Determine Availability of Uninterruptible Power Supply UPS

Electronics, Instrumentation & Electrical Database

Continued from:

• Determine if the safety of the selected Uninterruptible Power Supply UPS is acceptable.
  • of Selecting and designing a Uninterruptible Power Supply Uninterruptible Power Supply UPS

Determine if the availability of the selected Uninterruptible Power Supply UPS is acceptable: In managing a facility, the availability of equipment and systems is of the utmost concern. Simply stated, availability is the amount of time a piece of equipment is available to perform its function divided by the total time the equipment is needed. It is also defined as “uptime” divided by “total time.” Thus, if an air conditioner is required 12 hours each day, the availability would be 90 percent if it is out of commission an average of 1.2 hours each day. Normally, the required availability for Uninterruptible Power Supply UPS is 98 percent. Availability is a function of reliability and maintainability. The inherent or designed-in availability is usually expressed as follows.

Ai = MTBF / ( MTBF + MTTR )

Where:

Ai is inherent availability
MTBF is mean time between failure (a measure of reliability)
MTTR is mean time to repair (a measure of maintainability)

Reliability is the probability that the item will perform as intended for a specified period of time, under a stated set of conditions. It can be expressed in terms of the number of failures in a given time (the failure rate), or as the time between failures (for repairable items), or time to failure (for “one-shot” or non-repairable items. Maintainability is defined as the relative ease and economy of time and resources with which an item can be retained in, or restored to, a specific condition. This assumes maintenance is performed by personnel having the specified skill levels, using prescribed procedures and resources, at each prescribed level of maintenance and repair. It can be expressed as the probability that an item can be restored to operational condition in a stated time, the number of repairs that can be made in a specific time (repair rate), or the MTTR. From the equation for Ai, it is obvious that availability can be increased by increasing MTBF or reducing MTTR. For example, assume the MTBF and the MTTR of a single UPS unit are 500 hours and 20 hours, respectively. The inherent availability of a single unit configuration would be:

Ai = 500 / ( 500 + 20 ) = 0.962

The inherent availability of a two-unit configuration where only one unit is required would be:

Ai = A1 + A2 - (A1 x A2 ) = 0.999

The inherent availability of a two-unit configuration where both units are required would be:

Ai = A1 x A2 = 0.925

The availability could be increased by increasing the reliability or reducing the MTTR. The reliability could be increased by selecting a more reliable unit, derating the unit (i.e., use a unit capable of providing more power than needed - when used, it will be operating below its capacity thereby reducing stresses), or use redundancy . MTTR could be decreased by selecting an inherently more maintainable system or perhaps by improving diagnostics, training, or procedures.

Continue Determine if the selected Uninterruptible Power Supply UPS is maintainable.

References: “Joint Departments of the Army, Uninterruptible Power Supply System Selection, Installation, and Maintenance for Command, Control, Communications, Computer, Intelligence, Surveillance, and Reconnaissance (C4ISR) Facilities, 31 May 2002.”