Functional Safety 2014

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November 2014

Page 4 of 14

It is not easy or even impossible to obtain reliability data for systematic failures since the causes of

failure are widespread even within a particular industry. Currently there is a limited amount of

research failure data available for certain typical failures but these should only be used for reference

purposes.

In accordance with IEC61508-2 there are two types of system: Electro-mechanical systems are

classified as type A, i.e. they do not consist of any microprocessor or programmable electronic

functions (see 7.4.4.1.2 for detail); and Programmable Electronic systems are classified as type B (see

clause 7.4.4.1.3 for detail). Both types are subject to Systematic failures, though some types of

Systematic failure will only occur with type B systems (Fig. 1).

Fig.1 Types of failures

Safety Integrity

Safety Integrity is defined as “The probability of a Safety Instrumented Function satisfactorily

performing the required safety functions under all stated conditions within a stated period of time.”

IEC61508 states

“In determining safety integrity, all causes of failure (both random hardware failures and

systematic failures) that lead to an unsafe state should be included.”

With reference to a UK HSE study

on why control systems go wrong, most incidents happen because

of errors in than one phase of the safety lifecycle (analysis, realisation and operation). The analysis

also shows that the majority of the incidents were not caused by any failures of a device or control

system but resulted from systematic failures. Fig. 2 shows the percentage of primary causes

attributable to each phase of the lifecycle. The survey also shows that more than 80% of the failure

causes are attributable, in some degree, to human errors.

UK Health and Safety Executive, “Out of Control”, 2003