RELIABILITY

ABOUT RELIABILITY

■ Reliability in a Narrow Sense of the Term

In the industrial space, reliability is a measure of how long a particular product operates without failure.

■ Reliability in a Broad Sense of the Term

Every product has a finite service lifetime. This means that
1. Reliability ( narrow sense ), durability
no product can continue normal service infinitely. When a Long life time: MTTF, B10, R ( T )
product has broken down, the user may throw it away or Low failure rate: λ, MTBF

repair it. The reliability of repairable products is recognized Availability

Reliability
2. Maintainability
as "reliability in a broad sense of the term". ( broad sense )
MTTR
For repairable products, their serviceability or maintainability Preventive maintenance,
is another problem. In addition, reliability of product design predicted maintenance

is becoming a serious concern for the manufacturing 3. Design reliability

Human, Factor, redundancy, fool-proof,
industry. In short, reliability has three senses: i.e. reliability of
fail-safe
the product itself, serviceability of the product, and reliability
of product design.

■ Intrinsic Reliability and Reliability of Use

Reliability is "built" into products. This is referred to as intrinsic reliability which consists mainly of reliability in the narrow sense.
Product reliability at the user's site is called "reliability of use", which consists mainly of reliability in the broad sense.
In the relay industry, reliability of use has a significance in aspects of servicing.

Panasonic Industry Co., Ltd. Electromechanical Control Business Division

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RELIABILITY

RELIABILITY MEASURES

The following list contains some of the most popular ■ MTTF

reliability measures: MTTF is an acronym of mean time to failure. It indicates
the mean time period until a product becomes faulty MTTF
Reliability measure Sample representation
Degree of reliability R ( T ) 99.9%
normally applies to unrepairable products such as parts and
MTBF 100 hours materials.
MTTF 100 hours The relay is one of such objective of MTTF.
Failure rate λ 20 Fit, 1%/hour
Safe life B10 50 hours ■ Failure Rate
Failure rate includes mean failure rate and momentary failure
■ Degree of Reliability rate.
Degree of reliability represents percentage ratio of reliability. Mean failure rate is defined as follows:
For example, if none of 10 light bulbs has failed for 100
hours, the degree of reliability defined in, 100 hours of time Mean failure rate = Total failure count/total operating hours
is 10/10 = 100%. If only three bulbs remained alive, the
degree of reliability is 3/10 = 30%. In general, failure rate refers to momentary failure rate. This
The JIS Z8115 standard defines the degree of reliability as represents the probability at which a system, equipment,
follows: or part, which has continued normal operation to a certain
The probability at which a system, equipment, or part point of time, becomes faulty in the subsequent specified
provides the specified functions over the intended duration time period.
under the specified conditions. Failure rate is often represented in the unit of percent/hours.
For parts with low failure rates,
■ MTBF
MTBF is an acronym of mean time between failures. "failure unit ( Fit ) ＝ 10-9/hour"
It indicates the mean time period in which a system,
equipment, or part operates normally between two is often used instead of failure rate.
incidences of repair. Percent/count is normally used for relays.
MTBF only applies to repairable products.
MTBF tells how long a product can be used without the ■ Safe Life
need for repair. Safe life is an inverse of degree of reliability.It is given as
Sometimes MTBF is used to represent the service lifetime value B which makes the following equation true:
before failure.
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.

Panasonic Industry Co., Ltd. Electromechanical Control Business Division

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RELIABILITY

FAILURE

■ What is Failure?
Failure is defined as a state of system, equipment, or component in which part of all of its functions are impaired or lost.

■ Bathtub Curve
Product's failure rate throughout its lifetime is depicted as a bathtub curve, as shown below. Failure rate is high at the
beginning and end of its service lifetime.

1) Initial failure period 3) Wear-out failure period

The high failure rate in the initial failure period is In the final stage of the product's service lifetime
derived from latent design errors, process errors, comes the wear-out failure period, in which the life of
and many other causes. Initial failures are screened the product expires due to wear of fatigue. Preventive
at manufacturer's site through burn-in process. This maintenance is effective for this type of failure. The
process is called debugging, performing aging or timing of a relay's wear-out failure can be predicted
screening. with a certain accuracy from the past record of uses.
The use of a relay is intended only in the accidental
2) Accidental failure period failure period, and this period virtually represents the
The initial failure period is followed by a long period service lifetime of the relay.
with low, stable failure rate. In this period, called
accidental failure period, failures occurs at random Failure rate
1) 2) 3)
along the time axis. While zero accidental failure rate is
desirable, this is actually not practical in the real world.

m＜1 m＞1
m＝1

Time

■ Weibull Analysis
Weibull analysis is often used for classifying a product's
Failure rate

failure patterns and to determine its lifetime.

Weibull distribution is expressed by the following equation: m

m 63％
m m−1
(χ−γ)
f (χ) ＝ α (χ−γ) e− α

γ Time
α
• m: Figure parameter
• α: Measurement parameter The Weibull probability chart is a simpler alternative of
• γ: Position parameter complex calculation formulas. The chart provides the
following advantages:
Weibull distribution can be adopted to the actual failure rate 1) The Weibull distribution has the closest proximity to
distribution if the three variables above are estimated. the actual lifetime distribution.
2) The Weibull probability chart is easy to use.
3) Different types of failures can be identified on the
chart.
The following describes the correlation with the bathtub
curve. The value of the figure parameter "m" represents the
type of the failure.
1) When m 1: Initial failures
2) When m ＝ 1: Accidental failures
3) When m 1: Wear-out failures

Please refer to "the latest product specifications"

when designing your product.
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Panasonic Industry Co., Ltd. Electromechanical Control Business Division

industrial.panasonic.com/ac/e/ ー3ー Panasonic Industry Co., Ltd. 2022 ASCTB47E 202206