Generalities


    • Reliability index : MTBF

(Mean Time Between Failure)

MTBF = Total running time / number of downtimes.


    • Maintainability index : MTTR

(Mean Time To Repair)

MTTR = Total downtime / number of downtimes.


    • Availability rate

Availability = MTBF / ( MTTR + MTBF )

or Downtime / Scheduled running time.


    • Failure rate

failure rate = (1 / Availability) – 1

or Real running time / Scheduled running time.


The rates calculations are based on the downtimes during the opening hours which are defined in the topologies schedule.

If these downtimes overlap, for example from 8am to 11am and from 10am to 12pm for the same machine, only the highest and lowest hours are taken into account. So there is only one stop, from 8am to 12pm.


Example :

Happening

Duration

(hours)

1

Normal function

4

2

Stop

2

3

Normal function

3

4

Failure

1

5

Normal function

1

6

Failure

0.5

7

Normal function

1

8

Stop

1


From the example:

MTBF = (4 + 3 + 1 + 1) / 4 = 2,25h = 2h 15mn.

MTTR = (2 + 1 + 0,5 + 1) / 4 = 1,125h = 1h 7mn.

Availability = MTBF / ( MTTR + MTBF ) = 2,25 / (2,25 + 1,125) = 0.6667 = 66.67%

Failures rate = 1 / Disponibilité - 1 = 0.5 = 50%