US3693122A

US3693122A - Flux transfer trip device for electric circuit breakers

Info

Publication number: US3693122A
Application number: US142942A
Authority: US
Inventors: Henry G Willard
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1971-05-13
Filing date: 1971-05-13
Publication date: 1972-09-19
Anticipated expiration: 1989-09-19

Abstract

A sensitive magnetic trip device, adapted especially for use with an electric circuit breaker for operation by a relatively low power trip signal pulse, such as that developed by a zerosequence or differential transformer indicating presence of a ground fault condition. The device is also highly suitable for use with ''''static'''' or solid-state trip circuits operating in response to signals generated by current transformers associated with phase conductors of a multi-phase system. The trip device includes a magnetic armature normally held in a retracted position against the bias of a tripping spring by flux generated by a permanent magnet. The trip device includes a magnetic frame member and a ''''flux-diverter'''' interposed between the armature and the permanent magnet, providing an alternate path for flux from the permanent magnet. The permanent magnet flux is diverted from the armature path through the diverter path by magnetomotive force (MMF) generated by a signal coil. The diverter path includes an air gap normally causing the diverter path to be of higher reluctance than the path through the armature. Upon the occurrence of a trip signal of predetermined magnitude through the trip coil, magnetomotive force (MMF) generated by the trip coil opposes the flow of permanent magnet flux through the armature and diverts it through the diverter path, including the air gap, causing release of the armature. The armature is dimensioned with respect to the diverter so that the armature is driven to saturation by extra high current through the trip coil. This limits the flow of flux from the trip coil, thereby preventing demagnetization of the permanent magnet upon occurrence of such high trip signal currents. The ''''fluxdiverter'''' is formed and dimensioned so that in combination with the permanent magnet, the frame, and the trip coil, it minimizes stray flux and maximizes sensitivity and effectiveness of the device.

Description

United States Patent Willard [151 3,693,122 [451 Sept. 19, 1972 [54] FLUX TRANSFER TRIP DEVICE FOR ELECTRIC CIRCUIT BREAKERS Henry G. Willard, Wethersfield, Conn.

[73] Assignee: General Electric Company [22] Filed: May 13, 1971 [21] Appl. No.: 142,942

[72] Inventor:

s2 u.s.c| ..33s/174, 335/236 51 Int.Cl ..n01r3/12 5s FieldoiSeareh ..33s/174,179,170,229,23o,

Primary Examiner-Harold Broome Attorney-Robert T. Casey, Philip L. Schlamp, Frank L. Neuhauser, Oscar B. Waddell and Joseph B. Forman [57] ABSTRACT A sensitive magnetic trip device, adapted especially for use with an electric circuit breaker for operation by a relatively low power trip signal pulse, such as that developed by a zero-sequence or differential transformer indicating presence of a ground fault condition. The device is also highly suitable for use with static or solid-state trip circuits operating in response to signals generated by current transformers associated with phase conductors of a multi-phase system. The trip device includes a magnetic armature normally held in a retracted position against the bias of a tripping spring by flux generated by a permanent magnet. The trip device includes a magnetic frame member and a flux-diverter interposed between the armature and the permanent magnet, providing an alternate path for flux from the permanent magnet. The permanent magnet flux is diverted from the armature path through the diverter path by magnetomotive force (MMF) generated by a signal coil. The diverter path includes an air gap normally causing the diverter path to be of higher reluctance than the path through the armature. Upon the occurrence of a trip signal of predetermined magnitude through the trip coil, magnetomotive force ,(MMF) generated by the trip coil opposes the flow of permanent magnet flux through the armature and diverts it through the diverter path, including the air gap, causing release of the armature. The armature is, dimensioned with respect to the diverter so that the armature is driven to saturation by extra high current through the trip coil. This limits the flow of flux from the trip coil, thereby preventing demagnetization of the permanent magnet upon occurrence of such high trip signal currents. The fluxdiverter is formed and dimensioned so that in combination with the permanent magnet, the frame, and the trip coil, it minimizes stray flux and maximizes sensitivity and effectiveness of the device.

