US3365616A - Overvoltage protection system for electrical apparatus with current limiting fuse and removable oil fuse in series on line side - Google Patents

Description

PPARATUS E HERMANN FO US Jan. 23, 1968 CAL A MOVABL G. OVERVOLTAGE PROTEQTIO YSTEM R ELEO WITH CURRENT LIMI C: J E AND OIL FUSE IN 1135 ON LINE SIDE 5 Sheets-Sheet 1 Filed April 1, 1965 h-i nh W R Jan. 23, 1968 G. P. HERMANN 3,365,616

OVERVOLTAGE PROTECTION SYSTEM FOR ELECTRICAL APPARATUS WITH CURRENT LIMITING FUSE AND REMOVABLE OIL FUSE IN SERIES ON LINE SIDE Filed April "1, 1965 3 Sheets-Sheet 2 '60 K/KUIPRM/H/M/W/VG Fl/ff I I HSfCO/VPS I T/ME raw/m kcum/vrz/m/r/w/z/sz m Jim pm F055 3,365,616. CAL APPARATUS OVABLE J 1968 G. P. HERMANN OVERVOLTAGE PROTECTION SYSTEM FOR ELECTRI WITH CURRENT LIMITING FUSE AND REM OIL FUSE IN SERIES ON LINE SIDE- Filed April 1, 1965 3 Sheets-Sheet 3 [/2 @2255 Y G m/ Fflmwm 3,365,616 GVERVOLTAGE PRGTECTTON SYSTEM FGR ELECTRICAL APPARATUS WITH (JURRENT LHMETING FUSE ANB REMOVAELE 01L FUSE EN SERIES N LINE SIDE Gerald l Hermann, Pittstield, Mass, assignor to General Electric Company, a corporation of New York Fiierl Apr. 1, 1965, Ser. No. 444,535 5 Ciaims. ((31. 317-15) ABSTRACT OF THE DISCLQSURE An overvoltage protection system for electrical apparatus using serially connected fuses on the line side. A current limiting fuse is provided mounted in the high voltage or line bushing of the electrical apparatus. Connected in series with the current limiting fuse and the high voltage winding of the electrical apparatus is a low current oil fuse, removably mounted within the electrical apparatus. The low current oil fuse is connected in a removable fuse assembly which may be inserted in and removed from a well in the side wall of the tank of the electrical apparatus. Terminals are provided on the fuse weFl such terminals being connected to the current limiting fuse and to the high voltage Winding. Contacts are provided on the removable fuse assembly with the low current oil fuse connected between such contacts. The contacts make electrical connection with the terminals on the fuse well. The fuse assembly is so constructed that the fuse assembly must be locked within the fuse well prior to an electrical connection being made between the fuse well terminals and the fuse assembly contacts. The current limiting fuse and the low current oil fuse are coordinated such that the low current oil fuse will rupture on current faults equal to or less than the short circuit current of the electrical apparatus while the current limiting fuse will rupture on all current faults above the short circuit current of the electrical apparatus.

This invention relates to an electrical apparatus and more particularly to an overcurrent protection system for electrical apparatus.

As is well known to those skilled in the electrical apparatus art, it is desirable to provide protection for electrical apparatus, such as distribution transformers and the like, to prevent excessive currents from permanently damaging such apparatus. Excessive currents can occur in such apparatus due to overloading of the apparatus or due to internal faults that develop within the apparatus. Further, it is desirable that excessive currents from whatever cause be alleviated at the electrical apparatus subjected to them. If not so alleviated, the continued functioning of the apparatus will cause operation of various protective devices along the power line which energizes the electrical apparatus. The operation of these remote protective devices will generally cause disruption of service to areas other than those served by the electrical apparatus subjected to the excessive electrical currents.

In order to provide the desired protection of electrical apparatus, the usual practice is to provide a fuse device in the high voltage circuit of the apparatus and a circuit breaker in the low voltage circuit. As is pointed out in Patent No. 3,112,383 it is difiicult to properly coordinate the action of the high voltage fuse and the low voltage circuit breaker to adequately protect the apparatus from the various types of faults which may occur in such apparatus. To overcome such difficulties it has been proposed that a high voltage circuit breaker be combined with a high voltage fuse in the manner set out in Patent No. 3,112,383.

