Are You Designing Fire Alarm System In Accordance with the Code – 3?

Circuit Design and Survivability
The integrity or survivability of fire alarm circuits.
As noted in the previous section, the need to maintain circuit integrity (BS5839-1) or circuit survivability (NFPA72) shares commonality in concept and ideas which are receiving increasing attention in both codes. The idea of circuit integrity or survivability arises from the understanding that fire can develop before it is registered by the detectors and/or alarm raised. The interval between the start of a fire and its putative detection may very well damage components of the alarm system thereby increasing the response time to its discovery and eventual intervention by fire officers. Such scenario is increasingly a possibility due to the complexity of buildings and its internal space planning.
Circuit integrity in BS5839-1 is defined in prescriptive terms which can be summarised as follows:
Circuits containing detectors
~A fault, or faults, in one zone cannot prevent the operation of the system in other zones of the building.
~A single fault should not remove protection from an area greater than that allowed for a single zone (which has a maximum area of 2,000m²).
~Two simultaneous faults should not remove protection from an area greater than 10,000 m².
~Removal of detectors or call point from the circuit should cause indication of fault signals for immediate intervention by officers.
~Detectors designed to be removable from their bases should not affect the operation of manual call points.
~Malicious removal may be considered by the inclusion of lockable device with special tools for removal of detectors.
~The system should be designed to minimise disruption during maintenance and testing. It is desirable that provision be made allowing individual detectors to be tested without the need to sound an alarm or to disable the particular circuit.
~Isolation of all detectors or call points in single zone system is permissible but facility retained for allowing activation of general alarm from the control panel.
~Provision for isolation of detectors or call points for maintenance or testing should be such as to allow the operation of alarm sounders in response to the operation of detectors or call points that have not been isolated.
Circuits containing fire alarm sounders
~If alarm sounders are connected to the same wiring as detectors, then no alarm sounder should be affected by the removal of any detector.
~Any sounder that is necessary in order to reach the recommended audibility levels (65dB or 5dB above ambient noise level or 75dB in case of premises with sleeping resident) should only be removable or electrically disconnected from the sounder circuit by the use of a special tool and the disconnection should generate a fault warning at the control and indicating equipment.
Devices which are connected in a ring (usually though not always for addressable systems)
~Provided that the devices can receive or send signals in either direction, they will continue to operate even with a single circuit or high series resistance in the ring. Such faults should be indicated at the control and indicating equipment within 60 min of their occurrence.
~Short circuit on simple ring circuit (which cannot offer protection against such fault), should be indicated, without giving a false alarm of fire, within 100 s.
~Where sounders are used in simple ring circuits, the distribution wiring to each sounder circuit should be protected against overload due to short circuit by a fuse or similar device.
~Short circuit isolating devices are recommended for protection against cable faults in ring systems, where such device will isolate short circuit to sections of the circuit without affecting the whole circuit.
In most case, implementing measures to comply with the above requirement involve physical configuration of hard wiring and/or hardware which have to be addressed during design stage. Some examples are as follows:
~~Physical segregation of circuits between zone.
~~All sounders to be physically hardwired separately from detector circuits.
~~In case of ring circuit (usually though not limited to addressable system circuited in a loop), the above two measures may have to be adopted (i.e. physical segregation of circuits) despite the ability of addressable circuits to accommodate individual devices in a loop.
~~Alternatively short circuit isolating devices (either inbuilt into initiating or notification devices or installed discretely onto segments of the ring) may be used to demarcate segment of the ring to comply with the above requirements.
~~In a ring circuit, the start and return leg of the loop are physically routed separately.
~~Physical configuration of control panel, devices and circuits allow for fault indication in case of short circuit and removal of devices from the circuit (similar to class A and B circuit under NFPA72 and illustrated in Figures 1 and 2 below).
Class and style of circuit in NFPA72 as defined carries similar notion of circuit integrity as in BS5839-1.
Class Circuits are designated class A or B depending on its capability to transmit alarm and trouble signals during non-simultaneous single circuit fault conditions:
~Class A circuits are capable of transmitting an alarm signal during a single open or a non-simultaneous single ground fault.
~Class B circuits are incapable of transmitting an alarm beyond the location of the fault conditions specified for class A.
Style for initiating devices, notification appliances and signalling line circuits describe requirements in addition to the requirements for Class A and B circuits. Styles are designated for the various circuits depending on its ability to meet alarm and trouble performance during a single open, single ground, wire-to-wire short and loss-of-carrier fault condition.
~Initiating device circuit shall be Style A, B, C, D or E (table 1 in Appendix B);
~Notification appliance circuit shall be Style W, X, Y or Z (table 2 in Appendix B).
~Signalling line circuit shall be Style 0.5, 1, 2, 3, 3.5, 4, 4.5, 5, 6 or 7 (table 3 in Appendix B).
Further conditions on circuits prescribed can be summarised as follows:
~All styles of Class A circuits (except wireless circuits) shall be installed with outgoing and incoming conductors physically routed separately.
~Exceptions to above (separation of incoming and outgoing) are when:
distance of loop do not exceed 10ft (3m);
vertically run conductors are enclosed in 2-hour rated cable assembly or enclosure;
~~ in looped conduit/raceway single drop to individual devices is permitted;
~~ in looped conduit/raceway single conduit or raceway drops or tap-outs to multiple devices within a single room not exceeding 1,000ft² (92.9m²) in area shall be permitted.
Tables in Appendix B illustrates implementation of circuit by style.

Next Appendix B
(1) Abstract and Introduction
(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
(7) Fire Test -Figure2 3 t0 12
(8) Conclusion & Trends