FIRE ALARM SYSTEM DESIGN

Design of Fire Alarm Systems and Signaling Systems

Smoke detectors are one of the most important devices that you can have in your home and can mean the difference between life and death. For a smoke detector to operate correctly, install it correctly and keep up with monthly maintenance.

A project engineer would never intentionally design a building sub-system to be obsolete a year or two after completion - unless the whole project was a temporary venture. What factors must an engineer consider to ensure the longevity of a fire detection, alarm or signaling system? And, what is the useful life of a typical fire alarm or signaling system?
The answer to the second question is directly related to the first. For a properly designed system, its life cycle is defined only by possible changes in the required mission or by component failures and replacement availability.¹ To varying degrees, engineers can design to reduce component failures and provide for possible changes in a system's intended mission. There are many fire alarm systems still in service today that are 30 or more years old. They might not be as sophisticated as today's systems, but they continue to reliably perform their intended functions.
Like sprinklers, today's fire alarm and signaling systems are often designed for a specific mission. Building codes, fire codes and life safety codes have requirements for the use of fire detection, alarm and signaling systems in certain occupancy or use groups. The requirements in the codes are coordinated with other prevention and protection requirements to achieve some level of life safety and property protection contemplated by the code.
In some cases, code changes are made retroactively and would require either a new installation, if one did not already exist, or a change to an existing system. A good example are the code changes that took place to require sprinkler and alarm retrofits in many places of assembly as a result of the Station Night Club fire in 2003. Changes in occupancy and changes in owners' goals might also trigger changes to a fire detection, alarm or signaling system.
There are several ways that an engineer can help ensure that a system designed to meet today's needs has the infrastructure to be adapted to meet changes in its intended mission without requiring a total replacement. One of the best ways to provide a flexible infrastructure for a fire detection, alarm or signaling system is to ensure that there are an adequate number of circuits, properly distributed and of sufficient wire size, to permit changes and growth.
Most new fire detection and alarm system designs use addressable control units and addressable detectors for all but very small systems. In NFPA 72² terms, an addressable circuit is called a "Signaling Line Circuit" (SLC). Control unit manufacturers design their equipment to allow a single SLC to have as many as 200 addressable devices in various combinations of initiating devices and control devices (relays for example).
Design engineers would not normally specify the number of SLCs required for a project. Instead, they would show the number of initiating devices and specify the number of control points, and then allow the contractor or manufacturer to determine the required number of circuits.
However, to ensure room for changes and future growth, engineers should specify that SLCs not be loaded more than 80% - or some other limit. This could still result in 100 or more devices on a single SLC that passes through multiple fire areas of a building. While technically feasible, that is not good practice. Therefore, the engineer should specify that either separate circuits be used or that isolation modules be used to limit number of devices that might be out of service due to a fault at any one location. Some local codes might also impose similar limits.
Signaling line circuits also have a "growth" advantage in that they are permitted to be "T-tapped" - unless Class A circuits are specified. Class A circuits can remain fully operational even with a single wire break. On the other hand, a Class B circuit would not be operational beyond the open circuit. (See Figures 2 and 3) Class B SLCs, unlike other types of Class B circuits, are permitted to be "T-tapped" as shown in Figure 3 because they monitor the integrity of the wire by communicating with the devices. (Circuit classes will be described in greater detail in a future installment of this series.)
While it is possible to maintain Class A circuit integrity when adding to a system in an existing installation, it is generally less expensive to "T-tap" an SLC. Also, Class A circuits are not necessarily more operationally reliable then Class B circuits. Class B circuits can be more reliable, depending on the number of "T-taps," the wiring type, the type of fault and the number and location of isolation modules.
As with SLCs, Notification Appliance Circuits (NACs) should have a load limit specified by the designers. NACs are more likely to require changes during the life of a system. Therefore, a lower load limit of 70% or so is recommended. Design engineers should also specify that no one NAC serve more than one fire or smoke zone - even if the system is set up for general alarm. This would permit future "zoning" or selective communications options to be implemented by changing or reconfiguring the control equipment, which is easier and less expensive than rewiring a building.
Regardless of the specific load limits for the initial installation, the wire size for each circuit should be based on 100% loading so that it can properly handle future additions. Future changes to circuit length should also be incorporated in the initial installation by up-sizing the wires. For NACs, the wire size can be increased one or two sizes without any downside consequences - the only limit being the cost. However, for SLCs, the smallest wire size that meets the load and distance requirements is best. This is because larger wires have a greater capacitance, which degrades digital signaling and slows communications speeds. In most cases, No. 18 or No. 16 AWG (0.823 mm² or 1.31 mm²) wires are sufficient for SLCs. The panel manufacturer can provide resistance and capacitance limits.
Many facilities now install Emergency Communications Systems (ECS) to provide notification and information to occupants for more than just fire emergencies.² The back-bone of an ECS is the use of voice for the audible part of the system. This can have a growth advantage over conventional direct current audible appliances.
Conventional NACs used for horns are direct current circuits and are limited by the available current - typically one to two amps per circuit. NACs used for speakers are limited only by the available wire size and amplifier size. Thus, if properly planned, they have greater potential for future growth. The disadvantage is that separate circuits must be provided for visible signaling appliances. However, since the need for ECS is increasing, providing voice capability even when not required results in a more flexible system. In many installations, the cost can be offset by using the ECS for day-to-day communications and paging needs as permitted by NFPA 72.
Engineers should also address future component failures and replacement availability. The first step is to choose and specify the correct type of equipment for the environmental and situational conditions. In certain situations, mechanical protection, listed for use with the particular detector of notification appliance, might be required.² The availability of parts over time varies among different manufacturers and for different product lines. Engineers should work with manufacturers and distribution chains that have a track record for maintaining parts availability.
A system that can last forever and meet all future needs does not exist. However, by requiring certain features and upgrading from minimum requirements, engineers can design and specify systems that will be more likely to meet or contribute to future needs.
References:

