OK - agreed, that there is no mention now of skilled and instructed persons - I was just trying to get the OP to think about the risk hierarchy that he is trying to address with the design given the supply is a part of a life safety system


If the fire alarm supply cable is buried in zone, then is there a very real, credible risk of a "strike" to that cable in this environment - I suspect not


It may well be that the selected cable is already compliant in terms of an earthed metallic screen or sheath - it may take some calculations to prove that


It is a requirement for a lockable isolation position so it seems entirely credible to me that the 230V supply cable would easily be installed in a length of earthed conduit from the ceiling void to the lockable isolator and then to the panel, as a flush installation


Personally, I would try to avoid at all cost the provision of RCD protection to this circuit from a life safety safety perspective.


BS 7671 excludes fire alarm systems compliant with BS 5839-1 - is the 230V feed to the panel actually a 230V fire alarm circuit ? -  and not covered by BS 7671 anyway.


Regards


OMS

Apostolos Kasinalis:

OMS, sorry I am not so advanced in my knowledge around the regulations.

As I understand there is no provision in BS7671 to omit an RCD in case of a safety or a critical system. Am i right?

 

It depends on what it's wired in, and how as discussed and described in several posts above.


Personally speaking, I would use all of the available mechanisms to omit the RCD protection from a life safety system


Regards


OMS

Agree with OMS. Scrub the RCD!

MICC isn't an option? It'd be more expensive but would solve this issue and used to be the done thing for fire alarms, at least where I was trained (a university maintenance dept)

The fire alarm panel should be on a dedicated supply, in fire resistant cable and terminated in a double pole fcu needing a special tool. BS5839 overrides BS7671 (in this instance) and the recommendation is that a non RCD circuit should be used since this is a life safety system. Will most definitely be a L1 cat system with a minimum of 24hrs standby capacity. 

Regards

TS

Extract below from BS 5839-1:2017 [my emphasis], clause 25.2 Recommendations for mains power supplies:


" i)  The circuit supplying the fire detection and fire alarm system should not be protected by a residual current device unless this is necessary to conform to BS 7671. "


Unless using a fire rated BS 8436 compliant cable, running the non-BS 8436 fire rated cable in metallic conduit or running it surface mounted - RCD protection would be required unless the buried depth can somehow be increased to greater than 50mm (double sheet that wall?).


Happy to be corrected but personally I can't see that there's all that much wiggle room on the above.


I would state that I too would try and keep the circuit RCD-free, but to do so I'd be using one of the above methods - preference being concealed galvanised conduit drop from above.


P