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Availability

Reliability Engineering
mechanicalelectricalprocess

Definition

The proportion of time that an item is in a state to perform its required function under given conditions, assuming that the required external resources are provided. Availability combines reliability (how often it fails) and maintainability (how quickly it can be restored). Inherent availability considers only corrective maintenance; operational availability includes all downtime including preventive maintenance and logistics.

Context

Availability is the metric that matters most to operations because it directly relates to production capacity. In reliability engineering, availability is optimized by improving both reliability (reducing failure frequency) and maintainability (reducing repair time). Reliatic tracks availability as part of the reliability analytics dashboard.

Formula

A = MTBF / (MTBF + MTTR) for inherent availability; A = Uptime / (Uptime + Downtime) for operational

Examples

  1. A compressor with MTBF of 8,760 hours and MTTR of 24 hours has an inherent availability of 8760/(8760+24) = 99.73%.
  2. The operational availability of a production train is 96.5%, with the 3.5% downtime split between unplanned failures (1.2%), planned maintenance (1.8%), and logistics delays (0.5%).

Applicable Standards

  • IEC 60300-3-1
  • ISO 20815:2018
  • EN 13306

Related Terms

Reliability

The probability that an item will perform its required function under stated conditions for a specified period of time. Reliability is a quantitative measure expressed as a probability (0 to 1) or percentage. It is a function of the failure rate and time: for constant failure rate systems, R(t) = e^(-lambda*t). Higher reliability means lower probability of failure over the same period.

Maintainability

The probability that a failed item can be restored to operational condition within a specified period of time when maintenance is performed under stated conditions using prescribed procedures and resources. Maintainability is a design characteristic that reflects how easy, fast, and cost-effective it is to diagnose, access, repair, and verify the restoration of a failed item.

Mean Time Between Failures(MTBF)

The average time between consecutive failures of a repairable system or component, calculated as the total operating time divided by the number of failures during that period. MTBF is the primary reliability metric for repairable systems and represents the expected time of continuous operation between failure events. Higher MTBF indicates better reliability.

Mean Time To Repair(MTTR)

The average time required to repair a failed item and restore it to operational status, measured from the point of failure detection to the point of verified restoration. MTTR includes fault diagnosis, logistics (obtaining parts and tools), physical repair, testing, and recommissioning. It does not include administrative delays or waiting time unless specifically defined as Mean Time To Restore.

Where this appears in Reliatic

This concept is used in the Reliability Analytics module.