Electrical bushing having a replaceable stud

An electrical bushing includes a replaceable stud which connects the electrical bushing which is mounted on the housing for an electrial distribution unit to electrical equipment located outside the housing. The replaceable stud includes a first stud shank connectable to a conductor within the body of the electrical bushing and which is stronger than a second stud shank connectable to the electrical equipment located outside the housing. The replaceable stud, if damaged, can be removed from the bushing body without otherwise disturbing the bushing body or any connections between the electrical bushing and equipment positioned inside the housing.

TECHNICAL FIELD 
The present invention relates, in general, to power distribution equipment, 
and, more particularly, to insulation terminators used with said 
equipment. 
BACKGROUND ART 
Electrical bushings are generally used with electrical distribution 
equipment, such as a transformer having a transformer housing which may be 
filled with oil and a transformer coil unit located with the transformer 
housing. Generally, electrical bushings include a bushing body affixable 
to a wall of a housing for the electrical distribution equipment, and an 
electrical conductor secured to the bushing body to be rigidly affixed 
with the bushing body. One end of the electrical conductor is located 
inside the housing to be connected by a suitable lead to internal 
electrical components, such as a transformer coil unit. The other end of 
the electrical conductor is generally located outside the housing to be 
disposed for interconnection with an electrical line for electrical 
equipment located externally of the housing. 
Electrical bushings generally are of either a male type or female type, 
with the female type electrical bushing having a hollow interior portion 
adapted to receive a male electrical terminal for interconnection to 
external electrical equipment. Electrical bushings of the female type are 
often referred to as bushing wells. 
Conversely, male electrical bushings are not provided with connector 
terminals recessed within a well but generally the bushing and terminal 
project from the wall of a housing. 
Electrical bushings used with power distribution equipment of various types 
such as transformers, switchgear, switches and the like have always been 
susceptible to damage during handling, shipping, installation and 
maintainance operations. In order to minimize the likelihood of damage and 
to provide an economical electrical bushing for transformer applications 
which reduces the possibility of transformer oil leaking from the 
transformer housing around the electrical bushing, electrical bushings 
have been permanently mounted, as by welding, to the transformer wall. 
Such permanent mounting provides a sealed feedthrough connection for 
interconnecting the transformer unit to the electrical equipment located 
outside the transformer housing and also provides a secure mounting for 
the electrical bushing on the transformer wall. 
While permanently mounting electrical bushings has prolonged the life of 
these bushings, the breakage and damage problem is still a substantial 
one. Bushing terminal breakage or damage typically occurs in the field 
during installation of a transformer, or similar power distribution 
equipment, and normally, damaged bushings cannot be properly replaced in 
the field. This means that the entire unit must often be returned for 
repair. 
If a damaged electrical bushing is cut from a transformer in the field, 
access to the interior of the transformer housing must be attained to 
electrically connect a newly installed bushing. Furthermore, the process 
used to permanently mount the new electrical bushing on the transformer 
wall may occasionally not completely seal the transformer housing to the 
electrical bushing. A leakage problem will thereby be created, and such 
leakage may not be detected until the final stages of mounting the 
electrical bushing. 
Removably mounted electrical bushings have been developed to facilitate the 
repair or replacement of damaged bushing components in the field. Such 
removable bushings are disclosed by U.S. Pat. Nos. 3,790,698 to F. O. 
Engert; 3,803,523 to D. J. Farmer et al and 3,909,509 to J. L. Fisher. 
These prior art bushing assemblies may be easily removed from a 
transformer wall in the field for repair or replacement without requiring 
that the bushing be cut from the transformer. However, for the repair of a 
relatively small bushing component, such as a broken terminal stud, the 
entire bushing must be removed. This often requires new electrical 
connections to be made with internal transformer components, and if the 
new bushing is not carefully sealed when it is mounted on the transformer 
wall, the likelihood of leakage around the bushing will be increased. 
Since the terminal stud of an electrical bushing is the small component 
most likely to be damaged in the field, a need has arisen for a bushing 
construction wherein the terminal stud may be easily removed and replaced 
in the field without requiring the removal of the bushing from the housing 
of a power distribution unit. 
BRIEF DESCRIPTION OF THE INVENTION 
It is a primary object of the present invention to provide a novel and 
improved electrical bushing assembly for power distribution equipment for 
electrically connecting internal electrical components to external 
electrical assemblies. This electrical bushing assembly incorporates a 
removable stud type terminal which is electrically connected by an 
integral bushing conductor structure to internal electrical components 
when the bushing assembly is mounted in place on the housing for the power 
distribution equipment. 