1 1 Claims, 3 Drawing Figures zab PATENTEB EP 19 1912- FIGI FIGZ

FIG 5 INVENTOR HENRY c5. WILLARD BY ATTORNEY FLUX TRANSFER TRIP DEVICE FOR ELECTRIC CIRCUIT BREAKERS DESCRIPTION OF THE PRIOR ART two disadvantages. First, some or all of such devices have lacked the degree of sensitivity required for operation by relatively low power signals such as those developed by a zero-sequence or differential transformer indicating unbalance of current in two or more current paths thereby indicating presence of a ground fault condition. A similar disadvantage exists with respect to static trip devices responsive to abnormal phase current conditions. Secondly, some or all of such devices have sufiered from the disadvantage that if sensitive enough for operation by relatively small amounts of energy, such, for instance, as by 0.0005 joules, they are subject to possible demagnetization of the permanent magnet used if and when the trip signal becomes exceptionally large. In addition, some or all of such devices have suffered from the disadvantages of being relatively large in size, relatively complicated and consequently costly.

Examples of such prior art magnetic trip devices are shown in the following US. Pat. Nos:

1,427,368 Fortescue Circuit lnterrupter Aug. 29,

1922 2,130,871 Boehne High Speed Tripping System Sept. 20, 1938 2,915,681 Troy Magnet Assemblies Dec. 1, 1959 2,919,323 Drescher Electric Relay Dec. 29, 1959 3,253,098 Perry Mechanical Actuator With Permanent Magnet May 24, 1966 3,450,955 Johnson Circuit Breaker-With Magnetic Device Releasable To Effect Opening Of The Breaker June 17, 1969 It is an object of the present invention to provide a sensitive magnetic trip device for electric circuit breakers which shall be capable of operation by relatively small amounts of electrical energy, such, for instance, as 0.0005 joules, while producing a tripping force of substantial amount, such, for instance, as 5 pounds.

It is a further object of the invention to provide a sensitive magnetic trip device of the type described which is not subject to demagnetization of its permanent magnet by unusually high tripping signals.

It is a further object of the invention to provide a sensitive magnetic trip device of the type described which is relatively small in size, simple and inexpensive to manufacture.

In accordance with the invention in one form, a sensitive magnetic trip device is provided including a generally rectangular magnetically permeable frame member having a permanent magnet affixed to the inner surface of one end wall and an armature member slidably supported for movement through an opening in the opposite end wall and biased by a compression spring for movement outwardly through the frame. A flux-diverter member is interposed between the permanent magnet member and the armature, and has a base portion extending between opposed side wall portions of the frame with a small air gap provided between the ends of the flux-diverter member and each of the corresponding side walls. The flux-diverter member also includes a centrally located generally cylindrical portion extending toward the armature. In normal closed or inactive condition, the armature is in contact with the permanent magnet member, forming a generally central leg portion extending between central portions of the opposite end walls of the generally rectangular frame member. Flux generated by the permanent magnet member normally flows through the diverter member, through the armature member and back through the opposed side wall portions of the frame member to the opposite end of the permanent magnet member. A trip coil is positioned so as to surround the meeting faces of the armature and the flux diverter, and current is passed therethrough in a direction to produce magnetomotive force (MMF) opposing the flux of the permanent magnet member flowing across the interface between the flux-diverter member and the armature member. The occurrence of a trip signal of predetermined magnitude in the trip coil creates such opposing MMF which causes the flux from the permanent magnet member to take an alternate path through the flux-diverter base portion and through the relatively small air gaps between the ends of the diverter base portion and the opposed side walls of the frame member, and through the frame member back to the opposite end of the permanent magnet member. The armature member and the flux-diverter member are so proportioned that the occurrence of an unusually high tripping signal through the trip coil causes saturation of the armature member before saturation of the flux-diverter base portion occurs. This, together with the air gaps mentioned, assures that such extra high trip signal current will not cause demagnetization of the permanent magnet.

The invention will be more fully understood from the following detailed description, and its scope will be pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevation view of a sensitive magnetic trip device constructed in accordance with the invention.

