WO2009024176A1

WO2009024176A1 - Magnetic/hydraulic trip unit for molded case circuit breakers (mccbs)

Info

Publication number: WO2009024176A1
Application number: PCT/EP2007/007430
Authority: WO
Inventors: Wolfgang Erven; James Ferree; Michael Hein
Original assignee: Siemens Aktiengesellschaft
Priority date: 2007-08-21
Filing date: 2007-08-21
Publication date: 2009-02-26
Also published as: EP2179433A1

Abstract

The invention adds a feature to a magnetic /hydraulic trip unit, so that it does not change its time/current characteristic when mounted in different orientations. The inventive device comprises two cores (3, 4) of ferromagnetic material, that are movable in a translatory direction, and a spring arrangement (6, 7), so that the distance between the two moving cores does not change when the direction of gravity is changed.

Description

Magnetic/hydraulic trip unit for molded case circuit breakers (MCCBs)

The invention relates to a magnetic/hydraulic trip unit for molded case circuit breakers (MCCBs) .

Circuit breakers are switching devices capable of carrying currents under normal circuit conditions and breaking currents under specified abnormal conditions such as a short circuit. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset either manually or automatically to resume normal operation. The molded case circuit breaker is one of the two basic low voltage classes of circuit breakers. Low voltage circuit breakers usually have voltage ratings from 250 to 600 V AC and 250 to 700 V DC.

A circuit breaker typically comprises a latching mechanism. The latching mechanism has a handle, several mechanical links, and one or more springs. The operating mechanism is able to open or close a set of movable electrical contacts that are attached to electrically conducting contact arms. In this way the circuit breaker allows the flow of electrical current when the contacts are closed, and stops the flow of electrical current when the contacts open. The handle can be moved by a human operator or by an external motor operator between a closed position and an open position to cause the current-carrying contact arm of the circuit breaker to open and close respectively. The motion is transmitted between the handle and the contact arms by a set of springs and mechanical links. But typically a means is provided so the contacts will open automatically during an electrical overload. Typically one of the mechanical links is biased with a spring force against a latch. During normal opening and closing of the contacts via the handle, the link remains biased against the latch. But in the event of an overload, an actuator will release the latch and allow the mechanism links to suddenly move, which causes the contacts to open. The action of releasing a latch, sudden motion of mechanical links and opening of the electrical contacts is known as tripping.

The trip unit types for circuit breakers can be thermal overcurrent, thermal magnetic, magnetic, magnetic-hydraulic, or electronic. A trip unit with so-called "fuse characteristic" would be a desirable feature for customers, because it has certain advantages. For example, it is easier to achieve goals of selectivity and coordination when integrated into a distribution system of main and branch circuit breakers and fuses when all these devices have a "fuse characteristic". When all the connected devices in a distribution system have a fuse characteristic, selectivity and coordination can be achieved more naturally. Much less effort is required by the customer to adjust specific trip levels on the devices, and it is anticipated that lower cost devices can be used.

Magnetic circuit breakers are implemented using a solenoid/electromagnet whose pulling force increases with the current. The circuit breaker's contacts are held closed by a latch and, as the current in the solenoid increases beyond the rating of the circuit breaker, the solenoid's pull releases the latch which then allows the contacts to open by spring action.

Magnetic/hydraulic trip units have a long history in the industry and are well known. They incorporate a hydraulic time delay feature wherein the solenoid core is placed in a viscous fluid. The core is restrained by a spring until the current exceeds a certain threshold. During an overload, the solenoid pulls the core through the fluid to close the magnetic circuit, which then provides sufficient force to release the latch. The delay permits brief current rises beyond normal running current. Short circuit currents provide sufficient solenoid force to release the latch regardless of core position thus bypassing the delay feature. Several companies produce present day examples of magnetic/hydraulic devices. These devices have several design variables that allow the designer to create a wide range of possible time/current characteristics, including the possibility of a time/current characteristic similar to a fuse. The design variables include the sizing of the ferromagnetic parts, the viscosity of the hydraulic oil, the shape and area of the flow orifices for the hydraulic oil, the spring force and the spring constant.

