GB2149578A - Electric circuit breakers - Google Patents

Abstract

In a circuit breaker, closed loop 15 of electrically conducting material is so disposed that it encompasses a magnetic field generated by passage of short circuit current through conductor 17 of the circuit breaker and is so adapted that the forces resulting from the eddy currents induced in the loop 15 during the initial increase of short circuit current cause motion of the loop 15 which is transmitted by mechanical means 13 and 11 to trip bar 124 to cause tripping of the circuit breaker. In the absence of short circuit current, spring 18 holds the loop 15 in a rest position illustrated such that the mechanical means 13, 11 are out of engagement with each other. <IMAGE>

Description

SPECIFICATION Electric circuit breakers This invention concerns electric circuit breakers, which may be single-pole breakers or multi-pole breakers, of the type hereinafter referred to as "of the type described") comprising, for the or each pole, one or more movable contact arms, the arm or the arms being coupled to a pivoted trip arm by 9 linkage which incorporates a spring, with a pivoted dolly for movement of the latter in one direction (i.e.

the "on" direction) to cause contact-closing movement of the movable contact arm or arms, for respective movable contacts carried by the latter to engage with respective fixed contacts, and movement of the dolly in the other direction (i.e. the "off" direction) to cause contact-opening of the movable contact arm or arms, the trip arm being adapted, upon tripping of the circuit breaker, to be released from a latched position and to move to an unlatched position wherein the spring collapses the linkage.

Release of the trip arm is effected by a catch forming part of a trip bar which extends across the pole or all of the poles of the circuit breaker, and is acted upon by one or more release elements such as a bimetal strip or the armatures of a magnetic system.

In modern electricity supply systems the advantages of so called current limiting circuit breakers are recognized for the benefits they bring to the reduction of energy dissipated in a faulted circuit and consequential reduction in the damage caused.

Already known are arrangements in which current and energy limitation is achieved by extremely rapid detection of the short circuit condition followed by rapid separation of the contacts and generation of an arc voltage which, if of sufficient magnitude, will inhibit the increase of current and suppress its value to zero well before the naturally-occurring current zero in an alternating current system.

It is an object of the present invention to provide a tripping element for a circuit breaker which responds to the rate of change of current with respect to time, thereby ensuring earliest possible detection of the short circuit condition.

With this object in view, the present invention provides an electric circuit breaker of the type described, the pole or at least one of the poles of which incorporates a movable tripping element adapted to act mechanically upon the trip bar to cause circuit breaker tripping and characterised in that the movable element incorporates a closed loop of conducting material, so disposed that it will encompass a magnetic field generated by the passage of current through the circuit breaker and so arranged that the motion of the loop due to eddy currents induced in it during the initial increase of current upon passage of a short-circuit current is transmitted to the trip bar to cause circuit breaker tripping.

The invention will be described further, by way of example, with reference to the accompanying drawings which illustrate preferred embodiments of the circuit breaker, it being understood that the following description is illustrative and not restrictive in the scope of the invention. In the drawings:: Figure 1 is a schematic elevation illustrating, in a very simplified form, the essential components of a dolly-actuatable mechanism for opening and closing the contacts of an electric circuit breaker, these components being shown in the contacts-closed condition; Figure 2 is a view comparable with Figure 1, but showing the positions of the components in the contacts-open condition corresponding to the circuit breaker having been tripped; Figure 3 is a perspective view illustrating those parts, necessary for understanding the invention, of a first embodiment of the circuit breaker of the invention; Figure 4 is a perspective view comparable with Figure 3 but illustrating a second embodiment of the circuit breaker of the invention; and Figure 5 is a graph illustrating the relationships between current, magnetic field and force involved in the circuit breakers according to the invention.

Figures 1 and 2 illustrate very diagrammatically the basic components of the dolly-operated mechanism of an electric circuit breaker of the kind with which the present invention is involved.