FIG. 2 is a view similar to FIG. 1, but with certain of the parts being shown in section to more clearly show the construction.

FIG. 3 is a side elevation view, partly in section, of another embodiment of the invention.

Referring to FIGS. 1 and 2, the construction of a preferred embodiment is shown, as including a generally rectangular frame of highly magnetically permeable material and comprising two generally L- shaped magnetically

permeable members

10 and 12, interconnected by suitable means, such as by having a reduced end of each of the

members

10 and 12 extending through a corresponding opening in the opposite member and staked over as indicated at 13 and 14. Alternatively, other forms of fastening, such, for instance, as machine screws may be used. A generally cylindrical high induction permanent magnet member 16 is supported in close engagement with an end wall 17 of the frame member 12 by suitable means, not shown.

A generally cylindrical armature 18 of high magnetically permeable material is supported for reciprocal straight-line movement through a corresponding circular opening in the end wall 19 of the frame member 10. Movement of the armature 18 is guided in part by the end wall 19 of the frame member 10, and in part by insulating bobbin member 20, to be further described. The armature member 18 is biased for movement outwardly through the frame member by means of a compression spring 21 which bears against a generally cylindrical member 22 which is connected by a stem rod 23 to the armature 18. Movement of the armature member 18 outwardly through the frame member 10 is limited by suitable means, such as by a generally hatshaped stop member 24. The stop member 24 is affixed to the outer surface of the frame member 10 by suitable means, such as by screws 24A through the flange portion of the stop member 24. The central portion of the stop member 24 is dimensioned so as to receive the armature 18 when it is moved outwardly by the spring 21. Outward travel of the armature 18 is limited by its engagement with the inner surface of the central portion of the stop member 24. The stop member 24 has a hole 24B in its central portion, through which the stem rod 23 passes. The outer diameter of the central portion of the stop member 24 is dimensioned to closely fit inside the spring 21 to retain it in position.

A generally T-shaped flux-diverter member 25 is supported within the

frame

10, 12, and includes a generally rectangular transversely extending base portion 26 which extends into closely spaced relation with each of the

side walls

28 and 29 of the

frame members

10 and 12 providing small air gaps 26A and 26B. If desired, suitable nonmagnetic spacer members, not shown, may be placed in gaps 26A and 268 to aid in positioning flux-diverter 25. The flux-diverter member is supported in position by being abutted against an end portion of the permanent magnet member 16, and by having a generally cylindrical extension 30 which extends into the cylindrical central opening of the insulating bobbin 29.

In normal position, the armature 18 has one end thereof in abutting engagement with the cylindrical extension 30 of the flux-diverter 25. The normal flux path, therefore, for flux produced by the permanent magnet 16 is from one end surface thereof, into the flux-diverter member 25, from thence into the armature 18, from thence into the frame member 10, and through the

opposed side walls

28, 29 of the

frame members

10 and 12 respectively, to the end wall 17, to return to the opposed end wall of the permanent magnet member 16. This holds the armature in retracted position against the bias of the compression spring member 21.

For the purpose of causing release of the armature 18 from the retracted position, a trip coil 35 is provided which is wound on the bobbin member 20, so as to surround the interface of the armature 18 and the extension 30 of the flux-diverter 25.

In operation, the occurrence of a trip signal current through the coil 35 in the proper direction creates a magnetomotive force (MMF) which acts in a direction to oppose the flux created by the permanent magnet 16 across the interface of the armature l8 and the projection 30 of the flux-diverter 25. This causes the flux developed by the permanent magnet 16 to take the alternate path through the flux-diverter transversely extending base 26 and across the air gaps 26 and 26B and through a portion of each of the

side wall members

28, 29 of the frame, to the end wall 17 and to the opposite end wall of the permanent magnet member 16. The decrease of net flux across the interface of the armature l8 and the projection 30 of the flux-diverter member 25, when the trip signal current is of sufficient predetermined magnitude, causes release of the armature 18, which moves outwardly under the force of the compression spring 21, causing it to engage a trip member, such as member 40, associated with an electric circuit breaker, not shown. Following such tripping, and at a desired time, the armature 18 is reset by suitable means, not shown, to return it to its normal inactive position as shown in FIG. 1.

