Mini board in shed

Diggerdunne

I have a 10sq cable goin from board to mini board in shed. Have a 32A mcb at main board. I only have a lighting circuit and 2 socket circuits in the shed. Not much load on them. A washing machine and a freezer thats all. Whats the best setup at the mini board?
I was thinkin a 25A main fuse and then a 20A rcd and 2x20 mcb's and a 10A mcb. Is this the best form of discrimination or can it be done better.
Cheers..

unimog1968

That all sounds quite good. If I was nit picking I would say 10A for lights is too high. 6A is plenty, most lighting ccts in a domestic garage are only drawing about 1 or 2 amps. But in practise the cable is adequately protected by a 10A MCB. Presuming of course that you will use 2.5 for the 20A ccts and 1.5 for the 10A.

Diggerdunne

Yeah will be 2.5 and 1.5 respectively....
Am i correct in saying that it shud be a 32A rcd feeding it from the main board according to the regs or is a mcb ok?

unimog1968

I am not very familiar with the regs in the south but I do fail to see why you would need an RCD at both ends of the 10mm cable. I would think if you have a tripping current of 30ma on a suitable current rated RCD (say 32A) there is no need for one in the shed.

Bruthal

Diggerdunne wrote: »

Yeah will be 2.5 and 1.5 respectively....
Am i correct in saying that it shud be a 32A rcd feeding it from the main board according to the regs or is a mcb ok?

An MCB is fine for an swa out to a sub board. Wouldn`t want an entire sub board susceptible to RCD trippings.

Bruthal

unimog1968 wrote: »

I am not very familiar with the regs in the south but I do fail to see why you would need an RCD at both ends of the 10mm cable. I would think if you have a tripping current of 30ma on a suitable current rated RCD (say 32A) there is no need for one in the shed.

Its at the shed end that any RCD`s should be, when supplying a sub board.

2011

Diggerdunne wrote: »

I have a 10sq cable goin from board to mini board in shed. Have a 32A mcb at main board.

Sounds good. Clearly you need to check volt drop etc...

I was thinkin a 25A main fuse

OK, I will nit pick too seeing as you asked for the "best setup".
The 25A fuse is not required. You are already protecting the cable with a 32A MCB. Normally a sub distribution board would have a main isolator with the protection installed in the main board.

Discrimination will be far better now as the 20A MCBs have a 32A MCB upstream rather than a 25A MCB. As a rule of thumb discrimination is normally achieved with protective devices of the same type (B type MCBs in this case I would guess) having a rating factor difference of at least 1.5

In other words a 20A B type MCB could have a B type upstream MCB = or > 20 x 1.5 = 30 ⇒ 32A

and then a 20A rcd and 2x20 mcb's and a 10A mcb. Is this the best form of discrimination or can it be done better.
Cheers..

Nothing wrong with that.

2 x 16 or 20A B type RCBOs would be better again if you really want to go over the top.

Originally Posted by robbie7730

Its at the shed end that any RCD`s should be, when supplying a sub board.

+ 1

unimog1968

Yes I can see where you are coming from re; the position of the RCD. The difficulty of course is that it leaves the cable run with no additional earth leakage protection. Which could be important, depending on the installation.
Just for clarity, our purchase rate of MCBs v RCBOs is about 100 RCBOs to 1 MCB in the last 12 months.

Bruthal

unimog1968 wrote: »

Yes I can see where you are coming from re; the position of the RCD. The difficulty of course is that it leaves the cable run with no additional earth leakage protection. Which could be important, depending on the installation.

Earth leakage protection is to protect people from contact with live items, not to protect a cable. A properly installed sub main does not need this, as there will not be likely possible contact with it.

Just for clarity, our purchase rate of MCBs v RCBOs is about 100 RCBOs to 1 MCB in the last 12 months.

RCBOs are for final circuits though. You wont really see them on cables supplying other DBs.

M cebee

sub-main doesn't need an rcd
the uk has different rules on protecting circuits

paddy147

Do I dare post pictures???:D