Phased vs Simultaneous Evacuation in Fire Alarm Systems
When the alarm sounds, does everyone leave at once, or does the building empty in stages? For most buildings the answer is everyone at once, but for some larger or taller buildings a staged, phased evacuation is the safer and more practical strategy. The fire alarm system has to support whichever approach the building uses, and that shapes how it is designed and programmed. This guide explains the difference and how the alarm delivers it.
The essential point: simultaneous evacuation warns everyone at once; phased evacuation warns areas in stages, and the fire alarm must give the right signal to the right area at the right time.
Who this is for
This is for fire alarm engineers and responsible persons who need to understand how evacuation strategy shapes the fire alarm. The experience level assumed is competent engineer or an informed duty-holder. It explains the concepts; whether a building uses phased evacuation, and exactly how, is determined by the fire strategy and competent fire engineering, with the alarm requirements set out in BS 5839-1.
Two evacuation approaches
In simultaneous evacuation, the fire alarm warns everyone in the building to leave at the same time — the straightforward, common approach for most buildings. In phased evacuation, the building is evacuated in stages: typically the areas at greatest immediate risk, such as the fire floor and the floor above, are evacuated first, while other areas are alerted or held until their phase. Phased evacuation exists because evacuating a very large or tall building all at once can be impractical, and because appropriate construction and management can make a staged approach safe. It is a strategy choice, not simply an alarm setting.
How the alarm supports phased evacuation
Phased evacuation only works if the fire alarm can treat areas differently — sending an evacuate signal to the phase being evacuated and often an alert signal elsewhere — which means the system must control its outputs by area and by time. That behaviour is defined in the system's cause and effect and is typically implemented on an addressable system, or on a voice alarm system that can direct different messages to different zones. The design must ensure the right signal reaches the right area at the right moment, and that this is tested and verified, because a phased strategy depends entirely on the alarm delivering it correctly.
When phased evacuation is appropriate
Phased evacuation is not a default to reach for; it is appropriate only where the fire strategy, construction and management support it. That usually means larger or taller buildings with suitable compartmentation and an evacuation-management arrangement, where a staged evacuation is both safe and more practical than moving everyone at once. Whether it is appropriate for a given building is a fire-engineering judgement made in the fire strategy, not a decision taken at the alarm panel. The alarm system implements the strategy; it does not set it.
Voice alarm and the strategy
Where messages need to differ by area and phase, voice alarm (voice evacuation) comes into its own: clear spoken messages can distinguish alert from evacuate and guide occupants, which suits phased evacuation and large or complex buildings. Voice alarm is designed to its own requirements and integrated with the fire detection and alarm system within the overall strategy, under the general duty to provide appropriate means of warning and escape in the Regulatory Reform (Fire Safety) Order 2005. Whatever the approach, the principle holds: the alarm's job is to deliver the building's evacuation strategy reliably.
When not to rely on this alone
When not to use this article: do not use it to choose an evacuation strategy or programme phased evacuation for a real building. Those follow from the fire strategy, competent fire engineering and the current BS 5839-1. This guide explains the concepts and the alarm's role.
Professional disclaimer
This is an educational resource for competent engineers and informed duty-holders. It does not replace the current British Standards, the building's fire strategy or professional judgement. Confirm evacuation strategy and its implementation against the fire strategy and the current BS 5839-1.
Related reading
Read this with cause and effect in fire alarm systems, BS 5839-1 system categories and emergency voice communication and disabled refuge.