In accordance with the invention, the cross-sectional dimension of the armature 18 is selected with respect to the cross-sectional dimension of the base 26 of the flux-diverter member 25 so that the armature member 18 and the cylindrical portion 30 of the flux-diverter 25 are saturated by such flux associated with an excessively high trip current.

The saturation of the armature 18 and the cylindrical portion 30 of the flux-diverter 25 by such excessive currents prevents such currents from causing demagnetization of the permanent magnet 16, since the reluctance of the flux-path for flux generated by the trip coil 35 becomes very high, thereby reducing the flux available to flow through the permanent magnet 16 in a direction opposite to the inherent flux of the magnet 16.

In one embodiment of the invention having a form as illustrated in FIGS. 1 and 2, the device had the following characteristics:

Overall length of frame proper 10, 12 l Thickness of frame material 56 Width of frame material Permeability of frame material 16 2000 Diameter of permanent magnet 56" length of permanent magnet as" Strength of permanent magnet l l,000 gauss Permanent magnet material Alnico 5 manufactured by General Electric Co., Edmore,

Michigan Diameter of armature 18 5/16" Length of armature Air gaps 26A and 26B .030" Tripping coil 35-750 turns of No. 3i copper wire, resistance, 17 ohms d.c., 60 Hz impedance 42 ohms. Length of cylindrical portion of flux-diverter 25 (from end to plane of nearest surface of portion 26) W Thickness of portion 26 of flux-diverter 25 Strength of spring 21 in compressed condition 5 lb.

high trip current, such as 100 times the value required to trip, did not cause any measurable demagnetization of the permanent magnet. Thus, a device constructed as described above was mounted in a General Electric 1,200 ampere frame size circuit breaker. The secondary of a current-transformer having a secondary-to-primary current ratio of 200 to 5 was connected to the tripping coil of the trip device. A current pulse of 16,000 amperes (rms) was passed through the current transformer primary numerous times, each time causing tripping of the circuit breaker. No reduction in magnetic flux level of the permanent magnet could be detected.

The significant operational characteristics demonstrated in such tests were that an input trip signal having a power of only about 0.0005 joules produced release of the armature, delivering a tripping force of 5.0 pounds.

The illustrated construction of the frame of the preferred embodiment illustrated comprises two generally L-shaped members as described. Other formations may be utilized if desired, however, such, for instance, as a generally U-shaped frame member with a however, bridging end member. See, for example, the embodiment of FIG. 3, to be described. The construction illustrated, comprising two generally L-shaped members, is preferred, however, since it provides two similar members, simplifying manufacture and assembly.

In. FIG. 3 there is illustrated another embodiment of thev invention. This embodiment is essentially similar to that of FIGS. 1 and 2 except that the flux-diverter 125 is in contact with the

opposed side walls

128 and 129 of the frame and has a hole centrally thereof providing a generally circular air gap 125A of approximately onesixteenth inch. The interface of the permanent magnet 116 and the armature 118 is substantially co-planar with the surface of the flux-diverter 125, as shown. Also, the tripping coil 135 consisted of 800 turns of No. 33 copper wire.

Tests on the embodiment of FIG. 3 showed the following results:

Tripping coil current 0.20 amperes, giving 160 ampere turns.

Energy required to cause tripping, 5 milli-joules (0.005 joules).

While this embodiment is quite satisfactory for many applications, it will be observed that the form of FIGS. 1 and 2 provides an increase of sensitivity (decrease of energy required to trip) of about :1, or 10 times. This great improvement is believed to be due primarily to two things; (I) the location of the air gap at the outer ends of the diverter base (gaps 26A and 26B), and (2) the location of the interface of the permanent magnet and armature within the trip coil 35 as shown in FIGS. 1 and 2. These changes greatly reduce stray flux paths and improve the efficiency of the device accordingly.

