Locking structure for a truck of a switchgear drawout circuit breaker

A truck (12′) for mounting a circuit breaker (13) in a switchgear frame (14′) includes wheels (16), a racking screw (20), a nut (22) threaded with the racking screw. The nut floats linearly with respect to the truck. A spring (23) is provided between a portion of the truck and the nut such that during movement of the truck into the frame, rotation of the racking screw causes linear movement of the nut, with the nut pushing against the spring, with the spring pushing the truck to move the truck linearly. Locking structure is coupled with the such that when the truck is stopped and the racking screw continues to rotate, the nut will move to compress the spring, causing the locking structure to move from an unlocked position, disengaged with the frame, to a locked position, engaged with the frame.

BACKGROUND

The present invention relates to an electrical switchgear and, more particularly, to a drawout circuit breaker truck having locking structure for restraining the circuit breaker truck and circuit breaker thereon under fault conditions in a frame of the switchgear.

With reference toFIG. 1, conventional switchgear, generally indicated at10, includes a circuit breaker truck assembly, generally indicated at11. The truck assembly11includes a circuit breaker truck12that carries racked-in power circuit breaker13so that the circuit breaker13can be withdrawn from a frame14of the switchgear10for maintenance. In particular, the frame14includes at least one rail15. Wheels16(only one shown inFIG. 1) are mounted to at least one side of the circuit breaker truck12. The wheels16are supported by the rail15such that an inside portion of each wheel16of the circuit breaker truck12slides on the rail15. Under short circuit conditions, there are moments on the circuit breaker poles that cause the circuit breaker truck assembly11to twist about its horizontal and vertical axes. This movement may force the circuit breaker truck assembly11to jump off the rails15and raises the possibility of an arc initiation if the primary contacts separate from the bushings.

Thus, there is a need to provide locking structure to restrain the circuit breaker truck, and thus circuit breaker thereon, in place under fault conditions.

SUMMARY

An objective of the present invention is to fulfill the need referred to above. In accordance with the principles of an embodiment, this objective is obtained by providing a truck for mounting a circuit breaker in a switchgear frame. The switchgear frame includes rails. The truck includes wheels for engaging the rails when the truck is moved into the frame; a racking screw; and a racking screw nut in threaded engagement with the racking screw. The racking screw nut is constructed and arranged to float linearly with respect to the truck. A spring is provided between a portion of the truck and the racking screw nut such that during movement of the truck into the frame, rotation of the racking screw causes linear movement of the racking screw nut, with the racking screw nut pushing against the spring, with the spring pushing the truck to move the truck linearly. Locking structure is coupled with the racking screw nut and is constructed and arranged such that when the truck is stopped and the racking screw continues to rotate, the racking screw nut will move to compress the spring, causing the locking structure to move from an unlocked position, disengaged with the frame, to a locked position, engaged with the frame.

In accordance with another aspect of an embodiment, a method of locking a truck, carrying a circuit breaker, into a switchgear frame provides a switchgear frame including rails, and at least one lock receiving opening in the frame. A truck carries a circuit breaker. The truck has wheels; a racking screw; a racking screw nut in threaded engagement with the racking screw, the racking screw nut being constructed and arranged to float linearly with respect to the truck; a spring between a portion of the truck and the racking screw nut; and locking structure coupled to the racking screw nut. The racking screw is rotated to cause linear movement of the nut, with the nut pushing against the spring, with the spring pushing the truck to move the truck linearly along the wheels that roll on the rails. Movement of the truck is stopped while continuing to rotate the racking screw so that the nut will move to compress the spring, causing the locking structure to move from an unlocked position, disengaged with the lock receiving opening in the frame, to a locked position, engaged with lock receiving opening in the frame.

DETAILED DESCRIPTION OF AN EXAMPLE EMBODIMENT

With reference toFIG. 2an end view of a circuit breaker truck, generally indicated at12′, is shown in accordance with an embodiment of the invention. The circuit breaker truck12′ carries racked-in power circuit breaker13so that the circuit breaker13can be withdrawn from a switchgear frame14′ (FIG. 3) for maintenance. Wheels16are mounted to opposing sides of the circuit breaker truck12′. The wheels16are supported by rails15(FIG. 3) of the frame14′ such that an inside portion of each wheel16of the circuit breaker truck12′ slides on the associated rail15. The frame14′ can be of the type disclosed in U.S. Pat. No. 7,124,488, the content of which is hereby incorporated by reference into this specification.

