Fire detectors work by sensing one or more products of fire. The three most common detectors are:

• Smoke detectors
• Heat detectors
• Flame detectors

Smoke Detectors

Smoke detectors detect the visible or invisible smoke particles from combustion. The two main types are ionization detectors and photoelectric detectors.

Ionization Detectors:

The ionization detector contains a small radioactive source that is used to charge the air inside a small chamber. The charged air allows a small current to cross through the chamber and complete an electrical circuit.

When smoke enters the chamber, it shields the radiation, which stops the current and triggers an alarm.

These detectors respond quickly to very small smoke particles (even those invisible to the naked eye) from flaming or very hot fires, but may respond very slowly to the dense smoke associated with smoldering or low-temperature fires.

Photoelectric Detectors:

In a photoelectric smoke detector, a light source and light sensor are arranged so that the rays from the light source do not hit the light sensor. When smoke particles enter the light path, some of the light is scattered and redirected onto the sensor, causing the detector to activate an alarm. These detectors react quickly to visible smoke particles from smoldering fires, but are less sensitive to the smaller particles associated with flaming or very hot fires.

Heat Detectors

Heat detectors are normally used in dirty environments or where dense smoke is produced. Heat detectors may be less sensitive, but are more appropriate than a smoke detector in these environments. The most common heat detectors either react to a broad temperature change or a predetermined fixed temperature.

Heat detectors use a set of temperature-sensitive resistors called thermistors that decrease in resistance as the temperature rises. One thermistor is sealed and protected from the surrounding temperature while the other is exposed. A sharp increase in temperature reduces the resistance in the exposed thermistor, which allows a large current to activate the detector's alarm.

Flame Detectors

Flame detectors are line-of-sight devices that look for specific types of light (infrared, visible, ultraviolet) emitted by flames during combustion. When the detector recognizes this light from a fire, it sends a signal to activate an alarm.

Assure that all devices and equipment constructed and installed to comply with this standard are approved for the purpose for which they are intended. [29 CFR 1910.164(b)(1)]

Detector Selection:

Fire detectors should be selected based on the burning characteristics of the materials present and the nature of location they will be used to protect.

Smoke Detectors:

Ionization or photoelectric smoke detectors are designed to identify a fire during its smoldering or early flame stages and will meet the needs of most areas containing primarily wood, paper, fabric, and plastic materials. During combustion, these materials produce a mixture of smoke types with detectable levels of both large and small smoke particles. Smoke detectors are suitable for:

• Indoor areas with low ceilings such as offices, closets, and restrooms.
• Areas that are relatively clean with minimal amounts of dust and dirt.
• Areas that contain solid fuels like wood, paper, fabric, and plastic materials.

NOTE: Some locations are unsuitable for smoke detectors due to the potential for unwanted alarms -- work areas, kitchens, stairs, shafts, high air flow locations, areas that are dusty or dirty, as well as outdoor areas.

Heat Detectors:

Heat detectors are ideal for areas where flammable gasses and liquids are handled or any area where a fire will quickly cause a large change in the surrounding temperature. Heat detectors are also suitable for:

• Dirty, dusty or smoky environments.
• Indoor areas without winds or drafts that can prevent heat from reaching the detector.
• Manufacturing areas where large quantities of vapors, gases, or fumes may be present.
• Areas where particles of combustion are normally present, such as in kitchens, furnace rooms, utility rooms, and garages or where ovens, burners or vehicle exhaust gases are present.

Flame Detectors:

Flame detectors are best for protecting:

• Areas with high ceilings and open-spaces, such as warehouses and auditoriums.
• Outdoor or semi-enclosed areas, where winds or draughts can prevent smoke from reaching a heat or smoke detector.
• Areas where rapidly developing flaming fires can occur, such as petrochemical production, fuel storage areas, paint shops, and solvent areas.
• Environments that are unsuitable for other types of detectors.