While only two specific embodiments of the invention have been shown and described, it will be obvious that many modifications thereof may readily be made. I, therefore, intend by the appended claims to cover all such modifications as fall within the true spirit and scope of the invention.

I claim:

1. A sensitive magnetic trip device comprising;

a. a generally rectangular closed-loop frame of highly magnetically permeable material having opposed side walls, a back wall, and a front wall;

b. a generally cylindrical armature of highly magnetically permeable material supported for straightline reciprocal movement through a close-fitting generally circular hole in said front wall of said frame;

c. spring means normally urging said armature for movement through said hole outwardly of said frame;

(1. a permanent magnet member having one end in flux-conductive relation to said back wall of said frame one having its other end in flux-conductive relation with said end of said armature so as to normally hold said armature in a retracted position with respect to said frame against the bias of said spring means;

. an elongated flux-diverter member of highly magnetically permeable material supported within said frame member and extending between said opposed side walls of said frame at a point intermediate said front and back walls thereof, said flux-diverter providing an alternate flux path for flux from said other end of said permanent magnet to return to said one end of said permanent magnet member without passing through said armature, said alternate flux path including at least one nonmagnetic gap which is substantially shorter in length than the spacing of said permanent magnet from said side walls of said frame;

f. a tripping coil supported in said frame intermediate said back and front walls thereof and surrounding at least a portion of said armature, whereby current passing through said tripping coil in a predetermined direction creates magnetomotive force in said armature in a direction to oppose flux passing therethrough from said permanent magnet causing flux from said permanent magnet to be diverted to said path through said flux-diverter and through said non-magnetic gap, thereby reducing the net flux through said armature and causing release of said armature and movement of said armature outwardly of said frame under the bias of said spring means.

2. A sensitive magnetic trip device as set forth in claim 1 wherein said flux-diverter comprises a generally flat plate having its opposite ends in contact with said opposed side walls of said frame and wherein said armature contacts saidpermanent magnet at a juncture in a plane substantially co-planar with said flux-diverter, said flux-diverter having a generally central circular hole therein forming a circular air gap between said flux-diverter and said juncture of said armature and said permanent magnet.

3. A sensitive magnetic trip device as set forth in claim 1 wherein said flux-diverter comprises a continuous solid member having its opposite end portions in closely spaced magnetic gap relation to said opposed side walls of said frame, and wherein said permanent magnet member has one end thereof in contacting relation to a first generally planar intermediate surface of said flux-diverter and said armature has an end portion in contacting relation to an intermediate generally planar surface of said flux-diverter opposite from said first surface, whereby said air gaps between said fluxdiverter and said frame member are removed a substantial distance from the contacting surface of said armature and said flux-diverter member.

4. A sensitive magnetic trip device as set forth in claim 3 wherein said flux-diverter member includes a generally centrally located cylindrical extension at the surface thereof opposite said first surface, said armature having one end thereof normally in contact with the end of said cylindrical extension of said fluxdiverter member, and said trip coil includes a portion encircling at least a portion of at least one of said armature and said cylindrical extension portion of said fluxdiverter.

5. A sensitive magnetic trip device as set forth in claim 4 wherein said trip coil includes a portion encircling the meeting faces of said armature and said cylindrical extension of said flux-diverter.

6. A sensitive magnetic trip device as set forth in claim 4 wherein the cross-sectional dimension of said armature and the dimensions of said diverter are selected so that upon the occurrence of tripping signals of excessively high magnitude, at least one of said armature and cylindrical portion of said flux-diverter is driven to magnetic saturation before the remaining portion of said fluxdiverter is driven to magnetic saturation, whereby magnetomotive force generated by said tripping coil on the occurrence of excessively high tripping currents does not demagnetize said permanent magnet.

7. A sensitive magnetic trip device as set forth in claim 5 wherein said device also includes a bobbin member of low magnetically permeable material, wherein said bobbin member has a generally centrally positioned hole therein receiving said cylindrical extension of said flex-diverter and slidably receiving and guiding at least a portion of said armature member.

