Reliability, Availability, Maintainability

7/28/2019 Reliability, Availability, Maintainability

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RAM


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WHAT IS SAFETY ENGINEERING?


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Safety engineering is an applied science strongly

related to systems engineering and the subset System

Safety Engineering.

Safety engineering assures that a life-critical system

behaves as needed even when pieces fail.

The term "safety engineering" refers to any act of

accident prevention by a person qualified in the field.

Failure to identify risks to safety, and the according

inability to address or "control" these risks, can result in

massive costs, both human and economic.


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WHAT ARE FAULTS AND FAILURES?


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FAILURE UNRELIABILITY

IT BECOMES COMPLETELY UN

OPERATABLE

OPERATES BUT NO LONGER IN A

POSITION TO PERFORM THE

REQUIRED FUNCTION

UNSAFE FOR ITS CONTINUOUS USE


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A failure is "the inability of a system or component toperform its required functions within specified

performance requirements", while a fault is "a defect in

a device or component, for example: a short circuit or a

broken wire".

System-level failures are caused by lower-level faults,

which are ultimately caused by basic component faults


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CAUSES OF FAILURE


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DEFICIENCIES IN DESIGN

DIFICIENCIES IN MATERIAL

DEFICIENCIES IN PROCESSING

ERRORS IN ASSEMBLY

IMPROPER SERVICE CONDITIONS

INADEQUATE MAINTENANCE

VARIATIONS IN OPERATING &

MAINTENANCE CONDITIONS


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PHASES OF FAILURE AND METHODS

OF PREVENTION


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1. INITIAL FAILURES PROBABILITY OF DEFECTIVE

DESIGN, MANUFACTURE,ASSEMBLY

-----OPERATING THE ITEM FORSEVERAL HOURS & REPLACING THE

TYPE OF MATERIAL BECOMING

DEFECTIVE ---- WARRANTY


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RANDOM FAILURES: BY CHANCE

- REDUNDANCY

WEAR OUT FAILURES: AGEING

PROPER MAINTENANCE


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NATURE OF FAILURES


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AN ITEM MAY FAIL IN MANY WAYS, AN

UNDERSTANDING OF THIS FAILURES HELPIN TAKING APPROPRIATE CORRECTIVE

MEASURES FOR ACHIEVING BETTER

RELIABILITY

CATASTROPHIC FAILURES: A NORMALLY

OPERATING ITEM SUDDENLY BECOMES IN

OPERATIVE

Ex- BLOWING OF FUSE


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DEGRADATION(CREEPING OF

FAILURES)- BECAUSE OF SOMECHANGE OF PARAMETERS

Ex- CHANGE OF VALUE OF RESISTOR

INDEPENDENT FAILURES:

Ex- FAN BELT OF A CAR


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SECONDARY FAILURES: OCCUR AS A

RESULT OF PRIMARY FAILURE

EX:SPOKES OF A CYCLE BENT DUE TOTYRE BURST

MISUSE FAILURES:FAILURES

ATTRIBUTABLE TO APPLICATION OFSTRESSES BEYOND THE STATEDCAPABILITIES OF THE ITEM, OWING TOMIS HANDLING OR IMPROPER USE


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DIFFERENT MODES OF SAFEOPERATION


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A probabilistically safe system has no single point of failure, and

enough redundant sensors, computers and effectors so that it is

very unlikely to cause harm (usually "very unlikely" means, onaverage, less than one human life lost in a billion hours of

operation).

An inherently safe system is a clever mechanical arrangement

that cannot be made to cause harm obviously the bestarrangement, but this is not always possible.

A fail-safe system is one that cannot cause harm when it fails.

A fault-tolerant system can continue to operate with faults,

though its operation may be degraded in some fashion.


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Analysis techniques


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The two most common fault modeling techniques are

FAILURE MODES AND EFFECTS ANALYSIS

FAULT TREE ANALYSIS

These techniques are just ways of finding problems and of making

plans to cope with failures


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FAILURE MODES AND EFFECTS ANALYSIS

In the technique known as "failure mode and effects analysis"

(FMEA), an engineer starts with a block diagram of a system.

The safety engineer then considers what happens if each block of

the diagram fails.

