operate a 4 pole contactor, as am not too keen on an industrial looking switch by the hob!

Is there any overiding reason I should use one over the other?

 

[Bobster]

If I use 3 phase I will get a normal cooker isolation switch to operate a 4 pole contactor.

This would not count as isolation.

Would be better installing a true isolator in a hidden location e.g. cupboard.

If you have 3 phase available, this is the option I would go for. Smaller wire size, smaller conduit needed.

 

[Bogart]

Can I ask why would it not be considered isolation?

 

[Bobster]

In the same way a light switch isn't considered isolation.

Most contactors, unless "safety rated" don't have the required gap distance between contacts to meet isolation rules.

Also bear in mind, it would break the N at the same time as the lines, really lines need to break first, and connect last.

 

[Devonchris]

Are you sure there is an option for 3p and not just 2 line and N which can have two individual circuits or be bridged together?

 

[Bogart]

Assume you mean on the hob. No is definite 3 phase , 5 core flexible supplied in addition to usual 3 core.

 

[Bogart]

In the same way a light switch isn't considered isolation.

Most contactors, unless "safety rated" don't have the required gap distance between contacts to meet isolation rules.

Also bear in mind, it would break the N at the same time as the lines, really lines need to break first, and connect last.

So tell me how does a normal cooker switch "isolate" the appliance?
I can see nothing wrong in using a 4pole NC contactor. Only need an isolator switch in case of emergency and in that case as long as the lives and neutral are opened fine. As there is no actual electrical reg requirement for local isolation of an appliance I cannot see why my suggestion is not valid.

 

[Bobster]

So tell me how does a normal cooker switch "isolate" the appliance?
I can see nothing wrong in using a 4pole NC contactor. Only need an isolator switch in case of emergency and in that case as long as the lives and neutral are opened fine. As there is no actual electrical reg requirement for local isolation of an appliance I cannot see why my suggestion is not valid.

A normal cooker isolator puts a gap in the circuit of a set amount. There is a difference between functional switching and switching for isolation. However a functional switch in some circumstances can be both, an MCB for example.

Also, if you are going down this route, you'll want a normally open contactor. This way, if it fails, it is more likely to fail in a safe manner.

 

[Wilko]

Hi - my 20p is that a contactor will always be on, consuming a few Watts and possibly buzzing. So I wouldn’t use one for a domestic cooker. I’d try for a rotary switch in an adjacent cupboard.

 

[James]

A 4 pole NC contactor fed from a switch on the wall is about as far away from a safe means of isolation as I can imagine.

If you had an engineer round to fix a problem and the turned off the isolation switch, they would expect that there would be no way it could become energised again without them operating the switch.

However, if the coil failed or any of the control wiring was disturbed and became separated the cooker would become live again. There are so many ways this could go wrong.

You can get some reasonably small and neat 3 phase isolator, you don’t need to get some plastic rotary thing with a yellow handle.

 

[telectrix]

just fix this in the adjacent cupboard, or.... if you want to annoy the wife, fix it in plain sight. Tell her it's 18th Ed. regs.

 

[Bogart]

A normal cooker isolator puts a gap in the circuit of a set amount. There is a difference between functional switching and switching for isolation. However a functional switch in some circumstances can be both, an MCB for example.

Also, if you are going down this route, you'll want a normally open contactor. This way, if it fails, it is more likely to fail in a safe manner.

Only problem with a NO is that it will be constantly powered up unless one switches it off after every cooker use.

 

[Bogart]

Am receiving conflicting advice as to local isolation. Is a cooker as such required to have a local isolation point or is say the MCB in the CU good enough?

 

[DPG]

Am receiving conflicting advice as to local isolation. Is a cooker as such required to have a local isolation point or is say the MCB in the CU good enough?

It seems wise to have local isolation near to a cooking appliance.

 

[Bogart]

I would agree wise but is it a regulation?