CHAPTER 7 - ALARM SYSTEMS

Many buildings and complexes being constructed today are equipped with some type of intrusion detection and fire alarm systems. You, as a Construction Electrician, will be challenged to install, troubleshoot, and maintain these systems. Numerous detection and fire alarm systems are in existence today. In this chapter, we will discuss the function and operation of a typical detection system and of various fire alarm systems. When you are in charge of the installation or maintenance of either a detection or a fire alarm system, you should acquire reference material, such as manufacturer's literature. If such material is unattainable, look at NAVFAC MO-117, Maintenance of Fire Protection Systems, which provides an excellent description of several fire alarm systems. Design Manual 13.02, Commercial Intrusion Detection Systems (IDS), provides descriptions of various intrusion detection systems.

The purpose of any alarm system is to either protect life or property or to detect an intrusion. Alarm systems are set up to (1) give early warning so occupants may evacuate the building and (2) notify the fire department and/or security soon enough that they have time to react.

TYPES OF FIRE ALARM SYSTEMS

Building alarm systems may be local or local with base alarm system connections. They may be coded or noncoded and may operate either on line-voltage or low-voltage electric power. Their characteristics are described in the following paragraphs.

NONCODED ALARM SYSTEMS

A noncoded alarm system has one or more alarm-indicating appliances to alert the building occupants of a fire but does not tell the location or the type of device that has been activated (manual alarm or automatic protection equipment). The audible and/or visual alarm appliances operate continuously until they are turned off, until a predetermined time has passed, or until the system is restored to normal. The location or type of device originating the alarm condition can be determined by using an annunciator system. An annunciator is a visualindicating device that will be discussed later in this chapter.

CODED ALARM SYSTEMS

A coded alarm system has audible and/or visual alarm signals with distinctive pulsing or coding to alert occupants to a fire condition and to the location or type of device that originated the alarm. Coding the audible appliances may help personnel to distinguish the fire alarm signal from other audible signals. Clear and early recognition of the signal should encourage a more orderly and disciplined evacuation of the building. A common characteristic of coded alarm systems, especially of selective coded and multiplex coded systems, is that the coded alarm identification provided by the audible alarm signals is not repeated continuously. Normally, after four complete repetitions of the coded signal, the coding process ends.

THEORY OF OPERATION

In the event of a fire, a certain sequence of events has to occur for any alarm system to be effective. First, the fire has to be detected. This can be done by any of the following means: visually and by operation of a manual pull box, heat detectors, water pressure/flow switches, flame-actuated detectors, or smoke detectors. Any of these devices will initiate a signal to the fire alarm control unit, which is powered by a reliable power supply. (See fig. 7-1.) Second, the control unit accepts the signals from the initiating circuits and, through relays or other circuitry, provides the power to operate the indicated devices. These alarm devices may include, but are not limited to, horns, bells, chimes, flashing lights, or annunciators. Finally, operation of the alarm will alert personnel to evacuate and assist fire-fighting personnel in locating the fire, thus protecting life and property.

In the following paragraphs, we will discuss the principle of operation of the associated equipment that makes up an alarm system.

EQUIPMENT DESCRIPTION

Figure 7-1 shows how the basic parts of a local fire alarm system are interconnected. The devices in the diagram are grouped for convenience in labeling. Physical location and zoning of devices vary for different applications, and many systems do not have all the devices shown.