The use of security/fire-alarm systems is not only advisable, but is also man- dated in most populated towns and cities by national codes and local ordi- nances. This chapter introduces the basic types of security-system equipment and components.
Signaling Systems
Signaling techniques are not new. Methods were devised more than 5000 years ago to signal individuals and tribes of danger and of on-coming strangers: Indians used smoke signals to communicate with each other; other tribes used drums and animal horns; bells were used extensively during the settling of the United States to announce meetings and warn of fires and other dangers, and, of course, military troops have used flags and horns to communicate for thou- sands of years.
When electricity was put to practical use around the latter part of the 19th century, methods were devised to use electrical buzzers and bells (such as doorbells, entrance detectors, and manually operated fire-alarm signals) for signaling devices. However, at that time, electrical and electronic devices were usually limited to certain specialized structures such as banks and school buildings.
Today, all apartment buildings and town houses in almost every section of the United States must have an adequate number of smoke detectors installed to warn occupants of fire. Such buildings as nursing homes, schools, hospitals, and hotels are required to have an approved fire-alarm system installed, as well as fire sprinkler systems. The latter is usually designed to operate in con- junction with the fire-alarm system. Banks and similar institutions would not think of using a building without adequate security systems installed on the premises.
The list of applications is endless, and great opportunities await the trained security technician. To verify this, look in the Yellow Pages of any city phone
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directory under Alarm Systems, Burglar Alarm Systems, or Fire-Alarm Systems and note the number of businesses listed. Each firm probably has a dozen or more employees, and is eager to hire several more trained technicians. And, employees leave companies to start their own companies regularly.
Classification of Signal Circuits
A signal circuit used for a security or fire-alarm system can be classified as open circuit or closed circuit. An open circuitis one in which current flows only when a signal is being sent. Aclosed circuitis one in which current flows con- tinuously, except when the circuit is opened to allow a signal to be sent.
All security systems have three functions in common:
■ Detection
■ Control
■ Annunciation (or alarm) signaling
Many systems incorporate switches or relays that operate when entry, movement, pressure, infrared-beam interruption, or other intrusions occur. The control senses the operation of the detector with a relay and produces an output that can operate a bell, a siren, a silent alarm such as telephone dialers to law enforcement agencies, or any other signals. The controls frequently con- tain ON/OFF switches, test meters, time delays, power supplies, standby bat- teries, and terminals to connect the system together. The control output usually provides power on alarms to operate signaling devices or switch con- tacts for silent alarms (Fig. 5.1).
An example of a basic closed-circuit security system is shown in Fig. 5.2. The detection (or input) subdivision in this drawing shows exit/entry door or win- dow contacts. However, the detectors could just as well be smoke or heat detec- tors, switch mats, or ultrasonic detectors.
The control subdivision for the system in Fig. 5.2 consists of switches, relays, a power supply, a reset button, and related wiring. The power supply shown is a 6-V nickel-cadmium battery that is kept charged by a plug-in transformer unit. Terminals are provided on the battery housing to accept
12-V ac charging power from the plug-in transformer, which provides 4 to 6 V for the detection (protective) circuit and power to operate the alarm or out- put subdivision.
Figure 5.3 shows another closed-circuit system. The protective circuit con- sists of a dc energy source, any number of normally closed intrusion-detection contacts (wired in series), a sensitive relay (R1), and interconnecting wiring. In
operation, the normally closed intrusion contacts are connected to the coil of the sensitive relay. This keeps the relay energized, holding its normally closed con- tacts open against spring pressure, the all-clear condition of the protective cir- cuit. The opening of any intrusion contact breaks the circuit, which deenergizes the sensitive relay and allows spring force to close the relay contacts. This action initiates the alarm.
The key-operated switch shown in the circuit in Fig. 5.3 is provided to open the protective circuit for test purposes. A meter (M) is activated when the switch is set to CIRCUIT TEST. The meter gives a current reading only if all intrusion contacts are closed. All three sections of the switch (S1, S2, S3) make
contact simultaneously as the key is turned.
Opening of intrusion contacts is not the only event that causes the alarm to activate. Any break in protective-circuit wiring or loss of output from the ener- gy source has the same effect. The circuit is broken, which deenergizes the sensitive relay and allows spring force to close the relay contacts, thus sound- ing the alarm. Any short circuit between the positive and negative wires of the protective circuit also keeps current from reaching the relay coil and causes a dropout, which again sounds the alarm.
Other components of the alarm circuit in Fig. 5.3 include a second energy source, an alarm bell, and a drop relay (R2). When the keyed switch is at ON,
dropout of the sensitive relay (R1), and closing of its contacts completes a cir-
cuit to energize the coil of drop relay (R2). Closing the drop relay’s normally
open contacts rings the bell and latches in the drop-relay coil so that R2stays
energized even if the protective circuit returns to normal and opens the sensi- tive relay’s contacts. As a result, the bell continues to ring until the key switch is turned away from ON to break the latching connections to the R2coil.
