TUALATIN, OR, MAY 19, 1999

New White Paper Touts Critical Safety Interlock Switch Attributes

Factors engineers should consider in new machine design or retrofit specifications

Not all safety interlock switches presently installed on machines or being offered in the marketplace are as defeat resistant as their manufacturers claim. As a result many of these switches are and can be overridden by industrial plant workers, thus exposing them to injury, invalidating the machine manufacturersí warranties, and opening the plant to potentially costly injury litigation.

This is the premise of a just-published white paper by Sentrol*, entitled Non-Contact Safety Interlock Switches Provide Added Defeat Resistance - Important Factors to Consider in Choosing a Particular Product. The white paper presents key factors that engineers should consider when specifying safety interlock switches for industrial machines.

Many switches, the paper points out, are prone to override because they activate with an actuator having only a single magnetic field. As a result, placing an ordinary magnet on the switch closes its circuitry, preventing it from recognizing an unsafe condition. The more advanced switches incorporate two activation circuits that must change simultaneously before the switch will activate.

Highly defeat resistant switches will also incorporate an "H Factor", a predefined space between the switch housing and the actuator. The switch will not operate when the actuator occupies this space; so mounting the actuator directly to the switch will not override it.

Give preference, the paper states, to switches that can be wired in series, with relays that automatically monitor circuits for changes or tampering. Also to be considered are switches that provide variable sensing ranges, and can actuate through a variety of materials.

Environmental factors are important as well, particularly a switch's ability to operate in the presence of vibration, humidity, dirt and wide temperature variations. The most reliable switches are available in a variety of housings including explosion-proof die-cast aluminum, and chemically-resistant Kynar®.

Finally, factor in the manufacturer's standard of service. Look for a high level of technical support, such as up-front consultation; and exceptional responsivness, including a commitment to provide customized switch design. The mark of a top notch supplier is a willingness to interrupt standard production to assist a customer whose machine is down.

Sentrol Industrial is a market leader in the development and manufacture of safety interlock switches and position sensors for industrial applications. Specified on thousands of machines in various industries, Sentrol Industrial switches are designed to resist defeat attempts thereby helping prevent injuries and reduce liability exposure.

*Sentrol and Sentrol Industrial are now GE Security Industrial