Construction Method

You learned that low voltage power circuit breakers are essentially an assembly of parts on a metal frame or in an encased housing of insulating material. Because the makeup of both approaches was adequately discussed, the details will not be repeated. One point, however, that should be repeated centers around what applicable standards are required relative to the construction method. The frame construction used must hold all the circuit breaker parts in place and be capable of withstanding the tremendous physical forces and severe heating effects a power circuit breaker could be subjected to while performing its function. Standards do not specify the exact nature of the construction or the construction material. Those decisions are left to the circuit breaker manufacturer.

Obviously, the construction method and materials used must result in a strong, rigid design. For many years, the preferred approach was the open type metal-frame which had a number of pieces welded and/or bolted together. With the significant technological strides made in the areas of insulating material and molding processes, versatile rigid frame housings of high strength engineered thermoset composite resins have become available. Not only does the rigid frame housing type low voltage power circuit breaker meet the stringent requirements of ANSI, it exceeds them in a number of instances.

The Magnum DS circuit breaker uses a 3-piece construction:

A 2-piece engineered thermoset composite resin case completely encloses the current paths and arc chambers.
The operating mechanism sits on the front of the case and is electrically isolated from the current contact structures. It is in turn covered by an insulating front cover.



The rigid frame construction results in a more compact, lighter weight low voltage power circuit breaker. Previous circuit breaker designs with many parts that were once produced and attached individually to the frame can now be molded as an integral part of the rigid frame. The overall strength and rigidity of the engineered thermoset composite resin design can even contribute to higher performance capabilities by the power circuit breaker. This was the result with Magnum DS which has higher short time ratings (withstand) than previously available power circuit breakers, along with higher interrupting capabilities. In the way of review, remember that the short time rating consists of the following two components:

• Short delay current component (kA)
• Short delay time component (cycles)
In addition to the improved performance characteristics just mentioned, the rigid frame housing type low voltage power circuit breaker has individual arc chambers that (Figure 43):
• Insulate and isolate Arcing from other poles, the rest of the circuit breaker, and personnel
• Provide support for the current path pole assembly