Extract from BS 9999 2017 so others can take a view.

37.2.3.3  Primary and secondary power supplies

To reduce the risk of the loss of electrical supply to fire protection systems that are required to

operate continuously during a fire [such as those listed in h) below], a secondary power supply

should be provided. This supply should be of sufficient capacity to maintain supplies to all life

safety and fire equipment installations. The secondary power system should be designed to operate

safely in fire conditions. The means for the provision of a secondary supply should include the

overall electrical distribution system within the building, and also the power needs for other

equipment requiring a secondary power supply.

NOTE 1 In some cases, where the power demand from a system is low (such as control systems for

natural vent actuators), a secondary supply can be achieved by the use of back-up batteries.

NOTE 2 For legal and technical reasons, power supply companies have reservations about offering a

power supply from a second substation to provide protection against the occurrence of a fault

(unconnected with the fire) on the high-voltage distribution system. Accordingly, a generator or an

independent power supply needs to be provided if protection against faults is required by the

occupier.

The management procedures for the building should prohibit the isolation of circuits supplying

power to the above mentioned equipment during a fire emergency.

Power supplies should meet the following specific recommendations.

a) A secondary power supply independent of the primary power supply to the building, e.g. an

automatically started generator or a supply from another substation, should be provided which,

independently of the primary supply, is of sufficient capacity to maintain in operation for at

least 3 h the following:

1) any powered smoke control systems (including systems using pressure differentials);

2) any fire and rescue service communication systems; and

3) any other fire protection or fire-fighting equipment, except automatic fire detection and fire

alarm systems and evacuation lifts.

b) The secondary power supply should be capable of providing the power supply for items a1), a2)

and a3) within 15 s of the failure of the primary electrical supply.

c) Where the secondary electrical supply is to be taken from a separate substation (whether utility

or private) to that supplying the primary electrical supply, the following criteria should be met.

1) The electrical supplies to the two independent substations should be taken from two separate

high-voltage supplies, and not originate from the same substation.

2) The failure of one substation should not lead to the failure of the other.

3) The two independent substations should be adequately separated. Where the substations are

located within the building they serve, the following criteria should be met:

1. i) each substation should be enclosed within a fire-resisting structure having a minimum of 2 h

fire resistance;

1. ii) the two substations should be located in two separate parts of the building.

4) Supply cables from the high-voltage substations should enter directly the

high-voltage/low-voltage switchrooms and not pass through the building.

5) The two sets of supply cables should be adequately separated from each

other to avoid a single fault affecting both supplies.

d) Cables supplying current to the life safety installations should be installed in accordance with

BS 7671 and the manufacturer’s instructions. The cables should have an inherently high resistance

to fire and be protected where necessary against mechanical damage. Cables, switchgear and other

equipment transmitting the secondary power supply should be separate from those of the primary

supply, or be physically protected so that a breakdown, or any cause of breakdown, on one supply

would not lead to a simultaneous failure of the other supply.

NOTE 3 Further guidance on the selection of cables is given in BS 8519.

e) The primary and secondary power supply cables should be terminated in a changeover device

located within the plant room(s) housing the life safety and fire protection equipment, or in the

case of a firefighters lift, within the fire-fighting shaft.

NOTE 4 This is not to be confused with the lift well.

f) The changeover device should automatically effect the transition from the primary to the

secondary power supply if the primary supply to the particular plant fails.

g) Any electrical substation or enclosures containing any distribution board, generator, powered

smoke control plant, pressurization plant, communication equipment, and any other equipment

associated with life safety and fire protection systems, should be separated from the building by

construction with a duration of fire resistance of not less than 2 h.

h) Secondary power supplies should be provided for the following:

1) sprinkler pumps;

2) wet riser pumps;

3) firefighters lifts;

4) fire-fighting shafts (associated equipment and normal lighting);

5) fire-fighting intercommunications installations;

6) pressurization fans (air supply and pressure relief);

7) depressurization fans (air supply and pressure relief);

8) smoke control system;

9) evacuation lifts.

"normal lighting" wouldn't be provided by self contained emergency luminaires. Usual practice would be to use a static inverter within the firefighting shaft, fed by dual diverse supplies from the primary & secondary source, then use fire rated cables from the inverter to the luminaires.

Just to confuse matter though, BS9991 doesn't require "normal lighting" thereby allowing the use of self contained luminaires to provide 5lux. It all depends on which standard the Fire Strategy/Fire Engineer is using. As others have said already, the multiple BS's covering fire safety in buildings can be a confusing muddle, but it is critical to establish which applies to your particular site.

regards, burn

"normal lighting" wouldn't be provided by self contained emergency luminaires. Usual practice would be to use a static inverter within the firefighting shaft, fed by dual diverse supplies from the primary & secondary source, then use fire rated cables from the inverter to the luminaires.

Just to confuse matter though, BS9991 doesn't require "normal lighting" thereby allowing the use of self contained luminaires to provide 5lux. It all depends on which standard the Fire Strategy/Fire Engineer is using. As others have said already, the multiple BS's covering fire safety in buildings can be a confusing muddle, but it is critical to establish which applies to your particular site.

regards, burn