US3132223A - Fusible protective devices for cable protection - Google Patents

Description

y 5, 1964 P. c. JACOBS, JR 3,132,223

FUSIBLE PROTECTIVE DEVICES FOR CABLE PROTECTION Filed June 19, 1961 2 Sheets-Sheet l TEMPERATURE INVENTOR.

PHILIP G- JAGOBS JR BYNAMWW M 3,132,223 CABLE PROTECTION 2 Sheets-Sheet 2 P. C. JACOBS, JR

May 5, 1964 FUSIBLE PROTECTIVE DEVICES FOR Filed June 19, 1961 INVENTOR. PHILIP C. JACOBS, JR BY MW W United States Patent r 3,132,223 FUSIBLE PROTECTIVE DEVICES FOR 1 CABLE PROTECTION Philip C. Jacobs, Jr., Newtonyille, Mass, assignor to The Chase-Shawmut Company, Newburyport, Mass. Filed June 19, 1961, Ser. No. 118,048

7 Claims. (Cl. 200-120) have substantially the same degree of interrupting capacity 1 r and selectivity normally only encountered in currentlimiting fuses.

A current-limiting fuse of standard design is generally not suitable for use in cable networks.

It'is, therefore, another object of this invention to provide devices in the nature of a current-limiting fuse adapted for service in cable networks.

It is possibleto distinguish between two general classes offusible protective devices for the protection of cable networks. In devices of one class the fusible element is formed by a relatively large mass of metal having a relatively low fusing point. These devices are selective because of the great time lag involved in their operation. The main disadvantage of this type of protective devices is their relatively limited interrupting capacity. In addition thereto this type of devices involves relatively high watt losses. Devices of. the other above referred-toclasses include multiperforated ribbon-type fuse links of a metal such as silver, or-copper, having a high conductivity and a high fusing point and they further include a pulverulent arc-quenching filler as, for instance, quartz sand.

- This invention is concerned with the second type of fusible protective devices. Whilethis invention proposes to apply some of the principles underlying the design of a current-limiting fuseit represents a dnastic departure rfirom prior art devices for the protection of cable networks designed along lines similar to a current-limiting fuse.

United States Patent 2,770,757 to William S. Edsall, November 13, 1956, .Arrangement for the Protection of Cables Against Short-Circuit Currents and Against Overloading is concerned with a fusible device for the protection of cable networks designed along lines similar to those underlying the design of a current-limiting fuse.

The above Edsall device is a composite device of which one unit produces a circuit interrupting break on the occurrence of shortecircuit currents and another unit produces a circuit intenupting brea'k on the occurrence. of overloads. I

It is, therefore, another objectof this invention to provide simplified'fusible devices for the protection of cable networks designed along lines similar to those underlying the design of a current-limiting fuse but having one single integral unit for interrupting both .short-circuit currents and. overload currents. f

The above Edsall device is predicated on the concept that the "heat generated in the protective device should be minimizedand that the heat required for its operation shouldm ainly be furnished by the cablefor the protection of which it is provided, and the above Edsall device is further predicated on the concept that the time-current characteristic of the protectivedevice should follow as closelywas possible the smoking point characteristic, or the insulation damage characteristic, of the cable.

3,132,223 Patented May 5, l 964 ice concepts as far as the source of heat for operating the device is concerned, but deviates drastically from the Edsall concept of t etching the characteristic of the protective device with that of the cable. As long as it is the principal object of a cable protective device to extend the life of cables as much as reasonably possible, the characteristic of the protective device should necessarily match as close as possible the insulation damage characteristic of the cable. Considerations of continuity of service may, however, make it desirable to more or less drastically depart from the above Edsall doctrine of matching of characteristics.

It is, therefore, a further object of this invention to provide fusible protective devices for the protection of cable networks which are designed along lines similar to a current-limiting fuse but are adaptedto maintain continuity of service even at the expense of the life of cables as long as there is no immediate explosion or fire hazard.

These and other objects of the invention and advantages thereof will become more apparent from the accompanyequation ing drawings and the following description of a pre ferred embodiment of the invention.

