US2122794A - Electric circuit protection - Google Patents

Description

y 1938. B. E. GETCHELL I 2,122,794

ELECTRIC CIRCUIT PROTECTION Original Filed 06:. s, 1952 2 Sheets-Sheet 1 INVENTOR HEM/, rcua L,

('K/flrbRNEY ELECTRIC CIRCUIT PROTECTION Original Filed Oct. 3, 1932 2 Sheets-Sheet 2 Y WWI/vim rx/ve DEV/6E 5i 74 I x Patented July 5, 1938 UNITED STATES PATENT OFFICE ELECTRIC CIRCUIT PROTECTION Benjamin E. Getchell, Plainville, Conn., assignor to The Trumbull Electric Manufacturing Company, Plainville, Conn., a corporation of Connecticut 5 Claims.

My invention relates particularly to electrical protective switching devices for apparatus of the character described in my Patent No. 1,886,477 issued November 8, 1932.

Such constructions include manually controlled circuit making and breaking apparatus together With a thermostatically actuated device, including a heater or resistance element, for automatically opening the circuit upon overload. These devices are usually called enclosed or safety switches.

In installing switches of this type, it has been the usual practice to protect the switch from short circuit by means of a fuse of approximately four times the rating of the heater element but it is frequently desirable to install such switches in gangs of from five to fifteen with a single fuse in the circuit to protect the entire installation and such an installation requires a fuse of such large capacity that it will not protect the heater element from burning out in the event of a short circuit. Such ganging installations are permitted by the underwriters provided the fuse is arranged to blow and open the circuit quickly enough to .prevent the are caused by the automatically operated elements from damaging the switch mechanism or causing a fire hazard outside of the enclosing box. Usually, in the event of a short circuit, the heater element will be burned out and rendered inoperative but, in some instances, the intensity and volume of the arc will be great enough to actually destroy the operating mechanism of the switch.

The underwriters require that such a burning out must not cause sufiicient flash to ignite absorbent cotton outside of the enclosing ibox. They also require that the switch, per se, shall remain operable in spite of the burning out of the heater element.

In constructions of this character and with a large fuse in series with the heater the resistance of the heater, as a rule, is the greatest factor of resistance in the circuit and consequently holds down the current momentarily in proportion to its resistance and this reduction in current to a value much below the short circuit intensity, retards the blowing of the fuse and in the event of a'short circuit, the heater becomes incandescent and ionizes the air surround-.

- ing the heater and its terminals and a flash-over will take place which usually destroys the heater element and sometimes burns the adjacent parts so as to render the switch inoperative. Obviously, under these conditions, at least a new heater element must be provided after each short circult and sometimes the entire switch must also be replaced. 1

The principal object of my invention is '0 provide a practical and efiicient automatically operated means which will insure the blowing of the fuse instantly in case of a short circuit on the line and before any harm can be done to the switch mechanism or its correlated parts.

Another object is to provide an automatically operated device of this character which will not only protect the heater element and the switch parts but will also greatly reduce the fire hazard which is characteristic of devices of this type.

Another object is to provide a means which will shorten the length of the are caused by a short circuit so as to greatly reduce its explosive effect and destructive force.

Another object is to provide an automatically operated means for accelerating the blowing of a protective fuse so as to reduce the danger of damage to the switch or other correlated parts.

Another object is to provide automatically operated protecting means for the switch and. its correlated parts and including an easily renewable and relatively inexpensive heater unit.

Another object is to provide a heater unit so designed as to localize or confine ionization between its respective terminals in such a manner that the arc between the terminals will be of sufficient intensity to insure the instantaneous blowing of a correlated fuse but in which the duration and destructive force of the arc is not great enough to harm the switching mechanism with which it is associated.

I accordingly arrange the heater unit and its support in such a manner that, in case of a short circuit on the line, the circuit will be diverted or shunted from its normal path through the coils of the heater and a flash-over will occur between the opposed portions of the relatively heavy terminals of the unit and this flash-over or are will permit substantially full short circuit current to pass through the fuse and blow it before the are between the terminals of the heater unit assumes sufficient magnitude to injure the adjacent switch parts and without suflicient flash to cause injury inside of the switch box or to blow out through any normal openings in the box or its cover.

The opposed portions of the terminals of the heater unit are spaced apart one thirty-second of an inch or less for the purpose of making the length of the are very short and a slotted insulating plate or strip of mica or equivalent insulatin material is interposed between the opposed portions of the terminals, the purpose of the slot being to definitely locate the point at which the arcing will occur and also to limit the width and laterally spreading tendency of the arc.

The slot or opening in the mica or insulating plate which determines the point at which areing will occur is positioned adjacent the heavy portions of the opposed terminals and away from the relatively light and easily ionized coil of the heater.

From the foregoing, it will be clear that I have provided for a very short are between relatively heavy metal terminals, at a definite point and within an area so restricted that the flash of the arc is kept down to the minimum and thus prevented from spreading enough to cause damage to the coil windings or other parts.

Fig. 1 is a front view of a construction embodying my invention.

