IEEE 3006.9-2013 pdf download IEEE| Recommended Practice for Collecting Data for Use in Reliability, Availability, and Maintainability Assessments of Industrial and Commercial Power Systems
3. Definitions and acronyms
3.1 Definitions
For the purposes of this document, the following terms and definitions apply. The IEEE StandardsDicfionary On/ine should be consulted for terms not defined in this clause.
Some commonly used terms in system reliability analyses are defined here. These terms are also used in thowider context of system reliability activities. These definitions are referenced in several reliabilitypublications,and the formulas can be verified in the Reliability Analysis Center (RAC) Toolkit folcommercial practices, page 12, or MIL-STD-339. Definitions include the following:
availability: (A) (general) The ability of an item–under combined aspects of its reliabilitymaintainability, and maintenance support–to perform its required function at a stated instant of time orover a stated period of time. (B) (As a performance metric for individual components or a system) Thelong-term average fraction of time that a component or system is in service and satisfactorily performing itsintended function.(C) (As a future prediction) The instantancous probability that a componcnt or systemwill be in opcration at timme t
component: A piece of electrical or mechanical equipment viewed as an entity for the purpose ofreliability evaluation.
failure (f): The termination of the ability of a component or system to perform a required function.
failure rate (2): The mcan (arithctic averagc, also known as the forced outage rate) is the number offailures of a component and/or system per unit exposure time. The most common unit in reliability analyscsis hours (h) or years (y). Therefore, the failure rate is expressed in failures per hour (f/h) or failures per yeal(fy).
inherent availability (Ai): Long-term average fraction of time that a component or system is in serviceand satisfactorily pcrforming its intended function. Ai considers only downtime for rcpair of failures. Nologistics timc,preventative maintenance,etc., is included.
maintenance down time (MDT): The total down time for scheduled maintenance (including logisticstime, including spare parts availability, crew availability, ctc) for a given total period (TP) in hours.
mean down time (MDT): The average downtime caused by scheduled and unscheduled maintenanccincluding any logistics time. This is synonymous with mcan time to restore system (MTTRS) as found irsome publications.
mean time between failures (MTBF): The arithmetic mean of the times (observed or calculated) betweenrandom failures of a component or system.
mean time between maintenance (MTBM): The average time between all maintenance events, scheduledand unscheduled, and includes any associated logistics time.
mean time to failure (MTTF): The mean exposure time between consecutive repairs (or installations) of acomponent and the next failure of that component. MTTF is commonly found for non-repairable items suchas bulbs,etc.