Locking mechanism for a circuit breaker

Abstract

A locking mechanism (10) for a circuit breaker comprises: a contact arm (14) for supporting a contact (15) and configured to be moveable between a closed position and an open position, the contact arm (14) including on one edge thereof a striking surface (13) adjacent to a well (32); a spring (40); and a locking cam (16) biased by the spring (40), the locking cam including a cam rotator (18). The contact arm (14) is not is contact with the cam rotator (18) when the contact arm (14) is in a closed position, as the contact arm (14) is rotated from the closed position towards the open position, the striking surface (13) of the contact arm (14) contacts the cam rotator (18), and the cam rotator (18) is offset from a line that passes through a centre of rotation of the locking cam (16) in the direction of travel of the contact arm (14) towards the locking cam (16) such that force of the striking surface (13) against the cam rotator (18) causes the locking cam (16) to rotate and the locking cam (16) to move into the well (32) against the bias of the spring (40).

Description

[0001] Locking Mechanism for a Circuit Breaker

[0002] Field of the Invention

[0003] This relates to a locking mechanism for a circuit breaker.

[0004] Background to the Invention

[0005] Circuit breakers are one of a variety of overcurrent protection devices used for circuit protection and isolation. The circuit breaker provides electrical protection whenever an electric abnormality occurs. In a circuit breaker, current enters the system from a power line and passes through a line conductor to a stationary contact fixed on the line conductor, then to a movable contact. The movable contact can be fixedly attached to an arm and the arm can be mounted to a rotor. As long as the stationary and movable contacts are in physical contact, current passes from the stationary contact to the movable contact and out of the circuit breaker to down line electrical devices.

[0006] In the event of an overcurrent condition e.g., a short circuit, extremely high electromagnetic forces can be generated. The electromagnetic forces repel the movable contact away from the stationary contact. Because the movable contact is fixedly attached to a rotating arm, the arm pivots and physically separates the stationary and movable contacts thus tripping the circuit.

[0007] One type of circuit breaker is a moulded case circuit breaker MCCB. MCCBs use a temperature-sensitive device and a current-sensitive electromagnetic device to protect and isolate circuits. MCCBs can protect a wide range of electrical systems and equipment, and are commonly used in low and medium-voltage applications. MCCBs can handle higher levels of current. MCCBs typically have a rated current of 60 amps to 1200 amps, while MCBs typically have a rated current of less than 50 amps.

[0008] Summary of the Invention

[0009] A first aspect of the invention provides a locking mechanism for a circuit breaker, the locking mechanism comprising:

[0010] a contact arm for supporting a contact and configured to be moveable between a closed position and an open position, the contact arm including on one edge thereof a striking surface adjacent to a well;a spring; and

[0011] a locking cam biased by the spring, the locking cam including a cam rotator; wherein:

[0012] the contact arm is not is contact with the cam rotator when the contact arm is in a closed position,

[0013] as the contact arm is rotated from the closed position towards the open position, the striking surface of the contact arm contacts the cam rotator,

[0014] the cam rotator is offset from a line that passes through a centre of rotation of the locking cam in the direction of travel of the contact arm towards the locking cam such that force of the striking surface against the cam rotator causes the locking cam to rotate and the locking cam to move into the well against the bias of the spring.

[0015] The well may include a sidewall oriented with respect to a biasing direction of the spring such as to retain the locking cam within the well.

[0016] The locking cam may include a cam stopping face that is configured to contact a cam stop located at a side of the well that is opposite to the sidewall and thereby prevent further rotation of the locking cam.

[0017] The locking cam may include a cutout for a radial section of the locking cam that is located between the cam rotator and the cam stopping face.

[0018] The locking cam may include a second cam stopper configured to contact a lip of the well that is located between the striking surface and the sidewall to limit rotation of the locking cam after the locking cam has descended into the well.

[0019] The locking cam may include a first cam stopper having a profile matching a profile of a cam contacting surface such as to place the locking cam in a predetermined rotational position prior to the cam rotator being contacted by the striking surface. The profile may be flat.

[0020] The locking cam may be mounted on a spring pin that is constrained within a slot such as to restrict movement of the locking cam.