Protection of Fire Detectors:

Detectors that are installed outdoors and need to be shielded from the weather to work properly or may be exposed to corrosive environments must be protected from corrosion. Provide a canopy, hood, or other suitable protection (such as a non-corrosive coating) for detection equipment, requiring protection from the weather. [29 CFR 1910.164(d)(1)] In order to keep detection equipment operable, they must be located away from or out of contact with materials or equipment which may cause physical damage or be protected by a cage or metal guard. Locate or otherwise protect detection equipment, so that it is protected from mechanical or physical impact that may render it inoperable. [29 CFR 1910.164(d)(2)]

All detectors must be securely mounted to a solid surface, such as screwed to a junction box with a mounting plate or other appropriate method that prevents them from putting pressure or stress on attached wires or tubing. Assure that detectors are supported independently of their attachment to wires or tubing. [29 CFR 1910.164(d)(3)]

Number, location and spacing of detecting devices:

The number, spacing, and location of detectors must be based on design data obtained from 29 CFR 1910.164(f):

• Field experience or testing
• Engineering surveys
• The manufacturer's recommendations
• A recognized testing laboratory listing

General Guidelines for Placing Fire Detectors:

• Put at least one detector in each room, storage area, and hallway. You may need more than one detector per room for those that exceed the manufacturer's spacing requirements. For example, if your detector is rated for 30 feet, install detectors so they are evenly spaced with no more then 30 feet between detectors.
• Place the detector as close to the center of the ceiling as possible when only one detector is required in a room or space.
• Put at least one detector in each closet, elevator and dumbwaiter shaft, stairwell, and other enclosed spaces.
• Place a detector at the top of each flight of stairs.
• Place the detectors in the path of the air flow toward the return air duct when air supply or return ducts are present in a room or space.
• Place all smoke detectors at least three feet from ceiling fans.

Over time, dust, dirt, and other foreign material can build up inside a detector’s sensing elements, resulting in reduced sensitivity, which can limit the amount of warning time given during a fire. Dirty or dusty detectors can also result in unwanted alarms that can desensitize occupants to the alarm system or produce more serious behavior (such as disconnecting the system altogether).

To avoid malfunctions and unwanted alarms and to make sure your fire detection system will perform as expected in the event of a fire, you are required to:

• Operate and maintain your system in a working condition, making sure it is always turned on, except during repairs or maintenance. [29 CFR 1910.164(c)(1)]
• Test and adjust fire detectors and fire detection systems often to ensure that they operate correctly and maintain reliability. [29 CFR 1910.164(c)(2)] Detectors found to be unreliable and/or with reduced sensitivity must be replaced or cleaned and recalibrated.
• Assure that pneumatic and hydraulic operated detection systems installed after January 1, 1981, are equipped with supervised systems. [29 CFR 1910.164(c)(3)]
• Have a qualified person service, maintain and test all fire detection systems, including cleaning and necessary sensitivity adjustments. [29 CFR 1910.164(c)(4)]
• Have fire detectors cleaned of dust, dirt or other particulates at periodic intervals to assure their proper operation. [29 CFR 1910.164(c)(5)]

All fire detection equipment must be returned to normal operation as soon as possible after being tested, used, or accidentally activated. [29 CFR 1910.164(b)(2)]

NOTE: You are also required to have spare detection devices and components readily available in the workplace or from a local supplier to ensure prompt restoration of the system.

The following applies to fire detection systems used for specific applications:

• When a fire detection system is installed for the purpose of activating a fire extinguishing or suppression system, it must be designed to respond in time to control or extinguish the potential fire. [29 CFR 1910.164(e)(1)]
• When a fire detection system is installed as an employee evacuation alarm, it must be designed and installed to provide a warning for emergency action and safe escape of employees. [29 CFR 1910.164(e)(2)]
• Do not delay alarms or devices actuated by fire detectors for more than 30 seconds, unless the delay is necessary for the safety of employees. For example, if a fire suppression system uses a compressed gas that will flood an occupied area, it will be necessary to give employees time to escape. If such a delay is necessary, it must be addressed in an emergency action plan meeting the requirements of 29 CFR 1910.38. [29 CFR 1910.164(e)(3)]