However, existing magnetic/hydraulic trip units are inherently sensitive to the mounting orientation. This is because, in the low-level overload portion of the time- current curve, that is between about 130% to about 500% current, the magnetic forces are relatively small. In order for tripping to occur, the spring forces opposing motion of the magnetic core must be smaller than the magnetic forces. The force of gravity on the magnetic core is significant in comparison to the spring force. Therefore, if the direction of gravity is changed, then the total force on the moving core is changed by a significant fraction. This changes the velocity of motion of the magnetic core during a low-level overload event, which therefore changes the tripping delay.

Thus, when the direction of gravity is changed, these devices change their time/current characteristic. For example, if the device is mounted vertically and experiences an electrical overload of 200% rated current, it will trip after a time delay Δt. But if the device is mounted horizontally or upside-down and experiences the same 200% overload, its trip time will be greater or less than Δt .

Different solutions have been applied to overcome the inherent sensitivity of a magnetic/hydraulic device to the mounting orientation. First, manufacturers have required that customers must always install the circuit breaker in one particular orientation, for example vertically with respect to the surface of the Earth. Second, the change in the time- current curve due to different mounting orientations is predictable, so it is possible to provide derating factors for the customer. Third, the time-current characteristic can be drawn with a wide tolerance band. But all of these solutions are inconvenient and undesirable for the customer.

Also thermal/magnetic trip units have a long history, and are well known. These devices use a temperature-sensitive bimetallic actuator for the low-level overload portion of the time-current curve, and a magnetic device such as a solenoid or yoke/armature device to trip the circuit breaker on higher-level overloads and short circuits. A bimetallic actuator is inherently not sensitive to the mounting orientation. This is because the forces produced in the bimetal are very high in comparison to the weight of the bimetal. Also, the magnetic device is usually not sensitive to mounting orientation. This is because it is calibrated to trip only at higher-level overloads and short circuits. At higher levels of current, the magnetic forces are great in comparison to the weight of the armature or solenoid core. The spring force is equal to the magnetic force at the threshold tripping value. Therefore the spring forces on the armature or solenoid core are relatively great in comparison to the forces of gravity. Therefore the direction of the gravitational force does not influence the time/current characteristic.

However, thermal/magnetic trip units typically do not have time/current characteristics similar to a fuse. Bimetals must inherently be designed with a different heating characteristic than a fuse element. This is because the bimetal, in contrast to a fuse, must not melt during the overload. Therefore at the upper end of the overload region, at the threshold current of the magnetic device, for example between 500% and 1000% current, bimetals typically react more slowly than fuses. Yet, the magnetic device typically acts much more quickly than a bimetal. This is because the solenoid or armature is moving through air with very little damping, so it accelerates very quickly. Because of this, the time/current curve of a thermal/magnetic trip unit typically has a sudden step between the regions where the bimetal acts versus where the magnetic device acts. This step in the curve is not found in the "fuse characteristic".

Electronic trip units (ETUs) are well known. These trip units have a current transformer for sensing the level of current, and a microprocessor that makes the decision whether or not to trip. Typically the customer is able to adjust the long time delay, short time delay, and instantaneous trip levels of an ETU. With existing devices the customer could adjust the settings to approximate the characteristics of a fuse. Alternatively, new designs of ETUs could be programmed with a fuse characteristic. But ETUs have two specific disadvantages. First, they are costly. The costs of the current transformers, microprocessor, and actuating device are typically much greater than a magnetic/hydraulic device. Second, current transformers are not able to sense DC current, so ETUs are suitable for AC applications only.

Finally, it is obvious that fuses have a "fuse characteristic". However, fuses lack many of the features and advantages of circuit breakers. Only two of the most important differences will be mentioned. First, fuses cannot be immediately switched on after an overload or short circuit. Second, in a 3-phase circuit, fuses do not open all phases of the circuit, which means one or two phases will remain alive after the fuses open from an overload or short circuit event.

The object of the invention is therefore to provide a trip unit for molded case circuit breakers (MCCBs) with a time/current characteristic similar to a fuse, which does not change its time/current characteristic due to the influence of gravity, when the installed mounting orientation of the device is changed.

A device comprising the features of claim 1 achieves the object .

