Fire/Smoke Alarms
Fire and smoke alarms are essentially the same basic thing. They detect something, and alert you, and/or the services required.
However, they use different methods, and some of them do not contact outside of the location in which they are placed.
Smoke alarms within a residence can be wired, or non-wired. Wired smoke alarms can be only wired so that if one goes off, they signal the others within the residence to also go off. This allows for the notification of the occupants, allowing for the egress of the residence to a safer location. The other wired type, can also be wired into an alarm system. These can usually notify the different emergency services such as the fire department, EMS, or police. Obviously, if there is a notification of a fire and/or smoke, it will notify the fire department, but it can also notify EMS to have them just in case of a fatality or injury. A burglar alarm would be what is also wired into the system, and that allows for the notification of the police.
Some smoke detectors use a tiny laser, bouncing off of a mirror and being reflected back into a receiver. These are known as photoelectric detectors When that receiver detects a variation past a certain percentage, it goes off. That is why steam, or cooking fatty meats and oils on your stove can set it off. Other smoke detectors use ionization, or a combination of the two. Funny enough, these use radioactive material between two metal plates. The radioactive material causes the air between the plates to become an ionization chamber, allowing an electrical current to pass between the two metal plates. When the smoke disrupts the flow by absorbing alpha particles, the current is disrupted. The pattern is when a normal current, whether light, or electrical current is disrupted, this causes an alarm. Keep that in mind for the fire alarms.
If you’d like to know the best places to install the smoke detectors, here is a small list.
On every floor
Inside every bedroom
Outside of each sleeping area
Within the kitchen, or cooking area
Fire alarms within a commercial space are far more complex, but do the same basic thing. They can activate an alarm and notify the emergency services. The ways of detection are by only heat, or smoke.
Fire detection itself is a bit of a misnomer as they are detected by heat, or the resulting smoke from a fire. However, there are some in research and development that uses infrared to view flickering flames. These are still in the future, and as much as this is a fascinating technology, the ones used now only use the two other detectors. There are multi-functional detectors, but this comes at a steep price in comparison to single function detectors.
Heat detectors come in different types. These are usually best explained as one time use sorts of detection. For example, the fusible metals links used for fire dampers, or the inside of pre-engineered systems installed in restaurant hoods, melt at a specified temperature that allows for a change in stored mechanical energy. I.e. the damper door being pushed closed by a spring/gravity, or the line used to pull the pre-engineered systems links taunt. You can see more on pre-engineered systems on the resource page dedicated to them. The glass rods at the end of fire sprinkler heads are designed in the same way. They will break at a specified temperature. When they break, the pressurized water held back by the head of the sprinkler system then comes out, putting out the fire.
Heat sensors can be electronic, mechanically pneumatic, or mechanically bi-metallic. Electronic heat sensors contain a thermistor which changes resistance as the temperature increases. Bi-metallic mechanical alarms have two different metals made into a strip. When the temperature rises high enough, the metals twist and distort to make contact. Pneumatic detectors consist of a chamber filled with air surrounded by a movable diaphragm. When the temperatures rise, this makes the air within the diaphragm expand, and thus deforms the diaphragm. When the deformation occurs, it triggers contacts and sets off the alarm.
There are also rate of rise heat (ROR) sensors, alarms which go off when the rise of temperature exceeds 12-15 degrees Fahrenheit per minute.
The actual fire alarms installed within commercial locations have a centralized Fire Alarm Control Panel. (FACP) All of the different sensors connect to the main panel either directly, or through a secondary panel. The FACP is used to contact the emergency services, and outside monitoring services. The connections used are cellular, and internet based. They are required to be redundant, so that they are non-fail. This requires them to have battery power in case of power loss, and the ability to connect in case one or the other fails for whatever reason. There is plenty more of information, but this is usually held within NFPA, the manufacturer manuals, and the like.