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Defeat the Safety Interlock, Lose a Hand

Defeat the Safety Interlock, Lose a Hand

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The human element is often the weakest link in the pursuit of safety — something frequently illustrated by workers bypassing a critical safety device. Common excuses for such modifications include convenience, the desire for better throughput, false switch actuation, and poor switch reliability. Equipment manufacturers may not be able to control the actions of equipment users, but they can reduce the temptation to disable critical interlocks by striving to improve the human factor performance of their products.

A worker at a non-profit paper recycling facility lost a hand and part of his arm when he got caught between a moving compression platen and the access gate of a paper baling machine. Although the injured man stated that the platen fell suddenly and without warning, an examination of the hydraulically operated platen did not reveal any malfunction. Investigators concluded the worker had instead attempted to continue loading paper into the compression chamber after the compression cycle had initiated. 

Ciro Ramirez
Ciro Ramirez

The machine was equipped with a gate sensor which should have prevented platen actuation while the access gate was open. The sensor had been bypassed, allowing the compression cycle to start while it was still possible to reach into the chamber. 

The machine had been supplied by a commercial firm which was responsible for maintenance and the training of machine operators. It was in overall poor condition, indicating that little or no effort had been made to inspect and maintain it after delivery to the facility. The low level of technical knowledge demonstrated by the workers at the facility also indicated that they probably could not have bypassed the sensor. Thus investigators concluded the machine had already been modified before it was delivered and that the modification should have been discovered and corrected at that time.

The sensor that had been bypassed was an example of an interlock, usually electrical, that prevents machine operation when a guard is open. Doors on CNC equipment and gates that prevent access to the point of operation on punch presses typically come equipped with these safety measures. Such interlocked guards can prevent injury when the operator is careless or poorly trained. It is important for these devices to be inspected regularly and kept in good operating condition.

Emergency power-off devices, or EPOs, are a different type of safety device which let workers quickly shut down a machine in the event of a malfunction or other event. An EPO may be the familiar large red button or may take other forms. 

In another case of defeated safety measures, a worker was using a truck-mounted drilling rig to drill a hole when his clothing became entangled in the rotating drill, severely injuring him. Complete guarding of the drill is impractical because of the nature of the drilling operation. 

Two wobble switches which could stop the drill in less than one full rotation were mounted to either side of the drill tower. In an emergency, a worker need only touch one of these EPO switches to stop the drill. A long wand actuated each switch when deflected in any direction. Unfortunately, the wands on the switches for the incident rig had been cut off, making it impossible for the worker to stop the drill in an emergency.

It was never clear what the motivation was for cutting off the wands. One can only conclude that it must have seemed like a good idea at the time.

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This file type includes high resolution graphics and schematics when applicable.
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