Answer: BS 7671: 2008 (the 17th Edition) requires additional 30mA RCD protection of circuits in a bathroom, so any new circuits being installed in the new shower room will require 30mA RCD protection. If you are not installing or altering any other circuits, then the requirements are not applicable to this part of the installation.
If, however, you are changing the consumer unit, the protection provided is recommended to meet the new requirements, but the installation designer should consider Regulations 314.1 and 314.2, which require consideration of the inconvenience to the client and possible dangers arising from the operation of a protective device - i.e., in situations where an RCD is fitted as the main switch. I do not think, therefore, that putting a 30mA RCD at the main position to protect all circuits in this situation meets the intention of BS 7671: 2008 and would recommend (if consumer unit replacement is undertaken as part of this work) that either a high integrity 17th Edition consumer unit is installed or, alternatively, a unit with RCBOs to protect individual circuits should be fitted.
If you are simply installing the new shower room circuits and not altering the rest of the installations, the circuits you install will need to be RCD protected - which could be achieved if the existing consumer unit can be retrofitted with RCBOs for the circuits that you intend to install. Alternatively, install a separate RCD protected unit specifically for these circuits.
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