Page "Fire alarm notification appliance" Paragraph 10

Coding refers to the pattern or tones a notification appliance sounds in and is controlled either by the panel or by setting jumpers or DIP switches on the notification appliances.

The majority of audible notification appliances installed prior to 1996 produced a steady sound for evacuation.

In general, no common standard at that time mandated any particular tone, or pattern for audible fire alarm evacuation signals.

While less common than a steady sound, differing signaling methods were used for the same purpose.

These are named with respect to their distinctive structure and include, March Time ( usually 120 pulses per minute but sometimes at 90 pulses or 20 pulses per minute, depending on the panel ), Hi-Lo ( two different tones that alternate ), Slow-Whoop ( slow rising sweep upwards in tone ) among others.

Today these methods are confined to applications intended to trigger a response other than evacuation alone.

In 1996, the ANSI and the NFPA recommended a standard evacuation pattern to eliminate confusion.

The pattern is uniform without regard to the sound used.

This pattern, which is also used for smoke alarms, is named the Temporal-Three alarm signal, often referred to as " T-3 " ( ISO 8201 and ANSI / ASA S3. 41 Temporal Pattern ) and produces an interrupted four count ( three half second pulses, followed by a one and one half second pause, repeated for a minimum of 180 seconds ).

CO ( carbon monoxide ) detectors are specified to use a similar pattern using four pulses of tone ( often referred to as T4 ).