Supplier Requirements question from IET Safety Critical Systems Course

Posted on 02/09/2016

The IET Safety Critical Systems Course 2016 at Wyboston Lakes allowed me (David Green) and my colleagues from ESC to present on a number of topics relating to how to interpret and apply the IEC 61508 standard. The topics covered included understanding of technical requirements, management of the processes, assessment and competence issues.  As a result of my presentation on Functional Safety Management it became clear that passing requirements to suppliers was an area in which people started to think about what they do.

David Green

This requirement has been in the standard since its inception and I have come across different examples of how people pass information to their suppliers. These range from ‘I need a SIL2 system’ through to very detailed Safety Requirements Specification (SRS) documents for the equipment / systems being supplied.

Many people can be surprised when a Functional Safety Assessment is conducted by someone independent of the project, that what has been supplied is inadequate for their requirements.  This can result in companies, including those who thought that they had covered everything adequately, to have to conduct more work such as re-running random hardware reliability calculations or changing the installed equipment.

The Standards

IEC 61508-1:2010 requires that you pass on the requirements for your safety system to the suppliers that you ask to supply the equipment.

Clause 6.2.17 states:

‘6.2.17 Suppliers providing products or services to an organization having overall responsibility for one or more phases of the overall, E/E/PE system or software safety lifecycles (see 6.2.1), shall deliver products or services as specified by that organization and shall have an appropriate quality management system.’

What supplier requirements do I need to give to the supplier of MY safety system?

The ideal is to supply an SRS document for the equipment / system being supplied with as much data as possible (See clause 10 in IEC61511). However, this won’t cover everything.  You also need to specify the documentation you require to be delivered.

End users will also need to provide (not exclusively):

• SIL requirement (or PLr for machinery to ISO 13849);
• Probability of Failure on Demand (PFD) requirement;
• Maximum acceptable testing intervals for your installation;
• Failure mode actions (what do you want to equipment to do in fault);
• Services available (utilities, power etc..);
• General information regarding the application (including any specific requirements due to the process or environment e.g. corrosive/erosive, vibration, heat, humidity etc..)

Why should I bother?

It may appear that this is something that is adding lots of extra work to your procurement process. However, you need to consider whether the additional (and often costly) effort to fix something later in the process is something that you are willing to accept. It may be that this issue prevents the start-up of the machine or plant.

Maybe your supplier has supplied exactly what you asked for.

Are the suppliers that you use likely to ask for more money to correct the system due to YOUR inadequate specification?

What do you tell your suppliers?

Based on what you have seen in here, how good would you say your requirements specification to your suppliers are?

Would you be able to answer questions about YOUR safety systems when discussions are held with auditors and/or regulatory bodies?

Do you ever confirm that your suppliers Quality System is being audited and maintained?

What next

We hope this helps clarify any questions that came from this issue.  If you’d like any further advice, please contact us.

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• Phone: +44 (0) 20 3206 5200 (Environmental Resources Management [ERM] switchboard)
• Email: info@esc.uk.net

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