New York Power Authority (NYPA; White Plains, New York, U.S.) owns bulk power transmission lines in New York and has been exploring possible ways to limit cascade-type failures. H. Brian White, a major contributor to transmission-line design for more than 50 years, conceived a low-profile, mid-span failure-containment structure and suggested this design to NYPA.

Under White's guidance, Sam Raju, a senior engineer from NYPA, developed this concept into a detailed design for testing at one of NYPA's facilities. NYPA engineer Fred LaChance oversaw the field installation, and NGK Locke Inc. (Baltimore, Maryland, U.S.; www.ngk-locke.com) supplied special NCI insulators for the structure. In mid-October 2006, NYPA successfully tested this failure-containment structure proposed for deployment on existing lines thought prone to cascading after an initializing event.

In the short five-span section of a typical 230-kV wood pole line, the test structure was installed at one mid span and a conductor failure was simulated at one dead end. Full wire tension was maintained throughout the test by weights hung at the other end. When the wire was broken, the restraining structure did its job and effectively stopped more than a minimum of wire movement down the line. There was some relief of tension (in the assemblage).

To the comments that it all looks so simple, White responded that they all do after some hindsight, but added that it worked just as expected, because the necessary calculations were properly made.

White has studied and written about failure containment of longitudinal cascades for more than 40 years and has boiled down the subject of successful failure containment to a few essentials. Probably the most important of these is that it is necessary to stop the movement of the wire down the line, and that can be done by a full dead-end strain-type tower or else something like the little structure under test, whereby the wires are caught and arrested near their low point of the sag. Such structures can be installed after a line has been in operation and has been deemed susceptible to cascading. The anti-cascade structures can be installed without service interruption.

Merely reinforcing something like one in ten suspension structures to resist adequate longitudinal loads will result, as it already has, in a series of one up and nine down.

Raju and White believe most issues with this concept have been resolved; however, further study is needed to determine the most feasible and efficient of several options for the 3-phase structural arrangements that will depend on voltage class and extreme loading demands.”