Safety Function in a Machine Control System

A safety function is executed by the safety-related parts of the machine control system to achieve or maintain the equipment under control in a safe state with respect to a specific hazard. A failure of the safety function can result in an immediate increase of the risks of using the equipment; that is, a hazardous condition.

The safety function is often described with multi-part requirements. For instance, the safety function initiated by an interlocking guard has three parts:

1. The hazards protected by the guard cannot operate until the guard is closed.
2. Opening the guard will cause the hazard to stop if operational at the time of the opening.
3. The closure of the guard does not restart the hazard protected by the guard.

Different types of demands can be placed on a safety function; examples of various demands placed on the safety function are: the opening of an interlocked guard, pressing of an estop, stepping onto a safety mat, and so forth. In each case, an operator is demanding that the hazard either stop or remain de-energized if it is already stopped.

The safety-related parts of the machine control system implement the safety function. The safety function is not executed by a single device, for instance, considering the guard, the interlock on the guard sends a command to a logic device, which in turn, disables an actuator. The safety function starts with the command and ends with the execution.

Typically, the safety system must be designed with a level of integrity that is commensurate with the risks of the machine. Higher risks require higher integrity levels to ensure the performance of the safety function. Thus machine safety systems can be categorized into levels of performance of their ability to ensure the performance of their safety function, or to put in other words, their functional safety integrity level (SIL).

Related articles:

• Functional Safety in Instrumentation Systems (SIS, SIF & SIL)
• Methods of Protecting Instruments from Explosive Atmospheres
• What is Safety Related Control System (SRCS)?
• Hazardous Locations Classifications