[0021] Brief Description of the Drawings

[0022] In the drawings:

[0023] Figure 1 is an isometric view of a locking mechanism for a circuit breaker according to embodiments of the invention;Figure 2 is a side view of a contact arm and locking cams of the locking mechanism; Figure 3 is the same as Figure 2 but shows a crossbar present;

[0024] Figure 4 is a side view of a contact arm and locking cams of the locking mechanism and showing interior aspects of the crossbar;

[0025] Figure 5 is the same as Figure 4 but without showing springs;

[0026] Figure 6 is an isometric view of the contact arm and locking cams of the locking mechanism

[0027] Figure 7 is the same as Figure 6 but without showing springs;

[0028] Figures 8a to 8e are side views of the locking mechanism at different stages during movement of the contact arm and the locking cam;

[0029] Figure 9 is a side view of the locking mechanism showing reset components;

[0030] Figure 10 is an isometric view of what is illustrated in Figure 9; and

[0031] Figure 11 shows internally what is illustrated in Figure 9.

[0032] Detailed Description of the Drawings

[0033] A locking mechanism 10 for a circuit breaker is configured for locking a contact arm in a switch open position. The locking mechanism 10 comprises a contact arm 14 for supporting a contact 15 and configured to be moveable (by rotation for instance) between a closed position and an open position. The contact arm 14 includes on one edge thereof a striking surface 13 adjacent to a well 32.

[0034] A locking cam 16 is biased by a spring 40 into a certain position. The locking cam 16 includes a cam rotator 18 that causes the locking cam 16 to rotate and enter a well 32 as the contact arm 14 moves from a closed position. Rotation results because the cam rotator 18 is offset from a line that passes through a centre of rotation of the locking cam 16 in the direction of travel of the contact arm 14 towards the locking cam 16. The force of the striking surface 13 against the cam rotator 18 causes the locking cam 16 to rotate and the locking cam 16 to move into the well 32 against the bias of the spring 40. Once in the well 32, the locking cam 16 is retained in the well by force of the spring 40. The well 32 includes a sidewall 36 oriented with respect to a biasing direction of the spring 40 such as to retain the locking cam 16 within the well 32. The retention in the well 32 of the locking cam 16 results from it being needed to extend the spring 40 for the locking cam to be removed. If the locking cam 16 is provided with a spring to bias it to a certain rotational position, then retention in the well 32 of the locking cam 16 can be provided also by frictional resistance between the locking cam 16 and the sidewall 36. The retention in the well 32 of the locking cam 16 causes the contact arm 14 to be held in the open position. It remains there untilthe mechanism is reset. Locking the contact arm 14 in the open position prevents undesirable reclosing of the contacts to remake the electrical circuit.

[0035] The locking cam 16 includes a cam stopping face 26 that is configured to contact a cam stop 38 located at a side of the well 32 that is opposite to the sidewall 36 and thereby prevent further rotation of the locking cam 16.

[0036] The locking cam 16 includes a cutout 28 for a radial section of the locking cam 16 that is located between the cam rotator 18 and the cam stopping face 26. This facilitates capture of the locking cam 16 in the well 32.

[0037] The locking cam 16 includes a second cam stopper 24 configured to contact a lip 34 of the well 32 that is located between the striking surface 13 and the sidewall 36 to limit rotation of the locking cam 16 after the locking cam 16 has descended into the well 32.

[0038] The locking cam 16 includes a first cam stopper 22 having a profile matching a profile of a cam contacting surface 48 such as to place the locking cam 16 in a predetermined rotational position prior to the cam rotator 18 being contacted by the striking surface 13. The profile is advantageously flat.

[0039] The locking cam 16 can be mounted on a spring pin 20 that is constrained within a slot 21 such as to restrict movement of the locking cam 16.

[0040] The mechanism 10 includes a crossbar 12. This is a generally cylindrical member, in which the contact arm 14 is housed. The contact arm 14 is of substantially uniform thickness, and has features that form part of the locking mechanism 10, as discussed below.

[0041] The contact arm 14 is double-ended. Each end has a contact 15. Current can flow through the contact arm 14 when the contact arm 14 is in the closed position. When the contact arm 14 is rotated (clockwise in the figures) to an open position, the contacts 15 are separated from fixed contacts (not shown) to open the electrical circuit and prevent the flow of current through the contact arm 14.

