Functional safety is the part of a machine's safety that depends on a control system working correctly. A guard interlock that stops the machine when a door opens, a light curtain that stops a press when a hand breaks the beam, and a thermal sensor that switches off an overheating motor are all examples of safety circuits requiring functional safety. Each example performs a safety function, which ISO 13849-1 defines as a function of a machine whose failure can result in an immediate increase of the risk. Heat-resistant insulation or a fixed guard also reduce risk on a machine, but neither depend on a safety circuit to perform correctly.

Functional safety falls into the risk reduction steps of ISO 12100. When performing the risk assessment, the designer first follows inherently safe design rules, then adds safeguarding and and then information for use for residual risks. Although the best safeguards do not require safety functions (like a fixed guard), many safeguards rely on a safety function, and the parts of the control system that carry it out are called the safety-related parts of the control system (SRP/CS) according to ISO 13849-1. They run from the input, such as a door switch, through the logic to the output, such as the main contacts of a contactor opening and removing power to a motor. Because these components can fail, the machine designer must show that the probability of this failure is at an acceptable level, which is based on the hazard it is guarding.

Two standards set out how to do this for machinery. ISO 13849-1 is used for machines using off-the-shelf components such as e-stops, safety relays, and contactors. It defines the probability of safety functions by Performance Levels (PL a to e) and can be analyzed using a free software called SISTEMA. IEC 62061 stems from IEC 61508 and should be used for more complex and custom safety circuits. For example, if you decide to build your own safety controller instead of using an off-the-shelf unit, you would need to use IEC 62061's methodology to compute the probability of the component by taking into account all of its sub-components (resistors, capacitors, MOSFETs, etc.) It defines the probability of safety functions by Safety Integrity Levels (SIL 1 to 3).

Swanson Associates provides design reviews and consultation for both ISO 13849-1 and IEC 62061 (IEC 61508) circuits, depending on the design and components.