8. A sensitive magnetic trip device as set forth in claim 1 wherein said generally rectangular frame of said device comprises a pair of generally L-shaped members interconnected to form a closed loop of highly magnetically permeable material, said L-shaped members having substantially identical in form and dimension whereby to facilitate manufacture and assembly of said magnetic trip device.

9. A sensitive magnetic trip device as set forth in claim 1 wherein said armature and said flux-diverter are formed and dimensioned so that excessively high tripping current in said trip coil causes saturation of said armature before causing saturation of said fluxdiverter, whereby demagnetization of said permanent magnet by said excessively high tripping coil current is substantially prevented.

10. A sensitive magnetic trip device as set forth in claim 1 wherein said nonmagnetic gap in said alternate flux path through said diverter is large enough to normally cause flux from said permanent magnet to pass through said armature in its closed position in preference to said alternate path, but is dimensioned small enough to provide a flux path for flux generated by said tripping coil when energized by an excessively large tripping coil current to drive one of said elements comprising said armature and said cylindrical portion of said flux-diverter to saturation without causing dem a tizai n of said r anent m n t, such excess1v%: ux ollowing a p381 t rough saii aux-diverter and across said nonmagnetic gap in preference to a path through said permanent magnet in a direction opposing flux generated by said permanent magnet.

1 l. A sensitive magnetic trip device comprising;

d. a permanent magnet member having one end in flux-conductive relation to said back wall of said frame and having its other end in flux-conductive relation with one end of said armature so as to normally hold said armature in a retracted position with respect to said frame against the bias of said springs means;

e. an elongated flux-diverter member of highly magnetically permeable material supported within said frame member and extending between said opposed side walls of said frame at a point intermediate said front and back walls thereof, said flux-diverter being positioned between said permanent magnet and said armature and having its ends in closely spaced proximity to said side walls of said frame, the spacing between said ends of said flux diverter and said side walls being substantially less than the thickness of said frame walls;

f.- a tripping coil supported on said frame, said tripping coil when energized creating flux in said armature opposing flux from said permanent magnet whereby said flux from said permanent magnet is diverted through said flux-diverter member and across said spacing between said ends of said ends of said flux-diverter and said side walls thereby reducing the net flux through said armature and causing release of said armature and movement of said armature outwardly of said frame under the bias of said spring means, said flux-diverter also providing a closed path for flux from said tripping coil without such flux passing through said permanent magnet, whereby said tripping coil flux does not tend to demagnetize said permanent magnet.

Claims

1. A sensitive magnetic trip device comprising; a. a generally rectangular closed-loop frame of highly magnetically permeable material having opposed side walls, a back wall, and a front wall; b. a generally cylindrical armature of highly magnetically permeable material supported for straight-line reciprocal movement through a close-fitting generally circular hole in said front wall of said frame; c. spring means normally urging said armature for movement through said hole outwardly of said frame; d. a permanent magnet member having one end in flux-conductive relation to said back wall of said frame one having its other end in flux-conductive relation with said end of said armature so as to normally hold said armature in a retracted position with respect to said frame against the bias of said spring means; e. an elongated flux-diverter member of highly magnetically permeable material supported within said frame member and extending between said opposed side walls of said frame at a point intermediate said front and bacK walls thereof, said flux-diverter providing an alternate flux path for flux from said other end of said permanent magnet to return to said one end of said permanent magnet member without passing through said armature, said alternate flux path including at least one non-magnetic gap which is substantially shorter in length than the spacing of said permanent magnet from said side walls of said frame; f. a tripping coil supported in said frame intermediate said back and front walls thereof and surrounding at least a portion of said armature, whereby current passing through said tripping coil in a predetermined direction creates magnetomotive force in said armature in a direction to oppose flux passing therethrough from said permanent magnet causing flux from said permanent magnet to be diverted to said path through said fluxdiverter and through said non-magnetic gap, thereby reducing the net flux through said armature and causing release of said armature and movement of said armature outwardly of said frame under the bias of said spring means.

2. A sensitive magnetic trip device as set forth in claim 1 wherein said flux-diverter comprises a generally flat plate having its opposite ends in contact with said opposed side walls of said frame and wherein said armature contacts said permanent magnet at a juncture in a plane substantially co-planar with said flux-diverter, said flux-diverter having a generally central circular hole therein forming a circular air gap between said flux-diverter and said juncture of said armature and said permanent magnet.