[0042] The mechanisms at each end of the contact arm 14 are the same. For ease of explanation, only the mechanism at one end of the contact arm 14 is discussed in detail in the following.The locking cam 16 is mounted on a spring pin 20, as best seen in Figure 7. Ends of the spring pin 20 are supported in V slots 21, which are internal features of the crossbar 12, as is best seen in figures 1 and 4. As best seen in figures 4 and 6, contact springs 40a, 40b are connected to the spring pin on either side of the locking cam 16. The other ends of the contact springs 40a, 40b are coupled to a spring retainer 50, which is a feature of the contact arm 14. The contact springs 40a, 40b are held in tension, which applies a force to the spring pin 20 (and thus the locking cam 16) in the direction of the spring retainer 50. The spring pin 20 is prevented from moving any closer towards the spring retainer than is shown in Figure 1 because it is nestled at the bottom of the V slot 21. The contact springs 40a, 40b exert a force on the contact arm 14 at the location of the spring retainer 50 towards the position of the locking cam 16. Because the spring retainer 50 is the opposite side of the pivot point of the contact arm 14, the contact springs 40a, 40b assist in holding the contact arm 14 in the closed position and applying contact pressure to the contacts 15. Optionally, additional force for holding the contact arm 14 in the closed position is provided by a torsion spring 17, which is located at the pivot point of the contact arm 14.

[0043] The well 32 is formed on the contact arm 14. The well 32 is an indent or depression in the profile of the contact arm 14. The well 32 is on the uppermost edge of the left side of the contact arm 14, and is best seen in figures 1 and 4. The well 32 includes a sidewall 36 at the side that is closest to the pivot point of the contact arm 14. The locking cam 16 rests against this sidewall 36 when the mechanism is locked. The well 32 includes a lip 34 that connects the sidewall 36 to a generally flat upper edge portion of the contact arm 14, as best seen in figure 2. The generally flat upper edge portion of the contact arm 14 includes a striking surface 13, which interacts with the locking cam 16 during operation.

[0044] Opposite the sidewall 36, and further from the pivot point than the lip 34, is a cam stop 38. The spring retainer is adjacent to the cam stop 38 of the locking mechanism of the other end of the contact arm 14.

[0045] A spring (not shown) may be provided to bias the locking cam 16 into the rotational position shown in Figure 8a. Using a leaf spring to bias rotationally the locking cam is effective, but the spring could alternatively be a torsion spring mounted on the spring pin 20 and biasing the locking cam 16 clockwise, or it could be a tension or compression spring mounted between the locking cam 16 and the crossbar 12.The locking cam 16 includes a number of features. The cam rotator 18 is a protrusion that interacts with the striking surface 13 during operation.

[0046] The first cam stopper 22 is a flat surface that ensures a desired rotational position of the locking cam 16 when the mechanism is in the closed position. Specifically, the first cam stopper 22 constrains rotation of the locking cam 16 in the clockwise direction when the locking cam 16 is in contact with the cam contacting surface 48. Although not shown in the drawings, there may be a step between the first cam stopper 22 and the external surface of the locking cam that is adjacent to the first cam stopper 22 in the clockwise direction around the periphery of the locking cam 16.

[0047] The second cam stopper 24 is a protrusion on the surface of the locking cam 16 that contacts the lip 34 and serves to keep the locking cam 16 at a desired rotational position when the locking cam 16 is resting in the well 32, with the mechanism in the locked position.

[0048] The cam stopping face 26 is located adjacent the first cam stopper 22. The cam stopping face 26 is generally radial to the rotation axis of the spring pin 20. The cam stopping face 26 contacts the cam stop 28 of the well 32 shortly after the locking cam 16 enters the well 32, and prevents further anticlockwise rotation of the locking cam 16.

[0049] As can be seen most clearly in figure 1, the locking cam 16 includes a cutout 28, defined by cutout walls 30a, 30b. The distance between the cutout walls 30a, 30b is greater than the thickness of the contact arm 14, and the contact arm 14 fits into the cutout 28. When the locking cam 16 is located in the well 32 and at a rotational position that allows it, the spring pin 20, not the locking cam 16, contacts the surface of the contact arm 14 that constitutes the bottom of the well 32.

[0050] As best seen in figures 9, 10 and 11, driving links 42a 42b are connected to a driving rod 44. By applying a force to the 42a 42b, the crossbar 12 can be rotated (clockwise as shown) to effect unlocking of the mechanism. The crossbar 12 rotates around a crossbar pivot 46. The crossbar pivot 46 optionally is on the same axis as the torsion spring 17.

