US3004122A

US3004122A - Low-rated circuit breakers

Info

Publication number: US3004122A
Application number: US792068A
Authority: US
Inventors: Alexander R Norden
Original assignee: Federal Pacific Electric Co
Current assignee: Federal Pacific Electric Co
Priority date: 1959-02-09
Filing date: 1959-02-09
Publication date: 1961-10-10
Anticipated expiration: 1978-10-10

Description

Oct. 10, 1961 A. R. NORDEN 3,004,122

LOW-RATED CIRCUIT BREAKERS Filed Feb. 9, 1959 INVENTOR ALEXANDER R. NORDE/V ATTORNEY Un t d S at Pate fO 3,004,122 I LOW-RATED CIRCUIT BREAKERS Alexander R. Norden, New York, N.Y., assignor to Federal Pacific Electric Company, a corporation of Delaware Filed Feb. 9, 1959, Ser. No. 792,068 8 Claims. (Cl. 200-416) to trip in response to any current excess over 15 amperes.-

It will thus be appreciated that'such circuit breakers in practice cannot be of critical and delicate construction but must be relatively rugged. 7

Circuit breakers for this service have heretofore utilized a current-responsive bimetal in the overload releasing mechanism, this bimetal naturally providing a desired time delay characteristic, responding more quickly to a progressively more severe overload and responding very slowly to a very slight overload. Alternatively, more elaborate tripping arrangements have been used involving a magnetic coil and a retarded plunger moving Such magnetic in a liquid-filled dash-pot arrangement. circuit breakers can be made for virtually any desired current level, well below the 15 ampere standard commercial minimum rating for which bimetal-type circuit breakers are made; but they are relatively costly; and the required coil represents a substantial amount of in-' serted resistance which should be minimized, from the point of view of the protected circuit. While circuit breakers having bimetal overload release devices have been made under critically controlled conditions for lower values of current than 15 amperes, such designs have generally been quite critical and, accordingly, both' costly and delicate. I

The foregoing will be appreciated more fully when it is realized that the bimetal is commonly a part of an overload releasing arrangement in which current-responsive deflection of the bimetal is required to overcome the friction resulting from the relatively heavy latch pressure. The bimetal is heated by the flow of current through the bimetal, functioning as a resistor. Passage of a 15 ampere current through a bimetal has been found to provide a satisfactory deflection force for overcoming latch friction in releasing the circuit breaker.

If it should now be considered that a ampere circuit breaker should be built using the same mechanism as is employed in a rugged l5 ampere circuit breaker,

in order to obtain the same physical deflection resulting from a current in excess of 5 amperes, then the width of the bimetal should logically be reduced to of that in a 15 ampere bimetal. The resistance of the bimetal must be increased approximately nine-fold if the same tripping temperature is to be attained because the current decreases to one-third, and the heating is a func- A bimetal which is tion of the square of the current. both thin and narrow enough to meet this criterion would not have the necessary mechanical strength to perform as required in'typical circuit breakers used in invention is to provide a novel form of circuit breaker to the current passed through the circuit breaker.

distribution-circuit protection. An object of the present.

3,004,122 Patented Oct. 10, 1961 currents; but it will be readily understood that such re-.

opposite extremity of the bimetal is connected to a flexible braid so as to cause the current in the protected circuit to flow through the bimetal and to provide for thermal deflection. Actually the so-called fixed; end of the bimetal is carried by the movable contact member in the illustrative circuit breaker, which contact member is actually stationary while the circuit breaker is closed, therefore being in condition to sense and respond The bimetal in the illustrative circuit breaker may be of the same thickness, width and length as that of a circuit breaker rated three times as high (5 amperes as compared to 15 amperes, for example) but is divided longitudinally into multiple series-connected current paths which are quite close to each other physically so as to be in good heat-transfer relationship with one another. One lengthwise subdivision only is utilized for the mechanical functions of restraining and releasing the circuit breaker controlling latch.

