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Fire Safety for Healthcare Premises – System Technology for fire alarms – Part 141

August 17, 2020 12:36 pm

In LWF’s blog series for healthcare professionals, our aim is to give information on best practice of fire safety in hospitals and other healthcare premises. In part 140, LWF considered power supplies to the fire detection and alarm system in a healthcare environment. In part 141, we look at system technology available for healthcare buildings.

Conventional systems were mainly installed in healthcare buildings up to the 1980s, when newer technology led to the use of addressable systems. A conventional system will have zones of detectors and each zone comprises a single circuit, normally two-wire. Each manual call point and fire detector within the zone is connected in parallel across the circuit.

Each device on the zone circuit acts as a normally open switch, in electrical terms. When a call point is activated, or when a detector is triggered, the device/switch closes which creates a virtually short circuit across the pair of wires. This is known as low impedence and it is sensed by the control equipment and recognised as a fire signal from one of the devices in the zone.

Electrically-speaking, each zone is a radial circuit which terminates in the field at the ‘end of line’ device, such as a resistor. The resistor allows a small monitoring current to flow at all times and if a break occurs in the cable (an ‘open circuit’ fault) the monitoring current can no longer flow. At this point, a fault warning is given at the control and indicating equipment (CIE).

If a true short circuit were to occur, it would also be detected as a fault, because the impedence would be even lower than when a detector or manual call point operates. In this way, the system can detect and distinguish between an open circuit fault (very high impedence), a fire signal (low impedence) and a short-circuit fault (very low impedence).

Each detector used as a part of a conventional system is a ‘two-state’ device, also sometimes described as a digital device. It is called ‘two-state’ because it can be either normal (open) or fire state (closed). The control panel cannot distinguish between detectors within zones, so a signal will be received that there is a fire somewhere within a given zone.

While conventional systems are no longer in widespread use, they are still available and will be described as conventional systems to distinguish them from addressable systems.

In Part 142, LWF will look at how addressable systems work. In the meantime, if you have any questions about this blog, or wish to discuss your own project with one of our fire engineers, please contact us.

Lawrence Webster Forrest has been working with their clients for over 25 years to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact LWF on freephone 0800 410 1130.

While care has been taken to ensure that information contained in LWF’s publications is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of this information.

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