As is pointed out in that patent, in the case of a fault in the electrical apparatus the current will, in general, exceed the design limits of the transformer or electrical apparatus and will be such as to cause either the circuit breaker or the fuse to be activated to rte-energize the electrical apparatus. In the coordinated systems, as discussed in Patent No. 3,112,383, usually the fuse device on the high voltage side of the electrical apparatus is designed to operate in those instances where the current of the electrical apparatus exceeds the short circuit current of the electrical apparatus. Patent No. 3,112,383 notes that various types of electrical apparatus have a certain amount of inherent impedance due to the inductance and resistance of their windings. Thus if the secondary windings of such apparatus is shorted, only a given maximum current will flow in the primary windings when the rated voltage is applied to such windings. This maximum current is termed the short circuit current of the apparatus. However, in cases of fault in the apparatus, for example between the high voltage winding and ground, some or all of the windings may be by-passed depending upon the location of the fault. For these types of faults the current is not limited by the short circuit impedance of the windings and may reach magnitudes greatly in excess of the short circuit current of the apparatus. For faults of this nature it is desirable to remove the electrical apparatus from the line immediately, preferably by activation of the high current limiting fuse. However, for lesser secondary shorts or high overloads on the transformer which do not exceed the short circuit current of the transformer it is desired that another device be operative to remove the transformer from the line, such as, for example, the circuit breaker which is specified in Patent No. 3,112,383.

The circuit breaker and fuse combination of Patent No. 3,112,383 has been very successful in providing coordinated protection for electrical apparatus. However, as is well known by those working in the electrical field, there is a constant search in this field to obtain equal or better protection of apparatus at lower cost. It has recently been discovered that fuse devices may be combined in a protection system for electrical apparatus with coordinated protection at least equal to that obtainable with the fuse and breaker system, when both fuses are connected in series in the high voltage side of the electrical apparatus. A particular advantage of the combined fuse protection system is the lower cost of the system due to the elimination of the costly circuit breaker. As is well understood a fuse device is a much simpler mechanical mechanism than a circuit breaker and may be produced at relatively lower cost. A further advantage obtained by using a combined fuse protection system is that by making the low order current fuse removable the protection system can be set for higher current operation in emergencies merely by using a larger capacity fuse. A removable fuse may also be used as a load break switch to de-energize the electrical apparatus.

It is, therefore, one object of this invention to provide a protection system for electrical apparatus using a combination of fuse devices in the high voltage side of an electrical apparatus.

A further object of the invention is to provide a combined fuse protection system for an electrical apparatus such that one fuse will be effective on low order current faults of the electrical apparatus, while the other fuse will be properly coordinated to be effective on high current faults of the electrical apparatus.

A further object of this invention is to provide a combined fuse protection system for electrical apparatus where the low current fault fuse may be readily removable.

A still further object of this invention is to provide a combined fuse protection system which will automatically remove the primary current to an electrical apparatus whenever such current exceeds a predetermined safe value.

In carrying out this invention in one form a combined fuse protection system is provided for an electrical apparatus comprising a current limiting fuse for connection directly to the high voltage line which energizes the electrical apparatus. A low current fuse is provided connected electrically in series between the current limiting fuse and the high voltage winding of the electrical apparatus. The low current fuse is removably mounted in the electrical apparatus and both fuses are coordinated such that the low current fuse will rupture on low order current faults, while the current limiting fuse will rupture on high order current faults.

The invention which it is desired to protect will be particularly pointed out and distinctly claimed in the claims appended hereto. However, it is believed that this invention and the manner in which its various objects and advantages are obtained, as well as other objects and advantages thereof, will be better understood from the following detailed description of a preferred embodiment thereof especially when considered in connection with the accompanying drawings, in which:

FIGURE 1 is a diagrammatic illustration, partially in section, of a transformer device showing one form of this invention connected thereto;

FIGURE 2 is a representative curve showing the desired time-current characteristics of the combined fuse protection system according to this invention;

FIGURE 3 is a schematic diagram of the electrical circuit of the transformer of FIG. 1;

FIGURE 4 is a schematic diagram of the electrical circuit of a modified transformer with the protection system of this invention applied thereto;

FIGURE 5 is a partial sectional view of an alternate form of a low current fuse assembly according to this invention; and

FIGURE 6 is an exploded perspective view of a portion of the fuse assembly shown in FIG. 5.

Referring now to the drawings in which like numerals are used to indicate like parts throughout, there is shown a combined fuse protection system connected in series with an electrical apparatus such as a transformer for protecting such apparatus from overloads and fault currents. While the invention will be specifically shown and described with reference to the transformer as shown in FIG. 1, it will be understood that it is not limited to transformers but may be applied to all types of electrical apparatus.