[0051] The crossbar 12 includes a crossbar interior surface 48, which is best seen isometrically in figure 1. The crossbar interior surface 48 interacts with the first camstopper 22 to keep the locking cam 16 at a specific rotational position when the contact arm 14 is in the closed position.

[0052] The operation of the locking mechanism will now be described with reference in particular to figures 8a to 8e.

[0053] Figure 8a shows the initial position, in which the contact arm 14 is biased into the closed position and the contact 15 is biased against the fixed contact (not shown). Here, the first cam stopper 22 is flat against the crossbar interior surface 48, holding the locking cam 16 in the rotational position shown. The contact springs 40a, 40b are in their shortest lengths, and the spring pin 20 is at the bottom of the V slot 21.

[0054] In figure 8b, the contact arm 14 has rotated clockwise. This rotation is caused by an actuator (not shown), triggered by detecting an overcurrent or short circuit condition. In the figure, the cam contactor 18 has just made contact with the striking surface 13 of the contact arm 14. Because the cam contactor 18 is offset, the force provided by the striking surface 13 on the cam contactor 18 causes the locking cam 16 to rotate anti-clockwise. This is not prevented or resisted by the first cam stopper 22 nor the crossbar interior surface 48 because their shapes are chosen not to resist this rotation.

[0055] The force provided by the striking surface 13 on the cam contactor 18 also causes the locking cam 16 to move leftwards as shown. The movement is constrained by the presence of the spring pin 20 in the V slot 21, and so the locking cam 16 moves in the direction of the lowermost surface of the V slot 21. As the contact arm 14 continues to move upwards, the lip 34 enters the cutout 28. Depending on the speed of movement of the contact arm 14, the speed of rotation of the locking cam 16 and the speed of translational movement of the locking cam 16, the spring pin 20 may contact the lip 34. If there is contact, the lip 34 guides the spring pin into the well 32. At this time, the locking cam 16 is rotating anti-clockwise and it is descending into the well 32.

[0056] At the moment shown in Figure 8c, the spring pin 20 has not yet made contact with the bottom of the well 32. However, the cam stopping face 26 has made contact with the cam stop 38. This prevents further anti-clockwise rotation of the locking cam 16. In fact, it causes the locking cam to start rotating clockwise, as the spring pin continues to move towards the bottom of the well 32. The second cam stopper 24 at this time is in contact with the crossbar interior surface 48, which further prevents anti-clockwise rotation of the locking cam 16. Clockwise rotation of the locking cam16 can additionally be provided by the spring (not shown) that provides rotational bias on the locking cam 16 to the position shown in Figure 8a.

[0057] The locking cam continues to descend into the well 32 while rotating clockwise until the spring pin 20 reaches the bottom of the well 32, as shown in figure 8d. Further movement of the locking cam 16 in the direction away from the pivot point of the contact arm 14 is prevented by the cam stop 38, which defines a second sidewall of the well 32. At this time, the locking cam 16 remains in contact with the crossbar interior surface 48. Further movement of the contact arm 14 in the clockwise direction is prevented by a housing stopper (not shown). The difference in angle between the open and closed positions may be around 50 degrees.

[0058] Since further movement of the locking cam 16 in the direction away from the pivot point of the contact arm 14 is prevented by the cam stop 38, the force exerted by the contact springs 40a, 40b in the direction towards the spring retainer 50 acts to pull the locking cam 16 along the well to rest against the sidewall 36. The sidewall 36 is angled such that the force exerted by the contact springs 40a, 40b serves to force the locking cam 16 down into the well 32. This is achieved by the sidewall 36 being angled such that the corner defined by the sidewall 4 and the bottom of the well 32 is closer to the distal end of the contact springs 40a, 40b than is the part of the sidewall that abuts the lip 34. This then ensures that the locking cam 16 remains in the well 32. This locks the contact arm 14 in the open position and prevents the contacts from reclosing. If the locking cam 16 is provided with a spring to bias it to a certain rotational position, then retention in the well 32 of the locking cam 16 can be provided also by frictional resistance between the locking cam 16 and the sidewall 36. The frictional resistance results from a reaction force provided by the spring 40 biassing the locking cam 16 against the sidewall 36.

[0059] The second cam stopper 24 interacts with the lip 34 to prevent rotation of the locking cam 16 in a clockwise direction once the locking cam 16 is held against the sidewall 36 by the contact springs 40a, 40b.

