1.17 The power supply for a smoke alarm system should
be derived from the dwellings mains electricity supply.
The mains supply to the smoke alarm(s) should comprise a single
independent circuit at the dwellings main distribution
board (consumer unit). If the smoke alarm installation does
not include a stand-by power supply, no other electrical equipment
should be connected to this circuit (apart from a dedicated
monitoring device installed to indicate failure of the mains
supply to the smoke alarms see below).
1.18 A smoke alarm, or smoke alarm system, that includes
a standby power supply or supplies, can operate during mains
failure. It can therefore be connected to a regularly-used
local lighting circuit. This has the advantage that the circuit
is unlikely to be disconnected for any prolonged period.
1.19 Devices for monitoring the mains supply to the
smoke alarm system may comprise audible or visible signals
on each unit or on a dedicated mains monitor connected to
the smoke alarm circuit. The circuit design of any mains failure
monitor should avoid any significant reduction in the reliability
of the supply, and should be sited so that the warning of
failure is readily apparent to the occupants. If a continuous
audible warning is given, it should be possible to silence
1.20 The smoke alarm circuit should preferably not
be protected by any residual current device (rcd). However
if electrical safety requires the use of a rcd, either:
a. the smoke alarm circuit should be protected by a
single rcd which serves no other circuit; or
b. the rcd protection of a smoke alarm circuit should
operate independently of any rcd protection for circuits supplying
socket-outlets or portable equipment.
1.21 Any cable suitable for domestic wiring may be
used for the power supply and interconnection to smoke alarm
systems. It does not need any particular fire survival properties.
Any conductors used for interconnecting alarms (signalling)
should be readily distinguishable from those supplying mains
power, eg by colour coding.
Note: Smoke alarms may be interconnected using radio-links,
provided that this does not reduce the lifetime or duration
of any standby power supply.
1.22 Other effective, though possibly more expensive,
options exist. For example, the mains supply may be reduced
to extra low voltage in a control unit incorporating a standby
trickle-charged battery, before being distributed at that
voltage to the alarms.