A FALSE SENSE OF SECURITY

A printing press operator was proudly demonstrating a new printing press to his family at an open house intended to display the state-of-the-art safety devices engineered into the new equipment. One of the high-tech features was a photoelectric sensing device that was engineered to detect any object (such as the operator’s hands) that had violated the danger zone at the in running nip point to the printing rolls. The system was designed to immediately stop the rolls whenever an object was sensed. So proud of the design feature of the system was the operator that he demonstrated by thrusting his hand repeatedly into the danger zone. 


Ultimately, he succeeded in beating the system, and the printing press did indeed amputate the end of one of his fingers. Incredible as this case study may seem, this incident actually happened. One could question the judgment of the operator in tempting the machine in such a foolish way, but the tendency exists to trust the engineering implicitly. Thus, workers are exposed to hazards due to the false sense of security that engineering systems sometimes engender.Such false sense of security can even lead to new operator procedures that depend on the safety device to control the operation so that the work can be hastened. The best example that comes to mind is the hoist limit switch on an overhead crane. If the hoist load block approaches too close to the bridge, the hoist limit switch is tripped,

Pneumatic press rams pins operator’s hand on the upstroke; two-hand control safety device prevents the operator from reactivating the press to release her hand. Shutting

off the hoist motor. The idea sounds good, but the operator can take advantage of the device by depending on the switch to stop the load during normal operation.

The hoist limit switch is not intended as an operating control, but workers can and do use it that way. The only defense against such use appears to be proper training and safe attitudes on the part of the operator—that is, the psychological approach.

Finally, the engineered system can sometimes cause a hazard, as illustrated in the example that follows in which a pneumatic press rams pinned an operator’s hand on the upstroke. (See Figure 3.1.) The press was equipped with a two-hand control that was designed for safety’s sake not to allow the press to be actuated except by both hands of the operator. Ironically, the two-hand control created a hazard.This press was later redesigned to place a shield in front of the ram so that the operator would be unable to reach into the area above the ram. Foot controls for machines provide a good example of the conflicts that arise between the hazards that engineering controls are designed to prevent and the hazards that they create.


Accidental tripping is a problem with foot controls, so engineers have fashioned enclosures into which the operator must insert his or her foot before stepping on the control itself. The problem with these enclosures is that they make the process of activating the foot trip more complicated. More operator attention is required to move the foot in the right ways to get it inside the enclosure and then operate the pedal. Supposedly, this is good, because then a careless motion will not accidentally actuate the machine. However, because of the sometimes awkward additional motions that the enclosure requires, some operators position their feet so that they can keep a foot on the pedal at all times, so-called “riding the pedal.” Unfortunately, riding the pedal increases the likelihood that the operator will accidentally trip the machine, the very hazard that the foot pedal enclosure was intended to prevent.

This problem has been studied extensively by Triodyne, Inc. (ref. Barnett).

As another example, robots are being used to work in hot, noisy environments, to lift heavy objects, and to otherwise serve in places where humans might be injured or suffer health hazards. Most industrial robots are simply mechanical arms programmed by computer to feed material to machines or to do welding. But the mindless swinging of these mechanical arms can cause injury to workers who get in the path of the robot. The irony is that a hazard is created by the robot, the very purpose of which was to reduce hazards. One solution is to make the robot more sophisticated, giving it sensors to detect when a foreign object or person is in its path. Another solution is simply to install guardrails around the robot or otherwise keep personnel out of the danger zone.

In summary, the engineering approach is a good one and deserves the recent emphasis it is receiving. However, there are pitfalls, and the safety and health manager needs a certain sophistication to see both the advantages and disadvantages in proposed capital equipment investments in safety and health systems. Upon review of the preceding examples of engineering pitfalls, it can be seen that almost every problem can be dealt with if some additional thought is given to the design of the equipment or its intended operation. The conclusion to be reached is that engineering can solve safety and health problems, but the safety and health manager should not naively assume that the solutions will be simple.