Installed Fire Protection: Alarm Systems
One of the problems with alarms that do not provide communications features is that the occupants are left to interpret the significance of the alarm on their own.
ALARM systems, as the name implies, provide an alarm or warning that a problem exists. They may be signaling systems only, which require manual activation, or the more common variety that combines detection with signaling. Alarm systems are primarily used to enhance life safety of the occupants of the facility by providing early warning of a fire and thus allowing more time for escape. Alarms also improve the opportunity to contain the fire by permitting control efforts to be started while the fire is still relatively small.
There are many different types of alarm systems available. Our discussion in this brief article will focus on providing an overview of the major types and components commonly in use.
Local alarm systems provide warning to the occupants of the structure but do not transmit a signal to a central monitoring station or the fire department.
Most alarm systems are monitored either at a central location, such as the security office, at the site being protected, or by a contracted alarm monitoring company located off-site. Alarm signals are rarely transmitted directly to the fire department anymore.
Manual activation devices called pull stations are typically included in alarm systems. Pull stations should be positioned so they are easy for occupants to find and are typically located along routes of travel that would be used while exiting the building.
Fire Detection Devices
Common fire detectors include smoke, heat, and flame. Smoke detectors are available most commonly in photoelectric and ionization types. Systems based upon lasers projected through the area to be protected also are available. Heat detectors include fixed temperature, rate-of-rise, heat actuated devices (HADs), and fusible links. Flame detectors are usually infrared or ultraviolet.
Photoelectric smoke detectors operate based upon light scattering within the detection chamber of the detector. Light is projected through the chamber; if it strikes smoke, it will be scattered. This light reflected off the smoke in the chamber is detected by a photocell. Ionization smoke detectors (the most common in home use) detect the particles in smoke. As smoke passes through the chamber, the particles are ionized. These particles may then be detected by charged plates in the detector. The beam-type detector operates when the beam is interrupted by obscuring smoke between the laser emitter and receiver.
Heat detectors are most commonly either fixed temperature or rate-of-rise. Fixed temperature units are designed to activiate at a specific, preset temperature. In areas where higher temperatures may be present, a rate-of-rise unit is the preferred choice. These units do not have a fixed specific temperature, but operate based upon the rate of change in temperature over time. A significant rise in temperature over a brief period of time is interpreted as a fire, and the detector will activate the alarm.
HADs and fusible links are usually connected to extinguishing systems rather than alarm systems. A HAD is a pneumatic system where the detection devices are connected by tubing. Rapid rises in temperature will raise the pressure of the air in the system and trip the alarm. This type of detector often is used with extinguishing systems such as deluge systems. Fusible links are placed in tensioned cable systems where the release of the tension activates the system. This is a common arrangement on kitchen fire suppression systems.
Detectors also are used on some extinguishing systems as a means of activating the system. There is usually little if any difference among detectors used to detect a fire for alarm systems and those that are intended to initiate an extinguishing system.
Signaling devices typically include bells, horns, and strobes. These devices are tied into the system and provide warning to occupants. They must be audible throughout the facility and easy to distinguish from other alarm or warning devices used on the property. They must be capable of being perceived above ambient noise.
To ensure this is true, the systems should be tested while the facility is in operation with normal equipment noise present. These signaling devices should be supplemented with a public address capability that allows the personnel who are managing the emergency to communicate directly to the occupants. This communications capability is important to ensure the occupants understand the exact nature of the emergency and are provided with specific instructions on the appropriate actions to take to protect themselves.
One of the problems with alarms that do not provide communications features is that the occupants are left to interpret the significance of the alarm on their own. For example, if the horns and strobes go off in a hotel at night, the occupants will usually check the hall of their floor. If no evidence of an emergency can be seen, they will typically assume the alarm is false or that the problem is small and not near them. These assumptions are not safe but are common. Being able to follow up signaling devices with specific announcements will improve people's response to the emergency.
Facilities should be equipped with annunciator panels that help with determining the source of the alarm. These panels are usually located in or near the lobby of the building or in the security office. They show the location where the alarm originated, either using labeled indicator lights or with a graphical display of the floor plan of the facility.
Control Panels, Connections with Other Building Systems
Alarm systems may be tied into other building systems to control aspects of how these systems respond to a fire in the facility. For example, elevators may be recalled to the ground floor and held there, magnetic door holders may be released to close fire doors in corridors, or ventilation system dampers may be closed to prevent smoke travel.
Alarm activation may also be connected to fire suppression systems. For example, sprinkler systems usually have water flow alarms that activate when water flows through the system. These devices alert the alarm monitoring station that a sprinkler system is operating. Alarm systems may also be set up to monitor portions of suppression systems to ensure they are in operating condition. For example, water control valves on sprinkler systems are often equipped with tamper devices that would transmit an alarm signal to the monitoring station if the water control valve was closed.
Alarm circuits should be supervised. This means the system is self-checking and will provide a trouble indication if any portions of the system are not operating properly.
Control panels in modern systems tend to be computerized and offer a high degree of flexibility in alarm configuration. This is a major advantage. The additional complexity of alarm systems does, however, require more technical competence for those who install and maintain them.
Fire alarm systems should be provided with emergency power that is capable of operating all portions of the system even if the building power is interrupted.
From a compliance perspective, fire detection systems are covered by OSHA 1910.164 and employee alarm systems are covered by 1910.165. The National Fire Alarm Code® NFPA 72 is also a widely accepted standard concerning fire alarm systems. Your local authority having jurisdiction determines which code requirements must be met at your location. In some areas, individuals who install and/or maintain alarm systems must be licensed.
Alarm systems offer significant advantages for protecting the occupants of your facility and providing early warning of fires. It is critical that systems be properly designed, installed, and maintained.
"Industrial Fire Protection Handbook" 2nd edition, R. Craig Schroll, (c) CRC Press 2002, ISBN: 1-58716-058-7.
This article originally appeared in the February 2003 issue of Occupational Health & Safety.