Referring specifically to FIG. I of the drawings, there is shown an electrical apparatus such as transformer 10 which comprises a core and coil unit 12 mounted in a tank member 14, the tank member 14 being sealed by a cover member 16. The interior of tank 14 is filled with a dielectric fluid, indicated at 18, which completely covers the core and coil unit 12, as shown. A high voltage bushing member 219 is mounted on the tank cover 16 and extends into the interior of the tank, as shown. As will be understood the upper end of bushing 20 is provided with a combined cap and terminal member 22 to which will be connected the source of electrical energy (not shown) which is to be applied to the electrical transformer 10.

A preferred form of the cambined fuse protection system of this invention is shown in FIG. 1 as comprising a current limiting fuse 24 electrically connected in series with a low current fuse 28. Current limiting fuse 24 is mounted in the high voltage bushing 20. One end of current limiting fuse 24- is connected electrically to the upper cap and terminal 22. The other end of current limiting fuse 24 is spring connected to a second terminal 25 on the lower end of the high voltage bushing 20. The current limiting fuse 24 may be a current limiting fuse of any desired type, such as the type disclosed in applica- 4 tion Ser. No. 127,608 which was filed July 28, 1961 in the name of Sidney R. Smith, Jr. and which is assigned to the same assignee as this invention.

Connected in series with the current limiting fuse 24 is a low current fuse device 23 connected between terminals 30 and 32 which are part of a fuse holder 34. Fuse holder 34 may be mounted in transformer 14) in any desired manner. FIGURE 1 shows fuse holder 34 attached to mounting means 35, which, in turn, is attached to wall 14. As can be seen from FIG. 1, a lead 36 from terminal 26 of the high voltage bushing 20 extends to terminal 30 in the fuse holder 34 and a second lead 38 extends from the terminal 32 of fuse holder 34 to the high voltage winding of the core and coil unit 12 of transformer 10. As will be understood, it is desirable that the low voltage fuse 28 be a removable fuse member such that it may e readily replaced when ruptured and may also be removed and replaced with a higher current fuse to provide emergency high current setting of the transformer ltl, when desired. As will be apparent, any method of removably mounting fuse member 28 in series between the current limiting fuse 24 and the high voltage winding of the electrical apparatus may be utilized. For example, the fuse member 28 may be mounted within the fuse holder 34 by means of a bayonet joint or it may be mounted by means of a thread type connection or it may be merely a friction connection, as desired.

As is shown in FIG. 1 of the drawing, the fuse member 28 is mounted in a fuse holder 34 provided with the terminals 30 and 32 thereon. The fuse 28 extends between terminals 30 and 32. Fuse 28 may be for example a cylindrical type fuse which may be mounted in any desired manner in the lower end of the fuse assembly 40. The fuse assembly 4% is mounted within a well or chamber 42 which opens through the tank wall 14 of transformer 10, in the manner indicated. Of course, as will be understood, the well or chamber 42 may be made in the cover of the device, rather than the wall, if desired. The fuse assembly 40 extends through the fuse holder 34 and in contact with the terminals 30, 32 on such fuse holder. In order to provide firm electrical contact, the lower terminal 32 may be spring mounted in the base portion of fuse holder 34, such as by spring 44, as indicated. As earlier mentioned the upper contact of the fuse 28 may make contact with terminal 30 either by means of a bayonet joint, a threaded connection or a friction connection as desired. At any rate, as will be understood, the fuse assembly 46, including fuse 28, may be readily removed from fuse holder 34 and the well 42 merely by removing the cap 46 on the fuse well and removing the entire fuse assembly 40. The cap 46 may be secured to the top of well 42 in any desired manner such as by a friction connection or a threaded connection as desired. From the above description it will be apparent that there is provided in the electrical apparatus 10 a combined fuse protection system which includes the current limiting fuse 24 connected to the line energizing the electrical apparatus 10 and connected in series therewith is the low current fuse device 28 which in turn is serially connected to the high voltage winding of the electrical apparatus. 0f course, it will be understood that when the electrical apparatus 10 is subjected to current faults of a low order, that is, current faults which do not exceed the short circuit current of the electrical apparatus, that the fuse device 28 will rupture. However, when the electrical apparatus is subjected to high order current faults, that is, those current faults which exceed the short circuit current of the electrical apparatus then the current limiting fuse 24 will blow prior to the rupturing of the fuse member 28. As will be understood the fusible member of the fuse 28 is designed such that it will coordinate with the current limiting fuse 24 so that a sufficient time will elapse before the rupturing of the fuse 28, allowing the current limiting fuse 24 to rupture on any high order current fault.

