Informational Brochure

Selection and Application of Outdoor Air Switches Introduction Outdoor Air Switches are an essential element of electrical power transmission and distribution systems. They provide positive, visible air gap isolation of equipment and line sections for safe examination, maintenance, and repair. In the closed position, air switches must provide adequate capacity to handle all normal and abnormal currents that flow in the system. Finally, air switches must provide for ease of mechanical or electrical operation even under adverse conditions such as heavy ice coatings or corrosive atmospheres. In order to properly maintain the integrity of an electric power system, careful attention must be given to the selection and application of air switches . Insulator Cantilever Strengths Insulator stacks for any given voltage rating are available with several cantilever strength ratings such as standard strength, high strength and extra high strength. To determine which stack rating to use, it is necessary to calculate the short-circuit forces per linear foot acting on the conductors which the insulators are supporting. The NEMA formula for calculating this force is as follows:

The important considerations are:

x Insulation level to be provided. x Continuous and momentary currents to be encountered. x Insulator characteristics required. x Electrical Clearances and space limitations. x Current interrupting requirements. Air switches are built in a variety of physical forms to accommodate the various requirements of electrical clearances and limitations. Also, when used as interrupter switches, various interrupting attachments are available. Information in this publication provides a basis for selecting equipment best suited to solve most common applications. fault forces, under test conditions that are as little as 35 percent of those calculated by the equation. It is generally agreed that for outdoor substation practice, a safe multiplier to be used with the equation results is somewhere between 0.5 and 0.6. It is our standard practice to use 0.6 multiplier unless otherwise specified by the user. Further, it is our policy to limit the actual forces on the insulators to a maximum of 60 percent of their published cantilever rating. For the example given, then, a more realistic fault force is 24# x 0.6 or 14.4 pounds per foot. As noted in the bus arrangement, this force results in a maximum cantilever load of 576# at the insulator position, which supports the longest bus section. At the 115-kV rating, the published cantilever strength post-type insulator stack is 1700 pounds. When used at 60 percent of rating, the unit would have a safe allowable strength of 1020 pounds, which is quite adequate to satisfy the maximum loading required in the example. It should be noted that short-circuit loadings on switch insulators are usually less than the loadings on adjacent bus insulators, since the conductors terminate on the switch insulators and only 50% of the span applies. Notes: 1. The above applies to both upright and inverted mounted arrangements using published cantilever values. However, for insulators mounted horizontally, it is standard practice to use 40% of the upright published cantilever rating for short circuit forces. 2. If the short-circuit current (RMS asymmetrical) is not known, then the momentary rating of the switches, or the

The theoretical equation, as presented, has been found to produce fault forces on busses that are in excess of those actually experienced. Because of the inherent inertia of the bus bars, the flexibility of insulator mountings and supporting structures, some investigators have measured

breakers can be used, whichever is smaller. Switch momentary ratings are 3-phase RMS asymmetrical. Breaker ratings are 3-phase RMS symmetrical , which must be multiplied by 1.6 to convert to RMS asymmetrical

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