It is necessary to predict how systems will fail. Some of the common problems that will occur are listed below.

Component jams - An actuator or part becomes jammed. This can be detected by adding sensors for actuator positions and part presence.

Operator detected failure - Some unexpected failures will be detected by the operator. In those cases the operator must be able to shut down the machine easily.

Erroneous input - An input could be triggered unintentionally. This could include something falling against a start button.

Unsafe modes - Some systems need to be entered by the operators or maintenance crew. People detectors can be used to prevent operation while people are present.

Programming errors - A large program that is poorly written can behave erratically when an unanticipated input is encountered. This is also a problem with assumed startup conditions.

Sabotage - For various reasons, some individuals may try to damage a system. These problems can be minimized preventing access.

Random failure - Each component is prone to random failure. It is worth considering what would happen if any of these components were to fail.

Some design rules that will help improve the safety of a system are listed below.


• 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.

• Proper programming techniques and modular programming will help detect possible problems on paper instead of in operation.

• Modular well designed programs.

• Use predictable, non-configured programs.

• Make the program inaccessible to unauthorized persons.

• Check for system OK at start-up.

• Use PLC built in functions for error and failure detection.


• Provide clear and current documentation for maintenance and operators.

• Provide training for new users and engineers to reduce careless and uninformed mistakes.

[an error occurred while processing this directive]