"This part of IEC 61810 covers test conditions and provisions for the evaluation of endurance tests using appropriate statistical methods to obtain reliability characteristics for relays. It should be used in conjunction with IEC 61649. This International Standard applies to electromechanical elementary relays considered as non-repaired items (i.e. items which are not repaired after failure)@ whenever a random sample of items is subjected to a test of cycles to failure (CTF). The lifetime of a relay is usually expressed in number of cycles. Therefore@ whenever the terms ""time"" or ""duration"" are used in IEC 61649@ this term should be understood to mean ""cycles"". However@ with a given frequency of operation@ the number of cycles can be transformed into respective times (e.g. times to failure (TTF)). The failure criteria and the resulting characteristics of elementary relays describing their reliability in normal use are specified in this standard. A relay failure occurs when the specified failure criteria are met. As the failure rate for elementary relays cannot be considered as constant@ particularly due to wear-out mechanisms@ the times to failure of tested items typically show a Weibull distribution. This standard provides both numerical and graphical methods to calculate approximate values for the two-parameter Weibull distribution@ as well as lower confidence limits."