The Shunt Trip That Fires Before the Doors Open
A five-story office building has two traction elevators opening into a shared lobby, a rooftop machine room with one sprinkler head, and hoistway sprinkler heads at the top and in the pit. The fire alarm shop drawings show three things: the waterflow switch on the machine room and hoistway sprinkler branch immediately energizes the elevator power shunt trip relay; a heat detector in the pit initiates Phase I recall to the alternate level; and the lobby smoke detectors, one per floor, share an address with the corridor detectors covering the same floor. The elevator contractor has installed the shunt trip breaker and is waiting for the alarm contractor to 'make it work.' The building is fully sprinklered and is classified as a high-rise by the adopted code.
1. Evaluate the shunt trip sequence shown on the drawings and describe the correct one.
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The sequence as drawn is unsafe and non-compliant. Nothing may energize the shunt trip directly from a detection input. The correct order is:
- Detection (hoistway/machine room waterflow switch or heat detector) initiates Phase I recall. The car travels to the designated level, the doors open, and occupants get out.
- Only after the car has reached the designated landing and the doors have opened may the shunt trip operate and remove elevator power.
If the shunt trip fires first, the car can lose power between floors with passengers aboard - exactly the failure the recall sequence exists to prevent.
The relay logic must be interlocked so the shunt trip relay cannot be energized until a recall-complete condition is confirmed, and the elevator contractor must participate in the witnessed sequence test. Note the wrinkle for a shared lobby: a lobby detector alarm on the designated level sends the cars to the alternate level, so the interlock must follow the car to whichever level it actually reaches. Confirm the shunt trip circuit is supervised and that the waterflow switch has its own address.
2. The designer asks whether a smoke detector in the pit would be simpler than the heat detector, and which devices should trigger what. Answer both.
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Keep the heat detector in the pit. A smoke detector in the pit would take the elevators out of service on dust, dirt, and steam - a nuisance-recall machine. The condition of real concern is a fire in the pit that operates the pit sprinkler and dumps water onto the car, which is exactly what heat detection is for.
Typical device-to-function mapping:
- Machine room smoke detector → Phase I recall.
- Hoistway (top) smoke detector → Phase I recall.
- Pit heat detector → recall, and shunt trip after recall completes.
- Waterflow in pit, hoistway, or machine room → recall, and shunt trip after recall completes.
The single rule that is never negotiable: no input goes straight to the shunt trip without a completed recall and open doors.
3. Is sharing an address between the lobby detector and the corridor detectors acceptable? Justify your answer and state what you would require.
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No. The elevator lobby detector performs a distinct function and must be its own addressable device installed in the lobby.
- Its activation drives a specific, different action: recall of all cars in the group to the alternate level when the alarm is in the designated-level lobby, and to the designated level otherwise. A shared address makes that action ambiguous.
- A shared address means any corridor alarm on the floor produces the same elevator response, generating nuisance recalls and destroying your ability to troubleshoot after an incident.
- Acceptance testing requires you to activate the lobby detector alone and observe the recall - impossible with a shared address.
- A corridor detector that happens to cover the lobby area is not a substitute; lobby detection is required in addition to corridor coverage where elevator recall is provided.
Required: a revised device schedule with a dedicated lobby detector address per floor, isolated recall zones, and a witnessed sequence test from each lobby detector.