Conventional Initiating Devices
Pull Stations
Conventional Pull Stations are very simple in the way they work. Conventional Panels will simply activate when they detect a short circuit on the IDC (Zone). Because of this, Conventional Pull Stations work like a simple light switch. When activated, a switch inside the device causes a short circuit, activating the panel. The pull station is designed to be locked in activation mode and cannot be reset with out a screw or key.
Smoke Detectors
Smoke detectors work a little bit differently, because they require power to operate automatically. For two wire detectors, this means they cannot create a short circuit or they will not function. Instead, upon activation, they have an increase in current flow which the panel will still see as enough to activate, but the detector allows for enough power to keep it and sometimes other detectors active.
Four wire detectors work a bit differently, and more often than not are seen on security systems rather than fire, but it is still good to know. Four wire detectors take in both external power, and the IDC (Zone) wires separately. This means that when the detector activates, it causes a short on the IDC, but still has full operation power.
Heat Detectors
Conventional Heat Detectors are often a mixture of the two types of operation as listed above. Some may be fixed, some may be rate of rise, and some are both! Fixed detectors work like pull stations. When they reach a certain temperature, a link in the device melts away, and a spring causes the short. Many fixed heat detectors cannot be re-used, and must be replaced if tripped. Some newer detectors use a thermistor which when cooled, allows the device to be reset.
Rate of Rise, however, will use power of the IDC to activate. Similarly to the Smoke Detector, when the temperature increases at a specific rate, the device will go into an active state. These can be reset and reused when they cool down.
Most common, modern conventional heat detectors are a combination of both fixed and rate of rise. If the rate of rise portion activates, it can be re-used. However, if the fixed portion activates, the device will need to be replaced.
Others
Of course, there are other devices out there. Most of which work by creating a short circuit. This includes CO detectors, which take external 24 volt power to operate, and short a different circuit when activated. Water-flow detection, which will cause a short circuit when water is flowing. And tamper switches, which can cause an open circuit (or short if tied to a supervisory zone) when activated. There are many more out there, but most of the time the panel is just looking for a short circuit.