A plurality of circuit breaker assemblies are often mounted together such that the tripping mechanism of each individual breaker can commonly trip the remaining breakers. U.S. Pat. No. 3,288,965 to Klein and assigned to the common assignee of the instant invention describes such a common tripping breaker assembly and is incorporated herein for purposes of reference. Each individual breaker contains a movable contact assembly, an operating spring, a releasable member called a cradle and a latch member. The cradle tip is held by the latch member until the latch is moved from under the cradle by conventional thermal or magnetic means to release the cradle and trip the breaker. The operating spring directly connects the movable contact assembly to the metallic cradle and its tip rests directly upon the metallic latch. There is no intentional connection of the latch to the load terminal, therefore there is no intentional current flow through the cradle to the latch engaging surface.
When the breaker is subjected to short circuit currents much greater than the original design requirements, the contacts are separated by thermal and magnetic forces between them before the latch can be disengaged from the cradle by the conventional thermal or magnetic tripping means. The entire breaker housing is almost immediately filled with a conductive gaseous atmosphere which creates an unintentional circuit from the latch to the load terminal. When the latching surface of the cradle and the latch are separated by the conventional magnetic means, this unintentional circuit is interrupted and accompanied by arcing between the cradle and the latch which roughens the engaging surfaces and greatly increases the force required to trip the breaker during subsequent operations. One possible explanation for a part of this problem is that the cradle is connected to the movable contact arm by the operating spring. An insulator such as that described within U.S. Pat. No. 2,844,689 to Middendorf, was investigated but did not solve the problem. A proven explanation is the direct conduction path from the movable contact arm directly to the latching tip of the cradle, directly to the engaging surface of the latch, directly to the load terminal and all through the conductive atmosphere in parallel with the main inductive current path. The result of the roughened latching surfaces and accompanying increased force to trip is such that when a single breaker is called upon to trip several breakers within the common tripping assembly, its single operating spring cannot perform the required latching action.
The purpose of this invention is to provide means to prevent the occurrence of arcing between the cradle and the latch upon separation, and thereby preserve the required smoothness of their mating surfaces to retain the required common tripping action.
The aforementioned U.S. Pat. No. 2,844,689 to Middendorf describes a circuit breaker design wherein a substantial current flows through the mcvable contact arm and cradle to the latch before the breaker is tripped on overload. When the breaker is tripped, arcing occurs between the cradle and the latch as they become separated, causing their adjacent surfaces to become pitted. Middendorf found that the interposition of an insulating bearing between the movable contact arm and the cradle prevents the formation of an arc between the cradle and the latch at the instant of their separation upon tripping. The Klein breaker, which is fully described in the aforementioned patent, and to which reference can be made for a more detailed description of the cradle and latch assembly relative to the contact arm and operating spring, does not have a direct electrical path through the cradle and latch.