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AS1670 Calculating Power Supply Requirements for Fire Alarm Systems

3.16 POWER SOURCES

3.16.1 Primary power source

The CIE shall be energized by a reliable source of supply and shall be connected in accordance with AS/NZS 3000. The power source shall be either?
(a) an a.c. supply from an electricity authority; or
(b) a source equal in quality and reliability to Clause 3.16.1(a). The primary power source shall be capable of operating the system including the occupant warning system as per Clause 3.22(b).

3.16.2 Secondary power source

The system shall be provided with a secondary power source that is capable of operating the system. The occupant warning system as per Clause 3.22(b) should the primary power source fail.

The secondary power source shall consist of rechargeable stationary batteries, in accordance with the relevant part of AS 4029 compatible with the CIE.

NOTE: Automotive-type batteries are not normally suitable for stationary battery use.

Where the secondary power source is remote from the CIE enclosure, the secondary power source shall be protected for overload at the source.

3.16.3 Power source rating

All devices, facilities or equipment, external or internal, that utilize the fire detection and alarm system power source in either quiescent or alarm state shall be considered in the calculations of the power source rating.
The sum of the worst case of the following loads shall not exceed the power supply unit rating of the CIE:

  1. (a) The total load of the CIE with five actuating devices in alarm state in each of two alarm zones or the quiescent load of the CIE, whichever is greater.
  2. (b) Two fire suppression systems in an activated state, or 20% of such connected systems, whichever is the greater, where they are powered from the CIE.
  3. (c) For power supply units complying with AS 4428.5, the maximum battery charger current required to charge the battery within 24 h from fully discharged condition, to a capacity capable of maintaining the system for 5 h in normal working condition (quiescent) and 30 min in alarm condition.

NOTE: AS 7240.4 power supply equipment used in conjunction with control and indicating equipment complying with AS 7240.2 requires an equivalent, but different calculation. The requirement in AS 7240.4 will result in a maximum battery capacity being identified as part of the power supply equipment specification.

3.16.4 Battery capacity

The capacity of the battery shall be such that in the event of failure of the primary power source the batteries shall be capable of maintaining the system in normal working (quiescent) condition for at least 72 h, after which sufficient capacity shall remain to operate two worst case alarm zones and associated ancillary control functions for 30 min.
Where the power supply failure signal is externally monitored, the 72 h requirement may be reduced to 24 h.
When calculating battery capacity, allowance shall be made for the expected loss of capacity over the useful life of the battery. A new battery shall be at least 125% of the calculated capacity requirements, based on a loss of 20% of its capacity over the useful life of the battery.
The battery capacity requirement shall be determined as follows:

  1. (a) Determine the quiescent load current IQ.
  2. (b) Determine the alarm current IA.
  3. (c) Determine the capacity de-rating factor Fc of the battery when discharged at the alarm load rate taking into account the minimum operating voltage of the connected CIE using the battery manufacturer?s data. Where more than one CIE is connected to the battery, use the highest minimum of any of the CIEs. A value of 2 for FC is deemed to satisfy these requirements.
  4. (d) The 20 h discharge battery capacity C20 at 15°C to 30°C shall be determined as follows:
  5. C20 = 1.25[(IQ × TQ) + FC (IA × TA)]

where

C20 = battery capacity in Ah at 20 h discharge rate
IQ = total quiescent current
TQ = quiescent standby power source time, (normally 24 h)
FC = capacity de-rating factor
IA = total current in alarm state
TA = alarm load standby power source time (normally 0.5 h)
1.25 = compensation factor for expected battery deterioration

Where the load may vary, the worst case average over required period shall be used.

Where the average battery temperature is outside 15°C to 30°C the battery manufacturer?s data shall be used to determine any further compensation factor to be applied.

NOTE: For typical battery capacity calculations see Appendix C.

3.16.5 Batteries and enclosure

The battery enclosure shall be such that the batteries are readily accessible for inspection.
For non-sealed batteries, the battery enclosure shall not be above the enclosure for the fire indicator panel. The connecting leads to the battery shall be clearly labelled to reduce the possibility of reverse connections to the battery. The battery shall not be tapped for intermediate voltages and all connections shall be made using suitable connectors.

3.16.6 Ancillary loads

Ancillary control devices or isolation relays external to the CIE enclosure shall be installed within a protective enclosure and shall be marked or labelled with the words ?FIRE ALARM SYSTEM?.
NOTE: Normally energized ancillary loads, such as door holders, may be disconnected in the event of failure of the primary power source.