I am sorry Steve, I don't think nanny can let you do that...
It would be legal, however it would come under the scope of part pee.
Hence it would need inspecting by building control if you are not able to self certify the work, or treating with contempt and ignoring if you so desire!
Surely that's what the OP means - the two CU's must be in series, not in parallel otherwise how do you isolate the garage system on order to work on it's CU.
At least that's how I wired mine and that seemed rather obvious ! Based on the fact that the supply is just an ordinary 2.5 T &E, it's off a 15A fuse. I would rather you didn't ask any more about the system though - like how does it get to the garage.
Either way is acceptable and the relative merits have often been discussed here.
If the garage is fed in parallel (from a service connector block in the meter tails) there has to be a separate switch-fuse at the house end, otherwise you'd be relying on the supplier's fuse to protect the submain cable to the garage, which would be completely unacceptable. Said switch-fuse provides the isolation (and strictly speaking should be capable of being locked In the off position to protect anybody working in the garage from any 'surprises').
Electricians should be able to understand the Regs, or at least implement them properly.
The only real "issue" I can see is if the garage supply was from the RCD'd side of a split load board, or a whole-house RCD. It would not then be possible to achieve proper discrimination between the house RCD and any RCD in the garage, and a fault on the garage installation would cause nuisance tripping of the house RCD.
But then the regs are bloody stupid anyway, why would it be sensible to float the earth to a swimming pool control panel, and have a separate earth spike?
No, it is in fact quite common (and acceptable) to feed an outbuilding CU submain from an existing CU. The alternative would be to feed it from a service connector block in the meter tails via a switchfuse.
Why do you think that?
By float I take it you mean to leave the earth disconnected at the end the submain?
This is standard practice when making the outbuilding installation a TT earthed installation protected by its own RCD and a local earth rod.
It can be undesirable to export the house earth - especially over long cable runs, or in situations where it is difficult or impossible to ensure that all extraneous conductive parts can be bonded to the same earthing system.
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