With reference toFIG. 3, the truck12′, absent the circuit breaker13for clarity of illustration, is shown disposed in the frame14′. A conventional lead or racking screw20is provide, preferably of the type disclosed in U.S. Pat. No. 6,689,968 B2, the contents of which is hereby incorporated by reference into this specification. A racking screw nut22is in threaded engagement with the racking screw20in the conventional manner. The racking screw nut22differs from conventional racking screw nuts in that it is allowed to float linearly or along the longitudinal axis X of the racking screw20with respect to the truck12′. A heavy die spring23is provided between a portion of the truck12′ and the nut22so that that torque associated with rotation of the screw20is transmitted to the nut22via the threads, imparting linear movement of the nut22. The nut22pushes on the spring23and the spring pushes on the portion of the truck12′ to move the truck12′ linearly on the wheels16during racking into the frame14′. Since the spring23has a high spring rate, pushing of the nut22on the spring23does not compress the spring23.

The truck12′ includes locking structure, generally indicated at18. The locking structure18includes a first crank arm24and a second crank arm25that are each pivotally coupled to the opposing ends of the nut22via a pivot connection26. A first elongated lock bar28is pivotally coupled to the first crank arm24and a second elongated lock bar30is pivotally coupled to the second crank arm25. To provide some play in the linear movement of the lock bars28,30, an end member32is pivotally coupled to an associated lock bar28,30, via a pivot connection34. The end members32can be considered to be part of the lock bars28,30. The end members32are guided by supports35of the truck12′ for linear movement in a direction that is generally transverse to the direction of movement of the nut22. Each end member32has a free end36(defining an end of the associated lock bar) disposed adjacent to surfaces defining a lock receiving opening37in an associated sidewall38,38′ of the frame14′. In the embodiment, the openings37are in opposing sidewall38,38′ of the frame14′, but can be provided in the opposing rails15of the frame14′.

As shown inFIG. 3, in unlocked position of the locking structure18, the ends36of the end members32are in a retracted position, disposed outside of the openings37in the frame14′. The ends36are in this retracted, unlocked position since the spring23biases the nut22in a first direction (arrow A) which places the crank arms24and25in a first position, defining an unlocked position of the lock bars and associated end members32. The locking structure18is in the unlocked position while moving the truck12′, with circuit breaker13thereon, into the switchgear frame14′.

An important aspect of the embodiment is that a rail15that guides the truck wheels16must have a stopping block40that will stop the truck12′ at the mated position (ends36adjacent to openings37). Thus, the stopping block40is placed in at least one of the rails15behind the forward-most wheel16′ (FIG. 3) of the truck12′.

After the truck12′ is blocked from moving forward and the racking screw20is still rotating, with reference toFIG. 4, the nut22will compress the spring23, with the nut22moving in the direction opposite arrow A, causing the crank arms24,25to rotate simultaneously from the first position thereof to actuate the lock bars28,30and the end members32. This motion extends the ends36of the end members32outwardly and into the openings37, thereby defining an extended, locked position of the locking structure18. The nut22stops moving when a locking pin42on the top of the nut22moves into a locking hole44in the truck12′. The locking hole44is at the end of a slot46defined in the truck12′ and the locking pin42moves in the slot46until it reaches the locking hole44.

The nut22applies a thrust force via the spring23that is strong enough to overcome the mating forces of the breaker contacts, but provides enough compression distance to allow for adequate actuation of the lock bars28,30. In the embodiment, it takes 900 lbs. to compress the spring23one inch.

When in the locked position, if a short condition causes moments on the circuit breaker poles that cause the circuit breaker truck12′ to twist about its horizontal and vertical axes, the truck12′ will remain on the rails15and eliminate the possibility of an arc initiation.

Reversing the racking screw20will reverse the process, unlocking the locking structure18. Thus, rotation of the screw20causes linear movement of the nut22along the axis X, with the crank arms24,25, causing linear movement of the lock bars28,30and thus end members32in a direction transverse with respect to the axis X.

Although a pair of crank arms24,25and lock bars28,30(with end members32) are provided, it can be appreciated that only one crank arm24and lock bar28(with end member32) can be provided to lock the truck12′ with respect to the frame14′. However, providing two lock bars at opposite ends of the truck12′, as shown, ensures that the truck12′ will not be torqued due to a short condition occurrence.

Although not shown, the locking structure18can include a longitudinally extending rod coupled to at least one of the crank arms24,25, with the rod being coupled to another lock bar so as to lock a different portion of the truck12′ to the frame14′. For example, the truck12′ can include a vertically extending back-plate so that the back plate (defining a top portion of the truck12′) can be locked to the frame14′ with the additional lock bar. The locking structure18thus advantageously provides additional positional stability during short circuit events without any additional steps to be performed by the operator during racking and un-racking of the truck12′.