Fire Alarm and Detection Systems

A brief history

One of the first alarm systems was the American 'Fire Alarm Telegraph' in the 1850s. As a community system, it relied on individuals to trigger the alarm to summon help.

The first patented automatic detection system was developed in America in 1890 followed by the first heat detector which was patented in Britain in 1902. The first smoke detector was invented in 1930. Further detection development did not really take off until 1939. It was helped by the development of the cathode ray tube which enabled the detector’s signal strength to be strong enough to activate an alarm.

Modern-style smoke detectors became available by 1951. These were large and expensive and mainly used to protect commercial properties. Battery powered detectors did not come about until the 1970s.

Today fire and smoke detection devices are connected. They communicate both with each other and a main control panel. The interconnectivity enables the location or 'address' for the initial detection to be identified. The system is called 'addressable fire alarm detection'. Automatic detectors are used to sense smoke, heat or carbon monoxide (CO) and then trigger an alarm response. This then sounds an audible alarm or illuminates a visual indicator to alert people to the possibility of a fire or dangerous situation.

Design and risk considerations

There are design and risk issues to take into consideration when considering a new or replacement fire alarm scheme for a historic building. You, or your chosen fire advisor, will need to ensure the fire alarm system is compatible with the agreed fire strategy that covers all aspects of the building’s fire safety features including compartmentation, means of escape and other fire safety features.

Risk assessments

In determining which fire alarm system, you or your chosen fire advisor should conduct a thorough risk assessment. This means looking carefully at the building and the people who use it to understand the risks involved.

Fire risk assessments involve:

• identifying the fire hazards
• identifying the people at risk
• evaluating, removing or reducing the risks
• recording your findings, preparing an emergency plan and providing training
• reviewing and updating the fire risk assessment regularly

If you do not wish to employ a specialist to do the risk assessment, there is a range of government guidance to help you including the 5-step checklist. The section on Standards, regulations and guidance lists the key documents.

Design considerations

Before designing a fire alarm system, you should carry out an evaluation of the historic building and its use(s) to inform the design brief which will in turn help consolidate the design, project control and costs. For example, a cathedral or large country house will need a different detection system compared to a small chapel or small home. You will need to consider:

• how the detection components will work
• the volumes of the internal spaces to be protected
• the distance to an exit
• the height of the building
• the number of people in the building at the time of an alarm being raised

Your evaluation should also address the following:

What activities are to take place in the area and how large is the space?

A simple example is an area where cooking takes place. Optical detectors, and the soon to be phased out ionisation smoke detectors, can both be set off by simple operations such as making toast and the release of smoke particles. The reason for this is because the optical detector works using the light scatter principle. An infrared LED pulses a beam of light into the sensor chamber every 10 seconds to check for smoke particles.

Ionization-type smoke alarms have a small amount of radioactive material (Americium 241) between two electrically charged plates, hence the requirement for them to be phased out. This arrangement ionises the air and causes current to flow between the plates. When smoke enters the chamber, it disrupts the flow of ions, thus reducing the flow of current and activating the alarm. To prevent false alarms, heat detectors are usually fitted in kitchens.

A heat detector is a device designed to respond to the thermal energy of a fire. The thermal mass and conductivity of the heat sensitive element in the detector head regulates the measurement of the increased heat so these detectors have a thermal lag. It is one of the reasons that they are only used where smoke is usual such as cooking in a kitchen.

If on the other hand you are considering a fire alarm system for large spaces such as a cathedral these detectors will not be suitable for the main worship area. Usually aspiration or air sampling detection systems are used in large spaces. They are more sensitive systems. Air is constantly drawn through a network of pipes and sampled for smoke particles. They can detect fires at a far earlier stage giving occupants more time to evacuate. The sensitivity of the system can be temporarily adjusted to lessen the prospect of a false alarm for uses such as burning incense in church services. The alternative is to have an automatically timed period where the system is switched off. It must allow for the smoke particles to disperse before being reactivated. This should all be done automatically and should never rely on personnel to reactivate the system.

Another system often used to protect large open spaces is optical beam detector. This system projects a beam of light and the alarm triggered when the beam is blocked or scattered by smoke particles. They are used to detect fires in buildings where standard smoke detectors would either be uneconomical or restricted for use by the height of the building. The two main components are a transmitter and reflector plate.

Are there any down stand beams, other high-level obstructions, or ornate ceilings?

It is tempting to try and disguise the presence of fire alarm system components. Often ceiling mounted detector heads are placed on the opposite side of the down stand beam to the main doorway so it cannot be seen. However, this can result in the detector not functioning either as quickly as it ought to or in some cases not at all. The smoke will drift under the beam and bypass the unit completely. It is also not advisable to position detectors into corners or onto walls (unless they are specifically designed to work vertically). In the case of wall mounted units, they should be mounted 100-300 mm down from the ceiling.

It is always advisable to take the smoke detector away from any obstruction and use some other method such as matching the background colour to help blend the unit into the background. All alterations to the finish of a detector should be carried out by the manufacturer to ensure that the devices function correctly and that guarantees are not invalidated.

In the case of an ornate ceiling, the decoration can be used as a guide as to where to sympathetically place detector heads. For example, in a cross-hatched patterned ceiling, the detectors would be positioned in the centre of squares at appropriate distances to provide cover for the room.

