GB2149215A - Electric circuit breakers - Google Patents

Abstract

Upon passage of short circuit current in a circuit breaker through a conductive loop constituted by a go conductor 15 and a closely- adjacent return conductor 16 to which fixed contact 20 is connected, repulsive forces generated between the conductors 15 and 16 cause motion of the fixed contact 20 in the contact- opening direction of associated movable contact 21, which motion is limited by abutment of one end of slotted element 31 with pin 33, thereby to enhance contact pressure during short-circuit current flow prior to contact separation and ensuring rapid initial contact separation thereafter. In the absence of short circuit current, spring 34 ensures that the fixed contact 20 is retained in the illustrated rest position with the opposite end of the slotted element 31 abutting the pin 33. <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 a 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 by means of a trip bar common to all poles, to be released from a latched position and to move to an unlatched position wherein the spring collapses the linkage.

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. One commonly-used method of achieving rapid contact separation, particularly in smaller size circuit breakers (100 Amps or less), is the use of a so-called "hammer trip" mechanism.This operates by accelerating the armature of a small solenoid, energized by circuit current, towards the movable contact in such a direction that its impact causes the movable contact to separate from the fixed contact against the action of its resilient loading means. Movement of the armature also serves to trip the mechanism, thereby retaining the contacts in the open condition. Also known, for larger sizes of circuit breakers, are arrangements of conductors making use of the electro-magnetic forces which arise between them under short circuit conditions to directly separate the moving and fixed contacts.

A disadvantage of the "hammer trip" system occurs when the time required to accelerate the inert mass of the armature prior to contact separation becomes excessive with larger and heavier contact systems. A problem occurring with direct electro-magnetic separation can be contact welding at certain levels of current below the full short circuit level. This is not generally a problem where the current increases at a high rate and continues to do so through the level at which separation occurs. However, on alternating current systems the variation in instantaneous current values, due to both differing magnitudes and initiation points on the alternating voltage waveform, can produce a condition where the separating forces are barely sufficient and the contacts may re-close after a brief period of arcing with attendant danger of welding.A further disadvantage of direct electro-magnetic method concerns the relatively slow initial contact separation which allows the arc, so formed, to linger at a small contact gap, thereby resulting in excessive contact erosion.

This is not a significant problem with the "hammer trip" method where the impact of the armature causes rapid initial separation.

It is an object of this invention to provide a novel contact system with positive high speed separation under high short circuit conditions, in a particularly simple and effective manner.

A further object is to provide a system which functions in a manner known to reduce the tendency to contact welding and provide a system in which the response to short circuit conditions is, to some extent, a function of the rate of change of current with respect to time, thereby achieving even faster contact separation at higher levels of short circuit current.

With these objects in view, the present invention provides an electric circuit breaker of the type described of which the pole or at least one of the poles incorporates, for said pole, a fixed contact construction including a current-carrying loop providing a go element and a return element carrying the fixed contact which go and return elements are in close proximity to each other and are flexibly connected at a point remote from the fixed contact, the return element being arranged to have a limited movement in substantially the same direction as the opening direction of the movable contact, so that passage of short circuit current will create electro-magnetic repulsive action in the go and return elements to move the fixed contact in the opening direction of the movable contact, to prevent uncontrolled separation of said contacts.The arrangement of the invention may be further characterized in that said return element incorporates a fixed-contact carrying portion which is substantially at right angles to the effective current path in the movable contact, together with a further short portion adjacent to the point of contact of the fixed and movable contacts and substantially parallel to the effective current path in the movable contact. The return element may conveniently be lightly spring loaded against a stopface forming one limit to its travel and the stopface forming the other limit may be of relatively substantial construction and in close proximity to the fixed contact point to ensure sudden termination of movement, with consequent rapid contact separation.

