Underground Residential Distribution (URD) Cable Replacement

The electric utility industry in the United States grew significantly the 1960s and 1970s (Refer to Figure 1) and this led to the installation of a large number of electric utility plant assets for generation, transmission, and distribution.

Concurrent with this spike in growth, suburban locations nationwide desired the more aesthetically appealing aspect of underground utilities, with some communities even mandating that all new development use underground facilities.

Rapid Suburban Growth

The result was a noticeable increase in the installation of underground cable and due to the sheer magnitude of work it required entirely different design and construction approaches commonly referred to as Underground Residential Distribution (URD). URD cable installation differed from the then-common method of installing underground cable in several ways:

• It was directly buried (not in the more typical manhole-and-conduit configurations),
• Insulation was of solid dielectric instead of paper-insulated lead-sheathed cable, and
• Insulation was concentric neutral because it was mostly single phase construction (the neutral could be wrapped in a sheath around the conductor instead of as a separate conductor).

Changes Led to Negative Consequences

At an early point in the deployment of this then new technology the industry experienced some negative consequences. While the very earliest installations tended to perform properly, three developments took place that caused challenges in subsequent years:

• The selected solid dielectric material – specifically unjacketed, un-stranded, and with high-molecular weight polyethylene (HMWPE) – proved to be failure-prone,
• The thickness of the insulation was reduced from 220mil to 175mil, and
• The burial was accomplished in such a way as to allow rocks and damaging bends to compromise the cable.

As a result, beginning in the 1980s and continuing up to the present utilities found that cable that was purported to have a 30-year average life was failing in a much shorter time frame. Therefore, URD cable replacement programs became (and are still) a regular part of almost every electric utility’s budget. In lieu of whole replacement, many utilities have adopted rules-of-thumb that after two or three splices on a section of primary cable have been made between two pad-mounted transformers, then cable should be replaced rather than further further repaired through splicing.

A subsequent wave of failures also occurred in the utilities that

• Switched from HMWPE to cross-linked polyethylene (XPLE) yet maintained the 175mil insulation and unjacketed design;
• Used higher voltage URD cable, and
• Installed early versions of cable-in-conduit from a roll that had the cable and its conduit pre-combined.

 

URD Cable Replacement: Low Impact on Reliability, High Impact on Customer Satisfaction and Staff Productivity

The impact on electric system reliability (as measured by SAIFI and CAIDI) of an electric utility’s entire URD replacement program, which may total between hundreds of thousands of dollars to millions of dollars, is minimal. URD cable runs tend to involve at most 50 customers (oftentimes as few as 10 customers), so the number of customer interruptions during any one event is insignificant. Even if an electric utility experiences a few hundred URD cable failures per year, the impact to SAIFI is typically trivial. Therefore, the main driver behind utilities replacing failure-prone URD cable is customer-and cost-focused:

• Repeated URD failures lead to an increase in customer complaints
• Repair costs (on a per cable basis) tend to increase as once a URD cable starts to fail, the time between failures accelerates.
• Staff productivity suffers as URD failures are rarely predicted leading to emergent repair activity at the expense of other planned work.

1. energyauthority says:

   May 27, 2010 at 11:23 am

   Sure – glad to have it.