Basic Terminology/Definitions

Question 1

Reliability terminology

Answer 1

The reliability of a component or system = R (t)

• It is defined as the probability that a component or system remains running from start (0) to finish or however long (t) assuming that it was operating at 0.
• It is a measure of the probability of successful performance of the system over a period of time.
• Reliability assessment is initially carried out for components which are assumed to have settled down into steady state, or useful life, phase.

Question 2

Reliability hazard rate

Answer 2

Reliability
-The reliability characteristics of most
components will follow the ‘bath tub’ curve.

e. g:
1) Burn in
2) Useful life
3) Wear out

Question 3

Unreliability

Answer 3

The unreliability of a component or system = F (t)

• Similar to reliability. It is defined as the probability that the component or system fails from time 0 to time t considering that it was operating at time 0.
• R (t) + F (t) = 1.

Question 4

Availability

Answer 4

The availability of a component or system = A (t)

-Is defined as the probability that the component or
the system is operating at time t, given that it was operating at time 0.

Question 5

Unavailability

Answer 5

Unavailability = Q (t)

• Is defined as the probability that the component or system is not operating at time t, given that is was operating at time 0.

A (t) + Q (t) = 1

Question 6

Unavailability and Unreliability

Answer 6

Unavailability = Q (t)
Unreliability = F (t)
Q (t) less than or equal to F (t)

Non-repairable components:
Q (t) = F (t)