How Does A Fire Alarm System Work

Fire Alarm

The purpose of any fire alarm system is to warn people inside the danger zone — i.e., everyone within range of the noise and/or light signal emitted by the sensor(s) activated. This includes employees and other facility personnel who may not have time to find safety elsewhere but must evacuate immediately due to faulty wiring, electrical overloads or fire conditions. It should also include nearby residents whose homes could easily become engulfed during a blaze because of faulty construction materials or poor maintenance practices.

Some systems provide only one type of notification such as sound and lights while others combine multiple signals to produce more effective alerts. Some use wireless devices instead of hardwired connections. And some even offer advanced features like video surveillance cameras and voice messages left with police dispatch centers. Once you understand what kind of fire alarm system works best for your needs, it will help you decide which components to purchase.

In this article, we’ll look at several different types of fire detection technologies and explain how each functions. We’ll discuss why it might make sense to install them and compare advantages and disadvantages between various configurations. Finally, we’ll conclude with a few advanced options available today.

Let’s start with traditional smoke detectors and then go over some fire protection tips.

Heat Sensing Systems

A heat sensing system detects excessive temperatures above safe levels based upon either absolute temperature differences or changes in relative humidity over time. Both methods typically rely on metal oxide semiconductor field effect transistors (MOSFET). MOSFETS conduct electricity very efficiently. When exposed to high temperatures, however, electrons begin moving through the device faster than normal causing its resistance to increase. By monitoring the output voltage across the FET gate resistor, users can determine whether current is passing normally or abnormally fast. If the latter occurs, then the cause probably lies outside the unit itself rather than internal short circuits. Heat sensing units come in two varieties: stationary and portable. Stationary models hang directly beneath ceiling fans, air duct outlets or near furnaces and water heater tanks. Portable versions attach to walls or floors via wires running behind baseboards or recessed into cabinets.

Stationary Heat Detector

Stationary heat detector panels measure surface temperature in square feet. For example, Honeywell Residential Safety Alert PIR Smoke Detector Panel T1 Series measures 18 inches wide by 24 inches tall by 3 inches deep. These panels contain built-in digital thermometers measuring both indoor and outdoor ambient air temperature. In addition, most feature integrated circuit chips capable of detecting rapid increases in temperature caused by burning insulation or overheated appliances. Users simply set the panel against the wall or floor where they want it to monitor and turn red whenever the interior reaches unsafe levels.
An alternative to standard heat detectors is an “intelligent” model called a thermostat controller.

Unlike conventional heat alarms, intelligent controllers integrate microprocessors along with heating elements, wire gauges and control valves. These sophisticated monitors maintain constant room temperature regardless of fluctuations in external weather or HVAC equipment. Most modern electronic thermostats allow homeowners to program automatic shutoff times based upon preselected limits or manually select specific points in time for turning off individual heaters.

As discussed below, many smoke alarms utilize heat detection technology too. Let’s take a closer look at how they do it.

How They Work

When firefighters arrive at a house fire scene, they often encounter dangerous situations including hot flames shooting up stairways or out windows, heavy black smoke filling bedrooms or choking family members trying to escape. To deal with these hazards, certain communities require that all new home construction meet strict standards regarding evacuation routes and exit signs. One way to comply is to equip every residence with photoelectric smoke detectors.

Photoelectric smoke detectors consist of three major parts: a smoke detector, power source (usually batteries) and wired connection to a central control board. Whenever particles enter the ionization chamber, charged ions move toward oppositely charged electrodes creating an electric potential difference. As long as smoke remains in the ionization chamber, positive charges collect on the upper electrode. But once enough negative charge builds up on the lower electrode, ions break free from bonds with oxygen atoms and neutralize the polarity. This triggers circuitry aboard the control board which activates an LED flashlight bulb located atop the unit. Since LEDs don’t generate much heat, they emit bright white light visible far away. Photocells used in older smoke detectors respond to invisible infrared radiation generated by flaming wood embers and smoldering cotton fibers.