The invention will be described further by way of example with reference to the accompanying drawings which illustrate a preferred embodiment of the circuit breaker, it being understood that the following description is illustrative and not restrictive of the scope of the invention. In the drawings:- Figure 1 is a fragmentary part-sectional side elevation illustrating those parts of the said preferred embodiment necessary for understanding the invention, the movable contact being shown in the "on" or "contacts closed" position; Figure 2 is a view comparable with Fig. 1, but showing the two elements of a fixed contact of the circuit breaker at the limit of their mutually repulsive travel and with the fixed and movable contact about to separate; and Figure 3 is a view comparable with Figs. 1 and 2, but showing the movable contact having moved part way towards its fully open position.

The illustrated preferred embodiment of the circuit breaker conforming to the invention is a multi-pole breaker and comprises a moulded plastic casing 10 having a removable cover 11 and is shaped internally to provide separate compartments, of which the details of one are shown, for each pole. The individual components in each compartment are substantially similar and accordingly it will be understood that except where otherwise explained, the illustrated components are repated in each of the compartments.

Adjacent one end of the circuit breaker is a terminal assembly 12 for the connection of external conductors (not shown). The connection from the terminal assembly 1 2 has two branches 1 3 and 1 5 within the compartment.

The first branch 1 3 leads to the adjacent end of a stack of metal plates 14 the purpose of which is to quench any arc which may be formed on contact separation. The second branch 1 5 is constituted by one of two conductors 1 5 and 1 6 which flexibly joined at their furthest point 17 and constitute "go" and "return" elements respectively of a current-carrying loop, the two elements 15, 1 6 being in close proximity over the greater part of their lengths. The elements 15, 1 6 may advantageously each be formed of rectangular-section material disposed with the long dimensions of the section at right angles to the plane containing the loop.This enables the effective centres of the two conductor sections to be brought into closest proximity thereby maximising electro-magnetic repulsive forces which occur in the elements 15, 1 6 when current flows therethrough. This disposition also serves to minimise the space required within the compartment to accommodate the arrangement in a form having sufficient cross-sectional area of conductor to carry the necessary current without excessive heating. The return element 1 6 includes two further conductor portions 1 8 and 1 9 the latter carrying a fixed contact 20 which co-operates with a movable contact 21 attached to a movable contact arm 22.A further short conductor portion 23 serves to lead any arc, formed on contact separation, away from the contacts 20 and 21 and into the vicinity of the arc quenching plates 14 by a process which will be described in detail later.

The go element 1 5 is firmly fixed relative to the structure of circuit breaker casing by any suitable means. In the illustrated embodiment, this is achieved by means of projections 24, 25 and 26 which locate in respective slots (not shown) present in the casing structure.

The return element 1 6 is also provided with projections 27 and 28 which locate it firmly in a pivoted saddle member 29 having a pivotal axis 30 and capable of a small angular swinging movement, enabling the return element 1 6 to move correspondingly. Adjacent to the contact 20, the extent of possible angular movement of the element 1 6 is conveniently defined by a slotted element 31 integral with or firmly attached to the conductor portion 1 9 carrying the contact 20. Engaging into slot 32 of the element 31 is a pin 33 firmly located in the circuit breaker structure.

A spring 34, for example a compression spring, suitably located in a pocket 35 in the casing stucture, acts upon the return element 1 6 such that it is lightly loaded in a direction towards the go elements 1 5 and in closest proximity to it as defined by the possible angular movement of the return element 1 6 and saddle member 29 about the axis 30, limited by the engagement of the pin 33 with the end of the slot 32.

It will be appreciated that the loop, comprising the go and return elements, 15, 1 6 respectively, as just described, replaces the normal fixed contact arrangement of a conventional circuit breaker, by an arrangement providing the fixed contact 20 which is normally retained in a defined position relative to the structure of the circuit breaker. On the occurrence of a short circuit, however, the repulsive forces, set up between the two elements 1 5 and 1 6 of the loop by the passage of the large electric current, cause the fixed contact 20 to push the movable contact 21 towards its open position until the movement of the fixed contact 20 is suddently arrested by the pin 33 abutting against the end of the slot 32 as shown in Fig. 2.

