EN ISO 13849-1What is a safety related control system?A control system in a machine should beregarded as being safety-related if itcontributes to reducing the occurrence of ahazardous situation or if it is required tofunction correctly to maintain or achievesafety. The functions carried out by asafety-related control system are termedsafety functions.

Generally safety functions either prevent theinitiation of a hazard or detect the onset of ahazard. Safety-related control systemsshould be designed and configured to bereliable (bearing in mind the consequencesof any failure) and to perform the necessaryfunctions to achieve or maintain a safe stateor mitigate the consequences of a hazard.The main standard to use from a machinerysafety perspective is EN ISO 13849-1.

BS EN ISO 13849-1

Provides safety requirements and guidanceon the principles for the design andintegration of safety-related parts of controlsystems, including the design of software. Forthese parts of safety-related parts of controlsystems, it specifies characteristics that

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include the performance level required forcarrying out safety functions. It applies tosafety related parts of control systems,regardless of the type of technology andenergy used (electrical, hydraulic, pneumatic,mechanical, etc.), for all kinds of machinery.

BS EN ISO 13849-1 is intended to giveguidance to those involved in the designand assessment of control systems onaspects such as categories or performancelevels specification. As part of the overallrisk reduction strategy of a machine, adesigner will often choose to achieve somemeasure of risk reduction through theapplication of safeguards employing one ormore safety functions. Parts of machinerycontrol systems that are assigned toprovide safety functions are called safety-related parts of control systems and thesecan consist of hardware and software andcan either be separate from the machinecontrol system or an integral part of it. Inaddition to providing safety functions,safety related parts of control systems canalso provide operational functions (e.g. twohanded controls as a means of processinitiation).

EN ISO 13849-1EN ISO 13849-1EN ISO 13849-1EN ISO 13849-1EN ISO 13849-1EN ISO 13849-1EN ISO 13849-1

The initial part of the assessment requirescarrying out a risk analysis to identify thesafety performance level (PL) appropriateto the hazards associated with themachine, a procedure that is covered by ENISO 13849-1. The engineer will then designa control system to meet this PL, byconsidering the category, carefullyselecting the components used and,carrying out detailed calculations involvingthe mean time to dangerous failure forthese components, along with diagnosticcoverage and common cause failures.

ValidationHaving defined the performance level, thenext step is to validate this, to ensure thatthe design and specification of equipmentused will in fact meet the performance leveland the requirements of the standard.Validation is covered by EN ISO 13849-2which states:

“The validation shall demonstrate that eachsafety-related part meets the requirementsof ISO 13849-1, in particular:

• the specified safety characteristics of the safety functions provided by that part, as set out in the design rationale, and

• the requirements of the specified category [see ISO 13849-1, clause 6]

Validation should be carried out by personswho are independent of the design of thesafety-related part(s).”

The validation process should re-examinethe design, look at the implementation ofthe safety related parts of a control systemand, in some cases, verify its functionalityby testing. Validation must also take intoaccount the environmental conditions inwhich the machine will operate and be fullydocumented to show that it has beenproperly carried out.

SolutionValidation, while mandatory, is a far fromtrivial exercise. In fact, many machinemanufacturers may well find that they lackthe in-house resources and expertiseneeded to properly validate the safetyrelated parts of a control system in theirproducts. In such cases, the services of anexpert consultant, such as LaidlerAssociates, will be required. An additionalbenefit of using services of this type is thatthe requirement for validation to be carriedout by persons who are independent of thedesign process will be automaticallysatisfied.

Laidler Associates can provide advice andassistance with all aspects of the processof complying with EN ISO 13849-1 includinginitial risk analysis, validation anddocumentation.

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