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TR 62380 IEC:2004(E) – 15 –
The appropriate uprating factors cannot be given in this handbook: they depend on the prior
experience of a company and how new the equipment production process is (for example, for
unconfirmed failures, the uprating factor ranges from 10% to over 100%, depending on newness).
4.4.4 In cases where conditions are not yet known default conditions can be assumed.
According to 4.5.1, reliability prediction calculations should begin as early as possible, at the start
of the equipment design phase, even if not all the applicable conditions can yet be known: in this
case default values can be used provisionally, to help determine those conditions which are as yet
unknown. These default values will then be gradually discarded as the definitive conditions are
identified.
This method is far preferable to the simplified calculation method (for which all the values are
replaced by default values, including those, which are already known).
The calculations must therefore be prepared in such a way as to enable values to be modified
easily.
4.5 Uses and aims of a reliability prediction
4.5.1 Reliability prediction as an aid to equipment design
The most beneficial use of a reliability prediction is as an aid to equipment designers, In this case,
the help is based on determination of the stresses and factors influencing the reliability of each
component (temperature, input voltage, technical construction of the components, etc.). Predictions
based on this handbook will lead the originators of a new design to choose the best conditions and
the best component families, and to draw up component qualification or evaluation programmes.
If this important objective is to be met, it is essential for the reliability prediction to be begun at the
very start of design, by the design originators, and then revised as required. The work should be
carried out in close collaboration with the company's component quality experts.
4.5.2 Reliability prediction to assess the potential of new equipment
The predicted reliability can be compared with the reliability objectives or stated requirements.
4.5.3 Predicted reliability values as a basis for contractual reliability values
The contractual value of a failure rate must be determined on the basis of the predicted value; these
two values will not necessarily be equal: a number of contractual values may be assumed
depending on observation period or certain data may be modified provided it is justified. However,
in all cases, the predicted value should be taken as the base.
4.5.4 Where used in conjunction with other characteristics of a project (electrical characteristics,
weight, etc.), the results of a reliability prediction can be used to compare different project
solutions, such as when evaluating proposals from tenderers. Comparisons of this kind are possible
only if the data used is the same, hence the existence of a reliability data handbook.
4.5.5 The predicted failure rates for the individual items of a system are crucial when calculating
system dependability and reparability.
4.5.6 Reliability predictions can be used as a basis for evaluating stocks of equipment and spare
components required for maintenance (however, in this case, it is important to take account of
probable non-intrinsic failures, as was explained in 4.4.3). The purpose of a study of this kind is to
optimize stocks of spare parts (avoid stock outages, but also avoid excessive and costly stocking
levels).
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