FIELD OF THE INVENTION

The present invention relates to electrical switching devices.

BACKGROUND OF THE INVENTION

Electrical switching devices often include means to provide protection for electrical circuits from electrical fault conditions such as current overloads and short circuits. In response to abnormal currents, current in the circuit is interrupted by a mechanism operative for that purpose. Known art includes three basic types of current interruption mechanisms for three types of abnormal current conditions:

Thermal interruption mechanisms, such as bimetals, which protect against persistent, low level overcurrents, with slow response;

Electromagnetic interruption mechanisms, such as solenoids, which protect against higher level overcurrents, with faster response;

Blow-off interruption mechanisms, such as slot motors, which protect against massive, potentially catastrophic, overcurrents, with very fast response, comparable to that of a fuse.

It is required that the response to the latter two types of overcurrent conditions be as fast as possible for a number of reasons. In the case of a high overcurrent, even a single current cycle can pass enough energy to an electrical system to cause serious damage to electrical circuitry which is intended to be protected by the switching device. Additionally, even lower level overcurrents can cause arcing across electrical contacts as they open if they are not separated quickly enough. This is true even when the current is switched off in a normal, non-fault, condition. Arcing also causes overcurrent damage and erodes electrical contacts. An

additional problem resulting from arcing is that it generates metallic particles, soot, and other debris which can damage the switching device and nearby components and which can impair their operation.

The blow-off type interruption mechanism exploits a magnetic force acting directly on the current carrying element to open the circuit. It is very fast and generates a strong opening force so that it is current limiting, but requires a very high current to activate, typically 15 to 20 times the rated current of the switch.

U. S. Patent number 3,815,059 discloses a magnetic blow-off circuit interrupter that is fast and current limiting, but which has a number of limitations. The magnetic means disclosed therein is activated by an additional current coil from the electrical circuit, which introduces added expense and complication, as well as increased risk of arcing. The movable contacts disclosed therein are single contacts mounted on pivot arms which open into the magnetic circuit. The pivot action reduces the opening effectiveness of the magnetic force as the contact opens, limiting protection against possible damage due to arcing. Opening the contact into the magnetic circuit limits the range of motion for opening and requires the device to absorb the energy needed to stop the high speed opening; this cannot be accomplished without mechanical shock and vibration, which will limit the life of the device.

There is a need, therefore, for current limiting switching devices which can reliably operate at high speed for low magnetic switching currents.

SUMMARY OF THE INVENTION

The present invention seeks to provide an electrical contact assembly for use with an electrical switching device protecting an electrical circuit against damage due to abnormal current conditions in the circuit, which overcomes disadvantages of known art by providing a greater opening force at lower electrical current in the circuit, thereby providing faster switching and greater current limitation and hence, greater protection from abnormal current conditions as well as greatly reduced chance of arcing and damage therefrom. The present invention further seeks to provide these advantages with fewer and simpler required attachments to the electrical circuit than in known art.

There is thus provided, in accordance with a preferred embodiment of the invention, for use with an electrical switching device associated with an electrical circuit, an electrical contact assembly including a fixed contact subassembly, a movable contact subassembly, and a magnetic circuit which encloses part of the moving contact subassembly.

The fixed contact subassembly is associated with the electrical circuit and includes sets of fixed contacts and a number of current carrying buses electrically isolated from one another which are arranged with a crossover or X-form so that the segments of the current carrying buses are parallel to one another and carry electrical current in opposing directions. Segments of the current carrying buses are located in proximity to segments of the magnetic circuit so as to activate the magnetic circuit when the buses carry an electrical current.

The movable contact subassembly includes sets of movable contacts associated with the sets of fixed contacts, a contact plate, a contact carrier, and a biasing spring. The movable contacts are grouped in pairs with the contact plate to

form movable bridge contacts. The contact carrier is supported by a portion of the casing of the electrical switching device which has an opening with a profile similar to that of the contract carrier so as to constrain the motion of the movable contact subassembly thereby in a predetermined direction away from the fixed contact subassembly. The movable contact subassembly can move between positions where it is in contact with the fixed contact subassembly and the electrical circuit is closed and where is not in contact with the fixed contact subassembly and the electrical circuit is open The biasing spring, which is optionally adjustable, holds the movable contact subassembly in the open position in the absence of current in the electrical circuit.

