1. Field of the Invention
This invention relates to power switches for interrupting current in electric power circuits. More particularly, it relates insulated molded cases for such switches having a removable secondary cover which encloses recesses for auxiliary devices in the primary cover, and an assembly for removably securing the secondary cover over the primary cover.
2. Background Information
Power switches, such as circuit breakers, transfer switches and the like, used in low power (600 volts and below) electric power distribution systems, commonly have a housing molded of an electrically insulative resin and are, therefore, referred to as molded case circuit breakers, transfer switches and the like. The molded housing includes a base and a cover which together form an enclosure in which are mounted one or more switch poles, a switch operating mechanism and, where appropriate, a trip device. Typically, the molded case circuit breaker has a trip device that responds to short circuits and usually to persistent over current conditions also, and opens the main switch contacts interrupting current.
Often, it is desirable to provide additional features on the switch, sometimes as options. Such additional features include a remote trip, an auxiliary switch which provides an indication of the open or closed state of the main switch contacts, and/or a bell alarm which provides an indication that the trip device has been activated. In some installations, it is desirable to have the switch open on under voltage or off frequency conditions to prevent damage to a protected load.
These additional features can be incorporated within the switch housing formed by the base and cover. However, in some instances, the devices providing the desired features are received in cavities formed in the cover of the housing. In this latter case, a secondary cover encloses the cavities in what has become the primary cover. This secondary cover should be easily removable for inserting, removing or servicing the devices providing the additional features. Typically, the secondary cover is removably secured to the molded primary cover by screws. However, screws either require threaded inserts in the primary cover, which increases the cost and manufacturing steps or they are self tapping. When self tapping screws are used, the threads in the molded resin can be stripped out by repeated insertion and removal. In either case, the screws can be lost when removed.
There is a need, therefore, for an improved molded case power switch housing and particularly for an improved arrangement for removably attaching a secondary cover to the molded housing.
There is a further need for such an improved molded case power switch housing in which there are no loose fasteners to become lost or hard to locate.
There is another need for such an improved molded case power switch that does not require repeated insertion and removal of self-tapping screws into the molded housing.
There is yet another need for an improved molded case power switch housing which is economical to manufacture and easy to use.