1. "Mission Effectiveness and Failure Rates Drive Inspection, Testing, and Maintenance of Fire Detection, Alarm and Signaling Systems," Fire Protection Engineering, Bethesda, MD 20814, Summer 2002.
2. NFPA 72, National Fire Alarm and Signaling Code, National Fire Protection Association, Quincy, MA, 2010.

Importance of Smoke Detectors

A smoke detector's purpose is a simple one, to give you ample notification in case of a fire in your house. Without a smoke detector, by the time you realize that there is a fire, your house could be so badly engulfed that you cannot find a safe exit or the smoke can be so overwhelming that you suffocate trying to get out. The National Fire Protection Association reports that while 75 percent of homes have at least one working smoke alarm, between 2003 and 2006, 66 percent of fire deaths happened in homes with no working smoke alarm.

How Smoke Detectors Work

There are two main kinds of smoke detectors: photoelectric sensors and ionization sensors. Photoelectric sensors generate a beam of light focused on a light-sensitive cell, enclosed in the alarm. If the light beam is interrupted from smoke entering the detector, the alarm goes off. Ionization sensors work by having a small piece of radioactive material create an electric current between two plates. If smoke or hot air enters the chamber, the reaction is changed and the current is disrupted, causing the alarm to go off. Photoelectric smoke detectors work best with slow, smoky fires and ionization detectors work best with quick, hot fires.

Installing Smoke Detectors

You should install smoke detectors in every bedroom, in a hallway outside each sleeping area and at least one on every floor of the house, including the basement. Have each smoke detector interconnected with wires, so that when one goes off they all go off. Install both types of smoke detectors throughout your house, or purchase a combination detector that uses both technologies. Always install smoke detectors high on wall or on ceilings, as smoke will rise when it is created. If your home includes someone who is hard of hearing, install a detector made specifically for him as it has flashing lights and vibrates on the wall.

Maintaining Smoke Detectors

Every smoke detector has a test button, and you should press the test button to hear the alarm once every month to ensure the battery hasn't run out. If your alarm starts to chirp on its own, replace the battery right away. The National Fire Protection Association found that in 23 percent of fire deaths from 2003 to 2006, there was a fire alarm present that wasn't in working order.

Types of Smoke Detectors

here are two main types of smoke detectors: ionization detectors and photoelectric detectors. A smoke alarm uses one or both methods, sometimes plus a heat detector, to warn of a fire. The devices may be powered by a 9-volt battery, lithium battery, or 120-volt house wiring.

Ionization Detectors
Ionization detectors have an ionization chamber and a source of ionizing radiation. The source of ionizing radiation is a minute quantity of americium-241 (perhaps 1/5000th of a gram), which is a source of alpha particles (helium nuclei). The ionization chamber consists of two plates separated by about a centimeter. The battery applies a voltage to the plates, charging one plate positive and the other plate negative. Alpha particles constantly released by the americium knock electrons off of the atoms in the air, ionizing the oxygen and nitrogen atoms in the chamber. The positively-charged oxygen and nitrogen atoms are attracted to the negative plate and the electrons are attracted to the positive plate, generating a small, continuous electric current. When smoke enters the ionization chamber, the smoke particles attach to the ions and neutralize them, so they do not reach the plate. The drop in current between the plates triggers the alarm.
Photoelectric Detectors
In one type of photoelectric device, smoke can block a light beam. In this case, the reduction in light reaching a photocell sets off the alarm. In the most common type of photoelectric unit, however, light is scattered by smoke particles onto a photocell, initiating an alarm. In this type of detector there is a T-shaped chamber with a light-emitting diode (LED) that shoots a beam of light across the horizontal bar of the T. A photocell, positioned at the bottom of the vertical base of the T, generates a current when it is exposed to light. Under smoke-free conditions, the light beam crosses the top of the T in an uninterrupted straight line, not striking the photocell positioned at a right angle below the beam. When smoke is present, the light is scattered by smoke particles, and some of the light is directed down the vertical part of the T to strike the photocell. When sufficient light hits the cell, the current triggers the alarm.
Which Method is Better?
Both ionization and photoelectric detectors are effective smoke sensors. Both types of smoke detectors must pass the same test to be certified as UL smoke detectors. Ionization detectors respond more quickly to flaming fires with smaller combustion particles; photoelectric detectors respond more quickly to smoldering fires. In either type of detector, steam or high humidity can lead to condensation on the circuit board and sensor, causing the alarm to sound. Ionization detectors are less expensive than photoelectric detectors, but some users purposely disable them because they are more likely to sound an alarm from normal cooking due to their sensitivity to minute smoke particles. However, ionization detectors have a degree of built-in security not inherent to photoelectric detectors. When the battery starts to fail in an ionization detector, the ion current falls and the alarm sounds, warning that it is time to change the battery before the detector becomes ineffective. Back-up batteries may be used for photoelectric detectors.