You might have to add RCD protection to any LV cables that you bury into the wall less than 50mm unless it is in mechanical protection.
True - though BS5839 still contradicts itself on this point. It conversely asks for all wiring installed in compliance with BS7671, and then tells you (I forget the clause immediately) to ensure the fire alarm system is supplied on a dedicated fuse way without RCD protection!
This, the argument goes, is to minimise nuisance alarm sources. But then, the spur supplying the alarm system should also be cabled in fire retardant cable too......
Of course, the fire alarm cable, if indeed it's a full fire system, should only be carrying ELV at 24VDC, assuming it's a field cable and not the supply.
For emergency lighting..... again, the standard calls for fire retardant cable - and allows for wiring from a local lighting circuit..... essentially when you install emergency lighting to an existing circuit that's an MWC right away. There isalso the argument that it's barely worth using FP200 on locally added fittings - given the remainder of the circuit is most usually PVC anyhow.
I'm largely in favour of wholly separate circuits for emergency lighting, complete with test point, and triggered on failure of local lighting by means of detection at the MCB/fuse way. I acknowledge however, this is not always practical.
On the basis that emergency lighting also should comply with the wiring requirements of BS7671, I see only the normal issues with putting lighting circuits on RCD protected ways. Can we all say "RCD TRIPPED - must be another blown lamp".....
Final point - for ourselves, anywhere we bury cable for fire, lighting or similar, we put in in metal conduit and accessories, whatever voltage the cable is carrying - although many no longer do.
