Abstract
Safety must be the first priority in every industrial PLC design. Designers must take all reasonable steps to ensure that nobody is harmed in any way whilst the system is being operated. Designers must carry out a risk assessment procedure. In addition, the procedure must be documented for possible inspection at any time, to implement change in to the design to limit risks, as far as possible. They should ensure that where there are remaining risks, the use of safeguards must be implemented. In emergency, the emergency stop button should be wired to a safety relay circuit, one in which the relays use positive guidance contacts. When the emergency stop button is pressed, the circuit is designed to ensure that one relay will remain energized for a few milli-seconds longer than the other one will. The chapter explains various safety procedures, such as system start-up check. At the start up of the system after the emergency stop button has been operated or after a mains supply shutdown/failure, the safety relay should carry out an internal check to determine whether there is any fault within the safety relay.
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