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Efficient On-Line Testing of Electrical Cables Using LIRA®

Posted by AnnMarie Fauske on Jan 21, 2015 2:23:00 PM
By: James Raines, Manager CHAMP(Cable Health & Aging Management Program), Fauske & Associates, LLC
 
Electrical cables are the lifeblood for any industrial facility, from chemical and petrochemical plants to Figure 1: LIRA® testing of an underground electrical transmission circuitfossil and nuclear power generating plants. Understanding the condition of these cables, and
identifying potential issues - is essential to keeping plants up and running and avoiding costly down time. In order to understand a cable's condition, several parts of the cable need to be evaluated.
 
The key component of a cable’s construction is the insulation material which isolates the conductor from its surroundings.  The jacket of the cable provides physical protection during and after installation.  We notice that a simple internet search of cable aging, cable fault, cable failure, cable damage and so forth, does not yield a great deal of information on highly efficient test methods such as LIne Resonance Analysis (LIRA®). 
 
There are several cable test methods available to evaluate cable performance.  This cable condition monitoring test is unique in that it not only provides an indication of overall cable health, but also locates degraded areas.  A non-destructive examination tool that does not over-stress the cable; it uses a relatively small 5 volt peak-to-peak (Vpp) signal to perform its evaluation.   It performs both global and local assessments of the test cable.  Importantly, the test cable does not need to be disconnected and testing is completed in minutes.  Leaving the cable connected eliminates a potential problem source: the cable termination.
 
How Does It Work?
LIRA transmits a frequency modulated wave through the insulation material of the test cable. The impedance spectrum, phase, and amplitude of the returning frequency are analyzed to establish the overall health of the cable and identify locations where it has been locally degraded.  With this information, local repairs can be made, rather than replacing the entire cable.
 
In addition to localized insulation damage, it also provides an overall assessment of the cable’s global condition. This parameter, LIRA Delta-G (LDG), is establishing itself as a measurement that provides an indication of the cable’s global health. This information can be used to estimate the life expectancy of the cable and to plan for its replacement.
 
What Do You Need to Use It?
Testing a cable with LIRA is quick and easy. All that is needed is for the cable to be de-energized (it does not need to be de-coupled) and connection of the two LIRA test leads to two metal cores of the cable (for example, two conductors or one conductor and a shield). Depending upon the length of the cable under test, a typical test will take about three minutes. This test provides both a global assessment of the cable along with a localized assessment of cable degradation areas (if any).
 
Give Me An Example
The following is an example test result.  A 150 feet (46 meters) long, low voltage two conductor cable was tested with it. This cable was located close to a high temperature heat source and sustained thermal damage to the cable insulation at 71 feet (22 meters).  One of the key output plots is called the LIRA Signature plot, which is a normalized graph to account for attenuation effects (increasing with the signal frequency). The Signature plot for this example is shown in Figure 2.
 
 
LIRA_PlotFigure 2: LIRA Signature Plot – Thermally stressed cable

The abscissa (x-axis) of this signature plot shows cable length with the test leads connected at the origin.  The  test signal gain is shown on the ordinate (y-axis).  The gain represents the power ratio (given in decibels) between the reflected and incident wave at each cable location.  The magnitude of the test signal provides information about potential issues to which the cable has been exposed.  Typically, signature signals above an established threshold identify areas to be investigated.  These signatures can indicate splices or insulation-degraded areas.  For all LIRA tests there are peak values associated with the signal passing through the cable at the entry and exit points. As shown in Figure 2, a signal spike occurs at 71 feet, which corresponds to the location where thermal damage was identified.
 
For additional information on LIRA as well as other cable testing methods, please contact: James Raines, Raines@Fauske.com, +1 (630) 887-5241
 
LIRA® is a product and registered trademark of Wirescan AS
 
EQ and Cable Health & Aging White Paper
 
 

Topics: cable aging, cable failure, lira

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