Power Supply, Emergency Supply, Fail Safe Supply
Principles of reliability of power supply and similarity between both codes. Power supply for fire protection systems under both codes contain similar principles of ensuring reliability of power supplies. Primary supply source which are unreliable should not affect the operation during normal operation and response during fire conditions. Both codes advocate the backing up of primary (or main source) power supply with a standby supply with or without generator back-up.
Power supply prescribed in BS5839-1 can be summarised as follows:
Primary power connection:
~~Cables/apparatus directly connected to a public or private distribution supply should be in accordance with IEE Wiring Regulations (BS7671).
~~ Connection to the mains supply should be via an isolating protective device (e.g. isolating switch-fuse) reserved solely for the purpose. Isolating device should be suitably labelled with warning (in red) and may be enclosed in secure box to prevent unauthorised access.
~~The design of the system should ensure that residual current devices (RCD) are not necessary. In cases where RCD is unavoidable, interruption of the general building supply in response to a fault should not result in interruption of the fire alarm supply.
~~Continuity of supply to fire alarm system should be ensured.
~~Switching off supply due to reasons of maintenance, emergency, energy savings etc should not affect power to (except in unoccupied premises with a simple manual system).
~~In distributed power supply system, failure or disconnection of the supply to any individual unit should be indicated at the main indicator panel as a fault. Any switch that can disconnect the power supplies to all or part of the system should be suitably labelled with warning and coloured red.
Types of power supply
~~Normal supply should be derived from the public supply system, transformed or modified Where no public supply system is available, privately generated power may be used.
~~Standby Supplies comprise secondary batteries or secondary batteries augmented with standby generators.
Maximum alarm loads is defined as the maximum load imposed by the alarm system under fire conditions. It include the power required to operate sounders, detectors, fault warning and illumination of monitoring devices and all ancillary services powered by the fire alarm system.
Standby Supply Generally standby supply are as follows:
~~Comprises of a rechargeable battery and automatic charger. The battery should have an expected life of 4 years. Car batteries are not to be used.
~~The batteries should be labelled with their date of installation. Battery should charge up from its final voltage in 24Hrs.
~~For category M and L systems the battery should be able to support the system for 24hrs and then sound the alarm for 30 mins. If a back up generator is used the battery should be able to support the system for 6hrs and then sound the alarm for 30 mins.
~~For category P the 24 hrs plus half an hour ring applies a) providing the building is 'supervised' (staff monitoring at 6 hours interval) or b) power failures are automatically notified to a remote station for response from supervisor.
~~For category P the battery should support the system for 24 hrs longer than the building is unoccupied up to 72 hrs whichever is the less, plus half an hour ringing whatever applies. If the building is ever unoccupied for longer than the standby battery time and there is facility for remote transmission then the power fault should be remotely transmitted.
Power supply in NFPA72
can be summarised in the following terms:
Power source:
~~Fire alarm system shall be provided with at least two independent source of power supplies; one primary and the other secondary (standby).
~~Exceptions to above; when the primary source is supplied by a dedicated branch circuit of an emergency supply system or a legally required or optional standby system.
Notes: NFPA70 or the 'National Electrical Code' (NEC); Articles 700 defines Emergency Supply System as essentially for emergency loads (load during emergency condition); Article 701 Legally Required Standby System is a subset of Article 700 system but restricted for legally required load (communications, legal utilities etc), and Article 702 defines Optional Standby System for loads which may contribute to life safety but are not within the purview of legally sanctioned standby loads.
Primary Supply
~~Primary supply shall have high degree of reliability and may be either i) a light & power service (i.e. normal mains supply), or ii) an engine-generator set provided such generator are fully supervised by trained operator.
~~Connections to 'light & power' service shall be from dedicated branch circuits. Circuit should be mechanically protected. Circuit disconnector should red-marked, prominently labelled and accessible only to authorised personnel.
~~Overcurrent protection devices to protect against short circuit in ungrounded conductor shall be provided.
Notes: Overcurrent protection against short circuit protection will normally perrtain to fine-protection class fuses (IEC standard). Depending on the location of the circuit and prospective short circuit current, Miniature Circuit Breaker (MCB) may not be capable of interrupting short circuits above 6kA. However, this condition principally relates to the North American centre-tap 110/220V system which may produce higher short circuit currents compared to the IEC defined TN-S system which would be the norm for final circuit for power outlets in Malaysia.
Secondary Supply The secondary supply shall automatically supply energy to the system within 30 seconds, without loss of signal, whenever the primary supply system fails.
The secondary supply system shall have sufficient capacity to operate:
- 24 hours the complete system under maximum quiescent conditions;
- and then be capable of operating 15 minutes of full evacuation alarm operation at maximum connected load.
Secondary supply for emergency voice alarm communication system shall similarly operate 24 hours under quiescent conditions and shall be capable of operating the system for 2 hours during emergency conditions.
The secondary supply may consist of
- storage battery system, or
- standby generator system augmented with storage battery of 4 hours capacity (duration to power the fire alarm system).
Continuity of Power Supply
pertains to all cases of power transfer between primary and secondary source and can also be taken to cover power source connected from emergency or standby system. Continuity of supply must be maintained as follows:
~~power transfer must be automatic and generator must start up within 30 seconds;
~~standby batteries shall be maintain continuity of supply and to provide 15 mins. of power supply to the alarm system and to computer UPS forming part of the fire alarm system.
Next à Cable Types
(2) Overview of BS5839-1 and NFPA72
(3) Circuit Design and Survivability
(4) Appendix B – Circuits By Class and Style NFPA 72-2002
(5) Power Supply, Emergency Supply, Fail Safe Supply
(6) Cable Types, Fire Tests of Cables and Installation Practice
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