Failure on Time or Use basis

For each failure cause it can be set whether it will occur based on time, or on terms of use. If the failure  will cause downtime, or the unavailability of a system, the entire system will be considered unavailable. Since the entire system is not in use during the repair action, the other failure behavior can show two varieties:

• Failure based on time

  The fact that the system is not in use by any other disturbance has no effect on failure . The failure manifests itself on the basis of time; irrespective of the use of the system.

• Failure based on use

  The time that the system is not in use because of some other malfunction is not a part of the time until this failure. The duration of unavailability will be added to the set MTTF to calculate the next in time failure moment.

  For example: if a fault will occur in the third year, but the system is unavailable for a year, the first time of failure be not before 4 years.

Failure based on time

Example:

If a failure occurs once every third year, causing the system to be unavailable for a year, the first failure moment will be after 4 years. A subsystem can fail in two ways: the first failure (A) will occur in year 10, the second failure (B) in year 11. Both failures will occur no time basis. As an illustration consider the following: failure A will require repair with a duration of 4 years (costs 45), while failure B will require repair taking 5 hours (costs 15).

• Failure A:

  

  #1 -	MTTF 10 years	= 10 years
  #2 -	10 years + repair 4 years + 10 years	= 24 years
  #3 -	24 years + repair 4 years  + 10 years	= 38 years

  

• Failure B:

  

  #1 -	MTTF 11 years	= 11 years
  #2 -	11 years + repair 5 hours + 11 years	= 22 years
  #3 -	22 years + repair 5 hours + 11 years	= 33 years
  #4 -	33 years + repair 5 hours + 11 years	= 44 years

   

Combined the two failure (on time basis) will look like this:

These failures can also be set to fail on use basis. The effect will be that unavailability of the system because of failure A, will influence the time of failure because of failure B. As due to failure A the system will not be in use, this time will not be counted as use time to assess the moment of failure of failure B. The downtime of A will therefore be added to the MTTF of failure B.

Failure  A:  time basis

Failure B:  use basis

Failure  A+B:

Failure based on use:

Failure based on use will show a different failure behavior as seen in the previous example.

Failure based on time: