Reliability

Many methods exist in measuring Reliability:

• MIL-HDBK-217F
• Monte Carlo
• Markov chains and
• fault trees

Measured parameters include:

• MTBF = Mean Time Between Failure.
  • Statistically this means 63% will fail in MTBF time.
• MTTR = Mean Time To Repair
  • Statistically this means 63% will be repaired in MTTR time.
  • Thus to be 95% confident of a repair, the time taken is 3xMTTR.
• ë = Failure Rate =1/MTBFµ = Repair Rate =1/MTTR
• Availability= (MTBF-MTTR)/MTBF
• M = Maintainability =1-e-µt
• t = Repair interval

Reliability of an Electrical System component depends on:

• Redundancy
• Simple paralleling of equipment does not mean you have redundancy. If any interdependency exists then redundancy is either compromised or reduced.
• Discrimination
•  Where upstream electrical protection operates slower than downstream equipment specific protection.

  An example where this commonly fails is where a slow fuse or circuit breaker is used to protect one device. Upon a fault the upstream protection trips first and causes a loss of power to ALL equipment. Thus the fault impact is not contained to the one device.

• Maintainability

• Where servicing a system is difficult and usually requires a network of equipment to be made unavailable until works are complete.

  Redundancy can solve maintainability only where half the system is independent and can fully support all loads. This is not the only way to achieve good maintainability.