Elevator Recall & Shunt Trip
How the fire alarm system controls elevators in a fire, per NFPA 72 §21.3 and ASME A17.1. Detectors initiate Phase 1 recall — to the designated level, or the alternate level if the designated-level detector is the one triggered — and heat detectors near elevator-area sprinklers shunt-trip the power before water can reach the equipment. The elevator controller (A17.1) executes; the fire alarm supplies the signals.
Recall & shunt-trip triggers
| Trigger | Signal | Elevator action |
|---|---|---|
| Lobby smoke detector NOT at the designated level | Alarm | Phase 1 recall to the designated level |
| Lobby smoke detector AT the designated level | Alarm | Phase 1 recall to the alternate level |
| Machine room / machinery-control space detector | Alarm | Phase 1 recall + firefighters’ hat |
| Top-of-hoistway detector (only where a hoistway sprinkler exists) | Alarm | Phase 1 recall + firefighters’ hat |
| Elevator pit detector (only where a pit sprinkler exists) | Alarm | Phase 1 recall + firefighters’ hat |
| Heat detector within 24 in. of an elevator-area sprinkler | Alarm → shunt-trip circuit | Shunt trip: disconnect elevator power before the sprinkler discharges |
Recall, the hat, and shunt trip are three separate things
Recall gets the cars out of service and down to a safe floor — a life-safety move for occupants, driven by lobby, machine-room, and hoistway smoke detectors. The firefighters' hat is a warning to firefighters: it illuminates in the car when the detector that recalled the elevator is in the machine room or hoistway, signaling that fire may be threatening the elevator equipment itself, so operating the car under Phase 2 is risky.
Shunt trip is different again — it is about the sprinklers, not recall. Water and a running elevator do not mix: water on the brakes and controllers can cause uncontrolled movement. So where a sprinkler protects the machine room, hoistway, or pit, a fast-responding heat detector cuts the elevator's power before the sprinkler flows. The detector has to beat the sprinkler, which is why it sits within 24 in. of the head and is rated to respond first.
Common questions
How does elevator Phase 1 recall work?
A smoke detector in an elevator lobby (or the machine room or hoistway) sends the cars to a safe floor and holds them with doors open. A detector anywhere except the designated level recalls the cars to the designated level — the main level of exit discharge. If the detector at the designated level itself activates, the cars instead go to the alternate level, so they do not open onto the fire floor.
What is the difference between the designated and alternate level?
The designated level is the primary recall floor, normally the main exit level. The alternate level is a different floor the elevators serve, used only when the designated-level lobby detector is the one in alarm — because sending the cars to a lobby that is itself in smoke would defeat the purpose. This designated/alternate logic is the core of NFPA 72 §21.3.14.
What is elevator shunt trip?
Where sprinklers are installed in the elevator machine room, hoistway, or pit, a heat detector within 24 in. of each sprinkler disconnects the elevator's main power (shunt trip) before — or as — the sprinkler discharges. The heat detector is chosen to respond faster than the sprinkler, so the car is stopped before water can reach the brakes and controllers and cause uncontrolled movement. Reset is manual, at the machine room.
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