Some of the PLC Basic Safety Rules are
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a fail-safe design: Programs should be designed so that they check for problems, and shut down in safe ways. Most PLC’s also have imminent power failure sensors, use these whenever danger is present to shut down the system safely.
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proper programming techniques and modular programming will help detect possible problems on paper instead of in operation.
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make the program inaccessible to unauthorized persons
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use predictable, non-configured programs
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use redundancy in hardware
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directly connect emergency stops to the PLC, or the main power supply
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check for system OK at start-up
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provide training for new users and engineers to reduce careless and uninformed mistakes
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use PLC built in functions for error and failure detection
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use well controlled startup procedures that check for problems
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provide clear and current documentation for maintenance and operators
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modular well designed programs