UNIVERSITY OF SOUTH AFRICA (UNISA)
College of Science, Engineering and Technology
⋄
ASSIGNMENT 1
Year Module – 2026
⋄
Module Code: INE2602
Module Name: Reliability Engineering
Assignment No.: 1
Semester: Year Module – 2026
Submitted in partial fulfilment of the requirements for Reliability Engineering
at the University of South Africa.
,UNISA | INE2602 Reliability Engineering – Assignment 1
Question 1: Reliability Engineering Fundamentals
1.1 Definitions of Reliability Engineering and Reliability Metrics
Question: Define the terms: reliability engineering and reliability metrics, identifying their
purposes in industry.
Reliability Engineering is the discipline concerned with ensuring that a product, system,
or component performs its intended function without failure for a specified period under de-
fined operating conditions. Its purpose in industry is to minimise unplanned failures, reduce
maintenance costs, and extend asset life cycles, particularly in capital-intensive sectors such as
mining, manufacturing, and power generation (Moubray, 1997).
Reliability Metrics are quantitative measures used to assess, monitor, and improve the per-
formance and dependability of systems over time. Their purpose is to provide engineering
teams and management with objective data to guide maintenance strategies, procurement de-
cisions, and system design improvements (Smith, 2011).
1.2 Key Reliability Metrics
Question: Name and define the key indicators or metrics for reliability.
1.2.1 Failure Rate (λ)
The failure rate is the number of failures occurring per unit of operating time. It expresses
how frequently a component or system is expected to fail and is given by:
Number of failures
λ=
Total operating time
1.2.2 Mean Time Between Failures (MTBF)
MTBF is the average operating time between successive failures of a repairable system. It is
the reciprocal of the failure rate:
1 Total operating time
MTBF = =
λ Number of failures
Page 1 of 21
, UNISA | INE2602 Reliability Engineering – Assignment 1
1.2.3 Mean Time to Repair (MTTR)
MTTR is the average time required to restore a failed system to full operational status. It
measures maintainability and is calculated as:
Total downtime
MTTR =
Number of repairs
1.2.4 System Reliability R(t)
System reliability is the probability that a system performs its required function without fail-
ure over a specified time interval t under stated conditions:
R(t) = e−λt
1.2.5 Maintainability
Maintainability is the probability that a failed system can be restored to operational condition
within a specified time period, given that maintenance is performed under stated conditions.
It reflects how easy and fast a system is to repair.
1.2.6 Availability (A)
Availability is the proportion of total time that a system is in an operational state, capable of
performing its function:
MTBF
A=
MTBF + MTTR
1.2.7 Uptime
Uptime is the total duration during which a system is fully operational and performing its in-
tended function. It is the complement of downtime within any given observation period.
1.2.8 Downtime
Downtime is the total duration during which a system is not operational due to failure, main-
tenance, or repair. It directly reduces availability and productivity.
Page 2 of 21
College of Science, Engineering and Technology
⋄
ASSIGNMENT 1
Year Module – 2026
⋄
Module Code: INE2602
Module Name: Reliability Engineering
Assignment No.: 1
Semester: Year Module – 2026
Submitted in partial fulfilment of the requirements for Reliability Engineering
at the University of South Africa.
,UNISA | INE2602 Reliability Engineering – Assignment 1
Question 1: Reliability Engineering Fundamentals
1.1 Definitions of Reliability Engineering and Reliability Metrics
Question: Define the terms: reliability engineering and reliability metrics, identifying their
purposes in industry.
Reliability Engineering is the discipline concerned with ensuring that a product, system,
or component performs its intended function without failure for a specified period under de-
fined operating conditions. Its purpose in industry is to minimise unplanned failures, reduce
maintenance costs, and extend asset life cycles, particularly in capital-intensive sectors such as
mining, manufacturing, and power generation (Moubray, 1997).
Reliability Metrics are quantitative measures used to assess, monitor, and improve the per-
formance and dependability of systems over time. Their purpose is to provide engineering
teams and management with objective data to guide maintenance strategies, procurement de-
cisions, and system design improvements (Smith, 2011).
1.2 Key Reliability Metrics
Question: Name and define the key indicators or metrics for reliability.
1.2.1 Failure Rate (λ)
The failure rate is the number of failures occurring per unit of operating time. It expresses
how frequently a component or system is expected to fail and is given by:
Number of failures
λ=
Total operating time
1.2.2 Mean Time Between Failures (MTBF)
MTBF is the average operating time between successive failures of a repairable system. It is
the reciprocal of the failure rate:
1 Total operating time
MTBF = =
λ Number of failures
Page 1 of 21
, UNISA | INE2602 Reliability Engineering – Assignment 1
1.2.3 Mean Time to Repair (MTTR)
MTTR is the average time required to restore a failed system to full operational status. It
measures maintainability and is calculated as:
Total downtime
MTTR =
Number of repairs
1.2.4 System Reliability R(t)
System reliability is the probability that a system performs its required function without fail-
ure over a specified time interval t under stated conditions:
R(t) = e−λt
1.2.5 Maintainability
Maintainability is the probability that a failed system can be restored to operational condition
within a specified time period, given that maintenance is performed under stated conditions.
It reflects how easy and fast a system is to repair.
1.2.6 Availability (A)
Availability is the proportion of total time that a system is in an operational state, capable of
performing its function:
MTBF
A=
MTBF + MTTR
1.2.7 Uptime
Uptime is the total duration during which a system is fully operational and performing its in-
tended function. It is the complement of downtime within any given observation period.
1.2.8 Downtime
Downtime is the total duration during which a system is not operational due to failure, main-
tenance, or repair. It directly reduces availability and productivity.
Page 2 of 21