Drop relays often have additional contacts to control other circuits or devices. The extra contacts in the circuit in Fig. 5.3 are for turning on lights, triggering an automatic telephone dialer, etc. But the main two functions of the drop relay are actuation of the alarm and latching the coil to keep the cir- cuit in the alarm condition.
The majority of burglar systems use a closed-loop protective circuit. Typically, the system consists of an annunciator connected to a contact on each door and window and a relay connected so that when any window or door is opened it will cause current to pass through the relay. The relay, in turn, will operate to close a circuit on a bell, horn, or other type of annunciator, which will continue to sound until it is shut off, thereby alerting the occupants or needed agencies.
The wiring and connections for the open-circuit system are shown in Fig. 5.4. This diagram shows three contacts, but any number can be added. Closing one of the contacts completes the power circuit through the winding of the proper annunciator drops, the constant-ringing switch, the constant- ringing relay, the alarm bell, and the bell-cutoff switch. The current passing through the winding of the constant-ringing relay operates to complete a circuit placing the alarm bell directly across the battery or other power source so the bell continues storing until the cutoff switch is opened. At the same time, current in another set of wires operates a relay that closes an auxiliary circuit to operate other devices, such as lights and an automatic telephone dialer.
Contacts for closed-circuit operation are shown in Fig. 5.5A. The contacts are surface-mounted opposite each other, one on a stationary window or door frame; the other on the movable part of the window or door. When the window is raised, or the door is opened, the contacts break and sound the alarm. Contacts for recessed mounting (Fig. 5.5B) operate the same way as described for the surface-mounted contacts.
A spring-type contact for open-circuit operation is shown in Fig. 5.6. This device is recessed in the window frame or a door jamb so that the cam projects outward. When the window is raised, the cam pivots and is pressed in and makes contact with a spring that is insulated from the plate. The contact is connected in series with the power source and the annunciator; that is, one wire is connected to the plate and the other to the spring.
Fire-alarm systems
A fire-alarm system consists of the following:
■ Sensors
■ Control panel
■ Annunciator
■ Related wiring
They are generally divided into the following four types:
■ Noncoded
■ Master-coded
■ Selective-coded
■ Dual-coded
Each of these four types of alarm has several functional features so designed that a specific system can meet practically any need to comply with local and state codes, statutes, and regulations.
In a noncoded system, an alarm signal is sounded continuously until it is manually or automatically turned off.
Figure 5.5 Spring-type contact for closed-circuit operation.
Figure 5.6 Spring-type contact for open-circuit operation.
In a master-coded system, a common-coded alarm signal is sounded for not less than three rounds. The same code is sounded regardless of the alarm- initiating device activated.
In a selective-coded system, a unique coded alarm is sounded for each fire- box or fire zone on the protected premises.
In a dual-coded system, a unique coded alarm is sounded for each firebox or fire zone to notify the building’s personnel of the location of the fire, while non- coded or common-coded alarm signals are sounded separately to notify occu- pants to evacuate the building.
Figure 5.7 represents a riser diagram of a fire-alarm system. If a detector senses smoke or if any manual striking station is operated, all bells within the building will ring. At the same time, the magnetic door switches will release the smoke doors to help block smoke and/or drafts. This system is also con- nected to a water-flow switch on the sprinkler system. If the sprinkler valves are activated causing a flow of water through the pipes, the fire-alarm system will again go into operation, energizing all bells and closing smoke doors.
Smoke and fire detectors
Any product of a fire (like aerosols) that changes the ambient conditions in the building is called a fire signature and is potentially useful for detection
purposes. The principal fire signature used in residential smoke detectors is aerosol. Aerosols are particles suspended in air. The process of combustion releases large numbers of solid and liquid particles into the atmosphere. They can range in size from 10 m [a micron (m) is one thousandth of a millime- ter] down to 0.001 m. Aerosols resulting from a fire represent two different fire signatures. Those particles less than 0.3 m do not scatter light efficiently and are classified as visible. The invisible aerosol signature is usually referred to as the “products of combustion” and the visible aerosol signature as “smoke.” Invisible aerosol is the earliest appearing fire signature.
Types of fire-detection devices
Thermal Detectors: Thermal detectors are devices that respond to heat— typically 135°F. These units consist of a bimetallic element that bends to com- plete a circuit under high heat conditions. Because these units do not detect smoke or products of combustion, they are not recommended for living areas of a residence. However, they do have value for use in attics, unheated garages, and furnace rooms.
Flame Detectors:Flame detectors detect actual flames by sensing ultravio- let emissions. These devices would not be used in residential applications.
Gas Detectors:These units respond to certain gases (propane, carbon monox- ide, liquid petroleum, butane, and gasoline vapors) that would not be detected by a smoke and fire detector. Although these detectors do have some uses, they should not be used as a substitute for a smoke and fire detector. They will not respond to aerosols produced by the majority of residential fires.