In the drawings FIG. 1 is a diagrammatic representation of a portion of a cable network;

FIG. 2 shows diagrammatically or qualitatively the temperature distribution along a fuse structure which is inserted into aconventional fuse holder;

FIG, 3 shows diagnammatically or qualitatively the temperature distribution along a fusible protective device which isdesigned according to this invention;

FIG. 4 is in part a longitudinal section and in part a side elevation of a protective device embodying this invention; and

FIG. Sshows the same device as FIG 4 taken along Referring now to the drawings, and more particularly to FIG. 1 thereof showing a portion of a cable network, it has been assumed-that a short-circuit has occurred at the point S causing fault currents 1,, I and I to flow in the sound cables of the networl. If continuity of service is to be maintained the fuse 4 must blow to disconnect the short-circuited cable from the network, while fuses 1, 2 and.3 must remain intact. In other words, fuses 1, 2, 3 and 4- rnust operate selectively. Assuming the current drawn by load L to be zero, or that load is connected to the particular point of the network, then the totalcurrent I flowing from the three sound cables through fuses 1, 2 and 3 into the fault is given by the The largest of the three currents I I and I -which may be referred to as I -rnust' not cause the fuse by which it is being carried to blow, whereas I must be sufiiciently high to cause blowing of fuse 4. The ratio I /I *=r (2) selves to the protection of underground cable networks.

The present invention follows in general the above oneof the reasons why it was not possible, here tofore, to achieve a satisfactory protection of underground cable networks, V

FIG. 1 presupposes that fuses 1, 2, 3 and 4 are identical,"

fusing {i -dz of" the fuse carrying-the current I current-limiting fuses designed to have the smallest postion of the Underwriter and are subjected to excess currents of different magnitude. On occurrence of a short-circuit at the point S selective operation of the two fuses subjected to the highest currents I and I will occur if and when the condition'is met that the sum of the fusing fi 'dt and of the arcing ji -dt of fuse 4 carrying the current I I is smaller than the disintegration of organic cable insulation a matter of secondary importance, then a kind of current-limiting cable protector can be evolved along lines which will be outlined below more in detail. I

Fusible current-limiting protective devices intended to afford shorflcircuit protection as well as overload protec- I tion'should include multiperforated ribbon fuse link means sible let-through: currents and the highest possible se- 7 lectivity the fusing i -dt and the arcing fi -dt are substantially equal. Therefore the condition for selective operation can be stated by the following inequalityz wherein t,; is the fusing time and r tire ratio defined by 7 Equation 2. Hence It will be understood that the above calculation has been made assuming that the currents under interruption are relatively high-short-circuit current-eand the interrupt- V in g times sufficiently short to make is permissibleto neglect heat exchange phenomena and to consider all ji -dt values as being constants.

to .70. Therefore selectivity willbe maintained in the 1 loading, a relatively rapid progressive 'C., e.g. of silver or copper,

of a metal having a fusingpoint in the order of 1000 deg.

and overload-responsive fuselink-severing means having a substantially lower fusing point. Upon reaching their low fusingpoint the fuselink-severing means sever the current path through the fuse link means. A fuse link in form of a multiperforated silver ribbon having an overlay of tin-whose fusing point is 232 deg. C.-will be severed by a metallurgical reaction between silver and tin when the latter is heated up to and beyond its fusing point." If it is intended to provide a protective fusible device allowing, in case of severe overdisintegration of organic cable insulation and effecting interruption of the overloaded circuit only in case the cable becomes a fire hazard, then the overload-responsive fuse-.link-severing means must become operative onlyat higher temperatures than the fusing point of tin, preferably attemperatures In the current-limiting range as long asthe highest partial current 1 is about 70% but not more than 70% of the total fault'current I It is noteworthy that this high degree of selectivity is compatible with the high interrupting capacity inherent in current-limiting fuses.