Fig. 2 is a side view and vertical section showing parts of the switch mechanism and the thermostatic relay.

Fig. 3 is a vertical sectional view showing one of the switch units with the thermostatic strip device and heater in place.

Figs. 4, 5, and 6 are sectional views of the heater unit.

Fig. '7 is a diagrammatic representation of an electrical circuit showing one method of using my invention.

The conducting parts are preferably mounted upon an insulating base I I. The stationary contacts I2 and I3 may be of any suitable type and are adapted to be connected by a movable switch member l4 of any suitable type actuated manually by any suitable mechanism such as a lever I5 which is pivoted at I6 to a frame I! supported by the base.

The movable switch members I4 are carried by a cross bar I8 which is normally pressed outwardly by a spring I9. The latch 23 is pivoted at 2| and connected by a link 22 to the lower end of the lever 23. The lever 23 is connected at its upper end by a link 24 to the cross bar I8 and at an intermediate portion is a pin or roller 25 which slides in an inclined slot 26 in the operating, lever l5. When the outer end of the lever I5 is pressed upwardly the pin 25 is forced toward the right, as viewed in Fig. 2. The lower end of the lever 23 being stationary at this time of course the upper end with the link 24 is pushed inwardly so as to bring the movable switch member I4 into electrical engagement with the contacts I2 and I3 to close the circuit, the parts being held in this position by engagement of the pin 25 in the notch 21.

The normal electric circuit is completed from the contact I3 to the terminal 28 through the strip 29, anchor member 30 and heater coil 3I. This coil is mounted on the arm 32 but insulated from it by layers of mica 33, one end of the coil being secured at 34 to the arm 32 and the other end of the coil being secured to the rivet 35 which also serves to anchor the terminal strip 28 to the insulating block 36. This insulating block 36 also carries the anchor piece 30 which is secured to it by rivet 31.

The insulating block 36 is secured at one of its ends to the insulating base II by a screw 38 and the other end of the block is secured either directly or indirectly to the base II by means of screw 38' in such a manner that the block 36 and the attached heater 3| may be readily re moved as a completely assembled commercial unit.

The bi-metallic thermostatic strip 40 has one end secured to the anchor piece 4! which is secured to the base II by screw 42. The other end of the thermostatic strip is adapted to Warp or flex with changes in temperature due to the current flowing through the coil 3I. The trip member 45 is normally interposed in the path of movement of the latch member 20 and is carried by an insulating cross piece 43' which in turn is secured to an arm 43 which is pivoted at 44 so as to pivotally support the trip member as more fully shown and described in Patent 1,886,477. A rod 46 extends through the members 40 and 43 and may be in the form of a screw with a nut 41 so that the effective length of the rod may be adjusted. A spring 48 on this rod holds the parts yieldingly in a normal running position.

In case there is a sufficient overload the bimetallic strip 40 bends sufiiciently to release the trip member 45 from the latch 20 thus permitting the mechanism of the switch to operate and open the circuit.

The parts 30 and 28 are separated from each other preferably by a space or gap of of an inch or less. A layer of insulation 33 is preferably interposed between the two members 30 and 28 except at the point 50 where there is an opening or slot 5| of relatively small and narrow dimensions and, in this restricted space arcing may take place when the coils of the heater have come to a sufficient temperature to permit ionization to break down the resistance of the air gap and start an arc. The instant this arcing takes place the load is shunted around the heater and then passes between the opposed portions of the unit terminals and the impulse of this are permits a surge in the circuit through the fuse which will blow and open the circuit before the arc has assumed sufficient destructive force to injure the heater element, the switch or its operating parts.

This heater unit is preferably made removable so that in case it is damaged it may be readily replaced.

While these heater units are preferably readily replaceable, they may serve to are over many times to open the circuit in case of short circuits before being rendered inoperative or destroyed.

Referring now to Fig. '7, I have here shown a typical electrical circuit such as described in the first portion of this specification. Wires serve to conduct electric current from any suitable power source to the device ll which may be either fuses, as indicated or may be constituted by an overload circuit breaker of any suitable type. After passing through these fuses, the current then is led along main feeders 72 to a plurality of branch circuits 13-13. For convenience of reference I have indicated these various branches by the letters A, B, etc. These various branches may be identical or similar in type and there fore the following description, which applies to one of these branches, will serve to describe all of them.

The current in a single branch circuit travels along conductors 73 to the input connections 14 of a switch 75, provided with a handle 16, and output terminals TI. This switch is a diagrammatic representation of the switching portion, per'se, of the device shown in Figs. 1, 2, and 3. From terminal 11 the current travels through series resistor 3I, which is shunted by air gap 5I. These elements are the same as previously shown and described in connection with Figures 3-6. Conductors I8 serve to carry the current from this point to a translating device 19, which may be of any suitable type, such as an electric motor or the like.

