Failure Rates of Underground Cable Systems
Failures of cables, joints, terminations, and separable connectors
Topics / Updated Information
Composition of Underground Systems
Introduction
Electric distribution systems are large and complex, composed of many different functional components installed at different points in time. This diversity of age / design / function makes reliability analysis a complicated problem. Nevertheless, as distribution systems age and more distribution assets are placed underground, engineers and planners use failure rate information as inputs to their decision matrices. This page provides the most recent analysis of collected failure data from utilities.
Based on information from 19 utilities. The median average, excluding third party damage, for all components is estimated at:
2.89 failures / 100 conductor miles / year.
– Extruded cables account for more than 50% of these failures.
The utility systems, for which EPRI has received data, will have differing ages and compositions. Approximate timelines for the use of selected cable insulations in North America are listed in the 2023 EPRI Bronze Book. The figure below shows the density distribution of the distribution failure rate for all components.
Analysis
Utilities (Investor Owned, Cooperative, and Public Power) were requested to provide their estimates for numbers of failures, excluding third part damage, and their split between failures in cable, joints, and terminations. The figure below shows the density distribution of cable failure rates after adjusting for the length of each underground system. The spread of the data shows the diversity in failure rates experienced by different cable users, with the majority in the range of 0.7 to 4 failures / 100 miles / year.
Reported Cable Failure Rates
The current estimates of the median failure rates are tabulated below.
| Median Failure Rates (Failures / 100 conductor miles / year) | |
|---|---|
| Cable | 1.2 |
| Joints | 0.7 |
| Terminations | 0.2 |
It is anticipated that these estimates will be refined as more utilities provide data.
Notes:
- The designation of an accessory failure vs a cable failure has been made by each contributing utility. Thus, it is possible that different separation criteria were used.
- The cables and accessories within the utility systems, for which EPRI has received data, will have differing ages and compositions. Approximate timelines for the use of selected cable insulations in North America are listed below (further information in the 2023 EPRI Bronze Book).
- Analysis of the anticipated lifetimes has not yet been conducted as part of this work.
Insulation Technology Timeline (approximate usage in North America)
Discussion and Future Work
Underground assets follow the typical “Bathtub Curve” where early in their life some locations experience failure due to manufacturer and installation issues; then there is a long middle-life period of low and relatively stable failures; finally, the equipment begins to wear out and aging causes the failure rate to accelerate. Moreover, utilities operate a mix of designs (see estimate below) and installations each with their own longevity characteristics. Yet, the annual failure rates, experienced by utilities are a composite of all these factors.
Research is underway that will investigate ways to tease the age and mix factors apart such that members will have access to more granular data. In 2024, EPRI will provide opportunities for you to contribute and derive value from these studies for your utility.
EPRI Technical Contacts
Joshua Perkel
jperkel@epri.com
Nigel Hampton
nhampton@epri.com
Estimated Installed Cable System Population (by insulation type)
Bibliography
EPRI Underground Systems Reference Book 2023 Edition – Chapters 12 & 17
Lawson JH and Thue WA: “Summary of service failure of high voltage extruded dielectric insulated cables in the United States,” ICEI Boston, 1980, pp. 100-104