Abstract
A unique system is introduced for monitoring the viability of conventional position switch interlocks on movable barrier guards. At closely spaced intervals a test device motor causes the guard to open slightly to check if the interlock will change state in an attempt to interrupt control power to the protected machine. The machine will not in fact shut down during the testing phase because the interlock system will be isolated from the machine operating motor contactors during the test duration. Testing of a guard is annunciated just before and during the automatic testing phase. Any failures of an interlock or its testing system will also be annunciated until their causes are corrected. When a latching system is used in conjunction with the guard, a faulty interlock will cause the guard to be locked closed until it is convenient to effect a repair. Production is not interrupted because the interlock remains bypassed during this period. If an interlock is temporarily circumvented or if the interlock guard is sabotaged, the test device motor will not cause a change in state of the interlock during the test phase; again, system malfunction will be annunciated and/or the guard will remain latched and/or the machine will be shut down.
The proposed process provides the following advantages:
• A quantum leap in system reliability and safety is obtained using even the most ordinary “limit switch” type interlocks.
• The tamper-resistance of all known interlocks will be greatly enhanced; all popular forms of temporary interlock circumvention are eliminated.
• Long term sabotage which involves simultaneous guard removal and interlock bypassing is addressed for the first time; to defeat the proposed process involves the most outrageous restructuring of the control system.
• The system will safely endure adhesive environments that may cause interlock activators to stick in their “fail to danger” states.
• The monitored machine is almost never shut down nor production interrupted during or after the testing phase even if mischief is uncovered.
• The testing of large numbers of interlocks will not expose the machines’ electric power motors to the frequent restart demand associated with some conventional manual testing procedures.
• Without compromising safety, maintenance of even multiple interlock failures may be deferred until their repair is convenient.
• The assurance of working interlocks may enable OSHA to expand applications where maintenance is performed under “control stop” as opposed to the more elaborate “lock out.”
• The design of the new test and monitoring system may be executed and implemented using straightforward hardware and software that are firmly entrenched technology.