What is the floor to ceiling height and is a more sensitive detection method needed?

In large volume spaces with high floor to ceiling heights standard smoke or heat detectors may not function adequately or quickly enough.

For maximum or additional life protection automated fire alarm systems or maximum property protection automated fire alarm systems, known respectively as categories L1, L2, and P1, the maximum mounting heights permitted for 10% or less of the ceiling are:

• Smoke detectors. Maximum ceiling height 10.5 metres (12.5 metres for 10% of the ceiling area)
• Rate of rise heat detectors. Maximum ceiling height 9 metres (10.5 metres for 10% of the ceiling area)
• Fixed temperature heat detectors. Maximum ceiling height 7.5 metres (10.5 metres for 10% of the ceiling area)
• Carbon monoxide (CO) detectors. Maximum ceiling height 10.5 metres (12.5 metres for 10% of the ceiling area)
• Optical beam detectors. Maximum ceiling height 25 metres

Is wireless technology possible and has a signal strength survey been carried out?

Wireless fire alarm systems use secure wireless connections between the sensors and the panel. Wireless technology in listed buildings and scheduled structures, where suitable, can help limit the amount of permanent damage and/or loss of historic fabric such as cabling, chasing and hole drilling, and help reduce installation costs.

This type of installation will require a signal strength survey to be carried out and signal booster aerials installed as needed where the structure blocks or weakens the signal strength.

Is there a gas or heating oil supply that needs an automatic shut-off facility?

Automatic shut-off facilities are linked via a solenoid valve to the fire alarm system. They will shut off gas and oil supplies to a heating boiler or a standby electrical generator in the case of a fire being detected or a manual break glass unit being activated.

Has an assessment of the fire loading within the space been carried out?

The fire loading is used to estimate the potential size and severity of a fire. A fire load is defined as the weight of combustible material per square metre of floor space consisting of:

• Movable contents. Combustible furniture, equipment, goods, and supplies brought in for the use of the occupant
• Interior finishes. Exposed combustible materials permanently affixed to walls, ceilings, or floors plus doors, trim, and built-in fixtures

It is always best wherever possible to keep the moveable contents fire load as low as possible and to ensure that furniture and equipment are stored well away from escape routes.

Are there any noise issues that require visual warning methods to be employed?

During events like organ recitals, concerts, plays or where there is other background noise, it can be difficult for some people to hear the fire alarm. In such buildings, it is usual to provide both audible and visual warnings that the fire alarm system has been activated and that the building must be evacuated. A typical example is the installation of a xenon beacon near a church organ keyboard.

Have suitable locations for the main fire alarm panel and repeater been sorted and agreed with the local Fire and Rescue Service?

The fire alarm panels in a building are the main point of reference for both the building occupants and the Fire and Rescue Service. Repeater panels are more likely to be needed in large buildings with more access/exit points and a greater number of detection devices. An auxiliary panel repeats the same information as given at the main panel but from an alternative location.

The main purpose of the fire alarm panel is to provide information on which zone an alarm has been activated in. An addressable system will provide a precise location of the activated device because each detector and break glass unit have their own unique electronic address. A clearly labelled zone map of the building should be located adjacent or near the main panel and repeater panels. The map should also show final exits, escape routes, circulation areas and stairs.

Every panel will require a battery back-up supply in case of electrical mains supply failure. A minimum battery capacity should be able to maintain the system in full operational condition for at least 24 hours. After this period there should still be enough battery capacity to maintain an evacuate signal in all alarm zones for at least 30 minutes.

If a building is likely to be left unoccupied for periods of time, or not connected to an automatic alarm call centre, then the battery standby capacity should be sufficient to maintain the system in an operational condition for at least 24 hours longer than the maximum period the premises are likely to be unoccupied or for 72 hours in total, whichever is less. Again, after this period the system should still have enough capacity to provide an evacuate signal in all zones for a minimum of 30 minutes.

Is there an emergency response plan?

An emergency response plan ensures that people on site know how to respond in an emergency such as a fire and salvaging contents.

Emergency response plans document the actions to take during an emergency. Plans need to be specific to your building and its requirements. They must be easy to understand and follow in a fire emergency. A copy of the plan should be kept in a secure location on site and another copy held off site by a responsible person ready to be used in a fire emergency.

Guidance on how to formulate a plan and what it should include are given in our Emergency Planning and Fire Advice webpages.

Does the fire alarm system also need to operate specialist equipment like dropping a fire-resistant curtain?

The fire alarm system may also need to drop fire-resistant curtains, open smoke vents or louvres, or activate water mist or gas suppression systems.

The purpose of a fire-resistant curtain is to limit the initial development of a fire, to prevent the spread of fire, and to protect escape routes. An effectively fitted fire-resistant curtain can help to suppress the growth and development of a fire and smoke within a building. They also work to decrease levels of smoke. These are most effective in large open plan areas with little or no scope for adding fire doors.

Smoke vents or smoke louvres, such as windows or skylights, offer an escape route for smoke and hot air. Automatic opening vents are activated when sensors detect fire or smoke in a building and open to let smoke escape.

Water mist is a low water content variation on a sprinkler system that cools the fire down and stops the chemical reaction involved in combustion.

Gas suppression systems are used where water cannot be used. For example, valuable paper-based assets.