BACKGROUND TO THE INVENTION

This invention relates to circuit breaker and more particularly but not exclusively, to a magnetic circuit breaker incorporating a damper that renders the circuit breaker less prone to unwanted tripping due to vibration and shock.

In electrical engineering, a switch is an electrical component that can break an electrical circuit by interrupting the current or diverting it from one conductor to another. The most common form of switch is a manually operated electromechanical device with one or more sets of electrical contacts, which are connected to external circuits. Each set of contacts can be in one of two states. The set of contacts can be "closed", meaning the contacts are touching and electricity can flow between them, or "open", meaning the contacts are separated and the switch is non-conducting. The mechanism actuating the transition between these two states may take various forms, and can for example be toggle-type flip switch mechanism.

A circuit breaker is a special kind of automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload currents or short circuit faults. Its basic function is to detect a fault condition and interrupt current flow. Unlike a fuse, which operates once and then must be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation.

Many different kinds of circuit breakers are used in industry, and although all circuit breakers have some common features, the detail design and operation of the circuit breakers vary considerably depending on the intended application.

One kind of circuit breaker is a magnetic circuit breaker, A magnetic circuit breaker incorporates an electromagnet in the form of a solenoid. When a current passes through the electromagnet, a magnetic field is generated, with the strength of the magnetic field increasing as the electrical current increases. When the circuit breaker is in an ON condition, the contact set of the circuit breaker is connected. However, a displaceabie contact of the contact set is biased away from the stationary contact by way of a biasing arrangement {typically a spring), and is retained in the contact position (against the bias) by way of a latch or trip lever. As the current in the solenoid increases beyond the rating of the circuit breaker, the solenoid's pull releases the latch or trip lever, which lets the contacts open by spring action. More particularly, an armature located adjacent the solenoid is pulled towards the solenoid, and when the armature is displaced towards the solenoid the latch disengages the armature.

The armature is typically in the form of an elongate arm which is pivotably secured to the mechanism of the circuit breaker so that it can be displaced between a first position, in which it is spaced apart from the solenoid, and a second position, in which it is displaced towards the solenoid. In addition to the magnetic force exerted on the armature, the gravitational bias of the armature typically also urges the armature towards the solenoid. The magnetic force and the weight of the armature are counteracted by an oppositely directed spring force which is exerted on the armature by a suitable spring, for example a leaf spring. It will be appreciated that the balance of forces acting on the armature must be carefully calculated on order to ensure that the circuit breaker trips at the desired rating.

A problem associated with the above configuration is that circuit breakers utilizing the tripping mechanism described above are prone to unwanted tripping caused by externa! conditions, in particular shock and vibration. This may, for example, include situations where the circuit breaker is installed in the proximity of a railway, or where circuit breakers are used on resonating structures such high communication antennas of towers. Even a circuit breaker installed next to a contactor may be exposed to sufficient shock to result in unwanted tripping.

Two test procedures, IL-STD 202G and I EC 60068-2-6, are internationally recognized and widely used to verify the resistance of circuit breakers to tripping when subjected to conditions of mechanical shock and vibration. These standards follow the same test procedures whilst only varying the input test conditions. A standard vibration test consists of subjecting a specimen to sinusoidal vibrations over a given frequency range for a given period of time. The purpose of this test is to determine any mechanical weakness and/or degradation in the specified performance where the samples are subjected to specified displacement and tested over a wide range of frequencies.

