1. Field of the Invention
The invention relates in general to circuit breakers and more particularly to molded case circuit breakers having an adjustable rating.
2. Description of the Prior Art
Circuit breakers are widely used in commerce and industry to protect electrical apparatus from over-current conditions. The varying requirements of different applications require circuit breakers in a variety of ratings. Manufacturers and suppliers are thus required to produce and hold in inventory a large number of different types of circuit breakers. Considerable savings would result if a single circuit breaker could provide a variety of ratings.
Circuit breakers having adjustable ratings are described in U.S. Pat. No. 3,575,679 issued to James P. Ellsworth and John Zipay, assigned to the assignee of the present invention, and U.S. Pat. No. 3,593,234 issued to Allan P. Charbonneau. Circuit breakers described in the above-mentioned U.S. patents employ adjustment means having a variety of locking devices and biasing means to permit adjustment of the circuit breaker rating.
Safety problems can result however from a circuit breaker capable of covering a range of ratings. For instance, if a circuit breaker is properly adjusted and installed on a 200 ampere line, and conditions on the line cause repeated tripping, there is a natural human tendency to increase the tripping current level of the circuit breaker to prevent the nuisance of repeated trippings. This, of course, will result in a dangerous condition if the tripping current level of the circuit breaker is increased above 200 amperes. Thus, a compromise must be established between convenience and safety.
One answer in the past was to provide circuit breakers having interchangeable trip units. In order to change the rating of the circuit breaker the entire trip unit was removed in the field and a new trip unit of different rating installed. This was a complex procedure, however, and sometimes resulted in improper installation of the new trip unit. This in turn resulted in failure of the circuit breaker, causing extensive damage to the circuit breaker and to the apparatus being protected. Also, the problems associated with manufacturing and stocking a variety of unit ratings remained.
It would be desirable to provide a circuit breaker adjustable over a range to provide a variety of ratings which would avoid safety problems associated with adjustable circuit breakers, be of simple construction, and provide for convenient adjustment.