The engineer then draws up a table in which failures are pairedwith their effects and an evaluation of the effects.

The design of the system is then corrected, and the table adjusted

until the system is not known to have unacceptable problems.

It is very helpful to have several engineers review the failure

modes and effects analysis.


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FAULT TREE ANALYSIS

In the technique known as "fault tree analysis", an undesired

effect is taken as the root ('top event') of a tree of logic.

There should be onlyone Top Event and all concerns must tree

down from it.

Then, each situation that could cause that effect is added to thetree as a series of logic expressions.

When fault trees are labeled with actual numbers about failure

probabilities, which are often in practice unavailable because of

the expense of testing, computer programs can calculate failureprobabilities from fault trees.


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Usually a failure in safety-certified systems is acceptable if, on

average, less than one life per 109 hours of continuous operation

is lost to failure.

FAILURE RATE


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FAILURE RATE

The equipment reliability can be expressed with-

Failure Rate =Number of faults per unit time

= = 1/ M

The MTBF is given by M and is expressed in hours and

corresponding units of are faults per hour. The

component is extremely small and units may be

altered to give convenient numbers. Thus failure rates

may be quoted as a percentage per 100hrs, per 106

or per 10 9 hour.


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PROBABILITY

Probability = P = S/ n; Where , S =No. of results

and n = Possible results in all( failure and success).

Ps = Probability of success = a / a+b where a is the

probability of success and b is the probability of failure.

Pf = Probability of failure =b / a+b

Ps+Pf = a / a+b + b / a+b = 1 unit.


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Given the probability P of some event , the

probability of its complement, that is event will not

take place is (1 P).

If P = P1and P2 are probabilities of success in the two

events, the probability that both occur is, P = P1x P2 ,the probability that two trials will both succeed is P2

.

The product rule is directly applicable to series system ,

in which input of each unit is connected to the output of

previous

COMPOUND EVENTS(PROBABILITY)


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COMPOUND EVENTS(PROBABILITY)

In order for the complete system to operate correctly, each

unit must operate correctly. Thus probability of success or

in other word the reliability of the units are R1, R2, R3,

-----Rn.

The probability that they will all operate correctly, i.e, the

system will function,is given by R = R1xR2x R3------x

Rn.

Forn similar units of reliability Rr, this is R = ( Rr)n


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The product rule gives the joint probability that a number

of events will all be successful. For circumstances , we

require the probability that one or more events will besuccessful.

For example a box contaminating Eight - 0.1F , seven-

0.5F and Five- 1 F capacitors. Total No. of capacitors =20. If we pick capacitor randomly the probability of three

values -

P1 = 8/20 = 0.4(0.1 F), P2 = 7/20 = 0.35( 0.5 F), P3 =5/20 = 0.25 (1.0 F)

FAILURE ANALYSIS OF SERIES SYSTEM


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FAILURE ANALYSIS OF SERIES SYSTEM

If separate probability of a failure developing in the

components are P1, p2, P3, P4, P5 the joint probabilityof a fault occurring in one or more of the five components

is

P = P1 + P2 + P3 + P4 + P5; P1 To P5 Probabilitiesdepend upon duration of the test or prescribed operating

period.

For calculating reliability the failure rate is considered. Ifthe failure rates for the components are 1, 2, 3 etc.

The expected number of failures are: - n1 = 1xT for

component-1


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n2 = 2xT for component 2; Where T is the duration

of the test or operating period. Since we are considering

series system, any one of this faults will cause a system

failure. Thus: -

Ns = (1 + 2 + 3 + 4 + 5) T , is the total number of

faults expected during the interval T. Assuming that

1, 2, 3---- 5 are constant. Reliability R is given byR = 1 - Ns


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EXAMPLE OF SERIES SYSTEM

Numerical data are used to estimate reliability.The

system failure rate is calculated by adding together.

0.0125capacitor

0.00590Resistor

0.0525Silicon Transist.

0.0245Silicon diode

Failure rate in percent

per 1000Hrs

NumberComponent


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The total failure rate for each class of component

Silicon diode - 45 X 0.02 = 0.9

Silicon Transistor - 25X 0.05 = 1.25

Resistor - 90X 0.005 = 0.45

Capacitor- 25 X 0.01 = 0.25

Total failure rate in percent = 2.85 percent per

1000Hrs


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So failure rate = number of failure = 2.85/100 =

0.0285 failure in 1000Hrs.

