Post
by ericmark »
Mon Sep 05, 2016 1:30 am
I noted two real errors, 32A MCB or RCBO to cooker should have at least a 32A isolator not a 20A one.
The extractor should have a fuse not just an isolator.
Now to circuits and rings. The problem with a ring is high loads near the start or finish of the ring can cause an overload, in the centre of the ring there is not a problem. So in the appendix 12 of BS7671:2008 it suggests that any fixed appliance over 2 kW should have a dedicated supply.
We rarely do have dedicated supplies to the washing machine, tumble drier, and dish washer, what is more common is to supply a bank of 20A grid switches with two 2.5mm cables which in turn supply the units sockets. The reason for two x 2.5mm rather than a single 6mm is you simply can't get 6mm cable into the switch terminals, really they are cables in parallel rather than a true ring. Because both cables are same length they will share the load if other items are also added to the pair of cables, it does not really matter the point is distance to switch is the same for both cables, or close enough to mean neither leg is overloaded.
Using Ali-tube cable and having a dedicated and so marked socket for a fridge or freezer would clearly mean less chance of it tripping while on holiday, but as to using a RCBO with a high integrity consumer unit is debatable.
So with a frost free freezer you have a evaporator and heater and fan behind a panel and when cooling the motor and fan are running, and on defrost cycle the heater is running, the defrost cycle does not happen that often maybe twice a day, should the heater develop a fault the RCD part of the RCBO will likely cause it to trip. Resetting the trip will likely run the freezer for another 12 hours before the heater cuts in again. Resetting gives you time to do something, be it call some one to fix it or get new, you have time to sort the problem.
However this only works if you know it has tripped, my fridge/freezer and freezer both have little blue lights one when there is power to them, however I rarely notice if there not lit, however if the TV does not work, or the lights don't work then I notice that.
So not sure on the idea of a dedicated supply to freezer, as likely you will not notice when it trips, but on the other hand less likely to trip. But it is only worth a dedicated circuit when you have a high integrity consumer unit able to take RCBO's as where there are many MCB's onto one RCD then adding an extra MCB for the freezer would be pointless.
The tumble drier or washer drier is likely the biggest load in the kitchen after the cooker, and to have a dedicated circuit for the tumble drier or washer drier makes a lot of sense. These can draw over 2kW for an extended time.
However a washing machine without a drier may use 2 kW to heat the water, but the time the heater is on is short, so really not a problem, the same with the dish washer.
So dedicated circuit for tumble drier, oven and hot plate, rest on a ring. As to oven and hot plate the problem is you can't buy a 16A FCU, so feeding a oven with a 32A supply which is double it's rating is not really a good idea, all well and good if the oven if under 13A then you can fit a FCU, but if given the option I would have a 32A supply for hot plate and a 16A supply for oven. Where originally it was a cooker and this has been split to oven and hot plate then often no real option but use a double cooker connection unit and supply both from a 32A supply. But if you have option then I would use a separate supply.
So either 4 or 5 circuits for kitchen/utility room.
1) Hot Plate
2) Oven
3) Tumble drier or washer drier
4) Ring final
5) If using a high integrity consumer unit may be freezer.
In my house two rings supply the kitchen plus a dedicated cooker supply. Both rings also supply other areas of the house, and the kitchen is around the centre of the ring. Mother house a 50A MCB feeds the kitchen consumer unit using SWA cable and then 4 RCBO's feed the ring final, the hot plate, the oven, and to be frank not sure what the forth one feeds, likely the boiler. I see no reason to have the boiler on it's own circuit, but often they are.