The additional longitudinal electrically series-connected portions of the bimetal serve to raise the temperature of the space about the activeportion in proportion to the temperature of the active length of bimetals, in this way sharply. reducing the dissipationof the heat from.

additional mechanically inactive parts of the bimetal also provide both mechanical and thermal isolation between the conductive braid that is connected to the bimetal and the mechanically active length of the bimetal. The mechanically inactivelengths of bimetal which are integral withthe active length serve as a buffer against any mechanical stressesthat might otherwise be imposed by the braid on the active length of bimetal, also, loss of heat from the bimetal to thebraid (which would somewhat reduce the temperature rise of the bimetal, and its resulting current sensitivity) is avoided by interposing a substantial length of self-heating bimetal between the braid and the active length of the bimetal.

In the circuit breaker disclosed in detail below the present invention has special merit. The relatively heavy bimetal (considering the low-level current effective to' cause tripping) provides both the thermal deflection strength required to effect release of the contact operating mechanism and further provides the stiffness enabling it to act asa latch in restraining the contact opening mechanism bearing considerable endwise pressure in that phase of its operation. Nevertheless the invention has broader application in other tripping arrangements;

as in an arrangement wherein only the deflection strength of the bimetal is needed for shifting and thus releasing a separate latch that normally bears the pressure or where the bimetal is arranged to actin tension in restraining the contact opening mechanism.

the

V of bimetal useful as a substitute for the bimetal in FIG. 1.

Referring now to FIGS. 1 to 3 of the drawings in detail, there is shown an automatic circuit'breaker 1h provided with a casing constituted by the insulated body part 14 and a complementary cover of insulation (not shown) that are conventionally secured together in housing defining relation by rivets through holes 18. The circuit breaker mechanism comprises a movable contact arm 22 having pivot 21 in the form of a helical coil spring (viewed endwise) whose ends are captive in pockets in the confronting walls of the casing. Contact arm 22 carries contact 24 that cooperates with stationary contact Ztiunited with and forming a unitary part of solderless terminal 28. At its other end, movable contact member 22 carries actuator 34} on an insulated pivot. Actuator 30 is mechanically connected to handle 32. by a link 34. The movable contact member 22 is also provided with a current-responsive device 35 that restrains actuator 3% against moving clockwise. The overcurrent release device 35 as seen in FIGS. 1-3 is in the form of a onepiece bimetal that restrains actuator 30. Overcurrent release device 35 includes a bimetal length 36 that is physically and electrically united to movable contact member 22. For this purpose, rivet 38 projects integrally from contact member 22 and extends through a hole 40 at one end of the length 36 of the bimetal. At its other end, this length of bimetal $6 resists pressure of actuator 30 which thus tends to buckle or bow the bimetal portion 36' in the event that a heavy pressure is applied. When length 36 of the bimetal is heated, it deflects downward and, overcoming the pressure of actuator 30 and the resulting friction, it releases actuator 36 for opening of the circuit, breaker. Length 36 of the bimetal may thus be termed the active length of the bimetal.

The bimetal that here constitutes the release device includes two additional mechanically

inactive bimetal lengths

42 and 44 electrically in series with each other and with length 36 as a convolution which generally encircles the active length 36 of the bimetal. The free end of bimetal length 44 is joined to flexible braid-46, of many soft copper strands twisted together, and these extend to the plug-in male terminal 48 of the circuit breaker.

Portions

36 and 42 of the bimetal are connected to each other by means of an offset portion 48 which, as best seen in FIGS. 1 and 2, disposes

portions

42 and 44 in a plane somewhat below that of bimetal active portion 36, well clear of contact arm 22.

When handle 32 of the circuit breaker operates the

members

34, 30 and 22 into position to drive the movable contact 24 against stationary contact 26, the portion of the handle between its pivot and its connection to element 34 acts as one part of a toggle which applies thrust to element 34 during the. closing operation. Ultimately, however, when the circuit breaker is closed, thrust is'applied by element 34 along a line which is somewhat at. an angle (angle A, FIG. 1) to another line extending between the pivot of actuator 30 on member 22 and the pivotal connection between elements 30 and 34. This angle is an important factor determining the force that actuator 30 applies clockwise against the end portion of the mechanically active length 36 of the bimetal. If this 4 angle is made very small, then the circuit breaker action is very uncertain, bearing in mind the dimensional variations to be expected in manufacture of such circuit breakers.

