Oct- 29, 1946, " E. M. THIELERS ET AL 2,410,370 THERMAL CIRCUIT INTERRUPTER ' Filed Sept. 20, 1943 L ‘I M J K ‘1m a A Q Fig.2 O , N .L@ _ Fig.3 £1 ‘'5 R ' m, FI9. 4 XNVENTQRs . . E..M_TH\ELERS P§~°_ WO-W. BROBERG 'Bq ’ _‘ Ah“ Patented Oct. 29, 1946 2,410,370 "UNITED STATES PATENT OFFICE THERMAL CIRCUIT INTERRUPTER vErnst Martin Thielers, Stockholm, and Walter Otto Wilhelm Broberg, Nynashamn, Sweden, assignors to Telefonaktiebolaget L. M. Ericsson, Stockholm, Sweden, a company of Sweden Application September 20, 1943, Serial No. 503,118 In Sweden October 20, 1942 8 Claims. 1 (Cl. 200—123) . 2 Within telephone technic a fuse heat-coil has been used comprising a heat-producing winding acting upon an interruption point which consists of two metal parts, soldered together by an easily fusible alloy. As a rule,.the heat-producing coil shown in Fig. 2. In this ?gure, J and K are two springs, K being provided with a slot in which the narrow part of the heat-coil cover A may be inserted. This causes the springs J and K to be pushed apart and good contactis thus obtained. is inserted inv ametalcover and wound on a nar L is a spring which'in its normal rel-axed posi tion rests against a strip M and at its upper end has a slot in which the pin F is to be inserted. The contact at the strip M is then broken and row metal spindle bored lengthways, from one side so as to receive a pin which, by a fusible alloy, is soldered to the wall of the boring (in terruption poin . i , -' ' vWhen current ?owsthrough the heat-produc 10 the spring L presses against the pin head, this pressure generally amounting from 250 to 300 g. The described heat-coil operates as follows. The soldering metal which ?xes the pin F to the spindle C softens at a pre-determined rise of tem ing winding, heating of the easily-fusible solder follows which, at a pre-determined current, causes the solder to soften or tO-fllse-So that the pin, actuated by an elastic force, can be pulled out of 15 perature, caused by excess current, whereupon the boring thereby causing interruption of the the spring L is released thereby pulling out the pin. Current interruption follows and spring L is applied against the strip M, thus closing an alarm circuit. current. At the same time, the spring adapted to pull out the'pin may be brought against a con tact and close‘ an alarm circuit. » ~- . An arrangement of this kind is shown in Figs.'1 and 2 on the drawing, Fig. 1 showing the heat coil and Fig. 2 the heat-coil mounted in springs on a ?tting. ~ ~ . . The heat-coils are most frequently used in tele phone exchanges where they are placed in con nection with main distributing frames. ' goils combined with lightning protectors and ning protectors and fuses according to Fig. 3, 25 where the incoming line is marked N, the fuses Fig. 4 is a diagram representing the character istic curve of the heat-coils. 1 Fig. 5 is a sectional view of an embodiment of the invention. _ _ In Fig. 1 which shows a heat-coil ‘in, cross-sec 30 tion view, A isa metal cover, B an insulating sleeve in the‘lshapev of a: truncated cone, generally made of ebonite or-?ber, the middle of which is The ' heat-coils are frequently combined with light Fig. 3 is a diagram of a circuit with the heat O,» lightning protectors A, heat-coils P and the line to internal exchange devices Q. In this com bination the fuse generally has a limiting cur rent of 3 9... Whilst the heat-coil is adapted to interrupt at lower values. . - Due to its structure, the heat-coil has char acteristic curves differing from the characteristic curves of a fuse consisting of a fuse~wire. Fig. 4 gives the characteristic curves of one of the most frequently used kinds of heat-coils. It has threaded so that the spindle can be screwed in‘ to the insulating sleeve; E is insulating washers, in a limiting current of about 0.16 a. and, at a. load of 0.25 a., an voperating time of about 20 seconds and at 0.7 a. an operating time of 2 seconds. The heat-coil is thus considerably slower than a fuse - sulating the spindle from the cover A, F a .pin 40 containing a fuse-wire. The resistance of that with a, head; w-hichpin is inserted in C and ?xed by an easily-fusible solder (fusing point be coil is about 20 ohms. » ‘ is Owing to said characteristic curves the heat ing normally about 70° 0.). Gjis a heat-produc coil is adapted for use in combination with a fuse ing winding whose one end is in contact with the spindle C and whoseiother end is soldered to the 45 (fuse-wire) of about 3 a., the fuse-wire operating at high excess currents which generally are of ‘short duration, while the heat-coil is adapted The outer terminal clamp of the heat-coil is to interrupt at lower non-allowable currents formed partly by the cover A, partly by the pin F. which sometimes may be of longer duration. As a rule,\the heat-coils are mounted in so The heat-coil thus described has however sev called strips and fastened by means of springs, as 50 eral drawbacks.