Failure Due to Demand

failure occurring on demand

γ, ψ

failure of one item due to a change of its state triggered by an external event (the so-called “demand”)

EXAMPLE 1 Obtaining 2 when launching a dice is an event occurring on demand. The probability of this event

is 1/6. It does not depend on the elapsing time but only of the demand itself (i.e. the fact that the dice is launched).

EXAMPLE 2 The failure of an electromechanical relay (e.g. rupture of the spring) when it changes state depends

on the number of operations (cycles) rather on the operating time (see IEC 61810–2[49]) and this is the same for

the failure of an electronic device due to over voltage when it is switched or the blocking of a diesel engine when

it is started, etc.: these are typical examples of failures due to demands (or cycles).

Note 1 to entry: In this Technical Report two kinds of demand are considered: the periodic tests and the demand

for an actual safety action. The probability of a failure due to periodic test is a constant number noted γ and the

probability of a failure due to one actual demand of the safety action is a constant number noted ψ. Over a given

time interval, those probabilities of failure do not depend on the duration but on the number of demands or tests

occurring within this interval. The use of γ and ψ is explained in 7.3.

Note 2 to entry: This should not be confused with the “failure on demand” appearing in the term “probability of

failure on demand” (see 3.1.14, Note 2 to entry) used in functional safety standards[2] for low demand mode safety

systems. In those standards this means “failures likely to be observed when a demand occurs”.

Source: ISO/TR 12489:2013(E) Reliability modelling and calculation of safety systems. Global Standards

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