In practice the angle is such as to impose a substantial endwise pressure on the release device 35. This in turn means that the bimetal when heated by overload current through the circuit breaker is required to develop substantial deflection force in consistently and reliably overcoming the latch friction resulting from the latch pressure and the bimetal must be still so as to avoid bowing andwbuckling as a result of that end pressure. These mechanical considerations are satisfied yet response to unusually low current levels is achieved in the novel yet simple manner shown. The left-hand end of the bimetal length 36 is electrically connected and mechanically fixed to movable contact arm 22 (which of course is stationary when the circuit breaker is closed). The righthand end is thermally deflected to release latched member 30 which it normally restrains, and that latching end provides the physical support for the other lengths of

himetal

42 and 44 which thus need no further support or insulation. The right-hand end of length 44 of the bimetal is joined to copper braid 46. When the circuit breaker is operated for opening and closing the contacts, the

resilient bimetal lengths

42 and 44 minimize trans mission of stress from braid 46 (which has one end stationary at terminal 48) to the critical latch end of active bimetal length 36. Mechanical disturbance imposed at this point, especially where active'length 36 of the bimetal is slender for low-circuit response, could seriously vary the overload tripping level of the circuit breaker.

'

Bimetal lengths

42 and 44 manifestly do not contribute to the. deflection force in an overload release operation. Nevertheless they promote high sensitivity or response to low current levels by raising the temperature of the space about the active length of'bimetal so as to inhibit heat dissipation from that active length of bimetal. This means that the temperature and consequent deflection resulting from. a given current will be much higher with the inactive bimetal lengths than it would be if they were omitted. In an example, a circuit breaker having only an active length of bimetal tripped at about 10 am peres whereas a like circuit breaker with a bimetal having the shape in the. drawings, including an active length of equal cross-section and of this same bimetal stock' tripped at 4 amperes, latch element 36 extending equal distances across the ends of the bimetals when cold in those circuit breakers.

In FIGS. 1-3 the active length of bimetal is sheared from the lateral lengths, so that. insulating space is realized by oft-setting

lengths

42 and 44 from active length 36. ,By cutting a. narrow slot in the bimetal stock as in FIG. 3a the lengths of bimetal are provided with in-' sulating separation'and the off-set 48 of FIG. 2 may be avoided; but even in that instance an off-set may be desir' 1. A circuit breaker including a pair of contacts, a

movable. contact member carrying one of said contacts,

trip-free manually operable mechanism for opening andv closing said contacts and an overload release means for automatic opening of the. contacts in response to'over current conditions, said overload release means including a latched member, a length of bimetal arranged for latching said latched member, and at least one additional length of bimetal integral with said first mentioned length of bimetal and supported. only by said latching length. of;

bimetal, said additional length of bimetal being closely adjacent thereto and electrically in series therewith, said latching length of bimetal being united at one end thereof to said movable contact member and said lengths of bimetal being thereby mounted for bodily movement with said movable contact member, a connecting conductor of braid having a joint to said additional length of bimetal, said connecting conductor having a stationary portion spaced from said joint, said additional length of bimetal providing environmental heating to resist dissipation of heat developed in said first mentioned length of bimetal and said additional lengths of bimetal further providing mechanical isolation between said conductor and said latching length of bimetal.

2. A circuit breaker in accordance with claim 1 wherein said bimetal includes plural lengths of bimetal in series with and extending along said latching length of bimetal but on opposite sides thereof.

