DIY Doctor

Cooker Circuit With Kitchen Ring Main Running off?

Postby AjayK46 » Thu Dec 12, 2019 7:35 pm

I have not had an electric freestanding cooker in my kitchen for sometime but now need to get one. A radial circuit for the cooker is present with a cooker control switch with neon and the cable outlet plate so was hoping the installer would be able to connect up to this.

Looking at the consumer unit I have noticed that the 30A MCB for the cooker circuit also has the kitchen ring main off of it. Is this going to cause a problem / concern for the installer?

If the cooker circuit has to be put onto a dedicated MCB (There is space) can that be done within the older plastic consumer unit or would that constitute having the whole consumer unit changed for a modern metal one?
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Postby ericmark » Fri Dec 13, 2019 12:20 pm

The regulations are not law, and the question of if a MCB can be added is really down to the electrician doing the work, but I would fit one, but that does not mean it complies with the letter of the regulations.

There seems to be a problem defining a circuit, in my book is says "An assembly of electrical equipment supplied from the same origin and protected against over current by the same protective device(s)." this clearly would mean a FCU forms a new circuit, however the ring is actually called a "ring final" because it is the final circuit, so how can one have another circuit after the final one?

So the electrician has to use some common sense. It's not all down to following a rule book.

We have traditionally had the cooker switch with a 13A socket built into it, this stopped when we started fitting earth leakage devices because the cooker often had some leakage and would trip the RCD (name for modern earth leakage device).

So we started supplying the freezer and cooker without RCD protection, cooker because the mineral insulated elements could cause it to trip and cooker was fixed so earthed well anyway, and the freezer often also had a mineral insulated defrost element and clearly it tripping the RCD could result in spoilt food and food poisoning as a result, so risk assessment better not to have RCD protection.

However today that has all gone, the whole house is RCD protected, we no longer have circuits without RCD protection.

In my house all RCBO so each circuit has it's own RCD built into the MCB so a circuit to the cooker if there is a earth leakage fault will not trip the freezer.

So there is no rule to say sockets can't be fed from the cooker supply, however there is an appendix to BS7671 which says fixed items over 2 kW should not be fed from the ring final. The reason for 2 kW limit is a ring final can be over loaded if high powered units are used near the end of the ring final, in centre OK as even current on both legs, at end there is a problem.

So idea is a kettle even at 3 kW is used for such a short time there is no problem, an immersion heater at 3 kW may run for a few hours, so that is a problem, which then asks the question what about machines to dry cloths, the washing machine does not heat the water for long, although rated at 3 kW my son used a washing machine on a narrow boat with a 6 amp shore supply, as the heater was not no long enough to trip it.

However a washer/dryer or tumble driers is another story, if following the recommendation in the appendix then they should have a dedicated supply.

So if your kitchen ring is a lollipop design and it only supplies kitchen items I would not worry, and I would also connect the cooker, if however it is a kitchen/laundry room, then I would say this needs changing, however could still connect the cooker as the MCB means it will fail safe, but I would advise against it.

If the kitchen sockets are not in a ring final, but a spur, then I would look for the FCU to limit the supply to sockets, if the FCU is present then no problem, if not, I would disconnect the kitchen sockets, I would connect cooker, but would leave rest of kitchen sockets dead. Or I would swap the MCB for a 20A and have it only supplying kitchen sockets, and not connect cooker. However if this is the case, you really do need something doing.

My stand alone cooker can draw over 50 amp, however it is supplied from a 32A MCB, as in real terms all rings and the oven will never switch on together, however adding a load of kitchen items on the same 32A supply when the cooker can on it's own cause an over load and trip the supply does not seem a good idea, specially as in my case one has to go outside to reset a MCB/RCBO.

So if the kitchen complies with the regulations at time of fitting the ring, the only real problem your likely to have is if the supply is not RCD protected.

However some companies like PC world are know for installers to look for any excuse not to fit things, they want a cooker connection unit so they don't have to switch off the supply at CU, they just connect the three wires. In real terms you can't lock off a cooker switch, so should really turn off at the CU as they can fit a lock to the MCB's although unlikely most would bother. But they are well known for refusing to connect, I worked for a firm delivering and connecting cookers for a short time, think it lasted 2 weeks, I was not happy connecting cookers without having the equipment to test the supply, I worked with then a Corgi guy, I helped him with gas, he helped me with electric, I would say we only fitted 2/3rds of the cookers taken out, because we had found some reason not to fit them, mainly gas, a cupboard was 1/2 inch too close, and we got back in van without unloading.

If being fitted by an electrician as long as RCD protected should not be a problem, but if fitted by shop supplying, those guys are looking for an excuse not to fit.
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Postby AjayK46 » Sat Dec 14, 2019 1:31 pm

Many thanks for your very comprehensive reply. I think I will get the cooker connected up by an independent registered electrician rather than the shop that will deliver. That way if anything else needs doing it can all be done in one go.

Just concerned that if an additional MCB has to be added to the plastic type consumer unit that will require the whole consumer unit to be changed to a metal one?
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