Smoke alarms sharing same RCD as socket-outlets

[GaryMo]

Where does the idea of not placing smoke alarms on the same RCD as socket-outlets some from?

Is it from BS 5839-6 (design of fire detection installations for dwellings)?

Does it just relate to Grade F systems (no battery backup) and not Grade D systems (with battery backup)?

 

[Chri5]

I don't know.

But since appliances cause RCD's to trip it makes sense to keep a protective device away fro,a group of circuit on the RCD rail.

Since smoke detectors are prone to failure if you do a 5 x RCD test it makes sense.

Since a fire via an appliance could knock out the RCD it makes sense.

Since battery backup is limited, more so when the unit is detecting a fire it makes sense.

But as to a defined reg / rule / BS no- well I'd be interested to know.

 

[GaryMo]

I know all about sense and good practice.
Supplying a smoke alarm from the same RCD as socket-outlets isn't something I would do.

From what I've been reading, it doesn't relate to Grade D systems (battery backed), of which the majority seem to be.

Fire alarms in dwellings

Grade D (with battery backup)

The mains supply to smoke and heat alarms should either be a single independent circuit from the dwelling’s main distribution board or a separately electrically protected regularly used local lighting circuit.

Grade E (without battery backup)

The mains supply to smoke and heat alarms should be a single dedicated independent circuit from the dwelling’s main distribution board.
Smoke and heat alarms should be interconnected and, in this case, must be supplied from the same circuit.
The circuit supplying the smoke and heat alarms should preferably not be protected by an RCD unless one is required for reasons of electrical safety, then either the RCD should serve only the circuit supplying the smoke or heat alarms or the RCD protection of the fire alarm system should operate independently of any RCD protection for circuits supplying socket-outlets or portable equipment.

 

[flyingsparks]

OK so heres a scenario for you..
New dual RCD board. RCBO's are out of question due to a tight budget. Need to split socket circuits over both RCD's to avoid nuicanse trips so you still have some power should an RCD fail. Which RCD do you put the smokes on??

 

[GaryMo]

A square peg won't fit into a round hole....
In any case, assuming Grade D smokes are used then it's not really a problem.

 

[flyingsparks]

A square peg won't fit into a round hole....
In any case, assuming Grade D smokes are used then it's not really a problem.

Either...

(Whats F??)

 

[GaryMo]

F - battery only, no mains. Good for dual RCD boards with no high integrity ways!

 

[Chri5]

F - battery only, no mains. Good for dual RCD boards with no high integrity ways!

Battery only won't get through a LABC inspection for building work sign off

Flying- I sorry, but given the suggestion that paying £35+ extra for an RCBO (£25) and £10 for the larger CU with high integrity is beyond someones cost value is daft.

I design my clients system and if they argued over that, I'd walk out and not start the job.

£35 versus fire detection as it should be done, is a false and (frankly) daft economy.

 

[GaryMo]

Battery only won't get through a LABC inspection for building work sign off

LABC inspection not needed for consumer unit change

Flying- I sorry, but given the suggestion that paying £35+ extra for an RCBO (£25) and £10 for the larger CU with high integrity is beyond someones cost value is daft.

I design my clients system and if they argued over that, I'd walk out and not start the job.

£35 versus fire detection as it should be done, is a false and (frankly) daft economy.

This thread has come about from a friend changing a consumer unit at a property with a TT supply. A dual 5+5 30mA RCD board was chosen.

In this instance there was only one ring final though he still chose to put the existing Grade D smoke circuit on the same RCD. He's now going back to relocate it to the other RCD.

Imagine a couple of ring finals. No option of using a high integrity board here so common Grade D smokes will have to share the same RCD. It's not a problem though as I pointed out above.

 

[hairyben]

Since battery backup is limited, more so when the unit is detecting a fire it makes sense.

How limited? Given that power cuts increase fire-risk (candles!), no fire alarm system is fit for purpose without significant battery reserve.

I can see the sense in maintaining a fire alarm power supply (mr all-RCBO boards only speaking here), but can't see that pairing with a socket circuit should be less desirable than many other circuits.

 

[GaryMo]

but can't see that pairing with a socket circuit should be less desirable than many other circuits.

It doesn't seem to be when dealing with Grade D systems which are much more common that Grade E.

 

[securespark]

OK so heres a scenario for you..
New dual RCD board. RCBO's are out of question due to a tight budget. Need to split socket circuits over both RCD's to avoid nuicanse trips so you still have some power should an RCD fail. Which RCD do you put the smokes on??

You compromise just a tad. Have a High Integrity Split Board. 2x RCD plus a spare way. With surface wiring for the smokes, you can use an MCB only to supply them.