What is part of a safety control system

Last edit: 25/01/2024

In order to be part of a Safety Control System, components need Reliability data. Not all components have them; therefore the question is: what are the components that belong to an SCS? The picture underneath, taken from ISO 12100 annex A, shows that Sensors (Ex. interlocking devices), Logic systems (Safety PLC), and Power control elements (Contactors, Valves, etc.) are part of a Safety Control System. However, Machine actuators like Engines and Cylinders are not.

This definition is valid in the case of Emergency Stop functions. Different is the situation in case an Emergency start function is implemented. Here are the definitions from IEC 60204-1 [3] annex E:

[IEC 60204-1] Annex E: Explanation of emergency operation functions

Emergency stop: An emergency operation intended to stop a process or a movement that has become hazardous.

Emergency start: An emergency operation intended to start a process or a movement to remove or to avoid a hazardous situation.

There are situations where, in order to bring the machine into a safe state, it is necessary to activate a safety element. An example can be a motor break. Another example is, in a Continuous Casting line, the interruption of the steel flow from the tundish; that can be achieved by energizing a Solenoid valve that activates the movement of a cylinder.

In both cases, the machine actuators (the brake or the cylinder) are part of the safety system; however, that case is more the exception than the rule.