The movable contact 21 is carried by the movable contact arm which is pivotally mounted by means of a pin 36 to a carrier 37. This carrier 37 is itself pivotally located in the circuit breaker structure and is capable of rotation about an axis 38, being adapted to be moved by way of a spring-loaded linkage including a link 39 pivotally attached to it by the pin 40, from a dolly (not shown). The arm 22 is resiliently loaded towards the ON or contacts-closed position by means of the arrangement of a compression spring 41 and a link 42, as shown in Fig. 1. In the OFF and intermediate positions when the two contacts 20 and 21 are not touching (e.g. as shown in Fig. 3) the movement of the movable contact arm 22 in the ON direction is limited by a stopface 43 on the carrier 37, against which the arm 22 abuts.In the reverse, OFF or opening direction, the movement of the contact arm 22 relative to its carrier 37 is limited by a further stopface 44 against which the opposite face of the arm 22 abuts. It will be appreciated that the movable contact arm 22 is capable of a relatively large movement between the two stopfaces 43 and 44, which enables it to move from the ON position (in which the two contacts 20 and 21 are touching) towards the OFF or open position under the action of electro-magnetic forces without the necessity for the carrier 37 to move at the same time.

The effect of the loop elements 1 5 and 1 6 separating under the action of electro-magnetic forces may be conveniently improved by the addition of steel plates such as those indicated at 45. Each such plate 45 is formed as a closed loop completely surrounding the two elements 1 5 and 1 6 and is firmly located relative to the structure of the circuit breaker casing 10. The plurality of plates 45 serves to minimize eddy-current effects which could cause excessive local heating or possibly delay the build up of magnetic flux necessary to cause effective coperation.The plates 45 also serve to increase the magnetic flux produced by a given value of current which thereby enables loop separation (i.e. movement of the return element 1 6 away from the go element 1 5) to take place at a lower instantaneous current magnitude than that which it would without them. The steel pieces 45 may serve, in addition to the slot 32 and pin 33, to provide limitation of the travel of the return element 1 6 of the loop and further serve to prevent distortion of the two loop elements 1 5 and 16, under the extremely high forces which may occur at the highest levels of short circuit current.

The operation of the circuit breaker will now be described on the basis of the foregoing description. With the circuit breaker in the OFF position as shown in Fig. 3, the fixed contact 20 is held at the limit of its travel in the ON direction by the action of the spring 34 loading the movable return element 1 6 so that the pin 33 abuts the end of the slot 32.

When the circuit breaker is switched ON, the operating mechanism (not shown), through the link 39, carrier 37 and movable contact arm 22, drives the movable contact 21 towards the ON position, this motion continuing until the movable contact 21 abuts against the fixed contact 20. At this point, if the circuit breaker is connected in an energised circuit, a conducting path is established through the circuit breaker and current flow can commence. As the mechanism continues to move towards the completion of its travel, the movable contact arm 22 pivots relative to the carrier 37 about the axis of the pin 36.

This relative motion is transmitted via the link 42 to the compression spring 41 causing it to compress further, thereby resiliently loading the two contacts 20 and 21 together. On completion of the mechanism travel to the ON position (Fig. 1) this relative motion causes a small clearance to appear between the movable contact arm 22 and the stopface 43.

This clearance is necessary to ensure that in the event of repeated use causing wear and erosion of the contacts 20 and 21 they will always be resliently loaded together in the "on" condition of the circuit breaker, to maintain a low resistance conducting path through the circuit breaker.

When the circuit breaker is switched OFF or tripped (under overload or low short circuit conditions) the process just described takes place in the reverse sequence and for the purposes of understanding this invention need not be described in detail. It should be noted however that in all the closing oropening operations described so far, the fixed contact 20 remains in the same position relative to the circuit breaker casing structure 10 under the action of the spring 34.

Before describing in detail the operation under short circuit conditions, it is appropriate to explain, at least partially, the nature of electro-magnetic forces. As is well known to those skilled in the art, electro-magnetic forces act upon each element of a current-carrying conductor and are proportional to the product of the instantaneous current in the element and the component of the magnetic field at the element in a plane at right angles to the axis of current flow in the element. Thus, in a single pole of a multi-pole circuit breaker of which all conductors carry the same current, each element lies in a magnetic field produced by other adjacent elements and sine these magnetic fields are themselves proportional to the instantaneous current, the forces on each element are proportional to the square of the instantaneous value of the current passing through it.It is also known that the force existing between two current-carrying elements decreases as their separation increases.