The failure rate in 1 hour = .0285/1000 = =0.0000285

M = 1/ = 1/0.0000285 = 35100Hrs

A ship having the components of above failure ratemust operate 750 Hrs continuously until the ship

return.

So expected No. of faults in 750Hrs = n = (750x0.0285)/1000

= 0.0214 ; Thus reliability of each voice R = 1 n =

0.9786

REIABILITY OF PARALLEL SYSTEM


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REIABILITY OF PARALLEL SYSTEM

Suppose three systems are connected in parallel. Any oneof three systems, if works, the system will not fail . The

system will fail if all the three systems fail.Thus the

probability of failure are

Ps = P1 X P2 X P3; Where Probability of failures are

P1, P2& P3 for a specified interval of time. The system

reliability , assuming P is Small, is then

R = 1 - Ps = 1 P1 X P2 X P3

EXAMPLE OF PARALLEL SYSTM


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EXAMPLE OF PARALLEL SYSTM

A generating system having mean time between failures

of 5000Hrs. What will be the reliability of the system for

a 500 Hrs operating period if there are five identical units

and if three of them supply the required load?

The condition imply that if three or five or five machines

fail, the system will fail. The combined probability of

any one of three situations occurring is given by some of

three separate probability.

L P b h b bili f f il f hi d i


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Let P be the probability of failure of one machine duringthe 500Hrs interval.

The total probability of failure = Ps = 10 P3(1- P)2 +5 P4 (1-

P) + P5

Where, 2x5 P3(1- P)2 - Three faulty machines and oneworking.

5x1 P4

(1-P) - Four faulty machines and one working.Thus total probability of failure is -

Ps = 10 P3 - 15 P4 - 6P5 --------1

The mean time between failures for a single machine is5000Hrs. Thus the probable number nf of failures in a500Hrs period is

nf = 500/5000 = 0.1


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RELIABILITY


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EVERY FAILURE MUST BE REGARDED AS

SIGNIFICANT UNTIL ACTION HAS BEEN

TAKEN TO PREVENT ITS REOCCURENCE

RELIABILITYRELIABILITY IS THE ABILITY OF AN ITEM

TO PERFORM A REQUIRED FUNCTION

UNDER STATED OPERATING &ENVIRONMENTAL CONDITIONS FOR A

GIVEN PERIOD OF TIME


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The probability of survival, R(t), plus the probability of failure, F(t),

is always unity.

Expressed as a formula: F(t) + R(t) = 1 or, F(t)=1 - R(t).

The required function includes both a definition of satisfactory and

unsatisfactory operation (failure).The stated conditions are the total physical environment, including

mechanical, thermal, and electrical conditions.

The stated period of time is the time during which satisfactory

operation is desired.


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CONCEPTS : IT IS EXPRESSED IN TERMS OF

PROBABILITY Ex. 0.95 FOR 60 HRS

REQUIRED FUNCTION:EX. LIGHTING OF 10

CANDLESTIME: Ex. MISSILE

OPERATING & ENVIRONMENTAL

CONDITION: Ex. TYRE & ROAD CONDITIONS


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1. FAILURE RATE1. FAILURE RATE

EXPRESSED IN TERMS OF FAILURES PER

HOUR, 100 HR, 1000 HR OR % FAILURES PER

1000 HRS.

Ex: FAILURE RATE OF RELAYS HAS BEEN

CALCULATED AS 0.4623 PER 1000 Hrs, FROM

THE PAST EXPERIENCE. THIS MEANS THAT

OUT OF 10,000 RELAYS, 4623 ARE EXPECTED

TO FAIL DURING 1000 Hrs OPERATION


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2.PROBABILITY OF SURVIVAL2.PROBABILITY OF SURVIVAL

THE PROBABILITY OF SURVIVAL ISEXPRESSED AS A DECIMAL FRACTION ORPERCENTAGE WHICH INDICATES THE

PROBABLE OR EXPECTED NUMBER OFITEMS THATWILL OPERATE FOR AREQUIRED PERIOD OF TIME.

