Time-based Availability Considering operating and non-operating times
according to point 4.9 of the EN 13306 standard, time-based availability is:
During a given period of time, percentage of the time during which an item was able to perform when required.
The standard adds the following comment to the above definition:
Considering the required time, different ratios can be used to calculate time-based availability
The standard lists three „examples” under the comment. In this note, we deal with the first one, and the next two examples are the subject of the next two notes.
The Uptime, as illustrated in the attached figure, is the sum of the times during which the item is able to perform the required function. It includes the Operating Time (OT), the standby time (Standby), during which the device can be put into operation at any time, the Idle time (Idle), during which the device can be put into operation if a need for operation occurs, and the downtime due to external reasons (Downtime due to external causes) during which the device could perform the required function, but its commissioning is hindered by a lack of external resources.
In certain industries, an item in a standby state is often referred to as a „hot spare”. This is usually typical of redundant devices that are installed in parallel and have the same required function. For example: two pumps with the same technical parameters are installed in a technological line and the capacity of any of them is sufficient to serve the technological demand. Since continuous operation is vital, one pump is in operation and the other is in „hot standby”, which means that it is immediately connected to the technology thanks to an automatic start, if needed.
During the downtime due to external causes, the item would be able to perform the required tasks, so it would be functional, but its operation in the technology is hindered by the lack of certain external resources, such as the lack of cooling water or electricity.
The category Downtime (DT) includes the time required for repairs due to breakdowns (restoration) and the time required for preventive maintenance.