10.3 Fire Alarms and Fire-Fighting

Common fire detection and alarm systems

Fire detection systems are designed to discover fires early in their development, while there is still time for occupants to evacuate safely. Early detection also plays a significant part in protecting the safety of emergency response personnel. Property loss can be reduced and operating downtime minimised, because control efforts start while the fire is still small.

Detectors respond differently to fires, so where possible the detector should suit the risk:

• smouldering fires: optical smoke, optical beam obscuration or CO detectors
• fast-burning fires: ionisation detectors
• flammable liquid fires with little smoke: heat or flame detectors

Smoke detectors are very common. They:

• detect small smoke particles, are usually very sensitive and give early warning
• can give false alarms in a humid, wet, dusty or smoky atmosphere
• come in two main types: photoelectric and ionisation

Heat detectors suit certain applications. They:

• detect the excess heat produced by a fire, and are usually less sensitive and give later warning
• come in two main types: rate-of-rise and fixed-temperature
• may not detect smouldering fires that give off smoke but not much heat

Flame detectors are generally used in industrial settings, for example on boilers and furnaces. They:

• are sensors that detect and respond to the presence of a flame
• detect suitable radiation from flames where their field of view and the fire type allow; they are not general smoke detectors
• use ultraviolet (UV) or infrared (IR) technology to identify flames

Select a suitable warning system through the fire risk assessment and applicable requirements. Many workplaces need an electrically operated system with appropriately positioned call points; some small simple premises may use another adequate arrangement. Competent design must address the actual occupants and layout. It should have enough sounders for the warning to be heard clearly throughout the workplace. The sound should be distinct from other sounds in the workplace and, where background noise is high or an employee has a hearing impairment, it may be necessary to add a visual alarm such as a distinctive flashing or rotating light.

In more complex buildings, such as retail premises where evacuation is staged or phased, or where people are unfamiliar with the fire warning arrangements, consider installing a voice evacuation system. This could form part of a public address system and give both fire warning signals and verbal instructions in the event of fire.

Where a public address system is used together with a fire warning system, both should override any other function of the equipment, such as playing music. The public address element should give clear verbal instructions and should override the fire warning signal, which should be distinct from other signals in general use.

If an automatic fire detection system and a manually operated electrical alarm system are installed in the same workplace, they should normally be combined into a single integrated system. Voice evacuation systems should be integrated in the same way to prevent confusion.

Summary notes for Element 10 · Cheat sheet · Glossary. A study aid, not published or endorsed by NEBOSH.