The magnetic circuit has poles which surround the contact plate of the movable contact subassembly when the movable contact subassembly is touching or within a predetermined distance from the fixed contact subassembly The magnetic circuit also surrounds the current carrying buses of the fixed contact subassembly and, when the buses are carrying electrical current, is activated by the magnetic flux generated by the current in the buses When there is an abnormal current in the electrical circuit, such as in the case of a short circuit, the magnetic circuit exerts a force on the contact plate which quickly drives the movable contact subassembly away from the fixed contact subassembly to open the circuit. The dimensions of the magnetic circuit and of the contact plate are operative to provide maximal opening force at lower magnitude overload currents than in known art

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more fully understood and appreciated from the following detailed description, taken in conjunction with the drawings, in which:

Figures 1A and IB are schematic representations of an electrical contact assembly constructed and operative in accordance with a preferred embodiment of the present invention, wherein Figure 1 A is a side-sectional view of the electrical contact

assembly, taken along line X-X in Figure IB, and Figure IB is a side-sectional view of the electrical contact assembly of Figure 1, taken along line Y-Y in Figure 1 A; and Figure 2 is a cross-sectional view of the electrical contact assembly of Figure 1 A, taken along line Z-Z therein.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to Figures 1A and IB, there is shown, an electrical contact assembly, referenced generally 10, constructed and operative in accordance with a preferred embodiment of the present invention. Contact assembly 10 is associated with an electrical switching device having tripping means (not shown) and includes a fixed contact sub-assembly 1 1 having fixed contacts 17 and connected to an electrical circuit via a pair of current carrying buses 16 associated with magnetic circuit 27. Current carrying buses 16 are electrically isolated from one another and are formed with a crossover or X-form 18 so that the remaining segments of the buses 16 are generally parallel to one another and have electrical current flowing in opposing directions to one another near different segments of magnetic circuit 27. As will be understood by persons skilled in the art, when there is electrical current in the circuit, the arrangement of current carrying segments shown in Figure 1A is operative to provide the current required to activate magnetic circuit 27, with no need for additional current-carrying coils. Contact assembly 10 further includes a movable contact sub-assembly 12 which includes a contact plate 13 and movable contacts 14 which form a movable bridge contact and which, when in touching contact with fixed contacts 17 on fixed contact subassembly 1 1, as shown in the drawing, close the electrical circuit. Movable contact sub-assembly 12 further includes a contact carrier 15 to which contact plate 13 is connected and which, in turn, is supported and guided by a portion 19 of casing 29 of the switching device. Movable contact sub-assembly 12 additionally includes a biasing element 21 operative to urge movable contact

sub-assembly 12 away from fixed contact sub-assembly 1 1 and to hold movable contact sub-assembly 12 in the open, non-touching, position in the absence of current in the electrical circuit. Biasing element 21 applies a biasing force on movable contact sub-assembly 12 which is optionally adjustable. The closing force for movable contact sub-assembly 12, urging it to remain in touching contact with fixed contact sub-assembly 1 1, is provided by the electrical switching device when it has not been tripped (not shown).

Referring now to Figure IB, there is shown, in side-sectional view, electrical contact assembly 10 shown in Figure 1 A. In this view, magnetic circuit 27 is seen to enclose contact plate 13 of movable contact sub-assembly 12. In a preferred embodiment of the present invention, the gap width δ of the magnetic circuit is equal to or less than three times the thickness Δ of contact plate 13, and the thickness Δ of contact plate 13 is equal to or less than the height h of the plate. As will be understood by persons skilled in the art, this structure produces a maximal magnetic force in order to open the electrical circuit quickly in the presence of a current overload. This magnetic force, in Newtons, is given by the equation:

F = BIL where L is the length of the active portion of contact plate 13 (Figure 1A), I is the current in the plate, and B is the magnetic induction, in Tesla, which is given by the equation:

B = 1.75 x 10- ^{6 χ} (21/6) By limiting δ, the magnetic induction and hence the force, will be greater. Therefore, a force sufficient to open the electrical circuit can be generated with a lower magnitude of current in contact plate 13 and in the electrical circuit.

Referring now to Figure 2, there is shown a cross-sectional view of contact carrier 15 and a portion 19 of the casing 29 of the switching device enclosing the switching device. In a preferred embodiment of the present invention, a portion 19 of casing 29 (Figure 1A) has formed therein an opening 28 which has a profile similar to

that of contact carrier 15, so as to constrain any movement of contact carrier 15 and, hence, movable contact assembly 12 (Figure 1A) in a predetermined direction to most effectively separate movable contact assembly 12 from fixed contact assembly 11 in the case of a current overload.

Referring again to Figure IB, magnetic circuit 27 has an opening 23, the area of which is defined by the gap width δ, multiplied the gap height H. As shown in the drawing, the opening 23 extends above contact plate 13 when it is in the closed position. This allows the magnetic force urging movable contact sub-assembly 12 away from fixed contact sub-assembly 1 1 to continue to act on contact plate 13 as long as it carries electrical current, even if movable contact sub-assembly 12 is no longer in touching contact with fixed contact sub-assembly 11 , as occurs in the case of arcing. This provides an added measure of protection to the electrical circuit to limit arcing even if it should start to occur. Further, since, in a preferred embodiment of the present invention, movable contact assembly 12 is of the movable bridge contact type, contacts 14 and 17 are paired; this divides any arcing that may occur between two locations, thereby further reducing the possibility of damage therefrom

It will further be appreciated, by persons skilled in the art that the scope of the present invention is not limited by what has been specifically shown and described hereinabove, merely by way of example Rather, the scope of the present invention is defined solely by the claims, which follow