Wire Component Lifecycle
Author: Lectromec
Published: 06-20-2008
Wire
Component Life Cycle
Author:
Lectromec GSD Team
June 12,
2008
Component Lifecycle
In the
life cycle of components, including wiring and connectors that make
up the interconnection harnesses, there are three phases of the
component lifecycle that can be distinguished. They make up the
“bathtub curve” and are:
Infant
mortality: Failures caused by defects in manufacture and/or
installation. Handling wire failure during the installation and/or
manufacturing process was covered in another document. ./Images/Article62,Click here to review this .jpg.
Constant
(Random): Failures caused by external trauma (lightning strike, tool
accidentally dropped onto a harness, etc.)
Wear
Out: An increasing number of failures due to environmental aging of
the materials or the accumulation of minor damages to the object or
system.
Figure 1: The observed
failure rate (Blue) is a composite of the infant mortality failure
(Pink), the constant failures (Green), and the Wear-out Failure (Red)
rates. Time not to a linear scale.
Before a
wire maintenance program is initiated, it is important to determine
at what point along the ‘bathtub curve’ the different
harnesses are, and determine where this is with respect to the
transition from random to age related failures. To do this, a time
depend failure function that depend on wire properties, routing
considerations, and environmental conditions must be developed. This
function, when combined with harness criticality data and maintenance
scheduling, can be used to define the risk of failure and a
subsequent cable replacement strategy will be developed.
Cable
Replacement Schedule
A
simplified approach points to two types of maintenance: corrective
maintenance (after failure) and preventive maintenance (mainly before
failure). The decision diagram for the choice between corrective and
preventive maintenance can be seen below.
Figure 2: Decision diagram for corrective and
preventative maintenance.
Preventive
maintenance can be further subdivided into: time-based maintenance
(carried out at regular intervals of time), use-based maintenance
(carried out after a fixed cumulative use, operation, or load), and
condition-based maintenance (carried out at times determined by
(non-)periodic inspection or continuous monitoring of a component’s
condition). Selection of the type of maintenance is an important
first step in the maintenance process definition. Application of the
wrong step can lead to insufficient attention maintenance actions to
protect the systems or cause wasted resources by overestimating the
cost per failure.
How to
Approach the EWIS Problem
Whether
the wiring is on an aircraft, ship, or any other platform, the
Electrical Wire Interconnection Systems (EWIS) are typically routed
through high density areas with systems of differing degrees of
criticality mixed into the same wiring harnesses. To do a manual
investigation of the routing of each system and the consequences of
failure is often found to be cost prohibitive regardless of the
platform. In the case of the aerospace industry, Lectromec developed
an EWIS Risk Assessment Tool (RAT) to meet this need. The processes
used by the EWIS RAT™ can be used as a blueprint for evaluation
and maintenance needs of cable interconnection systems on any
platform. An investigation should evaluate the probability and
severity of failure(in the case of the EWIS RAT™, this is
called a Failure Matrix Report). This should take the available data
for the wiring and environmental conditions and calculate the failure
rates of the individual wires as well as the failure probabilities of
the bundle sections and complete harnesses.
The
process for the evaluation of the cable/wiring of most large systems
is rather complex; thus there exists a need for software that can
handle the processing and analytical needs associated with such an
effort. After the initial analysis of the system is completed with
the available data, the analysis of the maintenance data logs should
be conducted. It is important that this maintenance data be
evaluated by those with experience and knowledge with wire
degradation and electrical interconnect system investigations. To
supplement this, maintenance personnel should be interviewed to
gather additional information about the systems.
It is
often found that the available data is insufficient detail necessary
to develop a robust failure function that can model the
characteristics of all environments within the selected system. To
resolve this issue, a technique known as paired comparison with
expert opinion (PCEO) can be implemented to gather additional
information about the system. Lectromec successfully applied this
methodology to aerospace platforms and the same methodology to other
platforms. This data generated via PCEO can be combined with the
maintenance data to create a time and environment dependent failure
function. The failure function will then be used to create a time
dependent risk-hazard analysis that will define those cables that are
in the greatest need of maintenance action and/or replacement. The
replacement plans can be evaluated utilizing replacement optimization
strategies. This program can be delivered as the final replacement
schedule that includes the cable life expectancies of those items in
the system.
It is
because these systems are similar to what Lectromec has encountered
in the past, Lectromec can provide significant insights into the
aging of the wire and cables and the replacement strategies.
Conclusion
Whether
you are starting to define your wire maintenance plan or are looking
for a review of existing processes, Lectromec can provide experience
and guidance in the design, development, or re-evaluation of wire
programs. Implementation strategies could include the use of such
Lectromec systems as the EWIS RAT™ which, though originally
designed for aerospace applications, can be applied to ground and
naval platforms, as well as fixed location facilities.