US2293950A - Electric protective device - Google Patents

Description

Aug. 25, 1942. R. w. PRICE ELECTRIC PROTECTIVE DEVICE FilGd July 13, 1939 1 'II'III "I,

WITNESSES:

INVENTOR Robert WPrice.

Patented Aug. 25, 1942 azsassc suzc'rmo PROTECTIVE names Robert W. Price, Wilkinsburg, Pa., assignor to Westinghouse Electric 3; Manufacturing Company, East Pittsburgh, 2a., a corporation of Pennsylvania Application-July is, 1939, Serial No. earns (Cl. zoo-12s) 12 Claims.

My invention relates to electrical protective devices, and especially to that type which is designed to protect electric circuits as well as apparatus served thereby from damage arising from rela tively small overloads which may become harmful because of their continued duration, as well as from relatively heavy overloads or short circuits, but will not operate to open the circuit on harmless overloads, such as overloads of a magnitude or duration so small that no injury could result therefrom.

The proper protection of electrical appliances, such as small motors and lighting circuits, depends upon the selection of a fuse which will distinguish between harmless and harmful overloads. The standard commercial fuse does not possess sufficient time lag to provide protection on light continuing overloads, and at the same time not operate to open the circuit upon energizing the apparatus protected if it draws appreciable starting current. Thus such a fuse is either blown by the starting current drawn, or the apparatus lacks protection on light overloads which can become dangerous due to their continued duration.

For example, assume a small motor draws a full load current of 5 amperes and that its starting current is approximately 800% of full load current. It is obvious that a standard 5 ampere fuse would blow every time the motor is started. 0n the other hand, if a higher rated fuse such as a ampere fuse is used to protect the motor, it would not be protected on overloads of 100% which could seriously damage the motor if prolonged for any length of time.

The fuse to be described is designed to operate in 7 seconds on overloads of 650%, 29 seconds on overloads of 250% and in approximately 2700 seconds on overloads of 25%.

Thermal overload devices having an operating time lag greater than that of the ordinary type of fuse have been used heretofore, but in order to obtain protection from relatively great overloads and short circuits, it has been necessary in the past to provide a separate fuse connected in series with such devices It is, therefore, an object of my invention to provide in a single protective device the combined characteristics of a fuse and a time lag cutout.

A further object of my invention is to provide an electric protective device having relatively few essential parts, which maybe simply and economically constructed.

Another object of my invention is to provide an designed to give protection against heavy overloads and short circuits, which element is also designed to heat a fusible electric bond to give a time lag protection for relatively smaller continuing overloads.

Another object of my invention is to provide an electric protective device having a plurality of fusible portions to give protection for different harmful load conditions, which device has a single means for interrupting the circuit through the device, upon operation of any of the fusible por tions.

These and other objects of my invention will become more apparent upon consideration of the following specification and the annexed drawing in which:

Figure l is a central vertical sectional view of my device;

Fig. 2 is a view similar to Fig. 1 showing the circuit through the fuse interrupted at the fusible alloy juncture;

Fig. 3 is a transverse section taken on the line III-III of Fig. 1; and

Fig. 4 is a view similar to Fig. 1 showing the circuit through the fuse interrupted at the fusible portion of the heating element.

My protective device consists of a pair of opposed cup-shaped insulating members 2, 4. One of the members 4 is provided with an outwardly projecting flange 8 and a reduced tubular extension 9 having a flattened outer side portion 7.

The other insulating member 2 telescopes over the reduced tubular section 9 and abuts against flange 8. The insulating member 2 has an inner flattened side portion 5 to engage the flat portion 1 on the insulating member 4 to prevent relative rotation of the insulating members when the fuse is assembled. These members are formed of any desired insulating material, the outer member 2 being preferably formed of glass or other transparent material so that a visual indication of the condition of the fuse will be afforded. The insulating members are secured together by a side contact sleeve 6 having an inwardly projecting flange I0 engaging under the flange 8 on the insulating member 4 and threadedly engaging the insulating member 2. The insulating member 4 is provided at its bottom with an opening for receiving a center contact I2 of suitable conducting material which is secured in place in the opening in any suitable manner. As shown, the contact I2 has a flange l4 seated against the outer side of the insulating member 4 electric protective device having a fuse element and has outwardly bent portions It at the inner side of the insulating member 4 to secure the same in position.

