25.1 Commentary

1. Main Power Supply:

● Source: Derived from the building’s low voltage mains supply.

● Reliability: Must be capable of handling the largest load under normal, fire, and fault conditions.

● Design: Should be isolated from faults or maintenance impacts on other circuits.

2. Standby Power Supply:

● Requirement: To back up the mains supply, ensuring system operation during mains failures.

● Duration: For Category M and L systems, standby batteries typically need to last up to 24 hours. Extended duration might be needed for higher risk scenarios.

● Transfer: Should not affect system operation.

3. Power Failure Response:

● Visual Indicator: Essential to show power supply status, especially if a final circuit failure occurs.

● Category P Systems: Must maintain operation when premises are unoccupied. Consider longer battery durations, inspection arrangements, or fault signal transmission to an ARC if necessary.

4. Standby Generator:

● Category M/L Systems: Battery capacity may be reduced if the premises have an automatically started standby generator, provided it covers the fire detection system.

● Category P Systems: Standby batteries must be sufficient, as there’s no one present to detect generator failures.

5. Additional Equipment:

● Non-Essential Equipment: For non-essential indicating equipment, standby power can be less than the standard recommendations, but it must not compromise essential monitoring and control functions.

25.2 Recommendations for mains power supplies

The following recommendations are applicable to the low voltage mains supply to the system.

NOTE 1 This supply needs to be regarded as an integral part of the fire detection and fire alarm system, particularly for the purpose of certification of the system (see Clause 41), regardless of whether the electrical installation within the building is provided by the organization responsible for installation of the fire detection and fire alarm system.

1. Power Source and Isolation:

● Isolating Device: Mains supply should be from the load side of the building’s main isolating device. A separate supply should be provided that is not typically isolated during closed hours.

● Dedicated Circuit: The supply circuit must be exclusive to the fire system and derived close to the main isolating device. Avoid using card or coin-operated meters.

2. Local Isolation:

● Maintenance: Provide means for local isolation of the low voltage supply circuit serving the system’s power and control equipment.

● Isolator Labeling: Isolators and protective devices should be clearly labeled to prevent accidental disconnection.

3. Number of Isolators:

● Minimize: Keep isolating devices between the main supply and the system’s power unit to a minimum.

NOTE 2 Safe isolation is required under the terms of the Electricity at Work Regulations 1989 [10].The previous version of this standard recommended double pole isolation. This is no longer specifically recommended as, under the Electricity at Work Regulations 1989 [10], safe isolation requires verification that isolation has been successful (see Clause 45).

A number of switching devices do not meet the required isolation performance criteria. BS 7671 details whether a particular device can actually be used for the purpose of “isolation”.

4. Networked Systems:

● Large Buildings: If local distribution boards are used, minimize isolators between the main supply and these boards. Ensure infrequent need for isolation and low risk of inadvertent isolation.

NOTE 3 For example, the supply can comprise a dedicated circuit emanating from the first electrical distribution board in the building’s electrical distribution system.

5. Labeling and Access:

● Labels: Use durable, fade-resistant labels for isolators and switches, clearly indicating their function.

● Access Control: Isolators and protective devices should be in locations inaccessible to unauthorized persons or protected by special tools.

NOTE 4 It is recognized that in such an arrangement the recommendations of Clause 26 in respect of the fire resistance of the mains power supplies cables, other than the cables in the final circuit, do not apply.

6. Residual Current Device (RCD):

● Protection: Avoid using RCDs for the fire system unless required by safety standards. If used, ensure faults on other circuits do not isolate the fire system.

7. Power Supply Capacity:

● Alarm Load: The mains supply must be able to handle the system’s maximum alarm load, regardless of standby battery condition.

NOTE 5 A special tool in this context is a tool not likely to be carried by a member of the general public. Slot-headed screws would not be acceptable, since various articles can be used as screwdrivers.

25.3 Recommendations for fire detection and fire alarm system power supply units

The following recommendations apply to every power supply unit that forms part of the fire alarm system.

1. Seamless Transition:

● No Interruptions: Transition between normal and standby power should not interrupt system operation or cause false alarms.

2. Independence:

● Fault Isolation: A fault in one power supply should not affect the other. One protective device should not cause both supplies to fail.

3. Indication:

● Supply Status: A green indicator should show the status of both power supplies, positioned for easy monitoring.

4. Capacity:

● Maximum Load: Both normal and standby supplies must independently handle the system's maximum alarm load.

25.4 Recommendations for standby supplies

The following recommendations are applicable.

1. Standby Supply Requirements:

● Components: A secondary (rechargeable) battery with an automatic charger.

● Battery Type: Must last at least four years; automotive batteries are not allowed.

● Labeling: Batteries must be labeled with installation dates, readable without disturbance.

● Charging: Must recharge sufficiently within 24 hours after full discharge.

2. Battery Capacity:

● Category M and L Systems:

a) Standard: Must operate for 24 hours, then provide a 30-minute "Evacuate" signal.

b) With Generator: Must operate for 6 hours, then provide a 30-minute "Evacuate" signal.

● Category P Systems:

a) Manned/Monitored: Must operate for 24 hours, then run all alarms for 30 minutes.

b) Unmanned/Unmonitored: Must operate for 72 hours or the duration of expected vacancy, then run all alarms for 30 minutes.

3. Additional Equipment:

● Non-Essential CIE: If additional CIE is not critical for compliance, standby batteries may have less stringent requirements.

● Essential Use: If additional CIE is used for primary indication, standby batteries must meet the same capacity standards as essential equipment.

4. Transmission of Faults:

● Notification: Systems with ARC transmission should notify users of power supply faults, especially if likely to be unoccupied beyond standby duration.

NOTE 1 This is the requirement within BS EN 54-4, but it is included here as a reminder that connecting larger batteries than originally designed, for example due to an unforeseen extra load, can result in unacceptably long recharge times.

NOTE 2 If the premises are likely to be unoccupied for longer than the duration of the standby battery capacity at any time, and there is a facility for transmission of fire signals to an ARC, it would be of benefit to transmit power supply fault signals to the ARC, for notification of the user.

NOTE 3 BS 5839-6 recommends the provision of 72 h standby supply for Grade A systems, which are largely similar to this standard’s systems.

NOTE 4 If a circuit serving part of the fire alarm system (e.g. distributed power supply unit) is not served by the standby generator, the capacity of the standby battery ought to conform to 25.4e)1).

25 Power supplies