Current-limiting 'fuses'pro gressively decrease the duration of'the fiow of fault currents as the magnitude of available fault currents increases. The'total fusing and arcing fi -dz then approaches a' fixed value which, is substantially equivalent to a fixed predetermined temperature in the range of 300 deg. C. .An overlay of cadmium on a ribbon fuse link of silver or copper Will well serve the purpose of an overload-responsive fuse-link-s'eve'ring means, cadmium having a fusing point of 320.9 deg. C.

A'cadmiurn overlay or other overload-responsivelinksevering means becoming operative at a temperature in the order of 300 deg. C., if used inaconventional fuse or a current-limiting fuse does not give rise to any particular problems, but when such an overlay or other link severing means which becomes operative in the temperature range 'of'300 deg. C; is used in a fuse that inter-connects two lengths of cable, this gives rise to a .thermal problem. The

reason for this difference in behavior resides in the fact that in conventional fuses and current-limiting fuses there is a steep temperature gradient from the hot spot to points axially and radially spaced from the'hot spot, whereas such a steep temperature gradient, is lacking in a pro- 1 .tective device ofthe contemplated nature. Therefore the rise for a cable of'particular size. This'suggests that.

currentlimiting fuses be applied for the protection of cables provided thatthey can be adapted to meet all requirements of a cable protector.

The Standards for Safety Fuses UL' 198 Seventh Edi- V V Laboratories, Inc., April 1955 (reprinted July 1959,) specify the permissible maximum 7 temperature rise At above ambient air in degrees centigrade indicated inthe table below. 7

. Y At on ferrules Fuse rating in amps. or on blades in O.

Maximum-peak transient temperaturesmay be some-' what higher. I The National'Electrical Code 1959 specifies in article 310 thereof maximum operating temperaturesof cables.

casing of such a device will be subjected to very high temperatures. If the casing is made whollyfor in part of an organic insulating material that is allowed to deteriorate progressively in the samefashion as the insulation of the cable 'is allowed to deteriorate, then the casing may not be able to withstand the pressure incident to a severe circuit interruption. It is, therefore, necessary to avoid thermal impairment of the casing material 'even though thermal t impairment of thecable insulation is deemed permissible.

Hence the casing must be made of a mechanically strong heat resistant material, preferably of a silicone-resin-glasscloth laminate. F r

The fusible element or elements of a protective device intended to be connected into a cable should be able to rapidly'dissipate the heatre'sulting from relatively short overloads. This can be achieved by'providing the protective device with a. pair of terminal elements including blocks of solid metal, e.g. in form of a pair of plugs clos- ,ing' the ends of the tubular casing or fuse tube. Such terminal elements are Well known and used in the cur- V rent-limiting fuse art and are shown, for instance, in

1 US. Patent 2,658,974 to Frederick J. Kozacka, Novem- The maximum operating temperature of moisture] and heat resistant rubber cables has been specifiedto'be 60 C., and the maximum operating temperatureiof silicone v absestos cable hasbeen specifiedrto be C. Maximumpeak transient cable temperatures may be somewhat higher. It is considered safe practice to allow maximum- C.l75 admitting 'buta moderate. reduction of the useful cable life.. Ifcontinuityof service is ,considereda matter of prime importance,'and progressive, destructive I peak transient: temperatures of rubber'cables up to 'ber 10, 1953, High Current Carrying Capacity Current- ,Lim'iting Fuses. It is hardly'feasible to spot-weldfusible elements in ribbon-form to terminal elements'in the form of blocks of solid metal as, for instance, term n l elements in the'form of plugs of solid metal closingtheends of the'casing or fuse tube. The only feasible or-practical way of conductivelyconnecting fusible elements inribbon form to terminal elementsin the form ofjblocksof solid metal is by way of softsoldering. It is common practice in current-limiting fuse technology to usetin solder or similar solder on a tin basis for establishing solder joints between the axially outer ends of ribbon-type fuse link means and terminal elements involving a relatively large mass of copper. .Because of the differences between a normal current-limiting fuse anda cable protector of the kind under consideration, the use of tin solder, or of a similar solder on a tin basis, is not indicated in the latter case. This will now be explained more in detail in connection ,With FIGS. 2 and 3.

