Coincidence Detection in Fire Alarm Logic Explained

Coincidence detection is a fire alarm logic strategy in which an alarm signal is generated only when two or more independent inputs (typically two detectors in the same area) are in alarm at the same time, rather than when any single device alarms on its own.

The purpose of coincidence is to suppress nuisance alarms in environments where a single detector firing is more likely to be a false alarm than a real fire (for example, a transient cooking aerosol, a brief dust event, or an electrical glitch on one head), while still responding promptly when conditions are genuinely fire-like. Two heads alarming together is far less probable as a coincidence than as a real event, so coincidence logic dramatically improves the false-alarm rate at a small but defined cost to response time.

Coincidence is most commonly used to confirm an alarm before triggering a high-consequence output, such as a gas suppression release, full evacuation in a sleeping-risk environment, or automatic plant shutdown. It should never be used as a workaround for a poorly designed detection scheme; the appropriate fix for a chronically false-alarming detector is to relocate it or change technology, not to wait for a second one to confirm.

For the closely related two-shot pattern, see dual knock detection. For the panel logic that hosts coincidence, see fire alarm cause and effect.