Patent Application
20130092515
The present invention relates generally to molded case circuit breakers and, more particularly, molded case circuit breakers capable of withstanding short circuit conditions.
Molded case circuit breakers are commonly used in multi-metering panelboards for commercial and industrial applications that require hundreds, sometimes even thousands, of amperes of current to pass through the circuit breakers. In a short circuit condition, these current levels produce significant magnetic forces on the conductors. Substantially rigid busbars are typically used to connect the load side of the breaker to the load lines, but the line side of the breaker is typically equipped with lugs that can be connected to flexible cables leading to the power distribution lines. The magnetic forces produced by a short circuit is condition can cause whipping of the cables, which exerts significant forces on the lugs and the adjacent portions of the molded case. These forces can pull the cables out of the lugs, displace the lugs, and even cause cracking of the molded case. The cable whipping tends to become more severe as the length of the unsecured cables increases.
One way to reduce the movement of the cables during short circuit events is to “brace” the cables by tying all of the cables together with rope. This practice of “bracing” the cables is common on switchboards with high short circuit current ratings. (See UL 891 paragraphs 6.3.3, 9.2.4.2.4.1, G5.1 and Figure G5.1.) Bracing is primarily intended to prevent the cables from coming out of the lugs, but also reduces movement of the lugs and thus reduces the risk of fracturing the molded case of the circuit breaker. However, the use of cable “bracing” is not always appealing to the customers responsible for implementing it. It is also possible to design reinforcements into the molded case of the circuit breaker, but this increases the cost of the breaker, for a problem that is encountered in only a small percentage of the applications for such breakers. Thus, there is an ongoing need for improved techniques for protecting molded case circuit breakers from the effects of short circuit conditions.
Rather than using cable bracing or building reinforcement into the molded case, it has been found that external supports can effectively prevent fracturing of the molded case during short circuit conditions. By locating such supports adjacent the side walls of the molded case at opposite ends of the lug shroud, those side walls of the molded case are externally reinforced to resist forces that arise from short circuit conditions that tend to distort the walls of the lug shroud. The end walls of the shroud can move only a fraction of an inch before engaging the rigid brackets, and in certain embodiments the brackets also prevent flexing of the legs of the “pan” typically used to support a molded case circuit breaker. Holding the pan legs rigid holds the flat support plate of the pan firmly in place against the outer surface of the lug-carrying wall of the molded case. The overall effect is to externally reinforce the entire end portion of the molded case that is attached to the line cables, so that the forces applied to that portion of the molded case during a short circuit condition do not fracture the molded case.
The use of external supports permits any given molded case circuit breaker to is be installed with or without reinforcement for short circuit conditions, i.e., the reinforcement is optional and can thus be selectively used only in those applications in which the extra cost is warranted by the risk of short circuit conditions occurring. Thus, the same molded case may be used in all applications, thus reducing manufacturing costs by avoiding the need for different versions of molded cases.
In accordance with one embodiment, a molded case circuit breaker having multiple line connector lugs attached to an end portion of the molded case for connecting the trip unit to lines from a power source, is protected against fracturing during short circuit conditions by a pair of rigid support brackets attached to a rigid supporting structure and extending along a pair of opposite outer side walls of the molded case adjacent the lugs to resist outward movement of the side walls in the event of a short circuit condition that exerts distorting forces on the lugs and the portions of the molded case adjacent the lugs.
In one implementation, each of the support brackets is slightly spaced away from the adjacent surface of one of the opposite sides of the molded case, and an electrical insulator is provided between the support brackets and the adjacent surfaces of the opposite sides of the molded case. When the molded case is a “split” case that is molded in two parts, the brackets may be located entirely on one side, e.g., the lower side, of the split in the area where the lugs are located.
The foregoing and additional aspects of the present invention will be apparent to those of ordinary skill in the art in view of the detailed description of various embodiments, which is made with reference to the drawings, a brief description of which is provided next.
The invention may best be understood by reference to the following description taken in conjunction with the accompanying drawing, in which:
Although the invention will be described in connection with certain preferred embodiments, it will be understood that the invention is not limited to those particular embodiments. On the contrary, the invention is intended to cover all alternatives, modifications, and equivalent arrangements as may be included within the spirit and scope of the invention as defined by the appended claims.
Turning now to the drawings,
The particular breaker illustrated in
The circuit breaker 10 is installed on a rigid supporting surface 20 (see
In the illustrative embodiment, two screws 22a and 22b are used to secure the closed end portion 21a to the molded case 14, and four screws 23a-23d are used to secure the legs 21b and 21c of the pan to the rigid wall 20 of the enclosure. It will be understood, however, that other fastening means such as weldments or rivets could be used in place of the screws.
As can be seen in
Any forces exerted on the brackets 30 and 31 by the side walls 15 and 16 of the lug shroud 17 are transmitted to the respective pan legs 21b and 21c, which causes the closed end portion 21a of the pan to be urged against the bottom surface 14c of the lug shroud 17. In fact, the closed end portion 21a of the pan is placed in tension, which further increases the resistance to distortion of the lug-bearing bottom wall 14c of the molded case 14. Thus, the lug shroud 17 is externally reinforced on the bottom wall 14c, as well as the two side walls 15 and 16, of the lug shroud 17, which are all the exterior walls of the lug shroud 17 formed by the lower section 14b of the molded case 14, i.e., the section beneath the split 14′ in the molded case. This external reinforcement has been found to be effective in preventing fracturing of the molded case when subjected to short circuit conditions.
To ensure that the pan 21 and the brackets 30 and 31 are all electrically isolated from the molded case 14 and its lugs 12, a sheet 40 of electrically insulating material is is sandwiched between each of the brackets 30 and 31 and the respective side walls 15 and 16 of the lug shroud 17, as at 40b and 40c, and also between the bottom wall 14c of the shroud 17 and the closed end portion 21a of the supporting pan 21, as at 40a. This electrical insulation may be formed by a single sheet of insulating material that extends across the bottom wall 14c of the lug shroud 17 and is folded up at both sides 15 and 16 of the breaker 10 to extend along the side walls 15 and 16 of the shroud 17.
While particular embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise construction and compositions disclosed herein and that various modifications, changes, and variations may be apparent from the foregoing descriptions without departing from the spirit and scope of the invention as defined in the appended claims.
