Functional Safety 2016

Institute of Measurement and Control. Functional Safety 2016

Page 1

Leveraging Systematic Capability

through the SIS Lifecycle

Ian Curtis

Siemens Process Industries and Drives

Sir William Siemens House

Princess Road

Manchester

M20 2UR

Abstract

This paper considers the importance of systematic safety integrity and the term systematic capability

and looks at some of the steps taken by vendors to ensure systematic failures are addressed

effectively during the product lifecycle for a PES logic solver. It examines how some of the various

techniques and measures employed to meet the requirements of IEC 61508 can also subsequently

benefit systematic safety integrity when implementing and deploying a SIS. It discusses systematic

capability as it relates to IEC61511 Ed 2.0 and it also looks at some upcoming trends in the market

and how these might help combat systematic failures.

Background

Investigations into historic major process incidents often identify multiple systematic failures as being

one of the main contributing factors. The cornerstone of functional safety is the Safety Instrumented

Function (SIF) and it carries the target for safety integrity - normally expressed in terms of a safety

integrity level (SIL). Achieving a specific SIL requires that both hardware safety integrity and

systematic safety integrity are addressed. The IEC 61508 and IEC 61511 standards accord

importance to both aspects but there are significant differences in the approach taken for each.

Hardware safety integrity is the more tangible because it can be quantitatively assessed whereas

systematic safety integrity requires a more qualitative approach. As engineers we are arguably more

comfortable tackling hardware safety integrity - because it can be more readily measured and

controlled - but it is just as important that systematic safety integrity is also addressed throughout the

lifecycle.

Systematic failures

Systematic failures are, according to IEC 61508, “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.”

Some examples of precursors to systematic failures might include human error in:-

The safety requirement specification.

The design, manufacture, installation, operation of the hardware.

The design, implementation, etc. of the software.

IEC 61508-4 IEC:2010 Para 3.6.6