At its simplest a functional safety system detects a potentially dangerous condition and causes corrective or preventative action to be taken.
Typically a system will comprise a sensor which provides information on the value of a variable, a processor which compares the value with a predetermined limit and initiates action and an actuator which either corrects the variable or performs an emergency function.
Apart from the reference to danger, this description could be applied equally to the process controls for the equipment performing its function. In the past it was regarded as essential that the safety function operated independently from the process function. The advent of microprocessors enabled vast amounts of data to be collected and analysed in real time thus providing the possibility of sophisticated safety systems including such features as self-diagnosis. With many different parties involved in the specification, design, manufacture, installation, operation and maintenance of safety systems, the need for a standardised approach was recognised and IEC 61508 is the result.