Accordingly, the magnetic/hydraulic trip unit for molded case circuit breakers (MCCBs) according to the invention comprises a non-ferromagnetic longitudinally extending hollow structure filled with a viscous fluid, two movable ferromagnetic cores being provided inside said tube, the two cores being movable along a longitudinal axis of said hollow structure and arranged such that the two cores are spaced apart by a gap, two springs provided inside said tube and being connected to said cores on their respective distal ends not facing each other, a frame structure attached to said hollow structure having an upper ferromagnetic end above said gap and a lower ferromagnetic end below said gap, a ferromagnetic armature for actuating the trip unit and being arranged such that the armature is restrained from connecting said frame structure by a restoring force, a current conductor arranged such it is connected to a main electric current path of the circuit breaker, and arranged such that a magnetic field is produced by said current through said conductor to provide a force on the two cores along said longitudinal axis of said hollow structure .

The invention adds features to a magnetic/hydraulic trip unit, so that it does not change its time/current characteristic when mounted in different orientations. The device comprises two cores of ferromagnetic material, that are movable in a translatory direction, and a spring arrangement, so that the distance between the two moving cores does not change when the direction of gravity is changed. By providing at least two springs, the two cores are held symmetrically in any orientation of the device. Advantageously, the mass of the two cores and the spring constants of the respective two springs are provided with a value such that the size of the gap remains constant in any orientation of the magnetic/hydraulic trip unit.

The frame structure comprises a first upper ferromagnetic frame and a second lower ferromagnetic frame arranged spaced apart from each other. However, the frame structure may also be a single unit with an upper ferromagnetic part and a lower ferromagnetic part, connected by a non-ferromagnetic part.

The current conductor may be a coil being wrapped around said hollow structure and/or said frame structure. However, the current conductor may also have an s-shaped form or a loop form, where at least a part of the current conductor is wound around the hollow structure to produce a magnetic flux through said hollow structure.

The viscous fluid may be oil.

The longitudinally extending hollow structure may be provided by a tube. The hollow- structure may be made of brass.

The two cores may have a total length sufficient to bridge the distance between the upper and the lower ferromagnetic end of the frame structure. This has the advantage, that when an overload current flows through the device, the upper frame and the lower frame are connected to each other by the two cores being in contact with each other, allowing a magnetic flux to flow through the ferromagnetic elements to close the armature, which in turn release a latch, which then causes contacts of to open in order to stop a flow of current.

In a preferred embodiment, the two movable cores are positioned symmetrically about the middle of the gap. A further middle spring is preferably provided between said two cores and attached to said cores at their ends facing each other. By using in total three springs, all springs can be of the compression type.

The at least two springs are approximately equal in length and spring constant. Consequently, the two cores are approximately equal in mass.

The ferromagnetic armature is restrained to attach to said frame structure by a spring.

The armature is a multi-piece armature having more than one degree of freedom of motion and multiple springs for restraining the armature to attach said frame structure.

An end cap is provided at both ends of said hollow structure, and the hollow structure is previously to mounting the end cap provided open at both ends. An end cap may only be provided at one end of said hollow structure, its other end being closed. The ends of the hollow structure are sealed to prevent a leakage of the viscous fluid. The end cap may further be provided with means for adjusting the force of the at least two springs. Said means for adjusting the force of said at least two springs might be provided by a screw thread interface between an end cap and the hollow structure.

In normal operation mode corresponding to the flow of a rated current said gap may have a size such that it provides a reluctance path that prevents the magnetic field generated by said conductor from having enough intensity to attract the armature against the restraining spring force.

For currents up to about 500-2000% rated current said gap may have a size such that it provides a reluctance path that prevents the magnetic field generated by said coil from having enough intensity to attract the armature against the restraining spring force.

The invention will be further described by way of embodiments along with the accompanying drawings. The drawings show:

Figure 1 a prior art magnet/hydraulic trip unit at rest .

Figures 2 - 6 a prior art magnet/hydraulic trip unit during electrical overload.

Figures 7 - 8 a prior art magnetic/hydraulic trip unit during short circuit.

Figure 9 a sectional view of a preferred embodiment of the invention.

Figure 10 an end view of the preferred embodiment as shown in Fig. 7.

Figures 11 - 13 a tripping of the device according to the invention during electrical overload in a time sequence from left to right, wherein the arrows show the flow of the magnetic flux;

Figures 14, 15 a tripping during a short circuit in a time sequence from left to right.

Figures 16 - 18 the influence of gravity on an embodiment according to the invention.