3. A sensitive magnetic trip device as set forth in claim 1 wherein said flux-diverter comprises a continuous solid member having its opposite end portions in closely spaced magnetic gap relation to said opposed side walls of said frame, and wherein said permanent magnet member has one end thereof in contacting relation to a first generally planar intermediate surface of said flux-diverter and said armature has an end portion in contacting relation to an intermediate generally planar surface of said flux-diverter opposite from said first surface, whereby said air gaps between said flux-diverter and said frame member are removed a substantial distance from the contacting surface of said armature and said flux-diverter member.

4. A sensitive magnetic trip device as set forth in claim 3 wherein said flux-diverter member includes a generally centrally located cylindrical extension at the surface thereof opposite said first surface, said armature having one end thereof normally in contact with the end of said cylindrical extension of said flux-diverter member, and said trip coil includes a portion encircling at least a portion of at least one of said armature and said cylindrical extension portion of said flux-diverter.

6. A sensitive magnetic trip device as set forth in claim 4 wherein the cross-sectional dimension of said armature and the dimensions of said diverter are selected so that upon the occurrence of tripping signals of excessively high magnitude, at least one of said armature and cylindrical portion of said flux-diverter is driven to magnetic saturation before the remaining portion of said flux-diverter is driven to magnetic saturation, whereby magnetomotive force generated by said tripping coil on the occurrence of excessively high tripping currents does not demagnetize said permanent magnet.

9. A sensitive magnetic trip device as set forth in claim 1 wherein said armature and said flux-diverter are formed and dimensioned so that excessively high tripping current in said trip coil causes saturation of said armature before causing saturation of said flux-diverter, whereby demagnetization of said permanent magnet by said excessively high tripping coil current is substantially prevented.

10. A sensitive magnetic trip device as set forth in claim 1 wherein said nonmagnetic gap in said alternate flux path through said diverter is large enough to normally cause flux from said permanent magnet to pass through said armature in its closed position in preference to said alternate path, but is dimensioned small enough to provide a flux path for flux generated by said tripping coil when energized by an excessively large tripping coil current to drive one of said elements comprising said armature and said cylindrical portion of said flux-diverter to saturation without causing demagnetization of said permanent magnet, such excessive flux following a path through said flux-diverter and across said nonmagnetic gap in preference to a path through said permanent magnet in a direction opposing flux generated by said permanent magnet.

11. A sensitive magnetic trip device comprising; a. a generally rectangular closed-loop frame of highly magnetically permeable material having opposed side walls, a back wall, and a front wall; b. a generally cylindrical armature of highly magnetically permeable material supported for straight-line reciprocal movement through a close-fitting generally circular hole in said front wall of said frame; c. spring means normally urging said armature for movement through said hole outwardly of said frame; d. a permanent magnet member having one end in flux-conductive relation to said back wall of said frame and having its other end in flux-conductive relation with one end of said armature so as to normally hold said armature in a retracted position with respect to said frame against the bias of said springs means; e. an elongated flux-diverter member of highly magnetically permeable material supported within said frame member and extending between said opposed side walls of said frame at a point intermediate said front and back walls thereof, said flux-diverter being positioned between said permanent magnet and said armature and having its ends in closely spaced proximity to said side walls of said frame, the spacing between said ends of said flux diverter and said side walls being substantially less than the thickness of said frame walls; f. a tripping coil supported on said frame, said tripping coil when energized creating flux in said armature opposing flux from said permanent magnet whereby said flux from said permanent magnet is diverted through said flux-diverter member and across said spacing between said ends of said ends of said flux-diverter and said side walls thereby reducing the net flux through said armature and causing release of said armature and movement of said armature outwardly of said frame under the bias of said spring means, said flux-diverter also providing a closed path for flux from said tripping coil without such flux passing through said permanent magnet, whereby said tripping coil flux does not tend to demagnetize said permanent magnet.