Trainee Question RFC circuit ??

[BeatBox]

Hi

new to this site. I am a trainee electrician and have been trying to get some info on ring final circuits. My parents have had there kitchen done, well new worktop and doors all spray painted. They have installed a new larder unit and they are looking to plug in micro/oven/grill in the larder unit which there is a existing plug behind where the kitchen larder unit is.This socket is installed on kitchen/utility ring. It is a 5 bed house. With the following circuits
32amp cooker circuit
32amp downstairs ring
32amp upstairs ring
32amp kitchen/utility ring
6amp lighting upstairs
6amp lighting downstairs
20amp water heater
20amp garage supply.

so the micro/ oven looking to plug this in to kitchen/utility ring. But already on kitchen/utility ring is
Washing machine - 2kw
Tumble drier - 2.2kw
Dishwasher - 2.1kw
Kettle - 3 kw
Toaster - 1.5kw
Cooker hood - 80watt
Utility fan - 30watt

and I am looking to plug in the micro/oven/grill which is rated to 2.3kw overall. Would this be possible. I heard something regarding diversity on ring circuits that appliances will not be on at same time or pulling same current at same time. Is that correct ? The issue with running new cable in is its concrete floor and artex ceiling so would rather keep it on kitchen ring. Just hope this won’t be overloaded ?? Replies/advice will be very grateful

 

[Lucien Nunes]

People rarely switch everything on at once, and even if they do, many types of appliances don't use their full rated load all the time. Therefore, the likelihood of the total load exceeding the circuit rating is lower than it might appear. On the odd ocaasion that everything coincides, the likely duration is short compared to the thermal time constant of the cable and the tripping time of the OCPD. The larger the circuit and the greater the number of appliances, the more effective the diversity becomes.

Looking at your list, the kettle and toaster typically run for a few minutes at a time.
The dishwasher only uses its heater for perhaps 1/3 of its cycle time; at other times its load is negligible - maybe 80W for the wash pump. Similar for the washing machine, which might use 150W for the motor unless it is heating. The new combi oven could use perhaps 1200W continuously as a microwave, but long periods of microwave cooking tend to be at lower power settings where the magnetron cycles on and off, likewise for its oven element. The tumble dryer will use its full power while the washing is damp, but as it approaches dryness, a sensor dryer will ramp down the power.

Now try overloading the circuit; You've got to start the dryer, then get the washing machine and dishwasher started at the right times for their heaters to come on together part way through their cycles. At the moment the second (hot) wash starts, you have to boil the kettle and make the toast...
OK, so you manage to get the load up to 10kW or so for a minute, but before the cables have time to heat up, the toast is done, or the oven starts cycling on its thermostat, or whatever, and the load drops back comfortably within the 7.4kW available from the circuit.

Where possible, heavy loads such as heaters should be on their own circuits by design, to leave good margin for whatever happens to get plugged into the general purpose sockets circuit. But in practice, you have enough capacity in one 32A circuit to run a remarkable number of appliances under normal domestic operating conditions.

 

[BeatBox]

Thank you very much for the great reply. ???. Yeah going on the information provided the 32amp kitchen/utility ring main will be able to cope easily with the loads we already have connected and adding microwave/grill to it also. As they won’t be pulling full load all the time?

 

[pc1966]

You would be surprised at how tolerant the RFC is. My own flat had everything that can be plugged (except the kettle on the cooker outlet) on a single RFC from 20A MCB (not the usual 32A, a legacy from the original rewirable fuse rating) for for two decades and tripped only twice, in both cases when a fault took out the related plug's 13A fuse anyway.

 

[BeatBox]

That just shows you how tolerant the RFC is definitely. What kind of appliances were plugged into the ring all white goods and oven as well ?

 

[pc1966]

Everything. Though to be fair I'm not a big user of power so worst case of significant loads would probably be fridge, washing machine, tumble dryer, TV/HiFi/PC, and vacuum cleaner on simultaneously.

Oven was on the "cooker" feed (gas hob), as is kettle.
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Other things like microwave, toaster, stand lights, combi-boiler, etc, would typically not be on or of negligible draw.

 

[Lister1987]

I'd do a bit of earth leakage testing, just to get an idea how close (if it all) the circuit was to the trip current.

 

[Lucien Nunes]

That just shows you how tolerant the RFC is

This ability to allow a considerable amount of diversity is not specific to a ring circuit, as it applies equally well to a 32A radial circuit feeding the same sockets. It's a property of any circuit that can handle much more current than any of its individual loads, where those loads are switched on and off at random. The 32A socket-outlet circuit is in turn made possible by the use of fused plugs, which avoid the need to rely on the circuit OCPD to protect the plug and flex. Historically in the UK, the RFC was developed alongside the idea of a 30 or 32A circuit feeding 13A fused plugs in pursuit of improved flexibility and economy. But technically the fused 13A plug on a 32A circuit and the RFC are separate concepts with separate advantages and disadvantages.

I'd do a bit of earth leakage testing, just to get an idea how close (if it all) the circuit was to the trip current.

Given the list of appliances, what leakage would you be expecting? Why would you think it might be borderline in this case?

 

[davesparks]

I'd do a bit of earth leakage testing, just to get an idea how close (if it all) the circuit was to the trip current.

Can you expand on this please?

The list of appliances appears to be what you would normally or reasonably expect to have in a domestic kitchen on one circuit without any tripping issues.

Also what actual testing would you suggest?

 

[Lister1987]

Can you expand on this please?

The list of appliances appears to be what you would normally or reasonably expect to have in a domestic kitchen on one circuit without any tripping issues.

Also what actual testing would you suggest?

sure, I get that the list of appliances is "standard" but knowing how much leakage (and type of leakage) would put a spark in a better position to advise the customer.

You could have DC leakage effectivly blinding a type AC RCD resulting in a reduced chance of it operating at it should - but you wouldn't know about it (or rather how much) if you didn't test for it.

By testing for leakage (type and amount) you have the chance of futureproofing the install by changing the breaker type to deal with the expected leakage better. I don't have my books handy but I'm sure general rule was Type D can take more DC leakage than Type C, C can take more than B, B can take more than A and A can take more than an AC.

Am i making sense, hoping it doent sound like i'm talking out my arse, which isnt unsual i grant you.

 

[nicebutdim]

You could have DC leakage effectivly blinding a type AC RCD resulting in a reduced chance of it operating at it should - but you wouldn't know about it (or rather how much) if you didn't test for it.

By testing for leakage (type and amount) you have the chance of futureproofing the install by changing the breaker type to deal with the expected leakage better. I don't have my books handy but I'm sure general rule was Type D can take more DC leakage than Type C, C can take more than B, B can take more than A and A can take more than an AC.

Are you confusing designations for different characteristics?

I could equally be posting out of my backside, but thought that RCBOs were of types AC, A, B and F, with overcurrent trip curve being B, C or D.

It's unfortunate the same letters have been used to designate different characteristics.

 

[pc1966]

The type 'S' means delay, it can also be AC or A, etc, trip characteristics!

Yes, it is a complete mess...

I don't think they make RCBO with delay though, usually that is for incomer RCD, etc.

 

[nicebutdim]

Yeah, I'd edited to remove S type as I realised it wouldn't apply to RCBOs.

Hang on... They're also available with AC or A designation for over current tripping?

 

[Lucien Nunes]

sure, I get that the list of appliances is "standard" but knowing how much leakage (and type of leakage) would put a spark in a better position to advise the customer.

It's overkill here. This is a general-purpose socket-outlet circuit that should be able to take any normal selection of appliances without nuisance-tripping, not just the specific appliances that are present at the time of testing. In a domestic situation, it is rare for a final circuit to reach 15mA leakage (the minimum trip threshold of a 30mA RCD) without first reaching its load limit, so one does not normally have to account specially for leakage. In a situation where high leakage would be expected per kW of load, such as room full of computers, then the circuits would be designed to take account of both the total load and total leakage, with the loads divided between RCDs and high-integrity CPCs specced where needed etc.

 

[pc1966]

Hang on... They're also available with AC or A designation for over current tripping?

Not RCBO, but delay RCDs come in both flavours even if the suppliers are often not terribly open about it.

A while ago I had a rant on here about glass LBC fuses for a Hager TPN board's meter supply. For the same board I ordered the delay RCD incomer kit and it merits its own rant thread!

The catalogue clearly says it is type A, but the one I was sent was type AC and suspiciously dusty, and the AC nature is an issue as we have lots of electronics off the supply. To make matters worse, its code is not listed on the Hager site. So I want back to our trade supplier and they sorted it out but the story they forwarded from Hager UK was (a) the catalogue was a mistake (not that they are off loading unwanted AC RCD...) and (b) it is a special because they need the N terminal to the left for the TPN board's incomer layout, but their normal 4P RCD have it to the right.

FFS! They make both the board and RCD, can't they have made the board with N to the right!

So now we have a part that only exists as part of a whole kit if you need to order another
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In case you need to know Hager 300mA delay RCD:
CP484U is the AC right-N (now obsolete)
CP484G is the type A right-N
CP485U is the type A left-N
CP486U is the type AC left-N