Another object of the present invention is to provide a novel and improved 
electrical bushing assembly for a transformer which incorporates a 
removable stud type electrical terminal. This electrical terminal may be 
easily removed from the remainder of the bushing assembly in the field and 
replaced without disturbing the electrical connection or the seal between 
the transformer and the remainder of the bushing assembly. 
A further object of the present invention is to provide an electrical 
terminal stud for an electrical bushing assembly used on power 
distribution equipment which is adapted for use with either male or female 
bushing assemblies. Such electrical terminal stud is adapted to be easily 
removed and replaced without requiring the removal of the bushing assembly 
from the power distribution equipment of the making or breaking of 
electrical connections between the bushing assembly and power distribution 
equipment. 
A still further object of the present invention is to provide an electrical 
terminal stud for a transformer electrical bushing assembly which may be 
easily removed and replaced. The terminal stud includes two colinear shank 
sections separated by a flange unit which includes a structure adapted to 
be engaged buy a stud removal tool. One shank section is formed to have 
greater strength than the remaining shank section so that the weaker shank 
section will always be the first to break when the stud is subjected to 
stress. 
Yet another object of the present invention is to provide a novel 
replaceable terminal stud for use as part of a transformer electrical 
bushing, with the replaceable stud including two stud shanks which are in 
colinear relationship. One stud shank is formed to a larger diameter than 
the other stud shank so that the smaller stud shank will be the first to 
break under stress. The larger stud shank has screw threads thereon which 
mate with threads formed in the electrical conductor of the electrical 
bushing to facilitate easy stud replacement on and removal from the 
electrical bushing. 
These and other objects are accomplished by an electrical bushing embodying 
the present invention which is used with a transformer or other power 
distribution equipment and which can be permanently or releasably mounted 
on the wall of a housing for the equipment. 
The electrical bushing includes a bushing body having an electrical 
conductor rigidly secured therein with a first end located inside the 
power distribution equipment housing and a second end located outside the 
power distribution equipment housing when the electrical bushing is 
mounted on the housing. A replaceable stud is removably engaged with the 
electrical bushing body and serves to associate the electrical bushing 
with electrical equipment located outside the housing. This replaceable 
stud includes a first stud shank which engages the electrical bushing body 
and the electrical conductor to make electrical contact with the 
electrical conductor, and a second stud shank which is used to connect the 
replaceable stud with external electrical equipment. The two stud shanks 
are colinear, and a flange is affixed to the two stud shanks at the 
juncture therebetween. The first stud shank is formed to be of greater 
strength than the second stud shank. This may be accomplished by forming 
the first stud shank with a diameter larger than that of the second stud 
shank so that the second stud shank will break first under stress. The 
flange includes tool engaging means for engaging a tool used to attach and 
detach the replaceable stud.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring to FIG. 1, an electrical bushing 10 of the present invention is 
shown in position on the wall 12 of the housing for a power distribution 
unit, such as a transformer (not shown). 
The electrical bushing includes a unitary, one-piece bushing body 14, which 
can be formed of a suitable synthetic resin or other electrical insulating 
material and which extends through a bushing receiving opening 16 defined 
in the transformer wall 12. This bushing body is elongate and extends from 
an outer end 18 located outside transformer wall 12 to a body inner end 20 
located inside the transformer wall. The electrical bushing 10 is of the 
female type having an interior cavity 22 defined therein which is adapted 
to receive a male electrical terminal for connecting the electrical 
bushing 10 with exteriorly located electrical equipment. The cavity 22 
opens outwardly from the outer end of the bushing body 14, and the bushing 
body is formed to provide a boss 24 within the cavity 22. The boss 24 is 
located to project into the interior cavity 22 from a cavity end wall 26, 
and this boss terminates within the cavity at a terminal wall 28. An 
electrical conductor 30 is located within the bushing body and extends 
from a position adjacent the terminal wall 28 of the boss 24 through the 
boss and the remainder of the bushing body to the body inner end 20. This 
electrical conductor is rigidly affixed, as by bonding, to the bushing 
body, and has a connection means, such as threaded stud 32, which projects 
from inner end 20 and is adapted to be connected to a transformer coil 
unit located inside the transformer housing wall 12. 
A threaded blind ended bore 34 is defined in the electrical conductor 30 to 
extend inwardly from the terminal wall 28 of the boss 24. This bore may be 
threaded to receive a replaceable stud 36 which is removably mounted in 
electrical contact with the electrical conductor. 
The replaceable stud 36 is best shown in FIG. 2, and includes a first 
threaded shank 38 having a diameter and a thread pitch sized to engage 
threaded bore 34 to securely yet removably couple the replaceable stud to 
the electrical conductor 30. Replaceably stud 36 further includes a second 
threaded shank 40 which operates as an electrical terminal to connect the 
bushing 10 to external equipment. The second threaded shank extends 
substantailly colinearly with the first threaded shank 38, and is designed 
to provide a unitary electrical circuit from the second shank through the 
first shank to the conductor 30. 
