Smart Switches Think of Safety First

Jan. 15, 1999
Advanced monitor relays and actuators use clever circuits to prevent workers from defeating machine safety switches.

Susan Kloster
Robert Schonek
Sentrol Industrial Corp.
Tualatin, Oreg.

Safety interlock switches buried within industrial machines might annoy operators at times, but they were put there for good reasons. They prevent injuries to people and damage to machines as well as to the products they make. The switches halt machines while operators’ hands are in the path of doors, punches, and other moving parts, or when they could be exposed to harmful materials, high voltage, and similar hazards.

Safety switches may be occasionally defeated for valid reasons, such as routine maintenance, operator intervention during piece work, or in the event of a machine problem. More often, however, unauthorized personnel override interlocks simply to move an inconvenient protective shield or to run a potentially unsafe operation without interference. Overriding safety switches can invalidate the machine manufacturer’s warranty, or it may cost a company thousands of dollars in litigation when someone is injured.

These issues have fueled the demand for safety switches that resist defeating. Consequently, safety device manufacturers are offering families of switches with a variety of capabilities to ensure that safety is not a gamble.

Out of harms way
Many safety switches and actuators presently installed are not foolproof. A basic actuator typically provides a single magnetic field of arbitrary strength. Their companion safety switches can often be defeated by simply exposing them to an ordinary magnet or by removing and mounting the actuator on the switch housing. The magnet or actuator closes the circuit and prevents it from recognizing an unsafe condition.

A more recent design includes two circuits in the interlock safety switch that adds an extra measure of protection. For example, the two circuits must change state simultaneously in the presence of the actuator before the safety switch output actuates. Also, when an excessively strong field is applied from another device such as an external magnet, an ancillary reed switch opens the circuit to indicate tampering.

Some magnetically actuated reed switches are made even more defeat resistant by using magnetic poles and flux paths in a nontraditional manner. These advanced safety devices also contain dual circuits behind two magnetic reed switches. But the difference between this advanced dual-circuit switch and the one discussed in the previous paragraph is in the design of the actuator. This actuator provides two magnetic fields, one for each circuit at a specified strength to activate the switch. In addition, the two fields are oriented perpendicular to the sensor housing and each must align with the two reed-switch centerlines to allow machine operation.

Some versions also combine these dual circuits and flux paths with an antitampering reed switch. The reed operates only within a particular zone of sensitivity, called the H factor. The H factor is defined as the space immediately adjacent to the switch, a gap of about 0.25 in. between the switch housing and actuator. The switch does not operate when an actuator occupies this space. Thus, mounting the actuator directly to the switch cannot override it.

Noncontact safety interlock switches also can be wired in series with monitor relays to improve the safety margin. The relay monitors the circuits for any changes that indicate tampering or switch failure and turns off the outputs. In addition, both circuits must change states simultaneously before the relay can actuate. Variable-delay, monitor and control relays also handle applications that require an adjustable time response for a specific action, such as a slowly closing door.

In addition to superior defeat resistance, switches can provide variable sensing ranges and actuate through wood, aluminum, stainless steel, or other nonferrous materials. This allows concealing the switch in a machine for protection against tampering. Finding a suitable location for the switch is simple because precise alignment between the actuator and switch is not needed, direct contact with the actuator is unnecessary, and the sensing range is variable. And, because the switch can actuate through nonferrous material, it can upgrade numerous machines not presently outfitted with safety switches.

Finally, consider the switches’ environment. The dirt, vibration, humidity, and wide temperature range encountered in many factories requires a durable switch housing. A wide variety of reliable housings are available for most applications. For instance, die-cast aluminum is needed for some explosionproof switches, stainless steel is essential in food-processing machines, and Kynar housings are intended for machines adversely affected by chemicals and related products.

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