Safe machine design starts with Risk Assessment

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10 March 2012

Many mistakenly believe that the risk graph in the standard for safety-related parts of control systems,  EN ISO 13849-1 is “risk assessment” – it is not the primary risk assessment.

Risk assessment actually starts with the use of the standard “EN ISO 12100:2010 - Safety of machinery - General principles for design. Risk Assessment and Risk reduction.” It's only when you depend upon a safeguarding protective measure requiring control that the risk graph of EN ISO 13849-1 is used as a form of risk assessment specific to safety-related controls.


Many tasks performed by workers who operate or maintain industrial machines present high levels of risk. When building, retrofitting or interlinking machines, a systematic technical Risk Assessment should be conducted and documented, as required in the Machinery Directive (2006/42/EC) and the  general requirement for risk assessments detailed in the Management of Health and Safety at Work Regulations (MHSWR 1999).


Many mistakenly take the view that the risk graph in the standard for safety-related parts of control systems,  EN ISO 13849-1 (replaced EN 954-1 at the end of last year) is “risk assessment” – it is not. It is purely to determine the level to which risk will be reduced by a control function, where a control function is deemed necessary.  Before you get to this stage there are other more fundamental steps to take.

Below: The 'risk graph' (Fig A.1 within the informative Annex A of EN ISO 13849-1:2008)

The design of safety control systems does indeed play an important role in reducing risks, but risk assessment itself starts with the use of the standard “EN ISO 12100:2010 - Safety of machinery - General principles for design. Risk Assessment and Risk reduction.” 

The process includes

Statement of machine limits including technical specifications such as range of energy supply, speed of movement, operational frequency, and other limits related to environmental conditions etc. Note with the latest standards for safety related controls it is increasingly important to know these limits as later on when a safety control system is designed its performance may be degraded due  to wear which is directly related to operational frequency (referred to as nop).

Hazard identification - who could be hurt, how and when throughout all the relevant machinery lifecycle phases

Estimation of risk (quantification) and evaluation (if risk reduction is required) 

Hierarchical approach for risk reduction. The preference is to eliminate hazards so there is no risk, for example removing trap/nip/crush/drawing-in points. If after all this, some risks remain intolerable, the next step is to introduce safeguards and it is only at this stage you would consider guarding, and if this guarding requires interlocking then the safety-related control system standards become relevant. It is at this stage that the required level of performance  (PLr) or SIL of a safety function must be determined through the use of either EN ISO 13849-1 or  EN 62061. The PLr or SIL literally indicates the degree to which the safety function reduces the risk to an acceptable level. After safeguarding measures, the standard EN ISO 12100:2010 refers to complementary measures further reduce the residual risks to an acceptable level, through  such measures as training, signage, warning equipment (such as beacons). It is arguable that E-stops fall into this area since they should not be used as substitutes for proper safeguarding.

For builders of specific machine types there are so-called C-standards (such as the EN 415 series covering packaging machines) which also provide guidance on the risk assessment and risk reduction associated with these specific machines.

The last points to make about risk assessment are that it is an iterative process, and that it is often good to get a second pair of eyes to help you. Over-familiarity with a machine can leave you blind to hazards which may be obvious to others.

To learn more about this, sign up for one of the Machinery Safety Alliance seminars!



Tags: Hierarchical approach for risk reduction