Referring now to FIG. 2, this figure illustrates diagrammatically a conventional current-limiting fuse. The current-limiting fuse shown in FIG. 2 comprises a tubular casing 1' of insulating material, a pulverulentarc-quenching filler 2' within casing 1', a pair of cylindrical copper blocks 3 closing the ends of casing 1' and a pair of blade contacts 4', each integral with one of copper blocks 3. Multiperforated fuse link means 5' of a metal having a fusing point in the order of 1000 deg. C.silver or copper-conductively interconnect plugs 3'. To this end the axially inner surfaces of plugs 3 may be provided with grooves into which the axially outer ends of link means 5' may be inserted and which grooves may be filled with tin solder (not shown), or another soft solder which is comparable to tin solder. Each link means 5 is provided near the center thereof with an overlay 6' of tin. Overlays '6' are fuse-link-severing means which, upon fusion thereof, sever the-current path through fuse link means 5f by a metallurgical reaction involving the dissolution ofthe high fusing point base metal -silver or copperhr the low fusing point overlay metal tinand the penetration ofmolecules of the low fusing point metal into the molecular structure of the high fusing point metal. The ends of blade contacts 4 are inserted into a pair of fuse holder contacts 7'.. The curve T indicates temperature distribution in a direction longitudinally of the device when the latter is carrying its rated current. The curve T indicates the temperature distribution along the axis of the device assuming predetermined watt losses occurring therein. It is apparent fromcurve T that the center of. link means 5' has reached the fusing point of tin231.8*deg. :C..--at which temperature the link-severing process by overlay 6f is initiated. Assuming that the tin overlay: 6' isreplaced by a. link-severing overlay having a higher .fusingpoint thantin, eg an overlay of cadmium whosefusing point. is 320.9 deg. C. and further assuming that the wattage dissipated in the structure is increased to such an extent that the temperature of the hot-spot of the structure is raised to the fusing point of cadmium, this results in a temperature distribution as represented by. the characteristic T The temperature of the blade contacts 4'Whencarrying the rated current is about 75 deg. C. It is apparent from FIG. 2 that the temperature of the bladecontacts 4 rises but slightly if the load .is increased to such an extent as to result in temperature distribution curves T and T The particular temperatureadistribution illustrated in FIG. 2 isprimarily due to thefact that the temperature of fuse holder contacts 7* remains low even at currents which are a multiple. of the current rating of the "current-limiting fuse which is being supported bythem. i

In FIG; 3 the same numerals as in FIG. 2, however, with two primes added, have been applied to indicate like parts asin FIG. 2. Hence FIG. 3 does not call for a detailed description in regard to thestructure illustrated therein. Numeral 8" has been applied to indicate the ends of a cable inserted into tubular cable connector means 4" forming integral parts of copper plugs 3". T indicates the temperature distribution along the longitudinal axis of "the device whenit'he latter is carrying its rated current. T indicates the temperature distribution along the longitudinal axis of the device when carrying a currentwhich-if supplemented by the heating action of cables 8-is' sufiicieritly high to bring the hot-point of the structure to the fusing point of cadmium. When the hot-point of the structureraches the fusing point of cadmiumthe temperature of the insulation of cable 8 is way ferent. This difference. is particularly apparent when comparing the rise of temperature on blade contacts 4' with the rise of the temperature at the cable connectors 4". As shown in FIG. 2 an increase of the load current from the rated current to that current which causes blowing by a metallurgical reaction results in a very small rise in temperature at the fuse holder contacts 7'. FIG. 3 shows that a rise of the load current from the rated current to the current required to cause blowing of the cable protector by a metallurgical reaction results in a very high rise in temperature at the cable connectors 4". This difference is of considerable significance as will be explained below. V