Referring now to FIG. 2 of the drawing there is shown a representative curve showing the type of current characteristics of the current limiting fuse and the low current fuse. As shown in FIG. 2, line 50 is the operating curve of the current limiting fuse of the combined fuse protection system. As can be seen the current limiting fuse operates at substantially high currents over relatively long time periods. As is apparent from curve 50, as the current exceeds this high value, that is, attains a value which will exceed the short circuit current of the electrical apparatus, the current limiting fuse will operate in an extremely short period of time. Curve 52 is the curve of the low current fuse when the oil of the transformer is cold. As can be seen, for low order currents the fuse member will operate after a long period of time. As the current increases it can be seen that the fuse will rupture in shorter and shorter periods of time. The fuse line 52 crosses the fuse line 50 of the current limiting fuse at a point indicated at 54 in the drawing. At this crossover point the low current fuse will operate in substantially the same period of time as the current limiting fuse to remove the transformer from the line. However, at higher currents it will be seen that the curve 52 extends higher than the curve 50. Thus, the current limiting fuse will operate much faster than the low current fuse for any currents beyond those of the crossover point 54. As will be understood, in designing the current limiting fuse and the low current fuse it is desired to have the cross-over point substantially equal to the short circuit current value of the electrical apparatus utilizing the combined fuse protection system. Therefore, at any value of overcurrent less than the short circuit current of the electrical apparatus the low current fuse member will blow or rupture in accordance with the curve line 52. However,'when the value of current exceeds that at the cross-over point 54, which is substantially the short circuit current of the electrical apparatus, then the current limiting fuse will rupture thereby removing the electrical apparatus from the energized line.

A third curve 56 is shown in FIG. 2. This curve shows the operation of the low current fuse when the oil in the transformer or electrical apparatus is hot. As can be seen, the fuse device will rupture for low order currents in a shorter period of time when the dielectric of the electrical apparatus is hot. This provides an additional safe-guard in the electrical apparatus in that when the dielectric of the electrical apparatus becomes overheated it is normally an indication of some fault in or overload on the apparatus. Therefore, it is desired that the low current fuse device operate more rapidly to remove such electrical apparatus from the energized line.

FIGURE 3 is a schematic diagram of the electrical circuit of the transformer of FIG. v1. As can be seen the current limiting fuse 24 is connected at one end to the high voltage line indicated at 60 and the opposite end is connected to the low current fuse 28, which is readily removable from the transformer. Low current fuse 28 in turn is connected to the high voltage winding of the core and coil unit 12.

FIGURE 4 shows a similar type schematic diagram modified for use with a transformer utilizing two high voltage bushings. As shown in FIG. 4 of the drawing, when both sides of the high voltage winding are connected to an energized line, such as indicated at 62 and 64, a pair of current limiting fuses are provided. The current limiting fuse 24a in one high voltage bushing is connected to line 62 and the current limiting fuse 24b mounted in the other high voltage bushing is connected to the energized line 64. A low current fuse, such as removable fuse 28, is connected in series with the high voltage winding 24a and the high voltage winding of the core and coil unit 12a as indicated. In the other line a terminal board oil fuse 66 is provided in series with the current limiting fuse 24b and the other side of the high voltage winding of the core and coil unit 12a in the manner shown. This terminal board oil fuse 66 will be a fuse operating at a substantially higher current and/or temperature than that of the low current fuse 28. Considering the curves of FIG. 2, the curve of the fuse 66 would fall between the line 52 and 50. Therefore, under all normal circumstances the terminal board fuse 66 would not rupture on either the high or low order current faults. However, in the event of an unusual fault in a portion of the high voltage winding, then the terminal board fuse 66 would rupture.

Referring now to FIGS. 5 and 6, there is shown a modification of the removable low current fuse assembly. The low current fuse assembly shown in FIGS. 5 and 6 is especially useful where it is desired to use the fuse assembly as a load break switch. As will be understood by those skilled in the art, when a removable member, such as the removable fuse assembly, is used to break load, it is desirable that the removable member be in a latched position at the time the contacts are made or broken. This prevents any gas evolved during operation from forcing the removable member out of the electrical apparatus. In some instances such as fuse blowing during operation the gas generated may be such as to force the removable member out as a projectile, endangering the person operating the device. The modification shown in FIGS. 5 and 6 provides secure latching of the removable fuse assembly during operation of the contacts.

