Failure Rates of Underground Cable Systems
Topics / Updated Information
Composition of Underground Systems
Historical Service Performance of Extruded Insulations
Introduction
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 2 failures / 100 miles / year.
Reported Cable Failure Rates
The adjacent figure shows the distribution of failure rates for the different components, 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. As an example the majority of cable failure rates fall in the range of 0.7 to 2 failures / 100 miles / year.
| Median Failure Rates (Failures / 100 conductor miles / year) | |
|---|---|
| Cable | 1.3 |
| Joints | 0.4 |
| Terminations | 0.2 |
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.
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
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