The graph shown in Figure 1 shows how the test conditions or parameters are applied during the vibration test. A sharp peak in velocity is noticeable between 30 and 35Hz. This peak is known as the cross-over frequency and is a function of the maximum acceleration, the displacement amplitude and the frequency of excitation. This peak in velocity will negate any natural damping within the breaker. The 'natural damping' can be caused by friction between any two contact points. This means that there are many sources of natural damping within a circuit breaker {an in particular the analyzed Q-range breaker) and one can expect the velocity peak to cause problems. in addition, when a breaker is subjected to a vibration sweep, there are certain frequencies which excite the armature spring assembly. These natural frequencies are dependent on the mass of the armature, the spring constant and the frictional damping in the contact of the trip iever. When the armature spring assembly encounters its natural frequency it begins to oscillate. During this oscillation the armature works its way off the trip iever. This is due to the incline plane (a) on which the armature and trip iever interact. The incline plane (seen in Figure 2) makes it easier for the armature to move in one direction, than the other. This ultimately results in a faulty trip of the mechanism. Due to the fact that it is difficult to predict the fricttonal forces in the trip Iever, armature interface, it is also difficult to calculate the natural frequencies. However extensive testing has shown that there are at least two frequencies which tend to induce a faulty mechanical trip. The results of the vibration testing done to determine natural frequencies are shown in Figure 3. From the testing done, it can be seen that there are natural frequencies at approximately 33 and 78 Hz.

The experiments therefore supports the view that externa! vibration and shock stimuli will result in unwanted tripping in circuit breakers, and aids in identifying particular conditions that are problematic.

It is accordingly an object of the invention to provide a circuit breaker that will, at least partially, alleviate the above disadvantages.

It is also an object of the invention to provide a circuit breaker which will be a useful alternative to existing circuit breakers.

SUM A Y OF THE INVENTION

According to the invention there is provided a circuit breaker including: an electrical contact set comprising a first, stationary contact and a second, displaceable contact, the displaceabie contact being disptaceabie between a connected position in which the contacts abut, and a disconnected position in which the contacts are spaced apart;

a switching mechanism for displacing the second contact between the connected and disconnected positions, the switching mechanism including a handle arrangement which is displaceable between an off position, in which the second contact is in the disconnected position, and an on position, in which the second contact is in the connected position;

a biasing arrangement for biasing the displaceable contact towards the disconnected position;

an electromagnetic actuating arrangement which is displaceable between an operative position in which the displaceabie contact is retained, against the bias, in the connected position, and a trip position in which the displaceable contact is allowed to be displaced, under the bias, towards the disconnected position;

the electromagnetic actuating arrangement including:

a solenoid through which electricity flowing through the circuit breaker is conducted;

a pivotable armature located adjacent the solenoid, the armature being pivotab!y secured to a housing of the circuit breaker, and being displaceable between a norma! position in which it is spaced apart from the solenoid, and an actuated position in which the pivotable armature is displaced towards the solenoid due to an electromagnetic force exerted by the solenoid;

biasing means for exerting a biasing force onto the pivotable armature towards the normal position in order to counteract a gravitational bias of the pivotabie armature; characterized in that the circuit breaker includes a counterweight which is configured to induce a moment about the pivotabte armature in a direction opposite to a moment induced by the weight of the armature and the magnetic attraction of the solenoid.

There is provided for the armature to be in the form of a lever with a fulcrum of the lever disposed between the zone where the counterweight acts on the armature and the point where the magnetic force acts on the armature.

The lever may be a 1 ^st class lever.

There is provided for the counterweight to be separate from the armature, and for the counterweight to be slideab!y displaceab!e away and towards the armature.

The counterweight may be slideable inside a passage defined in a housing of the circuit breaker.

The passage may be located immediately above the one end of the armature, and may extend substantially vertically upwardly from the armature.

There is provided for the counterweight to rest on the armature under gravitational bias.

There is also provided for the actuating arrangement to include a trip lever which is pivotably secured to the housing, the trip lever having a first end that is adapted releasab!y to engage a receiving formation provided in the armature, and a second end which cooperates with the displaceabie contact and the switching mechanism. BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention is described by way of a non- limiting example, and with reference to the accompanying drawings in which:

Figure 1 is a graph showing test conditions applied during a vibration test;

Figure 2 is a dose up view of the interface between a trip lever of a circuit breaker and an armature of the circuit breaker, and emphasizes the propensity of armature creep;

Figure 3 is a graph plotting the dominant natural frequencies encountered in circuit breaker;

