Failure Due to Demand
failure occurring on demand
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) 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 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