Increasing Your Fire Alarm Literacy
Part four in a series that examines a vital link in the fire protection system.
Initiating devices send messages to the fire alarm control panel (FACP) that cause it to report and annunciate an alarm condition. The fire alarm system can be tripped by manual initiating devices only, by automatic initiating devices only, or by a combination of both.
A manual initiating device requires a human act to send an alarm message. Examples of manual initiating devices include manual pull stations; manual fire extinguishing system pull handles for kitchen hood and duct systems; and manual fire extinguishing system pull handles for C02, halon, or other specialty extinguishing system. Any of these devices, when manually activated, cause FACPs to report and annunciate an alarm condition.
An automatic initiating device does not require human intervention to operate. Examples include heat detectors, smoke detectors, exotic detectors such as flame or explosion detectors, automatic fire sprinkler flow switches, commercial kitchen hood and duct suppression systems, and halon systems and similar specialty fire extinguishing systems.
ZONES AND CIRCUITS
For all practical purposes, the terms zones and circuits, when applied to common fire alarm systems, mean the same. A common fire alarm system is one that has direct current-powered zones and common initiating and signaling devices.
Initiating devices are attached to a pair of wires, which in turn are attached to an FACP. Different types of initiating devices may be attached to the same zone. They are placed in parallel lines across the circuit wires. When an initiating device activates, it provides a short circuit across the circuit wiring. The FACP “sees” the short circuit, or closed switch, and determines that there is an alarm condition in that zone.
Observe carefully that the FACP does not indicate which device trips, only the zone in which the trip occurs. It is important to understand then that once a device trips in a zone, additional devices activating in that zone are not recognized until the first device resets or is reset. Therefore, if a zone experiences a faulty device trip, the entire zone is out of service until the tripped device is replaced or repaired.
There are two commonly used types of fire alarm zone circuits: Class A circuits are more reliable but less common; Class B circuits are less reliable but more common.
Class A circuits are not disabled when a break occurs in one of the wire connecting it to the FACP. Current flows to initiating devices from two directions. The advantage of Class A circuits is higher circuit reliability. The disadvantages arc limitations on circuit routing and an increase in the amount of wire required.
Class B circuits are more simple than Class A circuits. They are “deadended,” compared with the loop arrangement of Class A circuits. Class B circuits are easily disabled by a broken wire. Their advantages include increased flexibility in wire routing and less wire required for the circuit. The primary disadvantage is lower circuit reliability.
There are special fire alarm circuits called multiplex circuits, which operate differently from Class A and Class B circuits. Multiplex circuits relate to devices, not zones. They are commonly used in high-rise buildings and similar complicated occupancies. Each initiating device monitors itself for alarm or trouble. If either condition exists, it transmits this condition to the FACP by a special unique signal. Operation of these panels requires special fire officer training.
FIRE PROTECTION
INCREASING YOUR FIRE ALARM LITERACY
The end-of-the-line resistor (EOL) must be present in both Class A and Class B circuits. A common electroniccircuit resistor, its purpose is to establish some level of current through the zone wiring or zone circuit at all times, assuring the FACP that the wiring is intact. If there were no EOL it would not be apparent if and when the circuit wiring became damaged.
Zones and circuits receive the following signals:
Alarm. Alarm initiating devices send alarm signals to the FACP via a switch closure. litis creates a short circuit across the zone wires.
Supervisory. Opening one switch in a series of closed switches sends a supervisory signal to the FACP. For example, when the operation of a fire sprinkler system causes an open to occur at the switch on the valve control, the zone to which it is conclear-cut answer. In fact, if the designer desires, a set of control valves can be monitored by circuits identical to alarm circuits. An important point here is that zones or circuits must be clearly labeled “alarm circuit” or “supervisory circuit” so the firefighter knows what kind of zone he is dealing with.
nected sends a supervisory signal to the FACP. Why is an alarm signal generated by a closure across a set of wires and a supervisory signal generated by opening a switch? There is no
(Photo by author.)
FIRE PROTECTION
INCREASING YOUR FIRE ALARM LITERACY
Trouble. Trouble signals indicate failures within the fire alarm system circuitry itself. (See Fart 3 in this series in the April 1990 issue for a discussion of trouble indications in the FACP.) Zone trouble signals occur when a circuit wire is broken or is shorted to ground. The EOL is a critical part of the trouble circuitry.
Signals from automatic fire suppression systetns. The term fire suppression system covers a broad range of systems including automatic sprinkler systems, kitchen dry chemical systems, halon systems for sensitive electronic or computer rooms, and C02 systems for electrical switch gear or petroleum product storage. If an occupancy has an NFPA 72A fire alarm system, these special suppression systems are usually connected into the FACP as separate zones. Exactly how they are zoned depends on the authority having jurisdiction. Fire companies must recognize immediately which red lights on a fire alarm panel represent fire suppression systems located somewhere in the occupancy.
Automatic sprinkler systems also produce their own audible alarm — either an electrically operated bell or mechanical water-motor-gong near the flowing riser. Remember, the primary purpose of the bell or gong is not to warn occupants to evacuate, but rather to indicate that the sprinkler system has been activated.
Many sprinkler control valves are electrically supervised. If a valve is turned toward an improper position, a supervisory signal is displayed and an audible warning sounds on the FACP. Notice the term control valve-. Only valves that affect water supply to fire sprinkler systems are supervised; test valves are not. The fire alarm control panel must be clearly marked so firefighters can determine which supervisory zone is announcing itself. It is not uncommon to find up to five control valves on one supervisory zone.