As shown in FIG. 5 the fuse assembly is indicated as 40a, comprising an insulated fuse body 70 having a fuse 28a secured in the lower end of body 70. A pair of contacts 72, 74 are provided on the outer surface of body 70, as shown, and fuse 28a is connected between these contacts. Fuse assembly 40a fits within the fuse holder 34a which will be mounted within the electrical apparatus in any desired manner, for example by a mounting means such as is shown in FIG. 1. Of course, it will be understood, that the fuse assembly will be readily insertable into and removable from fuse holder 34a, through an opening in the electrical apparatus, such as fuse well 42 of FIG. 1.

Fuse holder 34a is provided with a pair of contacts 30a, 32a, which are connected to lead wires 36, 38, respectively. As will be apparent, contacts 72 and 74 on body 74 are positioned on body 70 so as to be movable into electrical contact with contacts 30a, 32a, respectively.

The upper end of fuse holder 34a is provided with a pair of slots 76, 78 which open into an internal groove 80, as is most clearly shown in FIG. 6. The fuse assembly 40a is provided with a pair of lug members, 82, 84, extending from the outer surface of fuse body 70. As can be seen from FIG. 6, lugs 82, 84 are positioned on body 70 to mate with slots 76, 78 when fuse assembly 40a is inserted into fuse holder 34a. The contacts 72, 74 are positioned on fuse body 70, with respect to lugs 82, 84, so that when fuse assembly 40a is inserted into fuse holder 34a, and lugs 82, 84, enter slots 76, 78, contacts 72, 74, will not establish a connection with the contacts 30a, 32a. However, when lugs 82, 84 are seated in groove 80, contacts 72., 74, will be in line with, although out of contact with the respective contacts 30a, 32a, as is clearly shown in FIG. 5.

As fuse assembly 40a is rotated within fuse holder 34a, lugs 82 and 84 will move into groove and out of the path of slots 76, 78. The first few degrees of rotation of fuse assembly 40a will therefore latch fuse assembly 40a into the fuse holder 3411. However, at this position contacts 72, 74 will still be out of contact with the contacts 30a, 32a. It is preferred that electrical connection be made between contacts 72, 74 and 30a, 32a after fuse assembly has been rotated approximately However, as will be understood the electrical connection could be made with a smaller or larger rotation of the fuse assembly 40a.

In order to insure a firm connection between the contacts 72, 74 and 30a, 32a, one or more stop means may be provided in groove 80. In FIGS. and 6 one stop means is shown as stop 86 in groove 80. As will be understood, stop 86 is so positioned that when engaged by lug 82;, an electrical connection will be made from contact 30a to contact 72, through fuse 28a, and contact 74 to contact 32a. Of course, it will be clear that when fuse assembly 40a is rotated to break the electrical connection that the fuse assembly 40a will remain latched within fuse holder 3411 until lugs 82, 84 are again in line with slots 76, 78. As will also be clear, means may be provided in groove '80 to insure rotation of fuse assembly 40a only in one direction. If desired, such means could be in the form of two partial grooves, or means could be provided in the groove in line with one side of slots 76, 78.

As shown in FIGS. 5 and 6 a knurled member 88 is provided for inserting, removing and rotating of fuse assembly 40a. Since the fuse holder 34a and fuse assembly 40a are preferably made of insulating material the fuse assembly may be directly manipulated by the lineman or repairman. However, if desirable, a hook for use with a switch stick could be provided in place of member 88.

While there has been shown and described the present preferred embodiment of this invention and a modification thereof, it will be readily understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention, particularly as it is defined in the appended claims.

What is claimed as new and which it is desired to secure by Letters Patent of the United States is:

1. An overcurrent protection system for electrical apparatus having a core and coil unit immersed in a liquid dielectric and at least one high voltage bushing attached to a tank enclosing the core and coil unit, said overcurrent protection system comprising; a current limiting fuse connected electrically in the high voltage bushing, an oil fuse connected electrically in series between said current limiting fuse and a high voltage winding of the core and coil unit, a fuse holder attached to the tank and extending therethrough, said oil fuse removably mounted within said fuse holder, said cur-rent limiting fuse and said oil fuse being electrically coordinated such that said oil fuse will rupture on current faults less than the short circuit current of the electrical apparatus, while said current limiting fuse will rupture on current faults greater than the short circuit current of the electrical apparatus.

