Availability of power

Availability means the amount of time a device is actually operating as the percentage of total time it should be operating. High-availability systems may report availability in terms of 9 minutes or hours of downtime per year. Special features allow the system to stay operational even when faults do occur.

A high availability system would disable the malfunctioning portion and continue operating at a reduced capacity.

Availability is typically given as a percentage of the time, a system is expected to be available, e.g., 99.999 percent (“five nines”) and is expressed as


MTBF Mean time between Failure
MTTRMean time to Repair


The major aspects which needs to be considered during design stage to have high availability of power for
critical loads are

• Reliability- The individual UPS and other power distribution systems must be reliable, as measured by field-documented MTBF (Mean Time between Failures).
• Maintainability- The system design must permit concurrent maintenance of all power system
components supporting the load with part of the UPS system while other parts are being serviced
• Fault Tolerance- The system must have fault resiliency to cope with a failure of any power system component without affecting the operation of the critical load equipment.

Reliability can be defined as the probability that a system will produce correct outputs up to some given time(t). Serviceability or maintainability is the simplicity and speed with which a system can be repaired or maintained; if the time to repair a failed system increases, then availability will decrease. Serviceability includes various methods of easily diagnosing the system when problems arise. Early detection of faults can decrease or avoid system downtime. The traditional focus has been on making the correct repairs with a little disruption to normalize the operation as possible. In principle with a high MTBF or low MTTR, the availability of the system can be improved.

Calculation of availability

For calculating the availability, we need to know its MTBF and MTTR.

The MTBF values are usually obtained from the vendors for all the components in the system and is based on the component’s configuration and design. The MTBF value is estimated by the manufacturer as the time after which a failure occurs in a hardware part resulting in the failure of the component. MTTR is based on the response time of the service provider or vendor and also includes the time taken to repair the fault or replace the failed component or module. The availability is calculated based on the MTBF and MTTR of the components in the design.

Components in series:

The two components A and B are considered to be operating in series if failure of either of the parts results in failure of the system.



Figure 1 Components in series

The combined system is operational only if both component A and component B are available. From this it follows that the combined availability is a product of the availability of the two components.

The combined availability is shown by the equation below: Availability = Availability of Component A X Availability of Component B

Components in parallel:

Two components A and B in a system are considered to be operating in parallel if the failure of either of the components does not result in the failure of the system. The system fails only when both the components fail.



Figure 2 Components in parallel

The combined availability of the system with components in parallel is shown by the equation below

Availability = (Availability of component A+ Availability of component B) – (Availability of component A X Availability of component B)

The implications of the above equation are that the combined availability of two components in parallel is always much higher than the availability of its individual components.

Comparison of component’s in series or parallel

The table below shows the availability and downtime for individual components configured in both series and parallel configuration.



Table 2 Availability of components in series & parallel