TYPICAL SOURCES OF ALARM TRIPS
Manual alarm circuits. Manual alarm circuits are reliable because they are simple. A manual pull box is nothing more than an electrical switch held in place behind a door. The door is held in place by a glass rod or plate. When the door is pulled, the glass restraint breaks and the switch releases.
False trips occur if a door is sprung because of abuse or if the glass rod is the incorrect size. The door should be held in place snugly by its restraint. Loose wires also can cause random alarms and trouble signals. These wires are behind the pull box. When the box is mounted, it is possible to force the wires one upon the other, creating a short circuit.
Heat detectors. There are three common types of heat detectors: fixed temperature, rate-of-rise, and a combination of the two. Fixed temperature detectors seldom cause false trips. They are not self-resetting. On the other hand, rate-of-rise detectors are often self-resetting—no human intervention is required to turn off the alarm and reset it. This feature can frustrate responding fire companies if the device self-resets before they arrive.
Rate-of-rise detectors tend to trip in situations during which the temperature changes rapidly. For instance, random trips can occur in an occupancy where overhead doors are opened and shut and the temperature difference between inside and outside is significant. High-risk locations for this type of detector are in kitchens over steam tables; near steam kettles, very hot dishwashers, and oven doors; and in incinerator rooms near incinerator doors.
Some common rate-of-rise detectors with a bell-shaped bottom have a hole in the heat-collecting bell. If the detectors are painted or become dirty, this small vent hole can easily become plugged. Painted detectors must be replaced and this hole must be kept clean.
Smoke detectors. Smoke detectors fall into two general categories: those that detect invisible products of combustion and those that detect visible products of combustion. Ion detectors, belonging to the former, are used in residential, commercial, and industrial applications. They detect charged ions produced in early stages of smoldering fires or flash fires, which do not produce heavy accumulations of smoke in their early stages of development.
Unfortunately, other products and conditions can fool the detector into sending an alarm signal: vapors and gases released by some cleaning products, especially those containing heavy oxidizers; friendly products of combustion such as cooking fires in residential occupancies; and roaches, spiders, and other insects that crawl inside the detector, just to name a few.
Ion detectors often are used in elevator lobbies. When people waiting for the elevator light cigarettes while standing under the detectors, they set them off. Finished floor-toceiling dimensions in buildings are decreasing, allowing architects to squeeze more floor space into the building volume, ‘litis also brings ceiling-mounted ion smoke detectors closer to the source of smoking materials. Thus care must be exercised in the placement and selection of elevator lobby smoke detectors.
Ion detectors near grade-level doors in high-rise occupanices are susceptible to random trips because of the rush of air that comes in through the doors when occupants open them. Outside pollutants are brought in by the chimney effect of tall buildings, which is especially noticeable if the air inside the building is warmer than the air outside.
Obscuration, light-scattering, and photoelectric detectors belong to the group that detects combustion products on the basis of their visibility. They operate when combustion products impede the passage of light in air or contribute to light reflections. While less prone to false trips than ion detectors, they tend to false-trip more than heat detectors.
FIRE PROTECTION
INCREASING YOUR FIRE ALARM LITERACY
Steam, haze, and condensation contribute to trips. For example, smoke detectors sometimes are mounted at the top of atriums in high-rise buildings. Warm, moist air inside rises to the top. The rapid cooling of night air or a cold night causes the inside moisture to condense and form a cloud-like haze at the top of the atrium. This condensation can contribute to unpredictable device trip. The same friendly fire sources that cause trips in ion detectors cause trips in this type of detector, and the same hazard exists from insects crawling inside them.
Automatic suppression systems. Because of the mechanical nature of suppression systems such as halon, and dry chemical, false trips do not happen often. However, automatic wet sprinkler systems do cause false trips under certain conditions.
Water leaks in automatic wet sprinkler systems contribute to false alarms. Flow switches with faulty time-delay features send a flow alarm every time there is a surge in the water inside the pipe. If a water suppression system is not drained properly and contains large amounts of air, the trip problem is amplified.
Some sprinkler systems have what is called a retard chamber instead of a flow switch with a pneumatic delay. This chamber begins to fill with water if the system main check valve opens for even an instant. Normally the water drains out of the retard chamber before it can fill with water and cause an alarm. If the retard chamber has a plugged drain hole, eventually the chamber fills with water. After that false alarms occur every time the water surges the main valve and alarm valve open. Retard chambers must show evidence of the ability to drain accumulated water.
It is not unusual to find that illegal taps have been made into sprinkler piping because the piping was conveniently located. Coffee machines, ice makers, and even toilets have been tapped into nearby sprinkler pipes.
When false alarms occur and the FACP indicates a fire suppression system, consider the possible causes. Also, develop a way to document alarm zones and times.
Initiating circuits extend out and away from the fire alarm control panel. They are like arms and fingers: When human fingers detect heat, a signal is transmitted back to the brain—in fact, when any heat is detected any place along the arm, a signal is sent to the brain. The FACP can be considered the brain of the fire alarm system. The human shouts when he is burned. The FACP uses signaling devices, bells, and horns to “shout.” Thus it is important for the firefighter to be able to understand these “shouts” and take appropriate action. ■
