How do Fire Alarms Work

With home security cameras and burglar alarms we sleep peacefully, with the confidence that we are protected from any unwanted intrusion into our homes. True, these home security systems (home alarm systems) keep us safe from external intrusion. But what about the threats that lurk inside our houses all the time as we go about doing our daily chores? One such threat is FIRE!

What is a Fire Alarm?

A fire may break out in one room and then spread stealthily to other rooms without any noise or warning. And by the time we realize, the raging flames may have engulfed the whole house. However, amazingly the device that gives us a warning signal against the break out of a fire is the small fire alarm that lies inconspicuously attached to the ceiling of a room, keeping a watch against fire as we sleep peacefully through the night. These devices have smoke or heat detectors that sense fire and set off an alarm that lets the residents of the house or building know that there a fire in within the premises. The fire alarm sounds off giving enough time to people to evacuate the building. Hence, since they were invented and popularized, they have been instrumental in saving a large number of lives. Let's see how do fire alarms work.

How do Fire Alarms Work?

There are mainly two types of smoke alarms (fire alarms) depending upon the type of detector that is used in them. In order to understand how each type of alarm works, one needs to know the mechanism of how each type of detector detects fire and sets off the alarm. The smoke detectors used are the ionization detector and the photoelectric detector. Most fire alarms use either one or both the types of detectors.

Ionization Detectors
Ionization detectors have a small quantity of americanum-241 that serves as the source of ionizing radiation. The alpha particles that are constantly released by americanum-241 strike with the oxygen and nitrogen atoms in the air. This knocks off electrons from the oxygen and nitrogen atoms resulting in the formation of positively charged ions. Ionization detectors also have an ionization chamber which houses two plates about a centimeter apart. The battery that is used to run the alarm applies a low voltage to the plates, due to which one plate gets positively and the other gets negatively charged. The positively charged ions of oxygen and nitrogen gas get attracted to the negative plate whereas the electrons released from the atoms of these gases move towards the positive plate. This generates a small continuous electric current within the alarm. So long as this current is flowing within the detector, the alarm is not set off. However, once the circuit is broken, the alarm rings out. In case of a fire when smoke enters the ionization chamber, the smoke particles attach to the positively charged ions and neutralize them. This results in a drop in the current that triggers off the alarm.

Photoelectric Detectors
In photoelectric detectors, the alarm is set off once the straight path of light beam inside the photoelectric chamber gets disrupted by smoke particles. Photoelectric detectors have a T-shaped chamber that contains an incandescent bulb or an infrared LED which acts as a light source. There is a lens in these fire alarms that collimates the light being emitted from the light source into a beam. There is a photodiode or some other photoelectric sensor at the base of the vertical arm of the T-shaped photoelectric chamber that acts as a light sensor. In the absence of smoke, the beam of light travels in a straight path without coming in contact with the light sensor. However, once the smoke particles enter the photoelectric chamber, the beam of light strikes them and get scattered to hit the light sensor. Once the light rays strike the light sensor the alarm is triggered off.

Points to be Considered

Over 85% of the houses in the United States have fire alarm systems installed in them. Given this fact, the number of fire related casualties should have been way below the recorded statistics. Does this mean that most of the accidents caused due to fire take place in the remaining 15% of the houses that do not have fire alarms? This interpretation doesn't sound feasible. The fact is that just installing a fire alarm does not guarantee safety from fire breaking out in the house. Once installed, these devices need to be regularly checked if they are in working condition to ensure fire safety. While some fire detectors work on batteries, there are others that are powered by the mains electricity of the house. In case of the battery-run fire alarms, one should be aware if the batteries need to be replaced. (The batteries used in fire alarms usually last for a year). In case of fire alarms that run on electricity, it is always advisable to have a battery back up so that the fire alarm can alert the residents of a house in case there is a fire breakout when there is no power supply.

Now that we know how do fire alarms work, we cannot help marvel at how this small device is effective in saving lives from being engulfed in the raging flames of a fire. Even though these devices may set off false alarms at times, it is always advisable to check for a fire in case a fire alarm is set off