Abstract
This paper is to demonstrate that digital protective relays are designed, developed and produced for long life, as the notion of life time includes several meanings and the matter is to find out how to differentiate useful life time from mission time, operational life time and commercial life time of the protection relay that we will also call "system", and to enable the user to properly understand these values. IEC definitions within the lEVocabulary are quite specific and can help to understand all these subtleties to avoid confusions and misunderstandings. It is frequently observed that useful life time and MTTF for example, both values featured by a number of years, are incorrectly tackled and this paper will develop the subject, showing how to define all these parameters by calculation models or return of field experience as defined in safety and dependability requirements.
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