Next, let’s consider how newer smoke alarms use radio waves to detect airborne particles.
According to Federal Emergency Management Agency statistics, about 1% of residential structures burn down annually. Of those burned buildings, nearly half were occupied. The majority of injuries occurred among household members, followed by residents, guests and pets. ­

Smoke Detecting Systems

Unlike heat alarms, which react quickly to rapidly rising temperatures, smoke alarms usually remain silent until thick gray billows of carbon monoxide fill rooms. Because of this lag, early symptoms of CO poisoning include headaches, dizziness, nausea, weakness and disorientation before victims collapse unconscious. Even after someone becomes fully conscious, he or she isn’t likely to leave without assistance since panic attacks and confusion hamper mobility. Fortunately, recent smoke alarms incorporate pneumatic pressure sensitive switches similar to the ones found on doorbells.

Pressurized gas released from these miniature cylinders spreads throughout the room producing tiny bubbles of nitrogen. As smoke enters the bubble, it displaces surrounding gasses making the resulting mixture heavier and denser than fresh air. When this happens, the switch closes briefly interrupting the flow of electricity supplied by small silver contacts embedded within the plastic housing. An onboard relay powers a loud 120 dB horn speaker mounted atop the unit. Sound travels farther underwater than through dry air so the presence of dense smoke carries clearer warnings than mere smoke alone.


Two additional innovations have improved the effectiveness of smoke detectors. First, many now incorporate low frequency electromagnetic fields. According to inventor Raymond Wiegand, U.S. Patent 4,856,729 describes a method whereby alternating currents flowing through thin sheets of ferrite create magnetic fields strong enough to attract bar magnets placed next to the sheet. Second, some newer designs employ laser beams. Although still experimental, prototypes represent a breakthrough since lasers operate reliably under extreme environmental conditions unlike incendiaries.

Now that we’ve examined the basic principles involved in activating various kinds of alarms, let’s see how combining multiple signals improves overall reliability.

Although heat alarms primarily serve to warn of imminent dangers to humans, fire departments sometimes deploy them to protect property as well. Certain cities mandate installation of heat alarms throughout commercial facilities. Their main function is to prevent downed wires sparking fires and infernos. During periods of drought, utility companies may also place heat alarms underneath reservoirs to keep fish alive and hydrate vegetation.

Multiple-Sensing Systems (Combination)

Since smoke alarms generally alert people indoors whereas heat alarms guard against possible harm outdoors, logic dictates that combining these two types of alarms would improve overall protection. However, doing so presents unique challenges. Whereas heat alarms depend entirely upon direct, smoke detectors need to differentiate. Also, heat alarms benefit greatly from longer ranges than smoke alarms. Therefore, incorporating smoke and heat detection capabilities requires linking the two different types of sensors together in ways that account for their respective strengths and weaknesses.

Most residential smoke detectors send false alarms frequently. Unfortunately, it tends to happen less often in larger residences thanks to better ventilation. On average, false alarms occur roughly twice per year in typical houses while apartment dwellers suffer upwards of 10 false alarms per month.

False Alarms

False alarms waste precious minutes needed to safely get out of hazardous areas. At worst, they delay rescue efforts and possibly lead to fatalities. So although installing more smoke detectors makes good sense, having too many can actually prove counterproductive. Too many smoke alarms consume valuable space, contribute to noisy hallways and complicate operation. Additionally, many jurisdictions prohibit installing extra bells and whistles such as automatic dialing services. Instead, regulators recommend limiting occupancy to fewer than four separate smoke detectors per dwelling.

For maximum efficiency, experts suggest placing smoke alarms alongside exits and on each level of the home. Placing them higher provides more coverage area but may impede evacuees’ ability to climb stairs. Ideally, placement should conform to NFPA 72 guidelines according to America’s Fire and Security. General recommendations dictate setting detectors approximately 6 ft apart horizontally and 12 ft apart vertically. The National Electric Code mandates spacing between adjacent smoke alarms never exceed 8 ft.
Now let’s examine how combining heat and smoke detection systems further enhances security.


Heat detectors can sometimes prevent fires from starting in the first place by sounding the alarm before anything happens. In other cases, they warn of developing danger before smoke can spread throughout buildings. Smoke detectors provide added protection against invisible CO levels and lack the limitations associated with heat alarms. Combining alarms provides more effective coverage than simply placing heat and smoke alarms in adjacent locations.

Although smoke detectors prove more effective when placed high, heat alarms work equally well whether mounted low or high. Whereas smoke alarms must be wired into an electrical system, heat alarms are battery-powered. Also, smoke detectors suffer from shorter ranges than heat alarms since the latter can detect temperatures far beneath the surface. Finally, combining both types of alarms produces a synergistic effect.



Spread the love
By Admin

Leave a Reply