A heating coil 20 of any suitable resistance material, such for example, as Nichrome, has one end thereof roughened and secured to the center contact l2 by any suitable means, such as for example, a mass of solder l8. The heating coil extends into an elongated heater cap 24, having a central bore 25 in which the heater coil is received. At the end of the bore in the heater cap, there is provided an eccentric aperture through the end of the cap for receiving the other end 26 of the heater coil 20. The end 28 of the heater coil is fixedly secured to the cap in any suitable manner such as, for example, by mechanically pinching the cap in the region of the part 26. At the open end of the bore and the heater cap, the heater element 20 is provided with a fusible portion 22 of reduced cross-section and formed, for example, by providing a nick in the heater element. A mass of fusible material 28, preferably any suitable alloy or eutectic metal having a low melting point, such for example, as

I a low melting point solder, is supported at the top of the heater cap and serves to anchor a resilient conductor 30, such as beryllium copper, to the heater cap. The conductor 30 is of such size and shape as to limit conduction of heat away from the heater cap. The conductor 30 is held under stress by the mass of fusible material and upon softening or melting of the fusible material 28, the resilient conductor 30 will move away from the heater cap to the position shown in Fig. 2. The resilient conductor 30 is secured between the insulating parts 2 and 4 as at 32 when they are assembled and has an end 34 extending into engagement with the side contact member 6. The heater cap is preferably formed of suitable metallic material and may, if desired, have an insulating coating for its bored part 25, such as an insulating lacquer, paint or the like. Inasmuch as the end 26 of the heater coil is connected to the heater cap 24, it need not extend to the fusible material as shown, but may terminate short of the'same. These features, however, do not alter the cooperation of the parts or their functions, as hereinafter described.

In the operation of my device, when the current through the device attains a certain value, which exists for a predetermined time, the heater develops suflicient heat to gradually raise the temperature of the mass of metal on the heater cap to the melting temperature of the fusible alloy, causing the latter to soften and release its hold on the spring, whereupon the spring separates from the heater cap and interrupts the circuit, as shown in Fig. 2. On heavy overloads and short circuits, the heater burns through at its fusible portion 22 before sumcient heat has been developed to raise the temperature of the heater cap and melt the fusible alloy. The resilient conductor 30 also operates to interrupt the circuit upon fusing of the fusible portion 22 of the heater element, as shown in Fig. 4, thus giving a wide gap to open the circuit both on small overloads and heavy short circuits. Thus it can be seen that this device has two distinct points of operation; the point at which operation takes place depend ng upon load conditions of the circuit in which the fuse is placed. On prolonged light overloads, the circuit is opened at the fusible alloy juncture. On heavy overloads and short circuits, the heater burns through almost irmnediately. In each case the resilient conductor 30 operates to interrupt the circuit. Due to the fact that light overloads will not burn through the fusible portion 22 of the heating coil and that the operation of the device under such overloads is dependent on the time necessary for the heater to heat the heater cap sufllciently to soften or melt the fusible alloy, it is apparent that upon such light prolonged overloads, this device will have a much greater time lag than the ordinary fuse of the same rating. However, on heavy overloads or short circuits the fuse operates immediately before suflicient heat can be developed to soften or melt the fusible alloy 28, thus providing a maximum of protection on both types of overloads.

It ,wm be noted that this circuit interrupting device is constructed to be inserted in standard threaded type fuse h'olders.

It can be further seen that I have provided a circuit interrupting device giving protection on short circuits and heavy overloads as well as on relatively small but prolonged overloads having relatively few operating parts and being of relatively simple, and economical construction.

While in accordance with the patent statutes, I have described in detail one embodiment of my invention, it will be obvious to those skilled in the art that many changes may be made therein, such as omission of parts, substitution of elements, and other changes apparent to one skilled in the art, and I do not wish to be limited to the particular embodiment disclosed, except as set forth in the appended claims.

