Elevators: Power Shunt Trip

In multistory buildings protected by automatic fire sprinkler systems, building codes require sprinkler heads to be located at the top of the elevator hoistway to protect the vertical shaft and the elevator car inside it and in each elevator machine room.

 

Most states and other jurisdictions have adopted American Society of Mechanical Engineers (ASME) A17.1 and A18.1 standards for the installation and operation of elevators, either as published by ASME or with additions or exceptions.

 

If a sprinkler head were to discharge water into the elevator hoistway or machine room during operation, it is likely that the elevator would operate unpredictably because of water on the control devices. Before power shunt trips were required for sprinklered elevator hoistways and machine rooms, elevators that became wet from sprinkler discharge were known to run with doors open, to miss floor landings, to stop between floors, and to crash into the elevator pit or into the top of the hoistway. 

 

A power shunt trip in an elevator operates simply and reliably. Heat detectors are located in the machine room and at the top of the hoistway, within 24 inches of each sprinkler head. These heat detectors are set at a lower temperature and are more sensitive to temperature change than the sprinkler heads, and will signal that sprinkler discharge is imminent. The elevator’s main power shunt trip breaker will trip and stop the elevator car wherever it is, even between floors, so that it will not malfunction when the sprinkler discharges. This power shunt trip can only be reset manually at the elevator controller. (Some jurisdictions may permit a delay in operation of the power shunt trip so that the elevator car can move to its primary recall floor level and open its doors. See National Fire Protection Association (NFPA) Standard 72 (2010), National Fire Alarm and Signaling Code, Article 21.4 and Appendix A.21.4.1; and NFPA Standard 70 (2008), National Electrical Code, Article 620.51-B, for details.)

The ASME and NFPA standards and codes consider the occupants of an elevator car that is stopped between floors or that is evacuated at the nearest floor level to be at less risk than they would be if the elevator continued to operate during fire sprinkler discharge.

The elevator power shunt trip does not operate without warning. The complete sequence of operation and warnings begins with smoke detector that activates the Phase I elevator recall for firefighter service:

 

1. Phase I Emergency Recall is activated by a smoke detector in an elevator lobby or landing or in a machine room or at the top of the elevator hoistway. (For details, see International Building Code 3004.3; NFPA Standard 1 (2009), Fire Code 11.3; NFPA Standard 101 (2009), Life Safety Code 9.4.3; and NFPA Standard 5000 (2009), Building Construction and Safety Code, Chapter 54.) Smoke detectors in these locations will usually activate before nearby automatic fire sprinklers begin to discharge. These smoke detectors should also set off the building fire alarm and may also send an alarm to the security service and/or the fire department dispatcher.  

 

2. As required by Phase I Emergency Recall, the elevator car will return immediately to its primary recall level, open its doors, discharge any passengers, and wait at that level with its doors open.

3. Arriving firefighters may initiate Phase II Emergency In-car Operation by using a key and pressing buttons inside the elevator car to close and open doors and to move the car between floors. Photo 1 shows a typical lobby elevator control panel with the key switch to select firefighter operation. Photo 2 shows a typical elevator car control panel, with the firefighter key switch and indicator lights in the red panel in the center and operating instructions in red directly above. In this way, firefighters can quickly move equipment to upper floors nearer the location of the fire and evacuate persons who are unable to self-evacuate using stairways.

4. If smoke is present at the top of the elevator hoistway or in the machine room, the smoke detector will activate a circuit in the elevator controller. This will cause the red firefighter helmet indicator light on the control panel (see photos) to begin to flash, indicating that smoke and fire are near the elevator machine or in the hoistway and that a power shunt trip is possible. The instructions on the panel in Photo 2 state, “WHEN [fire helmet] FLASHES, EXIT ELEVATOR.” Firefighters at this time have the choice of manually opening the elevator doors at the nearest floor and exiting the elevator or attempting to return the elevator car to the primary recall floor with the risk of becoming trapped in the elevator between floors when the power shunt trip breaker operates.

5. When significant heat is present at the top of the elevator hoistway or in the machine room, the heat detector near a sprinkler head will sense that sprinkler discharge is imminent and will cause the main elevator power to shunt trip without delay, without any action by the elevator controller. This power shunt trip breaker can only be reset manually in the elevator machine room. Disconnection of the elevator main power does not affect lights or communications with the elevator car, which are on separate circuits. 

The “flashing fire helmet” indicator light is present on the control panel of all elevators installed from 2004 to the present (and in older elevators whose controls have been made compliant with current building and elevator codes), which have automatic fire sprinklers in the hoistway and/or machine room. The lobby control panel shown in Photo 1 also has this feature.

 

If firefighters are alert and know the elevators in the buildings in their response area, they are unlikely to become trapped between floors by an elevator power shunt trip.  

The author acknowledges the valued assistance with the research for this article by Brian Rausch PE, at the Wisconsin Elevator Safety Program of the Wisconsin Department of Commerce.