The replaceable stud second shank 40 is designed to break under stress 
before the first shank 38. This may be accomplished by forming the second 
shank with a diameter smaller than the diameter of the first shank. This 
may also be accomplished by forming the second shank of conductive 
material which is weaker than the conductive material forming the first 
shank or by otherwise mechanically weakening the second shank. Thus, when 
the replaceable stud 36 is part of electrical bushing 10, damaging stress 
applied to the replaceable stud will break the second shank, thereby 
leaving first stud shank undamaged for proper engagement and disengagement 
with the threaded bore 34. 
Replaceable stud 36 further includes a flange 42 formed at the juncture 
between first and second stud shanks 38 and 40. This flange includes a 
first surface 44 oriented to engage the terminal wall 28 of the boss 24 to 
properly seat the replaceable stud. The flange further includes a second 
surface 46 for engaging connecting means applied to the second shank 40. 
The flange 42 is provided with removal tool engaging means, such as a 
plurality of apertures 48 (only one of which is shown), for engaging a 
suitable removal tool to permit disengagement of the replaceable stud 36 
from the electrical conductor threaded bore 34. Thus, the replaceable stud 
can be removed from the electrical bushing 10 without disturbing the 
placement or mounting of the electrical bushing body 14 on the transformer 
housing wall 12 and without requiring that the conductor 30 be 
electrically disconnected from any lead within the transformer housing. 
The apertures 48 in the flange 42 extend through the flange between the 
first and second surfaces thereof, and receive the prongs of a pronged 
removal tool. The flange may be configured in other ways to cooperate with 
removal tools of other types. 
Since the second shank 40 forms the electrical terminal which is subjected 
to external stress, it is important that this shank break before the first 
shank. Then the replaceable stud 36 can be removed and replaced by a new 
replaceable stud without otherwise disturbing the electrical bushing body 
14 and the mounting thereof on the transformer wall 12, or the connection 
of the electrical conductor 30 to the transformer coil unit. Seals between 
the electrical bushing body and the transformer wall 12 remain undisturbed 
and thus secure. 
The bushing body 14 is shown in FIG. 1 as being mounted on the transformer 
wall 12 by means of a mounting assembly 50 which includes a flange 52 
affixed to the bushing body 14. A clamping ring 54, is slipped over the 
outer end 18 of the bushing body to engage the flange 52, and is then 
suitable secured to the housing wall 12 by fasteners (not shown). A seal 
or gasket 56 is compressed between the flange 52 and transformer wall 12. 
The mounting assembly 50 is shown as releasably mounting the electrical 
bushing 10 to the transformer wall 12 in FIG. 1, but the electrical 
bushing can be permanently secured to the housing by welding, or the like. 
The replaceable stud 36 may be effectively used with male as well as female 
electrical bushings as will be noted from FIG. 3. Here a male electrical 
bushing indicated generally at 58 includes a bushing body 60 of electrical 
insulating material which projects from the housing wall 12 of a 
transformer. A conductor 30 secured within the bushing body extends 
through the housing wall and is electrically connected to electrical 
transformer components within the transformer housing. The bushing body is 
provided with an outwardly opening threaded bore 62 which receives the 
first shank 38 of the replaceable stud 36. The innermost end of the bore 
62 is closed by a bore end wall 64 through which the conductor 30 projects 
to make electrical contact with the first shank 38 when the first shank is 
in place within the threaded bore. Thus, in the male electrical bushing 
58, the threaded bore 62 supports the replaceable stud 36 and no bore is 
formed in the conductor 30. 
INDUSTRIAL APPLICABILITY 
The electrical bushing of the present invention may be effectively mounted 
upon a transformer housing to provide an electrical transformer terminal 
for connection to external electrical components. The bushing is adapted 
to receive a novel replaceable stud 36 which completes an electrical 
circuit to a conductor mounted within the bushing body. The bushing may be 
of either the male or female type, and may include a threaded bore in 
either the bushing body or in the electrical conductor mounted therein to 
receive a first threaded shank of the replaceable stud. A second shank of 
the replaceable stud forms the external terminal for the transformer and 
is formed to be weaker than the first threaded shank. The replaceable stud 
includes a flange positioned between the first and second shanks which is 
provided with a configuration to facilitate the use of a removal tool to 
remove the replaceable stud from the remainder of the electrical bushing. 
Thus, when the second shank is broken or damaged, the remainder of the 
replaceable stud may be removed and easily replaced with a new replaceable 
stud without disturbing the electrical bushing or any of the internal 
electrical connections to the bushing.