As previously pointed out, the fuse or overload protective device H is necessarily of a current carrying capacity approximately four times the total current carrying capacity of all the series resistors 3!. Since these series resistors are located in branch circuits and therefore can be looked upon as parallel load circuits. with respect to the main feeders l2, protective devices I! will, in the present instance, be of a current carrying capacity approximately twenty times that of any single series resistor 3 l When a short circuit, or some other accidental condition drawing an abnormal current occurs in any single branch circuit, the resistor 3| in that circuit may be effectively thrown directly across the leads 13 supplying that particular branch circuit.

From the current carrying relationships just outlined it will be evident that the main protective devices II will not operate so as to open the circuit to feeders 12 under this condition. Therefore, as previously pointed out, the abnormal current through resistor 3| may reach such proportions as to seriously damage this resistor and possibly its associated switch 15, without any protective action being exerted by protective devices 1 I.

When the above condition occurs, the heated gases from resistor 3! will travel through slot 5| to the point 50, where they will so lower the breakdown potential of the gaseous gap there existing as to allow the normal circuit potential to are thereacross. This arc across gap 5! will then eifectively shunt out resistor 3! from the circuit, thus increasing the current flow therein to a point limited only by the resistance which has occurred in that particular branch. This Very great rise in the current value will serve to operate the fuses or overload breaker located at H.

While I have shown a two-wire power system, it is to be understood that the general operative description above given would apply equally well to a three or four Wire distribution system such as employed, for example, with polyphase alternating current systems. Likewise while I have shown the resistor 3| and the gap 5| as inserted only in one lead of the power translating device, yet it is to be understood that such devices may be connected in other leads, especially in the case of polyphase circuits.

The heater unit is claimed per se in my Patent 1,996,720 and this application is a division from the application on which said patent is issued.

I claim:

1. An electric circuit including an overload protective device and independent means for determining the establishment of a heavy current surge, including an electrical resistance unit in series with said circuit normally limiting the current flowing therethrough upon the occurrence of an external short-circuit, two conducting electrodes connected to the respective terminals of said resistance unit and separated from one another by a gap sufliciently great so that the electromotive force present between said. electrodes under conditions of normal current flow through said resistance will not discharge across said gap, a passageway substantially open to gas flow between said gap and said resistance unit so that when an external short circuit occurs, said resistance unit will be heated and give rise to ionized gases, and so that said ases will be conducted to said gap and cause an electrical discharge thereacross, thereby effectively shunting said current limiting resistance and establishing said heavy current surge, thereby operating said overload protective device and opening said circuit.

2. An electrical circuit containing in series connection an overload protective device and an independent current limiting resistance, a gaseous discharge gap in shunt with said resistance and in close proximity thereto, said discharge gap cooperating with the balance of the circuit for permitting in said circuit a surge of current greater in value than the current capable of flowing through said resistance when a given electromotive force is impressed across the terminals thereof, said electromotive force causing said resistance to become heated to a degree sufficient to ionize the gas thereabouts by the passage of current therethrough, and means for confining said ionized gases adjacent said discharge gap so as to ionize the latter, whereby said given electromotive force will establish a discharge across said ionized gap and substantially shunt said series current limiting resistance, thereby permitting said surge of current to occur and to operate said overload protective device so as to open said circuit.

3. In an electric circuit, the combination in series connection of an overload protective device with a separate resistive and protective device, said device comprising a resistance unit, and two electrodes connected to the terminals of said resistance unit and physically located with respect to one another so as to provide a relatively short gaseous discharge path therebetween, said path having a length of the order of magnitude of one millimeter, and said resistance unit being so positioned adjacent said discharge path and with a free and unobstructed gas passage therebetween so that the heated gases produced by said resistance unit will readily and directly reach said path without passing through any solid material and lower the breakdown potential of said path, whereby an excessive flow of current through said resistance will cause the electrical breakdown of said gaseous path by means of the ionization of said path and by means of the simultaneous establishment of a higher than normal potential between said two electrodes, when said excessive current flows through said resistance, said breakdown of said path allowing a current flow therethrough, greater than the current flow possible through said resistance unit, whereby said overload protective device will function to open said circuit.

4. An electric circuit comprising in combination a source of electric energy and a device for consuming said energy, overload protective means, resistive means in series with said circuit for limiting the flow of current in the circuit, a pair of spaced electrodes having a gaseous path between them and through which current does not normally flow, said path being in shunt with said resistive means, means for ionizing gas adjacent said electrodes and means for confining the ionized gas adjacent the space between the electrodes whereby a gaseous discharge will be established between said electrodes, thereby shunting said series limiting means and causing the flow of a current in said circuit exceeding in value the current flow possible while said limiting means is in series with said circuit, whereby said overload protective means will operate so as to break said circuit.

5. In an electric circuit, a power source, a fuse, a current surge producing device and a power consuming device, all connected in series with one another, said current surge producing device including a gaseous discharge producing and confining device having a gas discharge chamber, two discharge electrodes and a gaseous filling within said chamber, said electrodes being separated by a gap of substantially the order of magnitude of one millimeter, an ionizing heater external to said gas discharge chamber but having communication therewith affording direct, close and unobstructed passage of gas from said heater to said discharge causing a heavy current surge and 10 causing said fuse to melt and open said circuit.

BENJAMIN E. GE'I'CHELL.

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