Medium-voltage (MV) cables are a vital component of your plant’s electrical power distribution system. Although their service life is expected to be long and they are generally highly reliable, MV cable systems can and do suffer from deterioration, defects, and faults, which can eventually result in complete breakdown and cable failure. Such failures can, in turn, lead to unplanned power outages, causing substantial losses including loss of plant production, significant equipment damage, and even personnel injury or death.
To protect people, operations, and the bottom line, you need insight into the condition of your MV cable systems. Fortunately, there are several tests and techniques that can help you keep a finger on the pulse of the health, performance, and reliability of these systems. However, choosing the right tests from myriad available options isn’t always a straightforward process. Let’s consider some of the more-popular testing options for MV cables and explore which test methods are best for a given installation or specific testing philosophy.
Industry standards: a great place to start
When it comes to selecting the best MV cable system tests for a particular facility or application, many plant managers turn to industry standards. In the United States, there are at least 10 standards that provide information about field testing MV cable and components. Among these are standards from the Institute of Electrical and Electronics Engineers (IEEE), the National Electrical Manufacturers Association/Insulated Cable Engineers Association (NEMA/ICEA), the Association of Edison Illuminating Companies (AEIC), National Fire Protection Association (NFPA), and the InterNational Electrical Testing Association (NETA).
These standards are continually being reviewed and updated, and they provide a good overview of the types of tests available. However, when choosing the best tests for your specific situation, you’ll want to consider the age and general condition of your cable system as well as the service level (loading/load cycles), operating environment, history, and criticality of equipment served. We’ll now take a closer look at each of these issues, beginning with understanding the lifespan of your MV cable system.
Understanding cable life
In general, an MV cable system is expected to provide reliable service for approximately 35 to 40 years. However, cable system components (conductor, insulation, shields, jacket, etc.) can be damaged mechanically during shipping, handling, or installation or by or other physical means such as excavation, settling of the earth, or support failure. Additionally, operational damage can occur due to severe load cycling, overloads, short-circuit currents, voltage transients, and lightning surges. The environment in which the cable operates also has a significant impact on lifespan. Exposure to extreme temperatures, moisture, heavy vibration, or chemicals can contribute to premature deterioration and/or failure.
Regardless of specific stressors, cable systems experience periods within their life cycle during which failure is more predominant. The first year of a cable’s operating life is considered the “infant mortality” period. If installation damage or manufacturing defects are not detected during initial acceptance testing, these problems will likely show up during this time.
Between initial operation and end of life, cable systems go through a period of relatively high reliability. Occasional failures may occur as a result of workmanship issues or uncontrolled factors, but in general, the cable system will exhibit its highest reliability during this period.
After a long period of reliable service, operational and environmental factors will lead to deterioration of cable components, creating an increased rate of failure. This is an indication that the cable system is approaching the end of its reliable life.
Different tests are recommended during different life-cycle periods. Here’s a look at the different types of cable tests and when to use them.
Types of MV cable testing
In general, there are two types of cable testing. Withstand tests are intended to stress the cable’s insulation and cause the cable system to fail at defects or areas of significant deterioration that could likely cause the cable to fail in the near future during normal operation. Condition assessment or diagnostic tests produce quantitative and trending data without causing (or intending to cause) failure or damage. These tests are used to determine the extent of deterioration or changes to the cable’s insulation system. Both types of tests play an important role in evaluating the reliability of the cable and determining if the cable is reaching the end of its life. Following is a closer look at the specific types of test within each category.