Referring firstly to Figure 1, a movable contact arm 100 ofthe circuit breaker is swingable about a fixed pivot 102 and carries a movable contact 104 engageable with a fixed contact 106 to provide a conducting path through the circuit breaker. Where the circuit breaker is a single-pole breaker, there will usually be one such arm 100 with its contact 104 and corresponding fixed contact 106. If the circuit breaker is a multi-pole circuit breaker, there will, usually, be one such movable contact arm 100 with its contact 104 and corresponding fixed contact 106 for each pole of the breaker, with the arms 100 coupled together for movement in unison.

Atoggle linkage comprising first link 108 and second link 110 pivotally connected to one another at 112 extends between the movable contact arm 100 (being connected to the latter at pivot 114) and a trip arm 116 (being connected to the latter at 118) which is swingable about fixed pivot 120 by reason of one end being connected to said pivot 120. The other end of the trip arm 116 is engaged beneath and held down by a latch or detent 122 projecting from a trip bar 124 which, in the case where the circuit breaker is a multi-pole circuit breaker, is common to all of the poles of the breaker.

An actuating dolly of the circuit breaker is represented by the line 126, and this dolly 126 is swingable through a limited angle about a fixed pivot 128 from an on position which is shown in Figure 1, to an off position which is not shown in the drawings but in which it is inclined to the left hand side of the figure by an angle approximately equal to its inclination to the right hand side shown in Figure 1. A strong tension spring 130 is connected between the pivot 112 of the toggle linkage 108,110 and a pivot 132 nearto the other end of the dolly 126.

It will readily be understood that in the on position of the dolly 126 of Figure 1 the action of the spring 130 is to hold the toggle linkage 108, 110 straight, and thereby press the movable contact 104 into engagement with the fixed contact 106. If the dolly 126 is swung to the left in Figure 1 about the pivot 128 until the line of action in the spring 130 moves to the left of the centre line through the pivots 112 and 118, the toggle linkage 108,110 is collapsed and the movable contact 104 is lifted rapidly away from the fixed contact.

If as shown in Figure 2 the latch or detent 122 is turned to release the trip arm 116, the latter can rise, allowing the toggle 108, 110 to collapse with the result that the movable contact 104 is raised to its open position, and the dolly 126 assumes an intermediate tripped position in which the extended line of action of the spring 130 passes through the dolly pivot 128. This provides for the possibility of the dolly 126 being moved manually to the off position so as to bring the trip arm 116 back to a disposition wherein it can be engaged again by the latch or detent 122 once any fault which may have caused tripping has been rectified.

Figure 3 illustrates the essential components of a first embodiment of the circuit breaker which is generally of the kind above described with reference to Figures 1 and 2, and which is constructed in accordance with the invention. As will be understood, in order to trip the breaker, the trip bar 124 has to be turned through a small angle in the direction of the arrow. The trip bar 124 normally rests in a "reset" position, which corresponds to that shown in Figure 1, under the action of a spring (not shown). It will be appreciated that, although not shown, additional tripping elements act upon the trip bar 124, including, for instance, bimetal-actuated thermal overload elements and. our attracted armature magnetic elements. The trip bar 124 incorporates, for the or each pole of the breaker, an extension 11 adjacent to which is pivoted an actuating element 12.

This actuating element 12 has two limbs, of which one limb 13 is an actuating limb so disposed that it can engage with the trip bar extension 11 to produce tripping rotation of the trip bar 124 by pivoting about axis 14. The other limb 10 of the element 12 incorporates a loop 15 of material having good electrical conduction properties, the two limbs 10 and 13 being rigidly coupled together. The loop 15 surrounds a core 16 of soft magnetic material such as iron or appropriate ferrite material, which is so constructed, if of iron, by lamination or otherwise, to minimise the presence of eddy currents within it.

Also surrounding the core 16 is a coil 17 forming part of the current path through the circuit breaker and through which flows all or part of the circuit current.

A spring 18 serves to load the pivoted actuating element 12 lightly such that its loop 15 rests in close proximity to the coil 17 and its actuating limb 13 is clear of the trip bar extension 11.The loop 15 is provided with sufficient clearance relative to the core 16 that the element 12 can pivot about the axis 14 at least as far as is necessary to ensure engagement between the actuating limb 13 and the extension 11 and consequential rotation of the trip bar 124 to the extent that the catch or detent 122 (Figure 1) will release the trip arm 116 (Figure 2).