Figure 4 is a plan view of a circuit breaker in accordance with one embodiment of the invention in an ON position;

Figure 5 is a plan view of the circuit breaker of Figure 4 in the OFF position;

Figure 6 is a plan view of the circuit breaker of Figure 5 in a TRiPPED position;

Figure 7 is a close-up view of the circuit breaker in the TRIPPED position;

Figure s shows the circuit breaker being reset from the TRIPPED position to the OFF position;

Figure 9 is a close-up view of the counterweight of the circuit breaker in an upper position; Figure 10 is a close-up view of the counterweight of the circuit breaker in a lower position; and

Figure 11 shows the displacement of the counterweight plotted as a function of time.

DETAILED DESCRIPTION OF INVENTION

Referring to the drawings, in which like numerals indicate like features, a non-iimiting example of a circuit breaker in accordance with the invention is generally indicated by reference numeral 10.

The bulk of the circuit breaker 10 shown in the drawings is similar to that found in the prior art, with the novel and inventive aspect residing in the way that a counterweight 60 interacts with an armature 42 of the circuit breaker 10. However, the operation of the remainder of the circuit breaker 0 will be discussed briefly in order to contextuaiize the invention.

Referring to figures 4 to 8, the circuit breaker 10 comprises a housing 11 which houses a first terminal 12 and a second terminal 13 which are in use connected to electrical conductors. The two terminals are selectively in electrical contact by way of a contact set comprising a stationary first contact 14, and a displaceable second contact 32. When the two contacts abut (Figure 4) the circuit breaker is in an ON position, and electricity is conducted through the conductor. When the two contacts are spaced apart, the circuit breaker is either in the OFF position (Figure 5) of in the TRIPPED position (Figure 6), in which condition the circuit is broken and no electricity is accordingly conducted.

A switching mechanism 20 is provided, and is used to toggle the circuit breaker 10 between the OFF and ON positions. The switching mechanism 20 includes a switching body 21 which is pivotably secured to the housing 11 , and which can pivot about a pivot point 23. A switch handle 22 extends from the switch body 21 (and also from the housing 1 ) and is therefore accessible to a user. An actuating extension 24 extends from the switch body 21, and is pivotably connected to an arm 31 of the dispiaceable contact arrangement 30 as described below. A cam formation 25 also extends from the switch body 21 , and in use assists in resetting a trip lever 50 of the circuit breaker when the circuit breaker is in the TRIPPED position.

The dispiaceable contact arrangement 30 includes a pivotabie arm 31 , of which a pivot end 33 is pivotably secured to the actuating extension 24 of the switching mechanism 20. The second contact 32 is located on an opposite end of the arm 31. An important feature of this kind of circuit breaker is that a trip lever 50 is required in order for the switching mechanism to function properly. The trip lever can be in a normal position (Figure 4 and 5), in which the trip lever 50 serves as a stationary anchor for a spring 34 forming part of the dispiaceable contact arrangement 30. When the trip lever is in the normal position, and the switching mechanism 20 is subsequently toggled between the OFF and ON positions, the contact set is simultaneously toggled between disconnected and connected positions. However, when the trip lever 50 is in an actuated position (Figure 6 and 7) the dispiaceable contact arrangement 30 defaults to the disconnected position, and cannot be moved to the connected position again until the trip lever has been reset to the normal position. This is then the TRIPPED state as shown in Figures 6 and 7.

