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3.1.5 MTBF Evaluation
3.1.5.1 Purpose
The purpose of this evaluation is to calculate the reliability of the Almos
supplied hardware.
3.1.5.2 References
Collection of reliability, availability and maintainability data for electronics and
similar engineering use - AS2529, 1982
Presentation of reliability data on electronic and similar components (or parts)
AS2530, 1982
Reliability and maintainability - Introductory guide - AS3930, 1992
Reliability Prediction Procedure For Electronic Equipment - Bellcore TR-NWT-
000332, Issue 4, September, 1992.
3.1.5.3 Definitions
MTBF - (Mean Time Between Failure) - Average time (expressed in hours) that
a component works without failure. It is calculated by dividing the total
number of failures into the total number of operating hours observed. Also,
the length of time a user may reasonably expect a device or system to work
before an incapacitating fault occurs.
MTTF - Related acronym (Mean Time To Failure) which is frequently used
interchangeably with MTBF.
3.1.5.4 Approach
Almos established via test data obtained under the operating environment
required for the REMOTE STATION hardware and by analysis of existing data,
that the supplied hardware will meet the mean-time-between failure (MTBF)
of 30,000 hours.
3.1.5.5 Reliability Data
The data used in this analysis was extracted from the population of equipment
installed for the Australian Bureau of Meteorology and relates to similar
hardware operating under similar conditions to those proposed for the
REMOTE STATION.
Upon request, a spreadsheet is available which provides the raw data used to
obtain the figure shown below as Total Hours.
TELVENT Low Level Windshear Alert System
Almos
57
LLWAS
Low Level Windshear Alert System
Data Item Value Used in Calculations
Oldest Unit (Years) 3.6 Yes
Machine Population 222 Yes
Total Hours 3,162,240 Yes
Failures 11 Yes
NFF 0 No
User Caused 3 No
Table 1 - Field Failure Data
3.1.5.6 Reliability Calculations
MTBF is Mean Time Between Failure and is typically expressed in hours. The
hours are calculated by dividing the total number of failures into the total
number of operating hours observed. For Almos, MTBF represents the average
number of hours a field population will work before a failure occurs.
3.1.5.6.1 General Assumptions
3.1.5.6.1.1 User caused failures
Any failure that was directly attributed to user negligence or abuse has been
shown in Table 1 but has not been included in the MTBF calculation.
3.1.5.6.1.2 No fault found
Units returned are subjected to extensive testing before release back to the
customer. Units with “No Fault Found” have been shown in Table 1 but have
not been included in the MTBF calculation.
3.1.5.6.2 Method 1
3.1.5.6.2.1 Basis of Calculation
Age of units - The population of equipment used in this analysis was supplied
over a period of time. It is assumed that the rate of supply of equipment is a
constant and the average age of all units is therefore half the age of the
oldest unit.
Operating Hours - The equipment is assumed to be in service for 100% of the
time. (24x7x365)
TELVENT Low Level Windshear Alert System
Almos
58
LLWAS
Low Level Windshear Alert System
3.1.5.6.2.2 Calculation
MTBF =
MTBF = = 318,227 hours = 36.3 years
3.1.5.6.3 Method 2
3.1.5.6.3.1 Basis of Calculation
Actual Operating Hours - The population of equipment used in this analysis
was supplied over a period of time. The actual year and month of supply for
each individual unit was used to obtain the age of the equipment. This was
used to calculate the total hours since supplied. (Refer to attached
Spreadsheet)
Operating Hours - It was assumed that each item was not installed for one
month after being supplied and thereafter has been in service for 100% of
the time. (24x7x365)
3.1.5.6.3.2 Calculation
MTBF =
MTBF = = 272,945 hours = 31.2 years
3.1.5.7 Results
The lowest figure provided by the two methods used has been selected as the
more accurate, being based on actual dates rather than an assumption about
the linearity of supply.
MTBF = 31.2 Years.
3.2 Supportability
The system has many built in test (BITE) and diagnostic features, which make
the maintenance of the system very efficient for trained personal.
Remote diagnostics means that engineers from Almos Systems headquarters
may dial into the system and analyse any faults that are being reported.
The Site Performance Evaluation Subsystem which is offered as a standard
tool, aids in the long-term accuracy of the LLWAS system.
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