A hidden failure is defined as “a permanent defect that will cause a relay or relay system to incorrectly and inappropriately remove circuit elements as a direct consequence of another switching event.” Furthermore, the defects associated with hidden failures must be capable of being monitored. The following scenarios represent what is not considered a hidden failure to help create boundaries for the definition: (1) a failure that results in an immediate trip without any prior event such as inappropriate tripping; the power system was planned to withstand the loss of any element and a trip of a single element can be tolerated; (2) a failure to trip, a subject of dependability, would just cause other protective elements to interfere and remove the abnormal condition, (3) a fault that isn’t monitored would be hidden from protective systems, but this would be considered ineffective protection design; (4) a defect or malfunction that occurs at the instant of a nearby fault may inappropriately trip another line but is also due to ineffective protection. Failure that cannot be monitored is not considered a hidden failure, nor is a defect of malfunction at the instant of switching effect or fault, because this failure wasn’t
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