A circuit breaker 10 if the electromagnetic kind includes an electromagnetic actuation arrangement 40 which is essentially designed to displace the trip lever 50 from the norma! position to the actuated position when actuated. The electromagnetic actuation arrangement 40 includes a solenoid 41 and an armature 42 which cooperates with the solenoid 41. The armature 42 is in the form of a pivotabie lever having a first end 42.1 that overlies the solenoid 41, and a second opposite end 42.2 discussed in more detail below. The armature 42 is pivotably connected to the housing 11 , and can be displaced towards and away from the solenoid. The armature is biased away from the solenoid by way of a spring, which also counteracts the gravitational bias of the armature. When a current flowing through the solenoid increases, a magnetic field induced about the solenoid increase proportionally, resulting in the armature to be pulled towards the solenoid. When the spring force is exceeded, the armature will be displaced, resulting in a first end 51 of the trip lever 50 being dislodged from a receiving formation 42.4 provided in the armature, and in the trip lever therefore being displaced to the actuated position, and in the displaceable contact arrangement 30 simultaneously being displaced to the disconnected position. In order to reset the circuit breaker 10, the handle 22 of the switch arrangement has to be pushed down all the way to the OFF position so as to cause the trip lever 50 to be urged back to the normal position by the cam formation 25 forming part of the switching mechanism 20.

All of the above forms part of the prior art, and there may be variations in detail. The invention is therefore not limited to the exact details of the circuit breaker 10 described above. The gist of the present invention entails the introduction of a counterweight 60 that counteracts the magnetic pulling force that is exerted on the armature 42. More particularly, the armature 42 is in the form of a first class or order fulcrum, with the second end 42.2 thereof extending beyond the fulcrum 42.3. A first class or order lever is a lever arrangement where the fulcrum is in the middle - i.e. the effort is applied on one side of the fulcrum and the resistance on the other side.

The counterweight 60 is located on top of the second end 42.2, and acts both as a stationary mass, as well as a dynamic impulse generating mass. The counterweight 60 is not connected to the armature, but is constrained to travel up and down in a channel (61, 62) in the housing immediately above the second end 42.2 of the armature 42. The design and operation of the counterweight is now described in more detail.

The function of the dynamic mass is to prevent the breaker from tripping due to mechanical vibration or shock, without affecting the breaker's normal functionality. The free sliding nature of the mass allows it to function either as a counter weight in a shock situation or a "jackhammer" in a vibration situation. Both of these functions are explained below.

In a mechanical shock situation the mass acts as a counterweight. The balancing of the forces around the pivot point will help to determine the impact of the mass.

m _A - Mass of the armature (Kg)

ag - Acceleration of gravity (10 mis ² )

T _s - Torque of the spring (Nmm)

Ff ~ Force due to friction (N)

a - Acceleration (m/s ² )

m _m - Mass of the Dynamic mass damper (Kg)

Sum of moments around the pivot point with no mass: Static:

-m _A a _g (13mm) + T _S + F _f (10mm)

-(0.003)(10)(13mm) + 2.7 + 0.166(10mm)

3.97 Nmm clockwise

Shock: (Solve for maximum shock)

-?n _A a(13mm) + T _S + F _f (10mm) = 0 -(0.003)a(13mm) + Constant = 0

~(0.039)a + 4.36 = 0

a = 112 m/s ²

a » 11 G's Sum of moments around the pivot point with the mass: Static:

-m _A a _g il3mm) + T _S + F _f (10mni + m _m a _g (Smm) -(0.003)(10)(13mm) + 2.7 -f O.X66(10mm) + (0.002)(10)(5mm)

3.87Nmm clockwise

Shock {Solve for maximum shock):

-m _A a(l3mm) + T _S + F _f (10mrn)+m _m a _g (_Smm) = 0 -(0.003)o(13mm) + (0.002) (Smm) + Constant = 0

-(0.029) + 4.36 = 0

a = 150 m/s ²

a ¾ 15 G's

Hence an improvement of approximately 35% is expected in a shock situation.

In a constant vibration situation the counterweight is allowed to move relative to the switching mechanism inside the breaker and can impact on the armature, even if oniy periodicafly. This dynamic impact has enough force to relatch the armature and thus effectively reset the mechanism. These periodic impacts have no effect on the overload and short circuit performance of the circuit breaker. Although the dynamic mass damper has a smaller moment arm and less mass than the armature, the impulse generated during the impact between the armature and mass still pushes the armature up to its stop, and ensures that the trip lever remains engaged with the armature in the normal position.

