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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)
PREVENTIVE MAINTENANCE(PM)
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PREVENTIVE MAINTENANCE(PM)
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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THANK YOU