Dual Knock Detection in Fire Alarm Systems Explained

Dual knock detection is a fire alarm logic strategy in which the panel requires two consecutive alarm signals from the same detector, separated by a brief reset and re-sample period, before raising a confirmed fire alarm.

Dual knock differs from coincidence in an important way: coincidence requires two different devices to be in alarm at the same time, whereas dual knock requires the same device to alarm, reset, and alarm again within a defined window. The purpose is the same in spirit (suppressing transient nuisance signals before raising a confirmed alarm), but the mechanism handles the case where there is only one detector in the protected area or where waiting for a second device would lose too much time.

The technique is most useful at the device level, particularly on multi-sensor heads where the chamber response can be re-sampled on a short timer to reject brief aerosol events while still catching genuine smoke that persists for more than a few seconds. As with coincidence, dual knock should be used to suppress documented nuisance patterns rather than to mask a chronically misbehaving detector; the right answer to repeated false alarms is to find and fix their cause.

For the closely related logic, see coincidence detection. For where the strategy is configured in the system, see fire alarm cause and effect.