The behavior of the mass was analyzed using some basic fundamentals of dynamics and making use of a few assumptions. In the operational position the mass will start off resting on the armature. Once the acceleration of the breaker during vibration is greater than that of gravity the mass will start to bounce on top of the armature within the breaker. As acceleration increases further the mass will bounce up and hit the housing. The housing will then force the mass back downward toward the armature. At low frequencies the mass will be in phase with the vibration of the entire breaker and will impact the armature on every oscillation, however as the frequency increases the breaker will change direction before the mass has travelled the distance from the housing to the armature. At this point the mass starts to behave erratically. However one can still be certain that the mass will periodically hit the armature. At high frequencies the armature is further away from its natural frequencies but the mass relatches it often enough to prevent faulty tripping, it becomes clear now that the mass requires a gap to move between the armature and shell, but it raises the question of how this gap affects performance. The size of the gap determines the frequency at which the mass changes over from an in phase movement to an erratic movement. It has been deemed that the mass works better while in phase and as such it is important to calculate the relevant gap required.

By analysing the nature of the vibration the optimal gap size was calculated. Referring now to Figure 11, if it is assumed that, at point A the mass is against the housing as seen in figure 9, then as the breaker decelerates to point B the mass will carry on at the same velocity (sliding within its housing) till it hits the armature as seen in figure 10 below. When the mass hits the armature it will change velocity to match the velocity of the armature. The magnitude of this change in velocity and the time over which it take place, make it possible to generate large enough forces to re latch the armature. The gap size can be calculated from a few oscillations at 33 Hz: V -Velocity (m/s)

G's - Acceleration of gravity (1G = 10 m/s ² )

f - Frequency (Hz)

t - Time (s)

Gap - Displacement (mm) The following calculations are done with respect to figure 11 :

At 33 Hz one oscillation takes -s - 0.03s

33

The maximum speed of the breaker and mass at point A will be:

V = 1.56 x G's x f

If we consider 2.5 G then:

V = 1.56 x 2.5 ÷ 33

V = 0.118 m/s

In order for the impact to be most effective, it must impact the armature somewhere between B and C. If the mass impacts the armature anywhere between A and B or C and D then the mass and armature are moving in the same direction and the impact of the mass will be less effective. Therefore, the mass must take between 0.0076 sec and 0.0227 sec to impact the armature.

Gap ~ V t

Gap = 0.118 x 0.00765

Gap = 0.9mm

Gap = 0.118 x 0.0227s

Gap = 2.7mm

It follows that at 33 Hz the optimum gap size ranges from 0.9 mm to 2.7 mm. Repeaiing this calculation for 78 Hz:

V = 1.56 x 2.5 ÷ 78

V = 0.05m/s

1

t =— s = 0.013s

78

0.0033 < t < 0.00975

Gap = 0.05 x 0.00335

Gap - 0.2mm

Gap = 0.05 x 0.00975s

Gap— 0.5mm

It follows that at 78 Hz the optimum gap size ranges from 0.2 mm to 0.5 mm.

From the above data one can see that the size of the gap is largely dependent on the frequency of the vibration. Because the optimal gap size does not overlap at all frequencies it is sometime necessary to come to a compromise between sizes. It is also important to realise that there wil) be some component of friction between the mass and its guides. This friction will slightly distort the calculations above.

The mass damper was developed with the objective to prevent the circuit breaker from tripping due to mechanical vibration or shock without any negative effect on the electrical functionality of the breaker. Although the damper can act as a counter weight, the key benefit is that fact that it transfers energy when the mass impacts with the armature.

This impulse is a function of mass and speed, and therefore the higher the mass the more energy is transferred to the armature. In this application brass was selected as it has no effect on the magnetic characteristics of the breaker. Changing the material would not cause the mass damper to fail, it would only mean thai other parameters such as displacement gap would need to be adjusted to optimize the system.

It wili be appreciated that the above is only one embodiment of the invention and that there may be many variations without departing from the spirit and/or the scope of the invention.