A Hazard by any other name…

18/11/2012 — Leave a comment

Although you would expect a discipline like safety engineering to have a very well defined and agreed set of foundational concepts, strangely the definition of what is a hazard (one such) remains elusive, with a range of different standards introducing differing definitions.

So as a start point here’s my list of definitions from various sources with a commentary on the key concepts that each definition introduces. The definitions are drawn from:

  • IEC 61508,
  • MIL-STD-882,
  • DEF AUST 5679,
  • DEF STAn 00-56,
  • Nancy Leveson’s Safeware, and
  • Peter Ladkin’s Computer Safety.

61508’s definition is the most generic of all the definitions, introducing only three concepts, potentiality, source and the concept of consequential harm. No mention of accident or a linkage of a hazard to an accident.

‘Hazard. Potential source of harm.’

IEC 61508

The US standard adds a shopping list definition of types of harm and ‘real’ as well as ‘potential’ sources of harm. The definition does not link the concept of hazard to that of an accident, nor identify a ‘system’.

‘Hazard. Any real or potential condition that can cause injury, illness, or death to personnel; damage to or loss of a system, equipment or property; or damage to the environment.’

MIL-STD-882D

The Australian 5679 standard introduces both the idea of a system as an actor and an internal system state/event that is a necessary (but not complete) condition for an accident to occur. There’s a strong if unstated implication that the ‘other events’ are external to the system.

‘Hazard. A system event or state that must occur, in combination with other events, for an accident to occur.’

DEF (AUST) 5679

00-56 introduces the idea of an initiating event (sometimes) for a hazard, again implied to be outside the system. Again the concept of a system as actor and an internal state (or physical situation) is introduced but here in the context of physicality. 00-56 also uses the concept a hazard being a necessary but not complete cause of an accident.

‘Hazard. A physical situation or state of a system, often following from some initiating event, that may lead to an accident.’

DEF STAN 00-56 Issue 4

Nancy Leveson’s definition is similar to the 5679 and 00-56 examples, however stated somewhat differently in that a system’s hazard state (or set of conditions) when combined with conditions in the environment will inevitably lead to an accident.

‘Hazard. A state or set of conditions of a system… that, together with other conditions in the environment of the system… will lead inevitably to an accident.’

Nancy Leveson (Safeware)

BTW I highly recommend Nancy Levesons Safeware to anyone working on the design of complex or software intensive safety critical systems.

Peter Ladkin uses phenomena (covering both states and events) rather than state or condition, he also introduces the concept of environmental and system hazards as a base taxonomy and states that a hazard must have probability of less than unity (certainty).

A phenomenon of a system, or its environment, or both, which substantially raises risk, although the likelihood of an accident still remains less than certain. A system hazard is a system phenomenon which is a hazard. An environmental hazard is an environment phenomenon which is a hazard.

Peter Ladkin (Safety of Computer Based Systems, Ch 6)

And for completeness here’s the dictionary definition of a hazard.

‘Hazard. 1. n. dice game; chance; danger, risk; (golf) bunker, water or other obstruction. 2. v.t. expose to hazard; run risk of; venture (guess etc.).’

The Australian Pocket Oxford Dictionary, Second Edition

I look at the definition of a hazard in more detail here, and discuss the philosophical reasons as to why we’ve arrived at this mess here.

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