Switches of this type are usually disposed in series with a power switch in the load circuit so that their own contact-closing and contact-opening operations occur under virtual open-circuit conditions. A group of such switches may be used, for example, to connect the open power switch to any one of several associated supply lines in order to establish a selected current path for a given load. These switches also serve to indicate, by their readily observable position, whether or not a particular circuit is connected to the power line or disconnected therefrom.
Even though a switch of the kind here considered opens and closes in the absence of load current, it may still be traversed by residual commutation or capacitive currents at relatively low voltage giving rise to unavoidable arcing on the opening stroke. When closed, the switch must be able to carry not only the normal load current but also, in the event of a short circuit, a greatly increased current which flows until the associated power switch is opened.
In the closure position of the switch, the load current flows through relatively displaceable first and second metallic terminal members which should engage each other with a minimum contact resistance. It is therefore important to protect their contact surfaces against erosion due to arcing, and it is known to provide these terminal members for that purpose with metallic guard elements of lesser cross-section which carry but a small fraction of the load current and become disengaged only after the two terminal members have become separated on moving into a disconnect position. Since these guard elements are the first to engage each other upon reclosure of the switch, any arcing on opening or closure will occur between them only. A simpler arrangement of this character comprises just one metallic guard element electrically and mechanically connected with the first terminal member and positioned to be engaged prior to that terminal member by the second terminal member upon closure, any arcing being limited in that instance to the zone of engagement between the guard element and the second terminal member.
Such an assembly of conductive elements is known, for example from German laid-open application No. 28 09 499 which discloses a so-called one-column disconnect switch wherein the first terminal member is a horizontal metal bar suspended from a power line while two second terminal members are constituted by a pair of scissorlike contactors gripping that bar in the closure position. The scissor linkage is displaceably mounted on a ceramic column and its contactors, in one particular instance, are receivable between two converging wire loops that are conductively connected to opposite ends of the bar and serve as guard elements therefor. A similar one-column disconnect switch, disclosed in German laid-open application No. 28 39 914, has the contactors of its scissor linkage equipped with guard elements in the form of slender rods which are resiliently supported thereon and coact with similar rods resiliently supported on the metallic bar to be gripped by these contactors. A switch assembly of the same general type is the subject matter of German laid-open application No. 28 47 377.
In all these instances the conductive guard elements lie so close to the associated terminal members--as is necessary to limit the opening and closure strokes of the switch--that there is a danger of the arc jumping over to these terminal members. With a disconnect switch serving an outdoor power line, such as the one-column switches referred to, this risk is intensified by the possibility that a gust of wind may drive a guard element toward ics supporting terminal member.