I claim as my invention:

1. In a protective device for electric circuits having a pair of contacts, electric heating means connected to one of said contacts, a conducting casing about the heating means and connected thereto, a movable conductor connected to the other of said contacts, and a mass of fusible material connecting said casing and conductor whereby said conductor is movable to interrupt the circuit through the device upon melting of the fusible material.

2. In a protective device for electric circuits, a casing member having an open end and a closed end, a heating member extending into the casing through said open end and having its inner end connected to the closed end of said casing member, a mass of fusible material supported on said casing and. electrically connected to at least one of said members, ang. a movable conductor secured in said fusible material, whereby the circuit through the device may be interrupted upon melting of said fusible material.

3. In a protective device for-electric circuits, a casing member having an open end, a heating member extending into the casing through said open end, a mass of fusible material supported on said casing and electrically connected to at least one of said members, a movable conductor secured in said fusible material, whereby the circuit through the device may be interrupted by fusing of the fusible material upon passage through the device of a current of predetermined magnitude, said heating member having a portion substantially at the open end of the casing member adapted to be fused upon passage of a current through the device of a predetermined greater magnitude.

4. In a protective device for electric circuits, a casing member having an open end, a heating member extending into the casing through said open end, a mass of fusible material supported on said casing and electrically connected to at least one of said members, and a resilient conductor secured in said fusible material and biased away therefrom whereby the circuit through the device is interrupted by said resilient conductor when the fusible material melts due to passage through the device of a current of a predetermined magnitude, said heating member having a portion substantially at the open end of the casing member adapted to be fused upon passage therethrough of a current of a predetermined greater magnitude, whereupon the resilient conductor will separate the fused portions of said heating member.

5. In a protective device for electric circuits having a pair of contacts, a mass of fusible material, a stressed resilient contact member secured therein and a heater member for the fusible material all connected between said contacts, one of said members having a fusible portion adapted to fuse upon the passage therethrough of a current of a greater predetermined magnitude than that required to heat said fusible material to its fusing point, whereby said resilient conductor will interrupt the circuit through the device upon fusing of said fusible material or of said fusible portion.

6. In a protective device for electric circuits having a pair of contacts, fuse means connecting said contacts including fusible material and a heating means therefor, a heat conducting member including a hollow portion for receiving said heating means and a relatively massive portion between said heating means and fusible material, whereby upon the passage of a current of a predetermined small magnitude through the device, the fusible material will be softened or melted by heat conducted from the heater to interrupt the circuit through the fuse, and the heater being provided with a portion adapted to fuse upon the passage through the device of a current of a predetermined larger magnitude.

able terminal, said conducting means including a hollow portion for receiving a heating means electrically connected thereto, and a relatively massive portion located between said hollow portion and said fusible material.

9. In a protective device for electric circuits, a casing member having an open end and a closed end, a heating member extending into the casing through said open end and having its inner end connected to the closed end of said casing member, a mass of fusible material supported on said casing and a stressed resilient conductor anchored in said fusible-material, whereby the circuit through the device may be interrupted upon melting of said fusible material by said resilient conductor.

10. In a 'protective device for electric circuits, a casing member having an open end and a closed end, a heating member extending into the casing through said open end and having its inner end connected to the closed end of said cas n member, a mass of fusible material supported on said casing, a stressed resilient conductor anchored in said fusible material, and said heating member having a portion adapted 'to be fused upon the passage of current therethrough of a value greater than that required to fuse said fusible material, whereby the circuit through the device may be-interrupted upon melting of said fusible material or fusion of said portion of said heating member, by said resilient conductor.

11. In a protective device, a casing, electric ing different fusing characteristics located, respectively, adjacent opposite ends of said casing and serially connected with said heating means, and one of said fusible means being arranged in heat-conducting relation with respect to said casing.

12. In a protective device, a casing, electric heating means in said casing, fusible means having different fusing characteristics located, respectively,-adjacent opposite ends of said casing and serially connected with said heating means, one of said fusible means being arranged in heatconducting relation with respect to said casing, and resilient means associated with one of said fusible means to cause separation of the unfused terminals of either of said fusible means upon fusion thereof.

ROBERT W. PRICE.

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