Referring again to FIG. 3, reference character T has been applied to indicate the temperature distribution along the axis of the device when carrying a current that is sufliciently low not to result in blowing of the fuse even though extending over a long period of time. .That particular current is, however, sufliciently high, to bring the terminal blocks 3" above the temperature of 231.8 deg. C., the fusing temperature of tin. If tin or a similar low fusing point solder were used to conductively. connect fuse links 5" to terminal blocks 3", this would result in progressive ageing of these solder joints and ultimately in formation of breaks atthe points where thesefsolder joints are located. Therefore it is not permissible as it is in case of conventional current-limiting fuses-to use tin solder for connecting the fuse links to. the terminal blocks. This must be achieved by means of a solder having a much higher fusing point than tin, preferably in the order of 300 deg. C. Cadmium silver solder serves this purpose well. Such a solder may have a fusing point in the order It will be noted from curve T that the solder joints between fuse links 5" and copper blocks or copper plugs 3" reach also a temperature in excess of the fusing point of tin when the current carried by the device is sufficiently high to bringoverlays 6" to the fusing point of cadmium, thus causing blowing of the cable protector. During the interrupting process the points of the fuse link where overlay 6" is located will exceed the fusingjpoint of cadmium and the solder joints between links 5- and plugs 3 may then exceed the fusing point of the solder of which these solder joints are made. A softening or even the fusion of the solderjoints between parts 5 and 3" incident to blowing has no tendency to impair the sound operation of the device, whereas the gradual deterioration of these solder joints by currents not sufficiently high to cause fusion of overlays 6" is likely to resultin a dangerous condition and, therefore, must be precluded by allmeans; l 7 Referring now to FIGS. 4 and 5-,. numeral 21 has been applied to indicate a tubular casing of a siliconefresinglass-cloth laminateclosed on the ends thereof by a pair of press-fitted copper plugs 23 held firmly in position-by transverse steel pins 29. Ajpair of tubular cable. connec tors 24 form integral parts of copper plugs 23. Cable connectors 24 may be silver brazed to copper plugs 23. To this end the axially outer surfaces of plugs 23 are provided with relatively wide circular grooves 33 into which cable connectors 24 are inserted. Cables 28 have bare ends inserted into .cable connectors 24 and held therein hy indentations 30 produced by an appropriate hydraulic press tool. The axially inner surfaces 31 of copper plugs 23 are pro yided with systems of relatively narrow radial grooves 32 into which multi-perforated ribbon typefuse' links 25 of silver are inserted. The axially outer ends of fuse links 25 are soldered to plugs 23; by means of joints (not

Claims (1)

1. A FUSIBLE PROTECTIVE DEVICE FOR THE PROTECTION OF CABLE NETWORKS COMPRISING A TUBULAR CASING OF HEAT-RESISTANT INSULATING MATERIAL; A PULVERULENT ARC-QUENCHING FILLER WITHIN SAID CASING; A PAIR OF TGERMINAL ELEMENTS CLOSING THE ENDS OF SAID CASING, EACH OF SAID PAIR OF TERMINAL ELEMENTS INCLUDING A BLOCK OF SOLID METAL; TUBULAR CABLE CONNECTOR MEANS ARRANGED IN COAXIAL RELATION TO SAID CASING FORMING AN INTEGRAL PART OF AT LEAST ONE SAID PAIR OF TERMINAL ELEMENTS; MULTIPERFORATED RIBBON FUSE LINK MEANS OF A METAL HAVING A FUSING POINT IN THE ORDER OF 1000 DEG. C. SUBMERSED IN SAID FILLER ADN CONDUCTIVELY INTERCONNECTING SAID PAIR OF TERMINAL ELEMENTS; OVERLOAD-RESPONSIVE FUSE-LINK-SEVERING MEANS OF A METAL HAVING A FUSING POINT IN THE ORDER OF 300 DEG. C. ARRANGED ADJACENT THE CENTER OF SAID FUSE LINK MEANS FOR SEVERING UPON FUSION THEREOF BY A METALLURGICAL REACTION THE CURRENT-PATH THROUGH SAID FUSE LINK MEANS, AND SOLDER JOINTS OF A METAL HAVING A FUSING POINT IN THE ORDER OF 300 DEG. C. FOR ESTABLISHING CURRENT PATHS OF LOW RESISTANCE BETWEEN SAID PAIR OF TERMINAL ELEMENTS AND SAID FUSE LINK MEANS.

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