Adding a small secondary consumer unit

I want to add a small secondary (two way) consumer unit to the main Consumer Unit, to give me easy access to the power supply in an attached shed.

The main unit has a split board, the protected part having an RCD. The small unit is a two way "garage unit" on which both circuits are protected by an RCCB. (I suppose it must be something like a shower uit)

*****Q1 Is there any difference between an RCD and an RCCB?

The Twin plus earth cable INTO the two way unit is easy to connect up but I'm not sure about the best way to connect the other end to the main Consumer unit supply:

1. I could connect it to a spare MCB:
a) on the protected side, but that would give "double" protection and might confuse the RCD and RCCB. In fact in this configuration the RCD on the main unit trips as soon as I run anything off the secondary unit.
b) on the unprotected side (since the secondary unit has its own RCCB for protection). But then the secondary RCCB trips as soon as I run anything off it. Clearly, running a secondary unit of any MCB in the main unit will not work.

2. I could instead connect the secondary unit to the main consumer unit's on/off switch (rather than into an MCB). It would then, in effect, be linked directly to the incoming power supply (via the CU switch). I have not yet tried this as it entails turning off all the power to the house for a few minutes but I can't see why it should not work. Given that a connection via an MCB in the main unit trips one thing or another, this third approach may be the only option.

****Q2Which would be the most appropriate way to wire in the secondary unit to the main one?

ambanekta said:

*****Q1 Is there any difference between an RCD and an RCCB?

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An RCCB is an RCD.

The Twin plus earth cable INTO the two way unit is easy to connect up but I'm not sure about the best way to connect the other end to the main Consumer unit supply:

1. I could connect it to a spare MCB:
a) on the protected side, but that would give "double" protection and might confuse the RCD and RCCB. In fact in this configuration the RCD on the main unit trips as soon as I run anything off the secondary unit.
b) on the unprotected side (since the secondary unit has its own RCCB for protection). But then the secondary RCCB trips as soon as I run anything off it. Clearly, running a secondary unit of any MCB in the main unit will not work.

Click to expand...

Is twin and earth adequate? What size cable? How is it installed? It would be better run from the non-RCD side in the main CU to provide discrimination between the two devices. The reason for the RCD tripping is you have a fault.

2. I could instead connect the secondary unit to the main consumer unit's on/off switch (rather than into an MCB). It would then, in effect, be linked directly to the incoming power supply (via the CU switch). I have not yet tried this as it entails turning off all the power to the house for a few minutes but I can't see why it should not work. Given that a connection via an MCB in the main unit trips one thing or another, this third approach may be the only option.

Click to expand...

I can see plenty of reasons not to do this, electrocution and fire are the first two!!
DO NOT DO THIS!!
Also, if you live in england or wales please note that adding a CU is a notifiable activity to your LABC under part P of the building regs.

The work that you describe is notifiable under Building Regulations Part P and, if you were to do it yourself, you must notify the local authority before starting any work.

The nature of your questions tell me that you have no idea how to approach this task. Some of your thoughts are downright dangerous. I would not want to even try and advise you.

My sincere suggestion is that you engage a qualified electrician to do this work for you and to certify and notify the installation to make it legal.

EdwardCurrent said:

A RCCB is an RCD which also provides protection against overload.

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Oh no it isn't

The work has been notified by means of a current Building Notice to the LABC.

The Twin + Earth is new, rated 40A and exposed (not buried). The whole installation is indoors.

The main consumer unit is stable (and had been for years). The problem arises when adding the secondary consumer unit, which is brand new. Replacing the RCCB makes no difference so I know there's no fault there. But it trips when anything is connected to either the 32A MCB or the 6A MCB so it is fair to conclude there is no fault with those either.

Next consideration is the wiring from the secondary CU. Unable to find a fault, I ran a simple short (1 metre) supply direct from the 32A MCB to a single socket as a test and it still trips, yet there is no fault with the 1 metre of cable, the socket, the plug or the lamp plugged into it.
It was this that caused me to think the problem lie in having the supply pass through an MCB (in the main unit), through an RCCB in the secondary unit and then another MCB in the secondary unit.

I would certainly welcome any additional constructive suggestions.

I note with thanks (and some relief if not a little confusion) that an RCCB is an RCD.

But does this also mean that an RCD is an RCCB?

(I mean an orange is fruit, but not all fruit is an orange - if you see what I mean). If they are the same thing, why are they labelled differently?

Many thanks again.

I think you need to have an electrician check your work is correct, more than likely a wiring fault.

More than likely a neutral is in the wrong place.

The theory is sound (and an electrician will need to inspect it on completion to sign it off) but if the main CU has never tripped in 5 years or so, the only reason it is tripping now is the addition of the secondary CU.

A 40A cable connects them and there is only a single socket attached to the secondary CU. The main unit does not trip when the secondary unit is connected until current flows from the secondary unit.

I think what I really need to be certain of it that the appropriate way to connect to the main CU is via a 40A MCB on the unprotected side. If that part is definitely correct (and the use of two RCD/RCCBs in line does not pose a problem), then I shall focus again on the wiring from the secondary CU.
Thanks, in advance.

Of course, another solution would be to run the shed lighting and ring circuits directly into two MCBs on the protected side of the main CU (and do away with the secondary CU altogether).

But I simply wanted to keep the shed isolated on a dedicated run.

Do you need a ring for a shed? - seems extreme! It is better to have a single means of isolation in an outbuilding.
A common mistake is to connect to the wrong neutral bar in a split load CU.

Surely the new CU should be located in the shed, not the house adjacent to a perfectly suitable CU


Why are you connecting two RCDs in series?

This is pointless, and not especially good design.

SOLVED.

By studying two responses in particular, I have found the problem - a short cut-off length of wire that had fallen inside the secondary CU, only visible by removing the RCCB.

I still have to decide whether to connect to the protected side of the split board in the main CU or to the unprotected side. My feeling, endorsed by the last post, is that it is pointless running two RCD/RCCB so I shall connect to a 40A MCB on the unprotected side.

The shed adjoins the building housing the main CU and hence nothing is outdoors but the secondary CU is inside the shed. It's quite large and has four double sockets - hence the small ring main.

Thank you Gentlemen. This topic can now be closed by the Mod.

Hopefully not before I can opologise for confusing an RCD with a RCBO.

Ed

The assumption about the orange analogy is somewhat correct:
A Residual Current Circuit Breaker is a Residual Current Device, by definition - it is possible to have a Residual Current Device that isn't a Circuit Breaker, but it's unlikely to find one in a domestic environment (I guess it'd be some sort of current limiting or other switching device that isolates part of a circuit)
You should run the secondary CU from the unprotected side, unless the protected side has a higher trip current than the shed one (unlikely on that installation), as it offers no more protection to use both, but can lead to the shed taking the whole house sockets out (which is a bleeder when the freezer is full of food and you're away on holiday...)