Overload switch

Abstract

In a transformer primary circuit breaker having a primary switch for opening and closing the transformer primary circuit, an operating handle mounted exteriorly of the transformer for opening and closing the primary switch, a trip mechanism operatively connected to open the primary switch under fault current conditions, the trip mechanism including a conductive metallic sensor element connected in the primary circuit and having a predetermined Curie temperature and a magnet mounted for movement into engagement with the sensor element and biased to open the primary switch when the temperature of the sensor element approaches the Curie temperature, the improvement comprising a shunt circuit connected to the primary circuit in parallel with the sensor element, a normally closed switch in the shunt circuit, a resistance element for limiting the current passing through the shunt circuit, and a tab mounted on the operating handle for opening the shunt circuit switch, a spring mounted in the operating handle for limiting the motion of the operating handle to thereby close the shunt switch, whereby a portion of the primary current will flow through the shunt circuit when the switch is closed. A bimetal switch can be included in the shunt circuit to open the shunt circuit at predetermined oil temperatures or excessive currents.