Ionization Detectors: Inside the ionization chamber, the radioactive source emits radiation, main alpha particles, which bombard the air and ionize the air particles, which, in turn, are attracted by the voltage on the collector elec- trodes. This action results in a minute current flow. If aerosols, such as prod- ucts of combustion or smoke, enter the chamber, the ionized air particles attach themselves to the aerosols and the resultant particles, being a larger mass than ionized air, move more slowly, and thus, per unit of time, fewer reach the electrodes. A decrease in current flow, therefore, occurs within the chamber whenever aerosols enter. The decrease in current flow is electroni- cally converted into an alarm signal output (Fig. 5.8).
An ionization type of detector responds best to invisible aerosols, where the particles from burning materials are in the range of 1.0 m in size down to 0.01 m. A tremendous amount of these particles are produced by a flaming fire as opposed to a smoldering fire, which produces large and small particles, but, because of low heat, the low thermal lift tends to allow particles to agglomerate into larger particles if the detector is some dis- tance from the fire.
High air flows will affect the operation of this type of unit by reducing the ion concentration in the detector chamber. In fact, with a high-enough air flow,
the unit will respond and alarm even though a fire does not exist. For this rea- son, locating ionization detectors near windows, direct air flows from air vents, and comparable areas should be avoided.
Ionization smoke detectors (Fig. 5.9) can be used in place of conventional smoke detectors or can be used in combination with standard smoke detectors. They are more sensitive than the conventional smoke detectors.
Photoelectric Detectors:A beam from the detector’s light source is projected across a chamber into a light catcher. The chamber is designed to permit access of smoke, but not access of external light. A photo-resistive cell or light- sensitive device is located in a recessed area perpendicular to the light beam. When smoke enters the chamber, particles will scatter or reflect a small por- tion of the light beam to the light-receiving device, which, in turn, will provide a signal for amplification to the alarm. Variations in design are sometimes used by manufacturers.
Some photoelectric detectors are adversely affected by dirt films. Any accu- mulated dirt, dust, film, or foreign matter on either or both lenses of the light source or the photocell will cause an opaque effect and the detector will then become less and less sensitive. Therefore, it will require more smoke in order to respond. Although the latest photoelectric models utilize solid-state light- emitting and receiving devices, which have a longer life than previous light devices, the problem of failure of the light source still exists. Underwriters’ Laboratories requires an additional audible alarm in case light failure occurs.
Figure 5.8 Diagrams of ionization detectors. The top diagram shows normal conditions. The bottom diagram shows aerosols, such as products of combustion or smoke, entering the sensor. In the latter condition, the alarm is activated.
Photoelectric units respond best to visible aerosols where the particles range from 10 m down to 0.3 m. These particles would be given off by a smolder- ing fire that produces very little heat (Fig. 5.10).
Ionization and Photoelectric Devices: Figure 5.11 can be used to illustrate both types of units—the difference is the use of either an ionization sensor or a photoelectric sensor in the reference chamber and detector portions of the circuit. Under normal conditions, the voltage across the reference chamber and the detection chamber is the same. However, when fire occurs, the detec- tion chamber then functions as described in the previous explanation. Thus, when there is sufficient voltage difference between the two chambers, the alarm is activated through the switching circuit.
Complete descriptive information and practical applications of smoke detec- tors are covered later in this chapter.
Components of security/ fire-alarm systems
Wire sizes for the majority of low-voltage systems range from #22 to #18 AWG. However, in some situations, it might be necessary to use larger wire sizes to prevent excessive voltage drop, for example: where larger-than-normal cur- rents are required for longer distances between outlets. Note:Voltage-drop cal- culations should be made to determine the correct wire size for a given application (see Chapter 2).
Most closed systems use two-wire #22 or #24 AWG conductors and are color- coded to identify them. A #18 pair normally is adequate for connecting bells or
sirens to controls if the run is 40 feet or less. However, many installers prefer to use #16 or even #14 nonmetallic cable.
A summary of the various components for a typical security/fire-alarm sys- tem is shown in the riser diagram in Fig. 5.12. Notice the varying types of sen- sors or detectors in this system.
Control stations
The control station is the heart of any security system because the circuitry in control panels senses a broken contact and then either sounds a local bell, a horn, or a silent alarm. Most modern control panels use relay-type controls to sense the protective circuits and regulate the output for alarm-sounding devices. They also contain contacts to actuate other deterrent or reporting devices and a silent holdup alarm with a dialer or police-connected reporting mechanism.
Figure 5.10 Basic operating principles of photoelectric detectors.
Power supplies
Power supplies vary for different systems, but, in general, they consist of rechargeable 6-V dc power supplies for burglar alarm systems. The power packs usually contain nickel-cadmium batteries that are kept charged by 12-V ac input from a plug-in or otherwise connected via transformer to a 120-V cir- cuit. High-quality power supplies have the capability of operating an armed