[0060] Figure 8e shows the final locked position of the contact arm 14. This position is stable, and the mechanism holds until there an external unlocking torque is applied on the contact arm 14 due to force on the links 42a, 42b.

[0061] To unlock the mechanism, one or both of the driving links 42a, 42b is forced upwards. This causes the crossbar 12 to rotate around its central axis, which causes the contactarm 14 to rotate relative to the crossbar 12. This causes the locking cam 16 to roll out of the well 32, allowing the contact arm 14 to rotate anti-clockwise. Because the crossbar 12 is rotated clockwise at this time, the contacts 15 do not reconnect with the fixed contacts. Once the driving links 42a, 42b are moved downwards, the crossbar 12 rotates anti-clockwise and the contacts then reconnect.

[0062] Although the locking mechanism is described in the context of an MCCB, it will be appreciated that the locking mechanism may be used in a different type of circuit breaker. It is particularly well-suited to circuit breakers in which a contact arm rotates from a closed position to an open position around a pivot. In the embodiments, the contact arm has contacts at both ends and has a locking mechanism associated with each end.

[0063] Key to the figures:

[0064] Mechanism 10

[0065] Crossbar 12

[0066] Striking surface 13

[0067] Contact arm 14

[0068] Contact 15

[0069] Locking cam 16

[0070] Torsion spring 17

[0071] Cam rotator 18

[0072] Spring pin 20

[0073] V slot 21

[0074] First cam stopper 22

[0075] Second cam stopper 24

[0076] Cam stopping face 26

[0077] Cutout 28

[0078] Cutout walls 30a, 30b

[0079] Well 32

[0080] Lip 34

[0081] Sidewall 36

[0082] Cam stop 38

[0083] Contact springs 40a, 40b

[0084] Driving links 42a 42b

[0085] Driving rod 44

[0086] Crossbar pivot 46Crossbar interior surface 48 Spring retainer 50

Claims

Claims1. A locking mechanism (10) for a circuit breaker, the locking mechanism comprising:a contact arm (14) for supporting a contact (15) and configured to be moveable between a closed position and an open position, the contact arm (14) including on one edge thereof a striking surface (13) adjacent to a well (32);a spring (40); anda locking cam (16) biased by the spring (40), the locking cam including a cam rotator (18);wherein:the contact arm (14) is not is contact with the cam rotator (18) when the contact arm (14) is in a closed position,as the contact arm (14) is rotated from the closed position towards the open position, the striking surface (13) of the contact arm (14) contacts the cam rotator (18), andthe cam rotator (18) is offset from a line that passes through a centre of rotation of the locking cam (16) in the direction of travel of the contact arm (14) towards the locking cam (16) such that force of the striking surface (13) against the cam rotator (18) causes the locking cam (16) to rotate and the locking cam (16) to move into the well (32) against the bias of the spring (40).

2. A mechanism as claimed in claim 1, wherein the well (32) includes a sidewall (36) oriented with respect to a biasing direction of the spring (40) such as to retain the locking cam (16) within the well (32).

3. A mechanism as claimed in claim 2, wherein the locking cam (16) includes a cam stopping face (26) that is configured to contact a cam stop (38) located at a side of the well (32) that is opposite to the sidewall (36) and thereby prevent further rotation of the locking cam (16).

4. A mechanism as claimed in claim 3, wherein the locking cam (16) includes a cutout (28) for a radial section of the locking cam (16) that is located between the cam rotator (18) and the cam stopping face (26).

5. A mechanism as claimed in any preceding claim, wherein the locking cam (16) includes a second cam stopper (24) configured to contact a lip (34) of the well (32)that is located between the striking surface (13) and the sidewall (36) to limit rotation of the locking cam (16) after the locking cam (16) has descended into the well (32).

6. A mechanism as claimed in any preceding claim, wherein the locking cam (16) includes a first cam stopper (22) having a profile matching a profile of a cam contacting surface (48) such as to place the locking cam (16) in a predetermined rotational position prior to the cam rotator (18) being contacted by the striking surface (13).

7. A mechanism as claimed in claim 6, wherein the profile is flat.

8. A mechanism as claimed in any preceding claim, wherein the locking cam (16) is mounted on a spring pin (20) that is constrained within a slot (21) such as to restrict movement of the locking cam (16).

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