Figures 19 - 21 alternative current paths in a device according to the invention. Figure 22 an alternative embodiment of the invention with only two springs located inside the oil- filled tube.

Fig.l shows a prior art magnet/hydraulic trip unit at rest. Fig. 2-6 show the prior art magnetic/hydraulic trip unit during electrical overload. Hydraulic Magnetic Circuit Breakers operate on the magnetic force produced by the load current flowing through a series connected current conductor 13. Preferably, the current conductor 13 is provided by a solenoid coil 13 which is wound around a hermetically sealed longitudinally extending hollow structure 9 containing an iron core 3, a spring 6 and a dampening fluid, here oil 5. Preferably and without any restriction, the longitudinally extending hollow structure 9 is a tube 9. At currents below the circuit breaker rating, the magnetic flux in the solenoid is insufficient to attract the core 3 towards the pole piece 14 due to the pressure of the spring 6 (Fig. 1) .

When an overload occurs, i.e. when currents above the circuit breaker rating flow, the magnetic flux in the solenoid 13 produces a sufficient pulling force on the core 3 to commence its movement toward the pole piece 14 (Fig. 2-6) .

During this movement, the hydraulic fluid 5 regulates the speed of travel of the core 3, thereby creating a controlled time delay, which is inversely proportional to the magnitude of the current through the trip unit. This time delay is useful in that respect that if the overload is of short duration as in the case of a start of an associated motor, the core 3 returns to its rest position of Fig. 1 once the overload disappears.

However, if the overload persists, the core 3 reaches the pole piece 14 after a time delay corresponding to the occurring current (Fig. 6) . As a consequence, the reluctance of the magnetic circuit drops considerably, so that the armature 11 is attracted to the pole face 14 with sufficient force to collapse the latch mechanism (not shown) and consequently trip the breaker (Fig. 5 - 6) . The contacts separate (not shown) , current ceases to flow, and the core returns to its rest position.

A short circuit operation is shown in Figs. 7, 8. With high values of overload currents or a short circuit, the magnetic flux produced by the coil 13 is sufficient to attract the armature 11 to the pole face 14 and trip the breaker even though the core 3 has only slightly moved (Fig. 7) or not moved at all (Fig. 8) . This is called the instantaneous trip region of the circuit breaker characteristic.

The trip point of a magnetic hydraulic circuit breaker is unaffected by ambient temperature, unlike thermal circuit breakers. As a consequence, after tripping, the breaker may be reclosed immediately since there is no cooling down time necessary. By the nature of the principle of operation, it is possible to obtain any variation of time/current characteristic as desirable.

Referring to the preferred embodiment in Fig. 9, the invention shares some basic functions of prior art devices. A ferromagnetic armature 11 is restrained by an armature spring 12. When a sufficiently high magnetic field arises, the armature is pulled towards a frame structure by overcoming the spring forces of the upper spring 6 and the lower spring 7, and as a consequence, the armature closes. This motion actuates a trip lever (not shown) . The frame structure preferably comprises two separate parts, an upper ferromagnetic frame 1 and a lower ferromagnetic frame 2.

At overload currents up to about 500-2000% rated current, an oil dashpot system is used to create a time delay that is inversely proportional to the current. A gap "A" exists between the ferromagnetic parts inside a tube 9 filled with viscous oil 5. The tube 9 is constructed of non-ferromagnetic material such as brass. The gap "A" constitutes a high reluctance path that prevents the magnetic field from having enough intensity to close the armature 11.

If an overload persists long enough, the parts inside the oil-filled tube 9 are attracted to each other against the spring forces. As the gap "A" becomes smaller, the overall reluctance of the magnetic circuit is reduced, the magnetic field increases in intensity, until the force on the armature 11 overcomes the spring forces, and the armature 11 closes and trips the circuit breaker.

On the other hand, if a high level short circuit occurs, the intense magnetic field will close the armature 11 immediately, before the parts inside the oil-filled tube 9 complete their motion.

The described function is similar to prior art devices.

However, in contrast to prior art devices with a one piece frame, one moving core, and one spring inside the oil-filled tube, the invention has a frame consisting of two pieces 1 and 2, two moving cores 3 and 4, and at least two springs inside the tube 6 and 7.