EX. 90%- 90 OUT OF 100 MACHINESEXCEEDED THE PROBABILITY OFSURVIVAL


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3.MEAN TIME BETWEEN FAILURES (MTBF)3.MEAN TIME BETWEEN FAILURES (MTBF)

APPLICABLE FOR REPAIRABLE ITEMS.

EXPRESSED IN HOURS

The MTBF of a system (givenby M), may be measured by

testing it for a total period (given by T) during which N

faults occurred. Each fault is repaired and equipment put backon test,the repair time being excluded from the total test time

T. The observed MTBF is then given by M = T/N

IF AN EQUIPMENT FAILS 6 TIMES OVER APERIOD OF 3000 Hrs, THE MTBF WOULD BE

3000/6=500 HRS.

THIS IS ALWAYS TAKENAS AN AVERAGE

TIME.


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4.MEAN TIME TO FAILURE(MTTF)4.MEAN TIME TO FAILURE(MTTF)

APPLICABLE TO NON REPAIRABLE ITEMS.

EXPRESSED AS AN AVERAGE TIME

IT IS THE TIME AN ITEM IS EXPECTED TO

FUNCTION BEFORE FAILING


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The observed MTTF is given by -

i=n

M =( ti)/ni=1

For Example: -If six units were tested until failure , and

the times to failure were 320 , 250, 380, 290, 310 and 400

hrs. The total test time would be 1950hrs and the MTTF

would be M = 1950/6=325Hrs


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MAINTAINABILITY


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THE ACTIVITY BY WHICH THE USEFULLIFE OF AN ITEM CAN BE EXTENDED BY

CARRYING OUT CORRECTIVE ACTIONS AT

SPECIFIED INTERVALS

GOOD MAINTENANCE AIMS TO KEEP

PRODUCTION MACHINERY & EQUIPMENT

IN EFFICIENTWORKING CONDITION ALLTHE TIME

MAINTAINABILITY


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DEFNITION: MAINTENANCE IS A COMBINATIONOF ANY ACTIONS CARRIED OUT TO RETAIN ANITEM IN OR RESTORE IT TO AN ACCEPTABLESTANDARDS.-BRITISH SPEC. 3811(1974)

MAINTAINABILITY IS A CHARECTERISTICS OFEQUIPMENT DESIGN 7 INSTALLATION WHICH ISEXPRESSED IN TERMS EASE & ECONOMY OF

MAINTENANCE AVAILABILITY OF THEEQUIPMENT, SAFETY & ACCURACY IN THEPERFORMANCE OF MAINTENANCE ACTIONS

MAINTAINABILITY

OBJECT OF MAINTAINABILITY


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OBJECT OF MAINTAINABILITY

TO DESIGN & DEVELOP SYSTEMS &

EQUIPMENTS WHICH CAN BE

MAINTAINED AT THE LEAST TIME &

AT THE LEAST COST AND WITHMINIMUM EXPENDITURE OF

SUPPORTING RESOURCES WITHOUT

ADVERSELY AFFECTING THE ITEMPERFORMANCE OR SAFETY.


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OBJECTIVES OF MAINTENANCE

TO EXTEND THE USEFUL LIFE

TO ASSURE OPTIMUM AVAILABILITY OF

THE INSTALLED EQUIPMENT

TO ENSURE OPERATIONAL READINESS OF

ALL EQUIPMENTS REQUIRED FOR

EMERGENCY.

TO ENSURE SAFETY FOR PERSONNEL

USING SUCH FACILITY


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FORMS OF MAINTENANCE

PREVENTIVE MAINTENANCE(PM)

CORRECTIVE MAINTENANCE(CM)



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TUNING OR ADJUSTMENTS

LUBRICATION

INSPECTIONCLEANING ETC

MAJOR PART OF PM IS INVOLVES

INSPECTION BY LOOK, FEEL & LISTEN

MAJOR ADVANTAGES OF PM


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MAJOR ADVANTAGES OF PM

LESS PRODUCTION DOWN TIME

LESS OVER TIME PAY FOR MAINTENANCE FOR

ORDINARY ADJUSTMENTS

FEWER LARGE SCALE REPAIRS

LESS REDUNDANCY REQUIRED

BETTER SPARE PART CONTROL- MIN.

