The impact of 600-V URD cable failures never received a great deal of attention, mainly because a single run of cable may only serve a few customers and, therefore, contributes little to system reliability indexes. Many utilities believe dig-ins cause the majority of their failures and reason there is little they can do to prevent them. However, a close examination of 600-V systems reveals some interesting results with respect to achieving a balance among system design, installation costs and failure rates.

Duke Power (Charlotte, North Carolina, U.S.) conducted a survey of 33 utilities that showed the number of failures per year varied from 47 to 7000. When the number of failures per year was based on the number of cable miles installed per year, the values converged to a range of 0.09 to 3.91 failures per mile (per 1.6 km). Assuming each failure costs US$750 to repair, the cost to the utilities in the survey would range from about $35,000 to $6 million, which illustrates that the direct cost of these failures has a major impact on incurred expenses.

While some utilities do a good job tracking 600-V cable failures, others do not track them at all. In the survey, 45% of the utilities reported their tracking systems would not accurately identify these failures and distinguish them from other types of outages. This leads to the assumption that some utilities actually experience more failures than they report and that their 600-V cable failures are a greater problem than they might realize.

The Best Materials and Practices

Many utilities that use cable-in-duct or install cable in conduit experience extremely low failure rates. These conduit systems provide good protection from some of the major sources of damage to the cable from other utilities, contractors and home owners. Conduit systems also prevent problems that are due to improper backfill and installation cable damage. In contrast, utilities that use direct-buried cable provide no protection from these sources of cable damage and, as a result, typically experience a greater frequency of failures.

With respect to conductor materials, aluminum conductor is subject to degradation if a breach in the insulation occurs. This allows an electrochemical process to initiate corrosion of the aluminum. This process continues until the metal corrodes to the point it no longer functions as a conductor. In contrast to the susceptibility of aluminum, copper is significantly more resistant to in-ground corrosion. When used as the conductor, it provides greater resistance to failure in case of damaged insulation. While 33% of the utilities surveyed use copper conductor for some of their circuits, only one utility used copper conductor exclusively.

Insulation on 600-V URD cables is typically about 65 mils in thickness; therefore, installation practices play a big part in the cable's future performance. Because the insulation is easily damaged, good cable-handling techniques are of such importance that installers should take special care not to drag the cable over rocks and debris, while ensuring that trenches and backfill materials are free of any hard objects.

When using conduits, crews must pull the cable carefully to prevent any scoring of the insulation by the edge of the conduit. As with any insulated cable, splices must be of the highest quality and properly waterproofed. The survey showed that 23% of the utilities indicated they had high occurrence of installation damage, which emphasizes the importance of proper handling during installation.

The Cost

A cost/benefit analysis illustrates if it is in the best interest of the utility to install the cable in conduit rather than directly in the ground. While the results of the survey show that all utilities could eliminate the majority of their 600-V cable failures if they were to use conduit, this solution may not always be cost effective. For example, Duke Power installs about 15 million ft (4572 km) of 600-V cable each year. The cost to install cable in conduit would amount to $19 million, which is several times greater than the cost to repair annual failures. Under the circumstances, when considering initial costs and annual repair costs, a direct-buried cable is the most cost effective.

In general, the utility must consider installation costs, failure rates, the cost to repair and customer satisfaction to identify the best practice regarding system design, installation and operations. The utility also must consider burial depth, cable markers, copper vs. aluminum conductors, and front-line versus rear-line construction. For some utilities, a conduit system that offers a very low failure rate may provide the best solution, while other utilities will find that a direct-buried system, with a higher failure rate, is more cost effective. Obviously, each utility must determine what it is that best meets its requirements.

Duke Power's Experience

A study of 600-V cable at Duke Power showed that these cables suffered failure very early in their service lives: more than one-quarter of all insulation failures occur on cable that is less than one year old; one-half of all failures occur on cable that is less than three years old; and more than three-quarters of all failures occur on cable 10 years old or less. The study showed that failures could be categorized with respect to frequency of occurrence as the pie chart illustrates.

The fact that these percentages exceed 100% is because more than one condition exists at the time of failure. Having identified these conditions, Duke Power is now in the process of formulating a set of programs that will minimize the risk to URD installations in the future. These programs include:

• Training both Duke and contract line technicians.

• Certifying line technicians to perform specific tasks.

• Performing installation quality audits.

• Requiring installers to sign off on their work by signing their names and the date of installation on all cables and splices.

• Obtaining a warranty from contractors on installation quality.

• Searching for splices that perform well regardless of installation technique.

• Utilizing one contractor to install facilities for several utilities on certain projects.

• Using self-sealing cable for specific applications.

Since the implementation of some of these programs, Duke Power already has observed a reduction in failure rates. The utility expects failures will continue to drop as it implements all of the programs. With more cable installations in the future, the success of these efforts becomes measurable in the near term as a consequence of the number of failures usually experienced in the early years of cable service. The success of this program depends on a decrease in the number of failures during the first few years of service.

Chris Fletcher received the BSEE degree from the University of South Carolina in 1974 with the BSEE degree. He joined Duke Power that same year and has held several field and office positions since then. Fletcher presently works in the Distribution Standards Group and is responsible for underground design and underground materials. He participates in several industry committees and is serving as chairman of the AEIC Cable Engineering Committee.