In the preferred embodiment there is also a third spring 8 between the two moving cores 3 and 4. By using three springs, all springs 6, 7 and 8 can be of the compression type, which allows a simpler assembly. However, it would be possible to build the invention with only two springs 6, 7 inside the tube 9 as shown in Fig. 22.

The coil 13 is located such that it is in series with the main current path of the circuit breaker. Current flowing through the coil gives rise to an excitation field that creates a flux in the magnetic circuit of the device. The coil 13 is wrapped around the tube 9 in the embodiment shown, but could conceivably be wrapped around the frame pieces 1 or 2 instead or in addition. A coil 13 is appropriate for small devices with low rated current. In larger devices a solid conductor would be used, as shown in Figures 20 and 21.

Fig. 10 shows an end view of the device from Fig. 9.

Fig. 11 - 13 show the operation of the invention during an electrical overload. The arrows show the flow of magnetic flux around the magnetic circuit. Initially, as shown in Fig. 11, the two moving cores 3, 4 are separated by a gap A, so that there is high reluctance in the magnetic circuit. But as the overload persists, the upper and the lower core 3, 4 are pulled together as shown in Fig. 12 and overcome the spring forces of the upper spring 6 and the lower spring 7. As the gap "A" decreases, the overall reluctance decreases and the magnetic field becomes more intense, and at a certain point there is enough force to pull the armature 11 closed as shown in Fig. 13. Because the cores 3, 4 are moving through a viscous fluid 5, there is a time delay prior to the armature 11 closing. In magnetic/hydraulic devices, it is typical to use a fluid 5 that does not significantly change viscosity with temperature, such as silicone oil 5.

The springs in the figure are compression springs. Each core is acted on by two spring forces. The spring below the core pushes upward on the core. The spring above the core pushes downward on the core. Figure 11 shows the initial position of the cores when the device is at rest. In Figure 11 the spring forces are balanced so the net spring force on each core is zero. But in Figures 12 and 13, the spring forces are opposing the displacement. The spring between the cores has a greater force than the other springs, so that the net spring forces tends to drive the cores away from each other. The magnetic forces tend to attract the two cores 3, 4 together. The magnetic flux is much stronger on the surfaces of the cores 3, 4 that face each other than on the surfaces that point away from each other, therefore there is a net magnetic attraction of the cores 3, 4 toward each other.

There is also a magnetic attraction between each core 3, 4 and the frame piece in which it is situated. But these magnetic forces are directed sideways, in which direction the cores are not free to move. Further, these forces act radially from the axis of motion of the corresponding core 3, 4. Since the frame pieces fully surround the core 3, 4, the sum of the radial forces on each core 3, 4 are essentially zero.

Fig. 14, 15 show a device operation during a short circuit. In this case the high currents produce an intense field, in spite of the high reluctance magnetic circuit. Therefore the armature 11 closes immediately before the two cores 3, 4 are pulled together. Fig. 14, 15 show the tripping in a time sequence from left to right, from Fig. 14 to 15.

Fig. 16 - 18 show the influence of gravity on an embodiment according to the invention, when the gravity direction changes according to four different mounting orientations of the circuit breaker. For gravity directions GC and GD as shown in Fig. 17, the upper core 3 and the lower core 4 are positioned symmetrically about the middle of the tube 9, with a gap A_C-_D- In the embodiment shown, the upper spring 6 and the lower spring 7 are approximately equal in length and spring constant. Also the upper and lower cores 3 and 4 have equal mass. When the mounting orientation and gravity direction is changed as in Fig. 16 or 18, the cores 3 and 4 move against the springs 6 and 7 under the influence of gravity and are displaced by distances Xi_B and X₂B, or X_iA and X₂A_^ respectively. It is a simple calculation to show that Xi_B equals X_2B, or that Xi_A equals X₂A- The result is that the respective gaps A_c-_D, A_B, and A_A always equal each other. The size of the gaps A does not change in different mounting orientations of the circuit breaker. This means the reluctance of the magnetic circuit does not change when the orientation is changed. The cores 3, 4 will have to move the same distance against the same spring forces prior to tripping, regardless of the circuit breaker orientation.

As a consequence, the time/current curve of the device is not sensitive to the mounting orientation. This is the primary advantage of the invention. It should be easily understood that the four gravity directions shown are not the only orientations with this advantageous characteristic, but that the invention works in any orientation.