It may therefore be readily understood that even in a relatively complicated arrangement of conductors substantially in the same plane (such as occur in circuit breakers generally of the type described) the forces acting on each element are, to a first approximation, inversely proportional to its distance from adjacent elements carrying the same current. It will further be appreciated that an extreme case occurs at contacting or abutting points (such as between the contacts 20 and 21 in the described construction) when the constriction of current flow through what is normally a microscopically small area causes large forces to arise between the contacts by virtue of the very close proximity of elements carrying large values of current.This is the origin of the so called "blow open" forces, which tend to separate the contacts under short circuit conditions and lie generally in the range 0.5 to 2 Newtons for a current of 1000 Amps, depending on the nature of the contacts themselves.

Reverting now to consideration of the present invention, with the circuit breaker in the ON position (Fig. 1) the occurrence of a short circuit causes a rapid increase in circuit current and therefore an even more rapid increase (proportional to the square of this current) in the forces between the adjacent conductor elements. In particular, the close proximity of the go element 1 5 and the return element 1 6 of the current-carrying loop, together with their relatively long length, causes the pivotally-mounted return element 16 to be violently repelled from the fixed go element 15, overcoming the relatively small force of the spring 34.As the return element 1 6 moves away from the go element 1 5 it must also move the movable contact arm 22 towards the OFF position against the action of the compression spring 41 via the link 42.

During this process, the force necessary to accelerate the moving contact arm assembly is transmitted via the two contacts 20 and 21 thereby opposing the blow open forces automatically. It will be appreciated that both the force tending to push the contacts 20 and 21 together (the accelerating force) and the force tending to separate them (the blow open force) are both proportional to the square of the same instantaneous current.It is therefore possible advantageously to exploit the basic arrangement by so proportioning the sizes, weights and spacings of the components involved in the motion, together with the forces of the springs, that the operation of the system approaches the ideal condition whereby, over a desired range of operational short circuit currents, the sum of the blow open force plus the repulsive force between the moving contact arm 22 and the conductor portions 1 9 and 1 8 is always less than the sum of the resultant spring and accelerating forces at the interface between the two contacts 20 and 21, which cannot therefore separate in an uncontrolled manner under the action of the blow open forces. Such uncontrolled separation is known to be a prime cause of contact welding.It will further be appreciated that by arranging for the interface between the two contacts 20 and 21 to be substantially in the same line as the axes of the two pivot pins 36 and 30, the relative sliding motion between the confronting faces of the contacts 20 and 21, during their movement, is minimised.

The motion of the fixed contact 20 and the return element 1 6 by which it is carried.

resulting from the flow of short circuit current, is suddently arrested by abutment of the pin 33 with the extremity of the slot 32. The movable contact 21 and its arm 22 continue to travel towards the OFF position by virtue of the momentum gained thereby during the initial part of the motion and as a consequence the two contacts 20 and 21 separate with a conveniently high velocity. Any arc formed on contact separation is thereby enabled to run rapidly up the extremity of the movable contact arm 22 and the fixed short conductor portion 23 towards the array of arc quenching plates 1 4. To those skilled in the art it will be apparent that the purpose of the relatively short conductor portion 23, acting in conjunction with the movable contact arm 22 is to provide the magnetic field necessary.

at the separating contacts, to propel the arc rapidly towards the arc quenching plates 14, as has been indicated diagrammatically at 50/52 in Fig. 3. Also, as the arc moves, the effective length of the movable contact arm 22 carrying current increases, and this has the effect of both increasing the magnetic field acting upon the arc and increasing the repulsion forces between the fixed conductor portions 1 9 and 23 and the movable contact arm 22 thereby assisting its motion towards the open or OFF position. The overall arrangement of the various conducting components is such that while the contacts 20, 21 are closed, the total forces on the movable contact arm 22 due to flow of current are not substantially increased above those unavoidably arising due to constriction of the current flow through the contact interface.In particular the conductor portion 1 9 is kept short enough to provide no more repulsion than that necessarily associated with the appropriate magnetic strength to ensure satisfactory initital arc movement. Further, the length of the portion 1 8 (parts of which adjacent to the arm 22 also provide some repulsion) in combination with the close spacing of the go and return elements 1 3 and 1 6 respectively ensures that the mutual repulsion between these elements is maximised while their combined effect upon the movable contact arm 22 is small, in spite of their length.

