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?