Dependability means the ability of the object to perform a desired function or task faultlessly in a certain environment on a planned time period. The concept of dependability can be divided roughly into two essential factors:
- The ability of the object to function without failing
- The ability to restore the object back to operation after failures
The meaning of dependability concerning machines and equipment has been understood more widely just during the past decade. Poor dependability tends to lead into unexpected failures and extra stoppages on the production process. The costs coming from the lost production rise easily to substantial level. In addition to lost production low level of dependability also causes unnecessary risks for the safety of the personnel and the surrounding environment.
With the help of dependability planning and management it is possible to control the risks and to improve the effectiviness of the production. Often this means increasing the share of the preventive maintenance and especially focusing the resources to prevent the most critical failures. It is also beneficial to examine the effect of different dependability improving investments and design solutions to the overall costs.
Availability and reliability as concepts are very closely related to dependability. Availability is the probability for the object to be intact when examined at a random moment. Availability as a concept can be considered as a synonym for dependability. Reliability describes the ability of the object to perform without failing. In other words reliability is the probability for the object not to fail at all during some certain time period.
The latter factor of dependability, i.e. the ability to restore the object back to operation, can be divided further into two separate factors. These are maintainability and maintenance reliability. Maintainability describes how well the object has been designed from the maintenance point of view. Supportability represents the ability of the maintenance organization to effectively perform a required task in certain environment on a required moment of time.
The picture below shows the most essential time-related factors concerning dependability:
- MTBF - Mean Time Between Failures
- MTTF - Mean Time to Failure
- MTTR - Mean Time to Restoration (often same than MDT - Mean Down Time)
- MWT - Mean Waiting Time
- MRT - Mean Repair Time
Time-related factors of dependability