The arrangement also maximises the acceleration of the combined content systems during the initial stages of their motion, since at that time the go element 1 5 and the return element 1 6 are in closest proximity to each other and produce maximum force. This ensures that the necessary minimum contact separation velocity is achieved at the earliest point in the short circuit sequence, thereby providing the best opportunity to achieve satisfactory current control and limitation.

Also the arrangement takes advantage of the phenomenon that a rapidly-changing current flowing around a conducting loop tends to concentrate on the inner surface of the loop, that is to say it seeks the path of lowest inductance while it is changing in value. Thus, the effective centrelines of adjacent loop conductors are closer together for a rapidly changing current than they would be for a current changing less rapidly or not at all.

This effective reduction in the distance between the adjacent conductor elements 1 5 and 1 6 of the loop causes the arrangement to produce a somewhat larger repulsive force between them when the current changes rapidly such as when a short circuit is initially increasing towards a high maximum value.

The effect will be enhanced in the arrangement described, since the dimension between the confronting faces of the conductor elements 1 5 and 1 6 is small (initially) in relation to the conductor dimensions themselves in the plane of the loop. The relative difference in inductance between the path of a current flowing in the outer limits of the conductor elements of the loop and that of a similar current flowing in the inner limits of such an arrangement is much greater than it would be if the distance between the inside faces of the loop conductor elements were large in relation to the conductor dimensions in the plane of the loop.

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 Claims. Thus, for example, the conductor elements 1 5 and 1 6 need not contain right angles, but could be otherwise shaped provided such elements remain in close relative proximity. Instead of the pin 33 and slot 32, any suitable arrangement could be employed to define the limits of possible movement of the "fixed" contact 20.

Claims (9)

1. An electric circuit breaker of the type described, of which the pole or at least one of the poles incorporates, for said pole, a fixed contact construction including a current-carrying loop providing a go element and a return element carrying the fixed contact, which go and return elements are in close proximity to each other and are flexibly connected at a point remote from the fixed contact, the return element being arranged to have a limited movement in substantially the same direction as the opening direction of the movable contact, so that passage of short circuit current will create electromagnetic repulsive action in the go and return elements to move the fixed contact in the opening direction of the movable contact, to prevent uncontrolled separation of said contacts.

2. An electric circuit breaker as claimed in claim 1 wherein the return element incorporates a fixed-contact carrying portion which is substantially at right angles to the effective current path in the movable contact, together with a further short portion adjacent to the point of contact of the fixed and movable contacts and substantially parallel to the effective current path in the movable contact.

3. An electric circuit breaker as claimed in claim 1 or 2 wherein the return element is held in close proximity to the go element by resilient means.

4. An electric circuit breaker as claimed in claim 1, 2 or 3 wherein the movement of the return element in the opening direction of the movable contact is limited by means attached to the return element abutting against a stop face.

5. An electric circuit breaker as claimed in any preceding claim wherein the go element and/or the return element of the loop includes a portion of substantially rectangular cross-section disposed with its long dimension at right angles to the plane containing the loop.

6. An electric circuit breaker as claimed in any preceding claim wherein ferromagnetic material is provided in the vicinity of the go and return elements to increase the magnetic flux produced by an electric current flowing in said elements.

7. An electric circuit breaker as claimed in claim 6 wherein the ferromagnetic material is arranged to provide limitation to the motion of the return element.

8. An electric circuit breaker as claimed in any preceding claim wherein a supporting structure is provided to prevent possible distortion of adjacent go and return loop elements under high current conditions.

9. An electric circuit breaker substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.