Adding supply to a RCD side of split Consumer Unit

[Highpike]

Hello

I am a bit of aspiring electrician and having been doing quite a bit of reading on electrics and circuit design. I thinking of setting myself the task of trying to re-vamp my kitchen with a new seperate ring and have started to design the circuit requirements etc. My question is principally about discrimination, but have added my designs to date to give you the full picture. (I live in a small one bed flat)

CURRENT SETUP:
Service Fuse = 60A (I think)
Main switch on CU = 100A MCB.

This main switch serves;
Lights (6A MCB)
Door bell (6A MCB)
30 mA RCD (In = 63A)

The RCD then serves:
Ring Circuit (32A MCB)
Cooker circuit with socket (16A MCB)

(Type B MCB's)

PROPOSALS:
To add to the 30mA RCD side of CU so that it serves:
New Kitchen Ring Circuit (32A MCB)
Rest of Flat Ring Circuit (32A MCB)
Cooker Circuit (16A MCB)
Out House Radial Circuit (sockets only) (32A MCB)
(Propose to have sub CU in out house)

Allowing for Diversity:
Socket circuits = 100% for first circuit, 40% for remaining circuits
= 32 + (40% of 32*2) = 57.6A

Cooker circuit = 100% up to 10A + 30% balance + 5A for Socket
Cooker = 2.8Kw = 12.2A @ 230V
= 10 + (30% of 2.2A) + 5A for socket = 15.66A

Total Current for RCD Side = 73.26A

Therefore propose to add a 30mA RCD rated at 80A.
BUT... and here is my question...

My discrimination table tells me that a 80A fuse will not discriminate with a 100A fuse. As I understand it, this could therefore technically blow my 100A main switch at the same time (i.e my lights), leaving me in the dark when according to the regs it should not.

Any suggestions helpful comments?

Thanks

Andrew

 

[Taylortwocities]

Maximum and diversity is a bit of a black art and not a precise science. Practically speaking you have a reasonably small property without any outrageuosly high loads (eg no 10.5KW shower).
Even your oven could be plugged into a standard socket.
I have seen much, much more run off a 60A supply.

Now, your studies should include what an RCD actually does. That RCD you propose has contacts rated at 80A, it will not trip when the current flow reaches 80A, the only thing that will make it trip is a (nominal) earth leakage of 30mA.
You already seem to have an RCD in your consumer unit, whay change it for another one???

Aspiring or not, you must consider this :
//www.diynot.com/wiki/electrics:part_p:diy_electrical_work_and_the_law

 

[Highpike]

Thanks for your comments, I understand the use and the way RCD's work and had also read a past thread re RCD's: that said:

//www.diynot.com/forums/viewtopic.php?t=6852

In this discussion it was stated that RCD's do not give Overload protection, however I would like to point out that this is wrong, RCD's DO give overload protection, only basic, but it is there.

I understand that it will not break at 80A exactly and there will be some kind of fusing factor, but that I thought was the underlying point behind discrimination tables (Total I2t must not exceed pre-arcing I2t of major fuse).

I however take on board your views that the 63A RCD should be adequate for my flat.

If I were to be very straight laced and stick to my maths and my tables, would the answer be to split the RCD load into two seperate RCD's (which would mean a new big CU box); or is the answer somthing else?

 

[skenk]

You should be fine with a 60A service fuse seeing as it's a 1 bed property with no shower. You won't 'blow' a 100a main switch . . . this is the rating of the switch, ie the max current at which is has been designed to operate safely. Your 60A service fuse would blow before 100A was flowing thru the switch . . .

Diversity to ring final circuits is calculated with reference to the maximum demand but this is not necessarily the rating of the circuit . . . without explaining further your two socket circuits likely dont add up to 57.6A . . . most likely much less unless you're running loads of high current stuff. Besides, as TTC says, diversity calc's are only guidelines and you only have a 1 bed property . . .

Your outhouse radial circuit would need to be wired in the appropraite size of cable to be on a 32A breaker (4mm?) why do you need this much for an outhouse??

To work out of you're gonna be ok with the new circuits and your 60A supply try adding up the power -in watts- of everything that is likely to be on at the same time, divide by 230 and if the answer is more than 60 then you could be in trouble.

Like TTC says, seen a lot more (ie the above plus at least an electric shower) on a 60A service fuse.

Don't know why you are referring to 80 and 100A fuses as they don't seem to be part of your system . . . a main switch or RCD is not a fuse. 63A RCD should be fine for your property (unless you are loading your two rings, cooker and outhouse radial near to capacity).