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1. Degree of Reliability
Degree of reliability represents percentage ratio of reliability. For example, if none of 10 light bulbs has failed for 100 hours, the degree of
reliability defined in, 100 hours of time is 10/10 = 100%. If only three bulbs remained alive, the degree of reliability is 3/10 = 30%. The JIS
Z8115 standard defines the degree of reliability as follows: The probability at which a system, equipment, or part provides the specified
functions over the intended duration under the specified conditions.
2. MTBF
MTBF is an acronym of Mean Time Between Failures. It indicates the mean time period in which a system, equipment, or part operates
normally between two incidences of repair. MTBF only applies to repairable products.
MTBF tells how long a product can be used without the need for repair.
Sometimes MTBF is used to represent the service lifetime before failure.
3. MTTF
MTTF is an acronym of Mean Time To Failure. It indicates the mean time period until a product becomes faulty MTTF normally applies to
unrepairable products such as parts and materials.
The relay is one of such objective of MTTF.
4. Failure Rate
Failure rate includes mean failure rate and momentary failure rate. Mean failure rate is defined as follows:
Mean failure rate = Total failure count/total operating hours
In general, failure rate refers to momentary failure rate. This represents the probability at which a system, equipment, or part, which has
continued normal operation to a certain point of time, becomes faulty in the subsequent specified time period.
Failure rate is often represented in the unit of percent/hours. For parts with low failure rates, "failure unit (Fit) = 10
-９
/hour" is often used
instead of failure rate. Percent/count is normally used for relays.
5. Safe Life
Safe life is an inverse of degree of reliability. It is given as value B which makes the following equation true:
1 - R(B) = t%
In general, "B[1 - R(B)] = 10%" is more often used. In some cases this represents a more practical value of reliability than MTTF.