Belowground Enclosures that House Electric or Gas Facilities are Critical to the safety of third parties, often pedestrians, as well as the protection of utility workers and equipment. With 5 million electric and gas customers in a 94,000-sq-mile (243,460-sq-km) service territory, Pacific Gas & Electric Co. (PG&E; San Francisco, California, U.S.) has more than 300,000 enclosures in the ground.

When an underground enclosure fails, for example, due to a broken cover or cracked sidewall, a utility is exposed to potential liability in addition to any repair and replacement costs. To minimize these issues and to ensure the reliability of the system, PG&E developed performance-based specifications for underground enclosures that will help guarantee system integrity over the long run.

In contrast to design guidelines, performance-based specifications detail how an enclosure will function in its application based on specific test criteria that the enclosure must meet. Performance-based specifications are at the core of PG&E's material-procurement program. This approach ensures quality enclosures based on performance requirements. The manufacturer has the freedom to incorporate product enhancements and new technology. The intent of the utility is to tell the manufacturer what is needed, not how to produce it.

WESTERN UNDERGROUND COMMITTEE GUIDELINES

PG&E's performance-based specification for underground electric enclosures was based on the work of the Western Underground Committee (WUC), Guidelines 3.3 and 3.6, which calls for three-position testing: lateral sidewall loads, vertical sidewall loads and vertical cover loads.

About 40 years ago WUC developed the three-position structural testing criteria that are still being used today. The WUC members had the foresight to develop guidelines for underground enclosures at a time when the underground system was relatively new. There were no industry standards, but members wanted consistency among enclosures to capture the economic benefits of interchangeability among components of different manufacturers. For example, interchangeability would allow for different extensions and covers to be used on different manufacturers' enclosures. While interchangeability was achieved in the guidelines, the WUC enclosure was essentially a non-traffic-rated enclosure. In the field, experience has shown that, while construction is still in progress, a contractor's vehicle can and will often go right across the top of an enclosure.

The WUC methodology was incorporated in the current standard of the American National Standards Institute (ANSI). Recently, ANSI adopted an updated standard based on a specification written by the Society of Cable Telecommunications Engineers (SCTE) for the performance of underground enclosures. The resulting document, officially referred to as ANSI/SCTE 77 2007 Specification for Underground Enclosure Integrity, supersedes ANSI/SCTE 77 2002 Specification for Underground Enclosure Integrity, which was the first national performance-based standard to not be material specific. This current standard helps ensure long service life, minimizes maintenance and reduces liability issues related to underground enclosures. With it, the underground utility engineer can be confident that an enclosure will perform as expected.

A PERFORMANCE-BASED SPECIFICATION

Prior to the 2002 ANSI Standard, PG&E decided to develop its own performance-based specification for underground enclosures using three basic criteria: safety, product performance and product cost. In addition, PG&E uses enclosures sized to meet specific operational needs in the field. For example, PG&E sizes enclosures large enough to splice cables without damage and to enable operation/physical movement of the cables without damage. Some utilities, including PG&E, have their own material testing programs for product acceptance, and they develop new products in cooperation with manufacturers. This is an example of how performance-based specifications can require a manufacturer to meet the needs of the utility.

A national performance-based standard would serve all utilities, large and small alike, because it could be written to eliminate a variety of problems that all utilities experience with underground enclosures. These problems include, but are not limited to: cracking and chipping, fractures of materials from extreme temperatures, sidewall deflection from soil and water pressure, and the need for heavy equipment to handle enclosures. With the PG&E performance-based standard, breakage of the enclosure installed in the field has been virtually eliminated.

A foremost consideration in developing the PG&E performance-based specification was to base it on performance, not on materials. Any material has the potential to perform well or poorly in certain applications. If the enclosure using a particular material is not engineered properly or constructed well, then the material alone will not prevent an enclosure from failing.

IMPORTANCE OF STRUCTURAL TESTING

An important requirement to incorporate into any enclosure performance-based specification is the previously mentioned three-position structural testing. This test is the best way to show the true strength of an enclosure. In a three-position test, a load simulating the weight of a car wheel is applied to three areas of an enclosure. The first position tests the lateral sidewall of the enclosure, which is the area that must withstand soil surcharges as vehicles approach. The second position tests the same loading when applied directly onto the vertical sidewall. The third position tests how the enclosure responds to the load when applied directly to the center of the cover. Some manufacturers use only center-of-cover test results to represent the strength of their enclosures. If lateral and vertical sidewall strength is not tested and disclosed, then the real strength of the enclosure is unknown. In that case, the utility should obtain its own test results.

The PG&E performance-based specification includes rigorous, meaningful testing requirements that encompass physical, environmental and internal equipment protection tests geared toward ensuring a long service life and reducing maintenance. It should be noted that ANSI/SCTE 77 2007 Specification for Underground Enclosure Integrity provides an excellent guide to the tests that utilities can adopt for testing enclosures.

Along with three-position testing, loading criteria for enclosures must be established based on the anticipated loads. The table on page 58 contains examples of loadings that are separated into tier levels defined by the application. Note the presence of both design and test loads. A test load is the minimum ultimate failure load, and while this number is good to know for understanding the enclosure material and design, it should never be used in specifications to represent the strength of an enclosure. Instead, utility engineers should insist on specifying a design load for each tier or type of application. PG&E uses a safety factor of 2.0 for design loads, but a more typical safety factor of 1.5, required by the ANSI standard, is generally quite sufficient. If the design load is not specifically enumerated, the manufacturer should be required to provide certified test data to substantiate the three-position design loads of their products.

In performance-based specifications for underground enclosures, safety is the most important benefit. Other benefits include not only decreasing the number of field failures, but also curtailing potential liabilities utilities are exposed to as a result of those failures. Enclosures that meet a good performance-based specification may cost a little more up front, but the overall cost savings are definitely worth the initial investment.

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James D. Sprecher is a principal electric engineer with Pacific Gas & Electric Co., where he oversees a team of engineers responsible for engineering specifications, construction standards and work procedures for PG&E's electric transmission and distribution system. He has a BSEE degree from the University of Wyoming and a master's degree in engineering management from Santa Clara University. JDS8@pge.com