As an engineer, the author has had many experiences over the years, where opinions were at odds with one another, when a group were discussing a specific topic. Discussions between engineers and professionals of a different persuasion or, discussions with clients, often become somewhat misinterpreted due to a different definition of a component or function of a safety related control system.

We have compiled a list of definitions below, that by experience, we have found on occasion, to have lead to disputes or differences of opinions, when it comes to report writing and the interpretation of some technical wording, particularly when composing proposal or other contractual documents.

Normal, the author would include a small definition section within a proposal or report, either by the addition of a spate page or by footnote.

The reader should take note as to the words should, shall and must have been used  throughout the definitions. These words have been taken directly from harmonised standards published within the European Journal, therefore shall be taken literally.

“Actuating control” is the mechanical mechanism within a control device which initiate power or motion and should be constructed and located so as to prevent unintended operation. The status of the actuating controls SHALL be clearly indicated e.g. power on, fault detected, automatic operation, etc.

“Automatic operation” is the state in which a machine assembly is executing its programmed task as intended.

“Control programme” is the inherent set of instructions which defines the capabilities, actions and responses of a machine assembly.

“Emergency stop”

“Energy source” can mean sources of energy from electrical, mechanical, hydraulic, pneumatic, chemical, thermal, potential, kinetic or other source of power. A means of isolating any hazardous energy sources must be provided, which means they shall have the capability of locking or otherwise securing in the de-energised position. Where there is a possibility of “stored energy“, a means of isolating for the controlled release of this hazardous energy shall be provided AND the component clearly affixed with a label to identify this hazard.








“Guard, interlocking”


“Limiting device” means the device or component that restricts the maximum space by stopping or cause to stop all machine moving parts or manipulating devices.

“Protection level” (PL)

“Protection level, required” (PLr)

“Safety distance”

“Safety rated” is the character of a machine assembly or control system, having a prescribed safety function with a specified safety related performance. This could be either via a Protection Level (PL) from EN ISO 13849-1 or a Safety Integrity Level (SIL) from IEC 62061.

“Safety rated monitored stop” specifically used in robotic systems where the robot is stopped, with drive power active, while a monitoring system with a specified specific safety performance, ensures that the robot does not move.

“Safety related function”

“Single point of control”, again normally used within robotic systems, but can be the case in complicated automated machine assemblies, is the ability to operate the robot or machines, such that initiation of motion is only possible from one source of control and cannot be overridden from another initiation source.

“Special protective device”