As an alternative, it is conceivable that the two moving cores 3, 4 could have different masses, and the upper and lower springs 6 and 7 could have different spring constants calculated in the same proportion, so that the distance between the cores 3, 4 remains the same under different conditions of gravity.

A simple armature 11 and spring 12 has been shown by way of example for this invention. However, it is anticipated that a more complex armature system could be used. For example, a different shape of the armature 11 could be used. Also, the armature 11 could comprise several pieces. A multi-piece armature 11 could have more than one degree of freedom of motion and multiple springs. These options could allow a further customization of the time/current curve. The simple example shown is not intended to limit the invention to this one type of armature 11.

In the present invention, the armature 11 is the actuator that releases a latch (not shown) . Many possible configurations of latching mechanism are known. For example, a link may be spring-biased against a D-shaped rotatable shaft. When the mechanism is latched, the rotatable shaft prevents motion of the link. When the shaft rotates, it moves to a position so the link is released and the mechanism trips. The shaft might have an arm extending radially that the armature pushes. Also, the armature 11 might have another arm extending from it that pushes against the shaft to rotate it. Also another possibility is that the armature 11 itself is integral with the shaft, so that rotation of the armature 11 directly rotates the shaft.

The end cap 10 as shown in Fig. 9 can be at one end only or at both ends of the tube 9. If the end cap 10 is at both ends, then the tube 9 is open at both ends. For example, such a tube 9 could be manufactured by extrusion and then cutting it to the correct length. If only one end cap 10 is used, then the tube 9 should be closed at the other end. For example, such a tube 9 could be manufactured by deep drawing. A sealing method is required to prevent the oil 5 from leaking out between the end cap 10 and the tube 9. This can be accomplished many different ways. For example, an epoxy sealant could be used.

The end cap 10 can include a feature for adjusting the force of the springs 6, 7 inside the tube 9. One possibility is to make a screw thread interface between the end cap 10 and the tube 9. By turning the end cap 10, the length of the springs 6, 7 could be adjusted. Alternatively an additional threaded part can be added to the end cap 10 to accomplish this purpose .

The cores 3, 4 are formed of ferromagnetic material, such as iron or steel. They might, for example, be cut and machined from a cylindrical piece of steel. Alternatively they could be made from powder iron or steel and manufactured by pressing and sintering. The springs 6, 7, and 8 are preferably formed of non- ferromagnetic or essentially non-ferromagnetic metal. Most likely they would be formed from stainless steel such as type 302, which is only slightly magnetic after forming. Alternatively they could be formed from brass. Alternatively they could be formed from a ferromagnetic material such as high-carbon steel, for example if testing shows the magnetic effects does not interfere with the function of the invention.

Figures 19 - 21 show alternative current paths in a device according to the invention. Fig. 19 shows the current part as previously shown in Fig. 9, i.e. the current flows through the current conductor 13 provided by a coil and produces a magnetic flux on the cores 3, 4 in the tube 9. However, as shown in Fig. 20, the current could also flow through a solid conductor 13 in form of a loop around the tube 9 or through an s-shaped or step-shaped solid conductor 13, where the tube 9 is placed adjacent the s-shape part of the conductor 13.

Figure 22 shows an alternative embodiment of the invention with only two springs 6, 7 located inside the oil-filled tube9, i.e. the middle spring 8 of Fig. 9 is missing. Otherwise, the shown embodiment works as described in the previous figures.

The invention adds a feature to a magnetic/hydraulic trip unit, so that it does not change its time/current characteristic when mounted in different orientations. The inventive device comprises two cores 3, 4 of ferromagnetic material, that are movable in a translatory direction, and a spring arrangement 6, 7, so that the distance between the two moving cores 3, 4 does not change when the direction of gravity is changed. List of reference numbers

1 Frame (upper)

2 Frame (lower)

3 Core (upper)

4 Core (lower)

5 Oil

6 Spring (upper)

7 Spring (lower)

8 Spring (middle)

9 hollow structure / tube

10 End cap

11 armature

12 armature spring

13 current conductor

14 pole

Claims

1. Magnetic/hydraulic trip unit for molded case circuit breakers (MCCBs) comprising: a non-ferromagnetic longitudinally extending hollow structure (9) filled with a viscous fluid (5); two movable ferromagnetic cores (3, 4) being provided inside said tube (9), the two cores (3, 4) being movable along a longitudinal axis of said hollow structure (9) and arranged such that the two cores (3, 4) are spaced apart by a gap (A); two springs (6, 7) provided inside said tube (9) being connected to said cores (3, 4) on their respective distal ends not facing each other; a frame structure attached to said hollow structure

