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ISS Safety Ltd Fulton House, Fulton Road, Wembley Middlesex HA9 0TF

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ISS Safety Ltd Fulton House, Fulton Road, Wembley Middlesex HA9 0TF

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									ISS Safety Ltd                  Fulton House, Fulton Road, Wembley Middlesex HA9 0TF




Summary EN (ISO) 13849
Safety of machinery - Safety related parts of control systems


Introduction
The new standard EN (ISO) 13849-1, 'Safety of machinery, Safety-related parts of control
systems, Part 1: General principles for design replaces EN 954-1 and will be harmonised to the
Machinery Directive in 2007. There will be a transition period up to 2009 during which EN 954-1
can still be used.

Generally summarising the requirements of EN (ISO) 13949-1 it requires a five stage approach to
the design of safety-related control systems.

1. Perform a risk assessment (see ISO 14121).
2. For the identified risks, decide the design measures at the machine and/or the provision of
   safeguards to provide the risk reduction.
3. For the identified risks, allocate the safety measure (Performance Level (PL)).
4. Devise a system architecture that is suitable for the Performance Level.
5. Validate and document the design to check that it meets the requirements of the initial risk
   assessment.

This last step involves using manufacturers' data for the reliability of the components and how
they are configured in the architecture. The results can then be compared with the charts in the
annexes of the standard EN (ISO) 13849-2 Safety of machinery - Safety-related parts of control
systems, Part 2: Validation, to produce the required parameters for cross-checking against the
original assessment.

General strategy for design
From the risk assessment (see ISO 14121) of the machine, the designer shall decide the
contribution to the reduction of risk which needs to be provided by each safety-related part of the
control system (see ISO 13849-1 annex B). This contribution does not cover the overall risk of
the machinery under control, e.g. not the overall risk of a mechanical press or washing machine,
but that part of risk reduced by the application of particular safety functions.

The key objective is that the designer ensures that the safety-related parts of a control system
produce outputs which achieve the risk reduction objectives of ISO 14121. This is not always
achievable, and in such cases the designer shall provide other safety measures. The hierarchy
for the strategy in reducing risk is given in ISO/TR 12100-1:1992, clause 5.

The category and other features e.g. physical position of parts, isolation, selected by the designer
for the safety-related parts will depend upon the contribution made by those parts to the
reduction of risk, the design and the technology. The designer shall declare:

   -   which category(ies) is being used as the reference point for the design;
   -   the exact points at which the safety-related part(s) start and which it ends;
   -   the design rationale, e.g. the faults considered, the faults excluded, within the design to
       achieve that category (ies).



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ISS Safety Ltd                   Fulton House, Fulton Road, Wembley Middlesex HA9 0TF




Process for the selection and design of safety measures


Step 1: Hazard analysis and risk assessment.
Identify the hazards present at the machine during all modes of operation and at each stage in
the life of the machine by following the guidance in ISO/TR 12100-1 and ISO 14121.

Assess the risk arising from those hazards and decide the approximate risk reduction for that
application in accordance with ISO/TR 12100-1 and ISO 14121.

Step 2: Decide on measures for risk reduction by control means
Decide the design measures at the machine and/or the provision of safeguards to provide the risk
reduction. Those parts of the control system which contribute as an integral part of the design
measures and/or in the control of the safeguards shall be considered safety-related parts.

Step 3: Specify safety requirements for the safety-related parts of the control system
Specify the safety functions to be provided in the control system.

Specify how the safety functions will be met and select the category(ies) for each part and
combination of parts within the safety-related parts of the control system.

Step 4: Design
Design the safety-related parts of the control system according to the specification developed in
Step 3. List the features included in the design which provide the rationale for the category(ies)
achieved.

Verify the design at each stage to ensure that the safety-related parts fulfil the requirements from
the previous stage in the context of the specified safety function(s) and category (ies).


Step 5: Validation
Validate the achieved safety functions and category(ies) against the specification in Step 3.
Redesign as necessary.

It is also necessary to validate the safety-related parts of the control system in conjunction with
the entire control system and as part of the machine. The requirements of such validation are
not within the scope of ISO 13849-1, but should be specified by the machine designer or the
appropriate Type C safety standard.

When programmable electronics are used in the design of safety-related parts of control systems,
other detailed validation procedures are required (see ISO 13849-1 section 8.4.2). Where
programmable electronics are used it may be difficult to determine with any degree of certainty,
in situations when a significant hazard can occur due to the misoperation of the control system,
that reliance on correct operation of a single channel of programmable electronic equipment can
be assured. Until such time that this situation can be resolved, it is inadvisable to rely on the
correct operation of such a single-channel device (according to IEC 60204-1:1992, 12.3.5).




                       Registered In England No. 5254536. VAT registration no. 778 0919 81.
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ISS Safety Ltd                    Fulton House, Fulton Road, Wembley Middlesex HA9 0TF




Figure 1 – Iterative process for the design of safety-related parts of control systems

                     Hazard analysis at machine
                  (ISO/TR 12100-1 and ISO 14121)


                     Risk assessment at machine
                  (ISO/TR 12100-1 and ISO 14121)
                       Step 1

------------------------------------------------------------------------------
                  Decide measures for risk reduction
                  (ISO/TR 12100-1:1992, clause 5)
-------------------------------------------------------------
   by design                                          by safeguarding
(ISO/TR 121-2:1992, clause 3) (ISO/TR 12100-1:1992, clause 4)

    Other             Control             Protective                 Other
  Measures            system            devices (part              Measures
     (not            (ISO/TR            of the control                (not
considered in         12100-               system)               considered in
 this part of      2:1992, 3.7)            (ISO/TR                this part of
 ISO 13849)                               12100-2:                ISO 13849)
                                        1992, 4.2.3)
                     By control             means

                      Step 2
------------------------------------------------------------------------------
               Specify safety requirements in terms of:

                  Characteristics of safety functions
                                 And
                   Realization of safety functions
                                 And
                     Selection of category(ies)


                      Step 3
-----------------------------------------------------------------------------

          Design safety-related parts of a control system                                      Verify
                         (clauses 4 and 6)


                     Step 4
-------------------------------------------------------------------------------

             Validate achieved functions and categories
                             (article 8)
                     Step 5



                        Registered In England No. 5254536. VAT registration no. 778 0919 81.
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 ISS Safety Ltd                             Fulton House, Fulton Road, Wembley Middlesex HA9 0TF




Figure B.1 – Possible selection of categories for safety-related parts of control systems


                                                                                      Category

                                                                       B        1        2         3   4
                       S1



                                                      P1
          1                          F1
                                                      P2



                       S2                             P1

                                     F2

                                                      P2




  Key
  1 - Starting Point for risk estimation for the safety-
      related part of the control system


 S Severity of injury
           S1 Slight (normally reversible) injury

              S2 Serious (normally irreversible) injury, including death



  F Frequency and/or duration of exposure to the hazard
         F1 Seldom to quite often, and/or short exposure time

              F2 Frequent to continuous and/or long exposure time


  P Possibility of avoiding the hazard
         P1 Possible under specific conditions

           P2 Nearly impossible


  Selection of categories B, 1 to 4

          Preferred categories for reference points

          Possible categories which may require additional measures

          Measure which can be over-dimensioned for relevant risk


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