Can you not just chase in a conduit or run surface?


If not and it has to be unprotected in the cavity, then RCD it but ensure it is a dedicated device protecting the fire alarm circuit only.


(Clause 25.2 (i) in BS 5839-1:2017)


Regards,


Pete

So it seems to be a choice between electrocuting someone or burning them alive ....

Disregarding the requirements, I would expect a fire panel to be provided with an internal battery supply to allow the panel to continue to operate in the event of a power failure (I think four hours is normal). This may provide an alarm for the loss of the main supply (depending on how the panel is configured), so the tripping of an RCD should not be as critical as you seem to imagine. The only problem is if the panel is in a location that is not permanently manned (e.g. an office block with no night security where the battery could be depleted by the time people are on site again).

On that basis I would suggest that there is no problem with using an RCD. However you will need to look at the pros and cons for the particular installation (plus of course the Regs.)

Alasdair

This sounds like a commercial job to me - so why do you specifically thing an RCD is required for the circuit ?


Regards


OMS

Because the cable is chased within 50mm of the surface of the wall, so according to BS7671 I need an RCD.

Apostolos Kasinalis:

Because the cable is chased within 50mm of the surface of the wall, so according to BS7671 I need an RCD.

 

So you said - however is the installation a domestic premises or can you reasonably expect it to be under control of a skilled or instructed person 


Can you also reasonably expect the metal foil of the FP 200 lay up to act as an effective CPC


Regards


OMS

This is a hotel we are talking about so it is kind of domestic.

I use a 3c2.5mm2 FP200 so my CPC is internal

OK - an hotel is most certainly not domestic - for starters you will almost certainly be providing a BS 5839 Part 1 fire alarm system (probably to L1 Category)


If you read the regulation group again, look for the differences between a domestic property where there is no duty holder and an hotel  - where there will most certainly be a duty holder


From there, determine if the presence of a duty holder suggests to you that the installation is under the control of skilled or instructed persons


Also, ascertain if your cable which has a foil sheath which is in homogenous contact with the CPC is of a type where it has an effective earthed metallic covering capable of withstanding the earth fault current at that point in the installation based on the prevailing fault level and selected protective device


Regards


OMS

OMS:

 

Apostolos Kasinalis:

Because the cable is chased within 50mm of the surface of the wall, so according to BS7671 I need an RCD.

 

So you said - however is the installation a domestic premises or can you reasonably expect it to be under control of a skilled or instructed person 


Can you also reasonably expect the metal foil of the FP 200 lay up to act as an effective CPC


Regards


OMS

 

 

OMS, where is the allowance made for commercial premises to have the mechanical protection of cables buried less than 50mm within walls omitted if under the control of a skilled or instructed person?

There is no issue with adding RCD protection to the circuit so why would it be omitted? Granted the function test of the RCD trip might cause some consternation but I'd be surprised if the panel couldn't allow for this and/or it be rolled into one of the weekly manual call point tests to avoid too much disruption.


Pete

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?

wiring to sensors, call points and sounders will be ELV, so can route anywhere.

Only the mains wiring needs consideration, and that is very similar to the rules for a lightswitch.


Namely that

If the route  is not concealed, RCD protection is not needed for any cable.

If the cable has a built in earthed screen or armour and the cable is of a type listed for that purpose, RCD protection is not required.

If the cable is protected by an earthed enclosure, conduit / trunking etc , RCD protection is not required.


Or it must be in rhe 'safe zones' and have RCD protection.


the former exception for wiring

‘under the supervision of a skilled or instructed person’  went in one of the amendments to the 17th, in 2015 -ish