Glossary of standard ISO 13849

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In this article, the definitions taken from EN ISO 13849 for application of the standard to the safety-related parts of the control system, SRP/CS.

Standard definitions


  • Machine control system:

System which responds to input signals from parts of machine items, operators, external control equipment or any combination of these and generates output signals causing the machine to behave in the intended manner.


  • Safety-related part of a control system (SRP/CS):

Part of a control system that responds to safety-related input signals and generates safety-related output signals.
The combined safety-related parts of a control system start at the point where the safety-related input signals are initiated (including, for example, the actuating cam and the roller of the position switch) and end at the output of the power control items (including, for example, the main contacts of a contactor).
If monitoring systems are used for diagnostics, they are also considered as SRP/CS.

  • Protective measure:

Measure intended to achieve risk reduction

Implemented by the designer: inherent design, safeguarding and complementary protective measures, information for use.

Implemented by the user: organization (safe working procedures, supervision, permit-to-work systems), provision and use of additional safeguards, personal protective equipment, training.

  • Safety function:

Function of the machine whose failure can result in an immediate increase of the risk(s).


  • Fault:

State of an item characterized by the inability to perform a required function, excluding the inability during preventive maintenance or other planned actions, or due to lack of external resources.
A fault is often the result of a failure of the item itself, but may exist without prior failure.


  • Failure:

Termination of the ability of an item to perform a required function.
After a failure, the item has a fault. “Failure” is an event, as distinguished from “fault”, which is a state.


  • Dangerous failure:

Failure which has the potential to put the SRP/CS in a hazardous or fail-to-function state.


  • Systematic failure:

Failure related in a deterministic way to a certain cause, which can only be eliminated by a modification of the design or of the manufacturing process, operational procedures, documentation or other relevant factors.

  • Mean time to dangerous failure (MTTFd):

expectation of the mean time to dangerous failure.


  • Performance level (PL):

Discrete level used to specify the ability of safety-related parts of control systems to perform a safety function under foreseeable conditions.


  • Required performance level (PLr):

Performance level (PL) applied in order to achieve the required risk reduction for each safety function.


  • Category (Cat.):

Classification of the safety-related parts of a control system in respect of their resistance to faults and their subsequent behavior in the fault condition, and which is achieved by the structural arrangement of the parts, fault detection and/or by their reliability.


  • Diagnostic coverage (DC):

Measure of the effectiveness of diagnostics, which may be determined as the ratio between the failure rate of detected dangerous failures and the failure rate of total dangerous failures.


  • Common cause failure (CCF):

Failures of different items, resulting from a single event, where these failures are not consequences of each other.


  • Mission time (TM):

Period of time covering the intended use of an SRP/CS.



More information:

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