Fire alarms are normally activated by heat or smoke detectors. However, they can also be activated with manual controls, or with flow sensors in fire sprinkler systems. There are two main ways in which a fire alarm can malfunction:

Activating by accident when there is no fire, causing a false alarm.
Not activating when a fire occurs, which is much more dangerous.
A false alarm is disruptive for a building, since it can cause an unnecessary evacuation and suspension of activities. To prevent false alarms during installation, fire protection engineers recommend installing all other system components before the communication lines. Frequent false alarms must be prevented at all costs, since they can confuse occupants. When there is a real fire incident, occupants may not evacuate believing it’s another false alarm.

Not activating during a fire is a much more serious issue in alarm systems: this leaves less time to evacuate, and the fire department is not notified immediately. A malfunctioning fire alarm system can also have legal consequences, and insurance coverage may be voided.

In addition to being reliable, a fire alarm system must offer durability. Consider that buildings last for decades, and fire protection systems should be equally durable to minimize repairs and part replacements.

Understanding the Basics of aFire Alarm System Design

As previously mentioned, fire alarm systems are designed to detect fire and smoke, and this activates signaling devices like horns and strobe lights. However, fire alarms can also activate with other inputs:

Manual pull stations, which can be activated directly by occupants. Manual stations are useful when fire or smoke are visible, but the alarm has not responded yet.
Fire alarms can also be activated by water flow sensors in fire sprinkler systems. Since sprinklers open with heat, water movement indicates that fire has triggered sprinklers somewhere in the building.
Fire alarm systems can also alert owners and facility managers when they are away from the building. Most importantly, fire alarms can alert the local fire department, ensuring a quick response. Automatic sprinklers are recommended to complement fire alarms, even when not required, since they can extinguish many fires on their own.

A fire alarm system has three possible states: normal, alarm and trouble. The current state is displayed permanently by the control panel.

Fire alarm systems are in the normal state most of the time, when no fires or performance issues have been detected.
The alarm state indicates that a fire has been detected - directly with smoke or heat sensors, or indirectly by measuring flow in the sprinkler system. The system also enters the alarm state when a manual pull station is used.
The trouble state indicates there is an issue that prevents the alarm system from functioning properly. A trouble state is often caused by an open circuit, which interrupts the power supply for system components.
A fire alarm system in the trouble state cannot detect fires reliably, and the cause must be found and solved as quickly as possible. If a fire occurs when the alarm system is in the trouble state, the building owner can face serious legal consequences, and the insurance company may refuse coverage for any damage caused.

Open circuits can be detected with an end-of-line resistor. The resistor is connected at several points between the last device and the control panel, while voltage and current are measured. The fault location can be pinpointed based on how voltage and current respond at different connection points.

A fire alarm system also enters the trouble state if there is a communication issue that prevents notifications. The system sends test notifications at regular intervals, to ensure all communication systems are working properly.