INVENTORYGREATER SAFETY FOR MAINTENANCE STAFF

& WORKING STAFF

LOWER UNIT COST OF MANUFACTURE


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FACTORS EFFCTING


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FACTORS EFFCTING

MAINTAINABILITY

DESIGN-RELIABILITY, COMPLEXICITY,

INTERCHANGEABILITY,

REPLACEABILITY, COMPATIBILITY,VISIBILITY & CONFIGURATION.

INSTALLATION-GENERALLY RELATE TO

HUMAN BEING-EXPERIENCE,TRAINING,SKILL &

SUPERVISION

Maintainability


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Maintainability

In telecommunication and several other engineering fields, the

term maintainability has the following meanings:

1.A characteristic of design and installation, expressed as the

probability that an item will be retained in or restored to a

specified condition within a given period of time, when the

maintenance is performed in accordance with prescribed

procedures and resources.

2.The ease with which maintenance of a functional unit can be

performed in accordance with prescribed requirements.


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Maintainability is defined as the probability of performing a

successful repair action within a given time.

In other words, maintainability measures the ease and speed withwhich a system can be restored to operational status after a

failure occurs.

For example, if it is said that a particular component has a 90%maintainability in one hour, this means that there is a 90%

probability that the component will be repaired within an hour.

In maintainability, the random variable is time-to-repair, in the

same manner as time-to-failure is the random variable in reliability.


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AVAILABILTY


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AVAILABILTY


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Availability = A = U/ U+D ; Where U = Up time ,

during which the machine is in working order; D = Downtime , During which the machine is faulty and being

repaired.

A = M/M+R = 1/ / (1/ +1/ ) = / + ; Where = failure rate = 1/M or M = 1/ and R = mean repair time

. Repair rate = = 1/R or R = 1/

Unavailability = B = D/ U+D

B = R/M+R = / +.

A+B = (U/U+D + D/ U+D) = 1


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Availability is defined as a percentage measure of the degree to

which machinery and equipment is in an operable and committable

state at the point in time when it is needed.

This definition includes operable and committable factors that are

contributed to the equipment itself, the process being performed, and

the surrounding facilities and operations.

This statement incorporates all aspects of malfunctions and delays

relating to equipment, process, and facility issues.

If one considers both reliability (probability that the item will not fail)

and maintainability (the probability that the item is successfully restored

after failure), then an additional metric is needed for the probability that

the component/system is operational at a given time, t(i.e. has not failed

or it has been restored after failure). This metric is availability.


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Availability Classifications

The definition of availability is somewhat flexible and is largely based

on what types of downtimes one chooses to consider in the analysis. As aresult, there are a number of different classifications of availability, such

as:

Instantaneous (or Point) Availability.

Average Up-Time Availability (or Mean Availability).

Steady State Availability.

Inherent Availability.

Achieved Availability.

Operational Availability.

Instantaneous or Point Availability, A(t)


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s eous o o v b y, (t)

Instantaneous (or point) availability is the probability that a system (or

component) will be operational (up and running) at any random time, t.

Average Uptime Availability (or Mean Availability)

The mean availability is the proportion of time during a mission or time

period that the system is available for use. It represents the mean value

of the instantaneous availability function over the period (0, T)Steady State Availability

The steady state availability of the system is the limit of the

instantaneous availability function as time approaches infinity

The instantaneous availability function will start approaching the steady

state availability value after a time period of approximately four times

the average time-to-failure.


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AVAILABILITY (INHERENT)

THE PROBABILITY THAT A SYSTEM,WHEN USED UNDER STATED

CONDITIONS,WITHOUT CONSIDERATION

FOR ANY PREVENTIVE ACTION IN ANIDEAL SUPPORT FACILITIES SHALL

OPERATE SATISFACTORILY AT ANY

GIVEN POINT OF TIMEAi = MTBF/MTBF+MTTR


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AVAILABILITY (OPERATIONAL)

Ao = MEAN TIME BETWEEN FAILURES/MTBF+MEAN TIME WAITING FOR

SPARES+ ADMINISTRATIVE TIME+ MEAN

TIME FOR REPAIRS


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Reliability, Availability, Maintainability

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