The operation of the invention relies upon the basic physical phenomenon of induced currents brought about when conducting material is subject to a changing magnetic field. When a short circuit occurs in the circuit in which the circuit breaker is installed, the current in the coil 17 rises very rapidly at a rate typically between 1000 and 10,000 amperes per millisecond. This current produces a proportional magnetic field in the core 16, the high rate of change of which induces a relatively large current in the loop 15, the induced current being proportional to the rate of change of magnetic flux. The induced current reacts with the flux to produce a force on the loop 15 proportional to the product of the rate of change of current and the current magnitude at any instant.The direction of the force is initially away from the coil 17 and results in rapid movement of the actuating element 12 to cause the circuit breaker to trip. During the ensuing interruption process, the circuit current falls to zero and this causes the force on the loop 15 to reverse (since the rate of change reverses) which will assist the spring 18 to reset the actuating element 12 to its rest position.

The arrangement shown in Figure 4 would advantageously be applicable to a circuit breaker of somewhat larger size or rating than that of Figure 1.

It comprises a current-carrying conductor in the form of a straight length of material 20 extending between the limbs of a generally U-shaped core 21. The actuating element 12 has two loops 22 and 23 each surrounding a respective limb of the core 21. It will be readily understood that the flow of short circuit through the conductor 20 causes each loop 22,23 to be repelled from the conductor 20. The resultant motion of actuating element 12 causes tripping of the circuit breaker in the same way as has above been discussed.

Figure 5 illustrates the general physical principles on which operation of the circuit breakers of the invention is based, it being understood that the operating occurs in the first phase of current flow during its initial rise to its peak value. It will also be evident that in whichever direction the current flow first occurs, the direction of motion of the loops 15 or 22, 23 is always initially away from the conductor 17 or 20 carrying the short circuit current.

It is to be understood that the invention is not confined to the precise details of the foregoing example and variations may be made to these details without departing from the scope of the invention as defined by the following claims. Thus, for example, the arrangement of conductors and magnetic circuit may be differently shaped and the transmission of the motion of the short circuited loop to the trip bar may be achieved in ways other than those described above.

Claims (9)

1. An electric circuit breaker of the type described, the pole or at least one of the poles of which incorporates a movable tripping element adapted to act mechanically upon the trip bar to cause circuit breaker tripping and characterised in that the movable element incorporates a closed loop of electrically conducting material so disposed that it will encompass a magnetic field generated by the passage of current through the circuit breaker and so arranged that the motion of the loop due to eddy currents induced in it during initial increase of current upon passage of a short circuit current is transmitted to the trip bar to cause circuit breaker tripping.

2. An electric circuit breaker as claimed in claim 1 wherein the movable element is resiliently loaded in a direction opposite to that in which loop motion results in circuit breaker tripping.

3. An electric circuit breaker as claimed in claims 1 or 2 wherein the loop encompasses a core of ferromagnetic material which serves to enhance the magnetic field generated by passage of current through the circuit breaker.

4. An electric circuit breaker as claimed in claim 3 wherein the core is so constructed by lamination that the occurrence of eddy currents within it is minimised.

5. An electric circuit breaker as claimed in claim 3 or 4 wherein the core is surrounded by a coil forming part of the current path through the circuit breaker and through which flows all or part of the circuit currentofthe pole or the respective pole.

6. An electric circuit breaker as claimed in claim 3 wherein the core is of ferrite material.

7. An electric circuit breaker as claimed in any preceding claim wherein the tripping element is pivotally mounted within the circuit breaker structure and the loop forms part of said element.

8. An electric circuit breaker as claimed in claim 7 wherein the tripping element includes an extension co-operating with the trip bar to cause circuit breaker tripping.

9. An electric circuit breaker substantially as hereinbefore described with reference to and as illustrated in Figure 3 or in Figure 4 of the accompanying drawings.