(9) having an upper ferromagnetic end above said gap (A) and a lower ferromagnetic end below said gap (A) ; a ferromagnetic armature (11) for actuating the trip unit and being arranged such that the armature (11) is restrained from connecting said frame structure by a restoring force; a current conductor (13) arranged such it is connected to a main electric current path of the circuit breaker, and arranged such that a magnetic field is produced by said current through said conductor (13) to provide a force on the two cores (3, 4) along said longitudinal axis of said hollow structure (9) .

2. Magnetic/hydraulic trip unit according to claim 1, wherein the mass of the two cores (3, 4) and the spring constants of the respective two springs (6, 7) are provided with a value such that the size of the gap (A) remains constant in any orientation of the magnetic/hydraulic trip unit.

3. Magnetic/hydraulic trip unit according to claim 1 or 2, wherein the frame structure (1, 2) comprises a first upper ferromagnetic frame (1) and a second lower ferromagnetic frame (2) arranged spaced apart from each other.

4. Magnetic/hydraulic trip unit according to any of the previous claims, wherein the current conductor (13) is a coil being wrapped around said hollow structure (9) and/or said frame structure.

5. Magnetic/hydraulic trip unit according to any of the previous claims, wherein the viscous fluid (5) is oil.

6. Magnetic/hydraulic trip unit according to any of the previous claims, wherein the longitudinally extending hollow structure (9) is provided by a tube.

7. Magnetic/hydraulic trip unit according to any of the previous claims, wherein the two cores (3, 4) have a total length sufficient to bridge the distance between the upper and the lower end of the frame structure.

8. Magnetic/hydraulic trip unit according to any of the previous claims, wherein the hollow structure (9) is made of brass.

9. Magnetic/hydraulic trip unit according to any of the previous claims, wherein the two movable cores (3, 4) are positioned symmetrically about the middle of the gap (A) .

10. Magnetic/hydraulic trip unit according to any of the previous claims, wherein a further middle spring (8) is provided between said two cores (3, 4) and attached to said cores (3, 4) at their ends facing each other.

11. Magnetic/hydraulic trip unit according to any of the previous claims, wherein the at least two springs (6, 7) are approximately equal in length and spring constant.

12. Magnetic/hydraulic trip unit according to any of the previous claims, wherein the two cores (3, 4) are approximately equal in mass.

13. Magnetic/hydraulic trip unit according to any of the previous claims, wherein the ferromagnetic armature (11) is restrained to attach to said frame structure by a spring (12) .

14. Magnetic/hydraulic trip unit according to any of the previous claims, wherein an end cap (10) is provided at both ends of said hollow structure (9) , and the hollow structure (9) is previously to mounting the end cap (10) provided open at both ends or wherein an end cap (10) is provided at one end of said hollow structure (9), the other end being closed.

15. Magnetic/hydraulic trip unit according to any of the previous claims, wherein the ends of the hollow structure (9) are sealed to prevent a leakage of the viscous fluid (5) .

16. Magnetic/hydraulic trip unit according to any of the previous claims, wherein the end cap (10) is provided with means for adjusting the force of the at least two springs (6, 7 ) .

17. Magnetic/hydraulic trip unit according to any of the previous claims, wherein said means for adjusting the force of said at least two springs (6, 7) is provided by a screw thread interface between an end cap (10) and the hollow structure (9) .

18. Magnetic/hydraulic trip unit according to any of the previous claims, wherein in normal operation mode corresponding to the flow of a rated current said gap (A) has a size such that it provides a reluctance path that prevents the magnetic field generated by said conductor (13) from having enough intensity to attract the armature (11) against the restraining spring force.

19. Magnetic/hydraulic trip unit according to any of the previous claims, wherein for currents up to about 500- 2000% rated current said gap (A) has a size such that it provides a reluctance path that prevents the magnetic field generated by said coil (13) from having enough intensity to attract the armature (11) against the restraining spring force.