‘ Primarily, the resistance is high coveratI-I. ' ' ‘H m " 2,410,370 3 which is particularly perceptible when a tele phone communication is established over several exchanges, 40 to 45 ohms for every fuse-point thereby being connected to the line. The con struction described involves a reduction of the resistance and at the same time increase of the operating current which is seen from the table below, taken from “Katalog over Telegrafverkets materiel” (Catalogue from the Swedish Telegraph Department) : Fusing current, ampcres at a Resistance fusing time of 20 seconds 0 .27 0.45 O .52 0.85 2 .00 Still more inconvenient is however the following observation made by the inventors. It has been found that the heat-coil has fairly narrow limits which with the heat-coil having a resistance of 20 ohms, amounts to about 0.16 and about 1 a. When the current exceeds 1 a. it may occur that (l) The insulation of the winding wire carbonizes and short-circuits occur without the pin 4 desirable to coat the winding with some ?xing agent (for instance thin “cellone” solution). The heat capacity of the winding must not be too high. Consequently, the wire diameter should be selected rather small as, for instance, with a coil having a resistance of 10 ohms it may be about 0.10 mm. It should also be observed that, at the points where the heat-coil rests against the springs, the surfaces of contact should be made as small as possible. For that purpose the pin head in its inner portion has a conical form. Further, the circular slot at the lower part of the cover may be conically shaped. When the heat-coil is fastened in its springs the above described springs I and K will rest against small surfaces on both sides of the lower ?ange of the cover. Easily-fusible alloys with various fusing tem peratures may serve as soldering metal. Nor mally, an alloy with a fusing point of 65-70" C. is used. The described heat-coil is adapted for use pref erably in telephone technics but may be used also in other ?elds, where its working properties may be found useful. We claim: 1. In a device of the character described, an electrically conductive cover, an electrically eon ductive tube arranged within the cover, insulat being pulled out and causing alarm. The ing means supporting said tube in said cover, a 30 pin soldered in said tube, a heating coil wound on heat-coil then offers no protection. (2) The winding wire is burnt Without the pin said tube for fusing the pin solder, the pin and cover being engageable with fastening springs and being pulled out and giving alarm. the heat capacity of the heating coil and parts When these heat-coils are used in combination with tube fuses of 3 a. capacities a ?eld of the 3 heated by said coil and heat losses of said insulat ing means and fastening springs being so reduced intensity of current of between 1-3 a. exists with that the interruption point for all excess currents in which no protectors, operating reliably, are over the limiting current attains its fusing tem available. perature at least as early as the leading coil at After a close research the inventors have suc tains a temperature injurious thereto. ceeded in constructing a heat-coil, whose resist ance amounts to less than half of that of the above described heat-coil type and which for all intensities of current above the limiting intensity breaks the circuit and causes alarm and which may be inserted in the same strips as those used for the present heat-coils. The invention is char acterized in that, by excess current, the point of interruption (i. e. the solder) attains the fusing or softening temperature earlier or quite as early as the heat-producing winding attains a tem perature injurious to the wire or its insulation. This is obtained by reducing the heat capacity as much as possible and by counteracting the heat-conduction from the pin to the bottom of the cover. Heat losses caused by radiation are left out of consideration by this invention. Fig. 5 shows an embodiment of the invention. R is a cover of metal, S a thin tube of metal (pref 2. In a device of the character described, an electrically conductive cover, an electrically con ductive tube arranged within the cover, relatively thin disc like insulating members supporting the tube within the cover, a .pinsoldered in said tube and a heating coil wound on said tube for fusing the pin solder. 3. An arrangement as claimed-in claim ,2 where in the thickness of the wall of said tube does not exceed .5 mm. 4. An arrangement as claimed in claim 2 wherein the internal diameter of said tube is ap proximately 1 mm. , 5. An arrangement as claimed in claim 2 wherein the heating coil is coated with a thin ?xing agent. 6. An ‘arrangement as claimed in claim 2 wherein the ends of the tube are ?anged and erably with good heat conductivity). T indicates insulation washers, for example of ?ber, which engaged with the disc like insulating members. are secured at the ends of the tube, the tube ends for this purpose being stamped to form a small ?ange. U is the pin arranged in the tube gaged with the disc like insulating members, said cover having depressions therein for retaining the insulating members in position. and being soldered with easily-fusible alloy. Finally, V is the heat-producing winding, whose ends are soldered to the tube or the cover, re spectively. The parts maintain their position by means of several annular depressions X formed in the cover. The winding is arranged so as to be wound in as few layers as possible (preferably one layer) and as close to the tube as possible. It is also '7. An arrangement as claimed in claim 2 wherein the ends of the tube are ?anged and en 8. An arrangement as claimed in claim 2 wherein the pin and cover are provided with por tions engageable with fastening springs, said por tions being shaped to minimize the area of con tact with said springs. ERNST MARTIN THIELERS. WALTER OTTO WILI‘IEL'M BROBERG.