Abstract
There is a wide range of criteria and features for evaluating reliability in engineering; but as many as there are, only one of them has been chosen to evaluate reliability of Digital Protective Relays (DPR) in the technical documentation: Mean (operating) Time Between Failures (MTBF), which has gained universal currency and has been specified in technical manuals, information sheets, tender documentation as the key indicator of DPR reliability. But is the choice of this criterion indeed wise? The answer to this question is being sought by the author of this article.