In uk.d-i-y, 4-LOM wrote:

So what do you do about lighting circuits in an outbuilding???

You make your own sensible risk assessment (a qualifier all too rarely
applied to "risk assessment"). Two extremes to illustrate. One: you have
a garage used pretty intensively as a workshop, with lots of spinning
sharp things. It's physically very close to the house, and the earthing
arrangements bring it within the same equipotential zone (i.e. you've
exported the house earth). (Oh, and the lighting switchgear happens to
be all-insulated, i.e. plastic, rather than metalclad). With these circs,
it's a no-brainer to have no RCD protection on the lights: the risk of
having the lights go out (making the spinning machinery more dangerous
in the dark) far outweighs the risks of anyone getting a fatal shock
from any fault in the lighting socket.

Other end of the spectrum: shed/outbuilding at the far end of nowhere,
TT earthing of uncertain earth resistance, outbuilding used for storage
and a socket for connecting the occasional strimmer or similar. Now it
makes no sense not to have all of the shed electrics on a single RCD.

Thoughtful design to balance risks in intermediate situations/extra
complications might involve additionally using: a whole-system time-delay
100mA RCD for the whole installation, with a 30mA RCD protecting the
sockets alone; separate RCBOs for lighting and for power; some light fittings
with battery backup so they stay alight when a whole-outbuilding RCD
trips out. Good judgment based on the specifics of an installation always
trumps slavish adherence to generic advice conservatively based on regs...

HTH, Stefek