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Doubling up on MCBs

Postby Sparkydelux » Tue Jan 08, 2008 10:43 pm


Wiring up a 21 way board and a bit tight for space.

I have upgraded from 6A to 10A on lighting circuits and doubled up on circuits. I was told today that this is not good practice and a common error. Tester told me that they cannot be tested as they occupy one circuit and therefore 2 different readings cannot be put on cert.

Is this true? i was thinking eg: Circuit 5: Have description of 1st circuit and directly under add the other readings (also lablled circuit 5). As long as there are sufficient readings it should be fine,right?

Why cant I have the 2 circuits in the MCB and just label up the phases with tagging or dymo label? It will still be indicated.

Any views?Any suggestions?

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Simply Build It

Postby 333rocky333 » Wed Jan 09, 2008 1:01 am

Confusing reg!
Take a look at reg 314-01-04

I read this to say you cannot.

I have seen this elsewhere and people say that You can bunch lighting and radial circuits but not rings.

What is classed as a " final circuit " as opposed to a "circuit"
The definition is in part two at the front of the regs.

People say that it counts as one " final circuit " , if both "circuits" are in the same mcb, but I dont agree.
And they say that the reg only relates to "borrowed neutrals" ie L and N across two different "circuits".

Incidently the iee quide book example of wiring lighting, actually shows two lighting circuits bunched together in one mcb !!, but whether its a printing error I dont know.

The old way of an extra spur from a ring mcb, also NO longer shows,
Which WAS technically a (20 amp) radial bunched with a (32 amp)ring.

Hope someone can settle this
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Postby ericmark » Wed Jan 09, 2008 5:35 pm

I have always been told one hole one wire. There is no reason why two cables should not be feed from same fuse or MCB but they should not be just bunched in together it must have separate way but not necessarily separate fuse. If it went from fuse to some linked terminals on a piece of din rail then it would be most likely OK as each circuit would have its own way and could be disconnected for testing.
Yes I know in the end its the same thing but on a 21 way board its not house bashing and it has to follow the rules a lot closer.
I felt the same when I moved and was told off for using a shifter spanner to tighten up glands as it might scratch them.
Changing MCB's can also get one into trouble if you have not measured and documented earth loop impedance prospective short circuit current and volt drop the latter is often a problem in commercial premises being so much bigger.
I have recommended MCB's to be reduced in size and or changed from C to B to comply many times. If after doing that and documenting it as I always did someone changed it back then there was a high chance their pass to work on site would be removed. i.e. sacked. It would be considered as gross industrial misconduct. Always either check existing paperwork or test your self and make sure there is paperwork to support what you have done.
i.e. Watch your back especially when independent testers are checking your work.

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