In general the invention relates to electric circuit breakers and in particular the stab or contact assemblies used to detachably interconnect the circuit breaker to current carrying conductors.
Whenever electric circuit breakers, in particular plug-in type 3-pole circuit breakers, are detachably fastened to a current carrying member such as a bus bar, it is important that the stab assembly used to make the interconnection maintain good electrical and mechanical contact between the circuit breaker and the current carrying member. This avoids arcing which can damage the current carrying member, the circuit breaker or the stab assembly. In addition, the design of the stab assembly should be such as to facilitate the assembly and repair thereof.
In order to meet these criteria the stab assembly is typically provided with a plurality of opposed contact fingers which are held in a carrier or cage in a spaced apart relationship allowing for the insertion of the current carrying member and a portion of a contact of the circuit breaker therebetween. Mechanical and electrical contact of the fingers with the stationary contact and the current carrying members are maintained through the use of biasing means such as compression springs or leaf springs. One example of such an assembly can be found in U.S. Pat. No. 2,751,471 issued June 19, 1956. There a slotted C-shaped cage is used to hold a plurality of opposed contact fingers in a spaced relationship. The cage is fastened to a carrier which in turn is secured to a contact of the breaker. Compression springs are used to maintain electrical and mechanical contact of the fingers with the carrier and the current carrying member onto which the breaker is connected. The current carrying member is provided with a specially shaped engagement edge which engages the contact fingers during insertion and withdrawal allowing the contact fingers to slide across the contact surface of the carrier. The contact fingers are provided with notches which limit the amount of this sliding travel. One drawback, however, with this scheme is that the carrier and the contact current carrying member must be in substantial alignment. Another disadvantage is that the current carrying member can be overinserted into the contact assembly causing stress upon the cage assembly.
Another example of an interconnecting stab assembly is presented in U.S. Pat. No. 2,029,028 issued Jan. 28, 1936. There a single compression spring via a system of linkages is used to maintain the electrical and mechanical contact of a pair of opposed contact fingers. Although this contact assembly allows for a limited degree of axial misalignment between the current carrying member and the breaker, overinsertion of the current carrying member causing the contact fingers to lift off the surfaces of the inserted conductors and overcompression of the springs can occur. In U.S. Pat. No. 4,160,142 issued July 3, 1979 the use of modular pairs of spring biased contact fingers is illustrated. Here however as with the previous example the compression springs may be overcompressed due to the insertion of the conductors therebetween. In addition with the latter two examples of contact assemblies the inward motion of the contact fingers is limited by the design of the contact finger. In the first example this inward motion is limited by the meeting of the finger against the carrier. It would be advantageous to provide a means to control this inward motion of the contact fingers caused by the biasing means which would be independent of the contact finger. By controlling this inward motion the spacing between the contact fingers can be controlled to increase or decrease the insertion forces required.