3. A circuit breaker including a pair of contacts, a movable contact member carrying one of said contacts, trip-free manually operable mechanism for opening and closing said contacts and an overload release means for automatic opening of the contacts in response to overcurrent conditions, said overload release means including a latched member, a first length of bimetal arranged to deflect in response to self-heating and thereby to trip said latched member, and at least one additional length of bimetal integral with said first mentioned length of bimetal and supported only by said first length of bimetal, said additional length of bimetal being closely adjacent thereto and electrically in series therewith, said first length of bimetal having one end thereof united to said moving contact member and said lengths of bimetal being thus mounted for bodily movement with said movable contact member, a connecting conductor of braid having a joint to said additional length of bimetal, said connecting conductor having a stationary portion spaced from said joint, said additional length of bimetal providing environmental heating to resist dissipation of heat developed in said first mentioned length of bimetal and said additional lengths of bimetal further providing mechanical isolation between said conductor and said first length of bimetal.

4. A circuit breaker including a pair of separable contacts, manually operable mechanism for opening and closing said contacts, and overload release means, said means including a bimetal arranged to cause automatic release of the circuit breaker, said bimetal having integral serially connected lengths of bimetal forming part of a current path through the circuit breaker, said lengths of bimetal being in close proximity to each other, at least one of said lengths being mechanically active in operating the overload release means in dependence on the current through the circuit breaker and at least one of said bimetal lengths being inactive mechanically in relation to the release mechanism but being effective due to self-heating to heat the space adjacent the active length of bimetal and thereby to inhibit heat dissipation therefrom.

5. A circuit breaker including a pair of separable contacts, manually operable mechanism for opening and closing said contacts, and an overload release means, said means including a bimetal arranged to cause automatic release of the circuit breaker, said bimetal having integral serially connected lengths of bimetal forming part of a currentpath through the circuit breaker, said lengths of bimetal being in close proximity to each other, one of said lengths being mechanically fixed at one end thereof and being mechanically active at the other end thereof for releasing the overload release means in dependence on the current through the circuit breaker and another of said bimetal lengths being disposed adjacent to said one length and supported substantially exclusively by said active end 01" said one bimetal for effecting environmental heating of the space about said one length and thereby to minimize heat dissipation therefrom and to provide mechanical ism lation between said active end of said one length of bimetal and the end of said other length of bimetal remote means including a release element and a bimetal cooperable therewith for effecting automatic release of the circuit breaker, said bimetal having integral serially connected lengths of bimetal forming part of the current path through the circuit breaker, said lengths of bimetal being in close proximity to each other, one of said lengths having one end thereof fixed'and the opposite end thereof cooperable with said release element and thereby being me chanically active for releasing said overload release means in dependence on the current through the circuit breaker and another of said bimetal lengths having one end thereof supported at said mechanically active end of said one length and a braid conductor connected to the other end of said other length of bimetal providing an electrical connection mechanically remote from said active end of said one length of bimetal.

7. A circuit breaker including a pair of separable contacts, manually operable mechanism for opening and closing said contacts, and an overload release means, said means including a release element and a bimetal cooperable therewith for effecting automatic release of the circuit breaker, said bimetal having three integral lengths serially connected to form part of the current path through the circuit breaker, two of said lengths being disposed at opposite sides of the third length and being mechanically inactive, said third length having one end fixed and its opposite end connected to one of said two lengths and said opposite end being mechanically active in efiecting operation of said overload release mechanism in dependence on the current through the circuit breaker, the second of said two lengths of bimetal having an electrical terminal connection physically adjacent to said active end of said third length.

8. An overload release device comprising a latched element, a first length of bimetal fixed at one end and having its opposite end arranged as a latch to resist endwise pressure of said latched element, a second length of bimetal extending integrally from said first length at said opposite end thereof and wholly supported thereby, and said second length extending laterally along one side of said first length of bimetal, a third length of bimetal extending integrally from and wholly supported by said. second length at the end thereof remote from said latch end of said first bimetal, said third length extending along the side of said first length opposite the second length of bimetal, said second and third length of bimetal being mechanically inactive, the remaining end of said third length of bimetal and said fixed end of said first length of bimetal having electrical connections thereto for self-heating control cur- Switzerland July 1, 1935