Availability

Availability is the share of required time in which a machine can work. EN 13306 offers three time ratios, and each leaves out different stops, so state which one you use.

All three leave out stops caused by missing outside resources, such as power or staff.


In the standard	Availability (EN 13306:2017, 4.7); time-based availability (4.9)
German	Verfügbarkeit; zeitbezogene Verfügbarkeit
Also called	Equipment availability; operational availability (Gulati); uptime, in our experience

How do you calculate availability?

Pick a period, usually a year, and take the required time: the hours the machine must be ready. Then choose a ratio. EN 13306 lists these three:

UT / (UT + DT)     up time over up time plus down time
OT / (OT + TTR)    operating time over operating time plus time to restoration
OT / (OT + DT)     operating time over operating time plus down time

Up time (UT): the machine is able to work. A stop for missing power still counts as up time.
Operating time (OT): the machine works.
Down time (DT): a fault or a preventive stop holds the machine. Waiting for a technician or a part counts.
Time to restoration (TTR): from a failure until the machine works again. Waiting counts. Preventive stops do not.

Ratio	Counts against the machine	Leaves out
UT / (UT + DT)	Faults, preventive stops	Stops for outside resources
OT / (OT + TTR)	Faults only	Preventive stops and every other stop
OT / (OT + DT)	Faults, preventive stops	Every other stop

The ratios answer different questions. The first asks how much of the required time maintenance left the machine able to work. The second asks what failures cost, with preventive work set aside. The first and the third differ only when the machine was ready but did not run, for example without power. The first counts that time as success. The third leaves it out.

Which availability should a plant report?

We recommend the first ratio, with all down time counted, as the plant's availability. It tells production how much machine time maintenance took. Report the faults-only ratio beside it: it shows what failures and waiting cost, with the planned stops removed. Keep the same ratio from month to month, or the trend means nothing.

What is a good availability?

No single value holds for all machines. Gulati calls about 95 percent the general expectation and 99 percent or more the target for critical assets. He says it rests on industry expectations and cites no study. Wireman lists a range of 65 to 99.9 percent and gives no fixed best practice: each plant sets the value from what lost output costs it. EN 13306 sets no target. None of the three ties a value to a study.

When does availability mislead?

Outside stops stay invisible. A power cut or a missing operator does not lower availability, so production may still see a stopped machine. The standard's up time includes those hours, so "uptime" is not the time the machine ran. See uptime and downtime.
OEE uses another ratio. In OEE, availability is run time over planned production time, and it counts every stop.
The textbook formula leaves out preventive work. Gulati calls MTBF / (MTBF + MTTR) inherent availability, a design value. With EN 13306's MTTR it equals the second ratio. For IMM-07: 658 h / (658 h + 5.0 h) = 99.25 %. Real availability is lower once preventive stops count.
An average hides the pattern. 45 hours of down time can be nine stops or one. Read availability beside MTBF and MTTR.

Sources