How do Fire Alarms Work?
How Do Fire Alarm Systems Work?
A fire alarm system works by detecting fire by one or more of its detection devices. These devices send a warning message to the fire alarm control panel which in turn sends a signal through a dedicated line to an alarm receiving centre. The ARC then contacts the fire rescue service.
There are two basic types of fire alarms or smoke detectors. One is equipped with a light beam, and if smoke is in the room, it will break the light connection, causing the alarm to go off. The other is based on connectors, and if smoke breaks the connection, it will set the alarm off. 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
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. As 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.