2. An overcurrent protection system for electrical apparatus having a core and coil unit immersed in a liquid dielectric and at least one high voltage bushing attached to a tank enclosing the core and coil unit, said overcurrent protection system comprising; a current limiting fuse connected electrically in the high voltage bushing, an oil fuse connected electrically in series between said current limiting fuse and a high voltage winding of the core and coil unit, an opening in the tank, a fuse holder mounted in said opening, said oil fuse mounted in said fuse holder, cooperating means on said oil fuse and said fuse holder for removably retaining said oil fuse in said fuse holder, said current limiting fuse and said oil fuse being electrically coordinated such that said oil fuse will rupture on low order current faults while said current limiting fuse will rupture on high order current faults.

3. An overcurrent protection system for electrical apparatus having a core and coil unit immersed in a liquid dielectric and at least one high voltage bushing attached to a tank enclosing the core and coil unit, said overcurrent protection system comprising; a current limiting fuse connected electrically in the high voltage bushing, and oil fuse connected electrically in series between said current limiting fuse and a high voltage winding of the core and coil unit, a fuse holder attached to the tank and extending therethrough, contacts on said fuse holder electrically connected to said current limiting fuse and said core and coil unit, contacts on said oil fuse for making an electrical connection with said fuse holder contacts, cooperating means removably securing said oil fuse in said fuse holder, said cooperating means acting to secure said fuse in said fuse holder prior to the making of said electrical connection between said fuse holder contacts and said fuse contacts, said current limiting fuse and said oil fuse being electrically coordinated such that said oil fuse will rupture on low order current faults while said current limiting fuse will rupture on high order current faults,

4. An electrical apparatus with a current limiting fuse and an oil fuse in combination for providing coordinated protection to the electrical apparatus comprising:

(A) An enclosed tank for an electrical apparatus (B) a fuse assembly including at least one bushing (a) said bushing secured to said tank and extending therethrough,

(b) a first terminal on one end of said bushing for connecting to a source of high voltage electrical energy,

(c) a second terminal on the other end of said bushing (d) a current limiting fuse member mounted in said bushing and electrically connected between said first and second terminals (c) said current limiting fuse rupturing on high current faults of the electrical apparatus (C) a second fuse assembly including a fuse holder,

(a) said fuse holder secured to said tank and extending therethrough (b) a first terminal on said fuse holder within said tank,

(c) a second terminal on said fuse holder within said tank (d) a removable fuse member extending through said tank in said fuse holder,

(1) first and second contact means on said fuse member respectively electrically engaging said first and second terminals on said fuse holder,

(2) an oil fuse link in said fuse member between said first and second contact means,

(e) said first terminal on said fuse holder electrically connected to said second terminal on said bushing,

(f) said second terminal on said fuse holder connected to a high voltage winding of the electrical apparatus,

(g) said oil fuse link rupturing on low current faults of the electrical apparatus,

whereby said current limiting fuse and said oil fuse will provide coordinate protection to the electrical apparatus enclosed by said tank.

5. An electrical apparatus with a current limiting fuse and an oil fuse in combination for providing coordinated protection to the electrical apparatus comprising:

(A) An enclosed tank for an electrical apparatus (B) a fuse assembly including at least one bushing (a) said bushing secured to said tank and extending therethrough,

(b) a first terminal on one end of said bushing for connecting to a source of high voltage electrical energy,

(c) a second terminal on the other end of said bushing (d) a current limiting fuse member mounted in said bushing and electrically connected between said first and second terminals (e) said current limiting fuse rupturing on high current faults of the electrical apparatus (C) a second fuse assembly including a fuse holder,

(a) said fuse holder secured to said tank and extending therethrough (b) a first terminal on said fuse holder within said tank,

(c) a second terminal on said fuse holder within said tank (d) a removable fuse member extending through said tank in said fuse holder,

(1) first and second contact means on said fuse member for electrically engaging said terminals on said fuse holder,

(2) an oil fuse link in said fuse member between said first and second contact means,

(e) cooperating means on said fuse holder and said fuse member for removably securing said fuse member in said fuse holder,

(1) said cooperating means acting to secure said fuse member in said fuse holder before electrical engagement of said fuse member contacts with said fuse holder terminals,

(f) said first terminal on said fuse holder electrically connected to said second terminal on said bushing,

(g) said second terminal on said fuse holder connected to a high voltage Winding of the electrical apparatus,

(h) said oil fuse link rupturing on low current 5 faults of the electrical apparatus,

whereby said current limiting fuse and said oil fuse Will provide coordinate protection to the electrical apparatus enclosed by said tank.

References Cited MILTON O. HIRSHFIELD, Primary Examiner. 20 R. V. LUPO, Assistant Examiner.

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