Comments about technological history, system fractures, and human resilience from James R. Chiles, the author of Inviting Disaster: Lessons from the Edge of Technology (HarperBusiness 2001; paperback 2002) and The God Machine: From Boomerangs to Black Hawks, the Story of the Helicopter (Random House, 2007, paperback 2008)

Friday, January 6, 2012

Deadly Elevator Accident: Interlocks under suspicion

Thoughts on information that's been released about the circumstances of Suzanne Hart's death December 14 in an elevator at the Young & Rubicam office building, 285 Madison Avenue, NYC. Here's a photo of the building entrance from NY Daily News, with an NYPD Emergency Service Unit heavy-rescue truck parked outside:
The office building opened in 1926. The accident happened in an elevator in the lower-floor bank, covering floors 1-12. The elevator is a traction device, which rolls along steel tracks while suspended from a steel cable or belt. The prime mover is an electric winch with electronic controls. 

Being licensed by NYC's Department of Buildings, and inspected by contractors, it's supposed to have multiple safety devices that prevent doors closing on people, and to prevent the car from moving without proper command.

December 13: A company called Transel Elevator comes to the building to perform routine work on the elevator. Here's a link to a Transel site. Transel advertises its reliance on non-proprietary control systems for construction and repairs.

December 14: Transel puts the car at 285 Madison Ave. back into service in the morning. It's unclear how much time passes, but at about 10 am the elevator car is on the ground floor, with two passengers inside, its doors open. As Hart begins to step in, a passenger presses the button for an upper floor. The car shoots upward when Hart is halfway in and pins her at the second floor, between the top of the hoistway opening and the floor of the car. The other two passengers in the car are trapped in this horrifying space for over an hour, until emergency workers get enough control of the machinery to extricate them. One has sued for emotional distress. 

There are reports that the impact was so violent the other elevators in the bank had to be checked for damage.

I looked through accounts of other mishaps involving uncommanded elevator movements, and what turned up. Here's a list of factors that investigators (including a firm hired by the Department of Buildings, and consultants hired by the parties) are likely to check.

Timing of events: After the elevator went back into service that morning, were any trouble reports posted before the fatality?

Electronic controls:  These include control panels up in the machinery room, near the hoist motor. Were jumper wires still in place from the repairs? Jumpers -- temporary wires with alligator clips at each end -- are a possible cause when electronics go crazy soon after repair work on it. It's not necessarily a problem to use jumpers for diagnosis while the car is out of service, but the machine must not be restored to service with jumpers still in place; they could short-circuit the interlocks that keep passengers safe.

Counterweights: Were these in proper balance, such that the car had no strong tendency to lurch upward when lightly loaded?

Overspeed governor: Probably not relevant here given the very short distance of travel, but these devices clamp the car to the rails when velocity exceeds a set value, commonly 125% of the maximum speed. They're standard on traction systems to guard against excessive downward speeds, in case of cable break. Some models protect against upward movement.

Doorway detectors and associated interlocks: Depending on the model, these can include photoelectric light beams, safety edges on the doors (the gadgets that look like retracting bumpers at the door edges), and infrared curtains. Any of these should have detected that Hart was in the doorway, which with safety interlocks should have kept the doors from closing. And with the car doors and hoistway doors standing open, the elevator shouldn't have moved at all. 

Thus the recent speculation about some kind of serious problem remaining in the electronics at 10 am.

1 comment:

  1. Doorway detectors and associated interlocks: Depending on the model, these can include photoelectric light beams, safety edges on the doors (the gadgets that look like retracting bumpers at the door edges), and infrared curtains.
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