Note: Descriptions are shown in the official language in which they were submitted.
3~ ~
-- 1 --
l }IIGH CONTINUOUS CURRE~T CAPACITY OIL
EXPULSION rUSE
Background
`This invention relates to high voltage distribution
circuit oil expulsion fuses in general and more particularly,
it is concerned with a fuse of this variety which as compared
with a single bore fuse of the same element material and
corss sectional area has greater ampacity as well as improved
arc-quenching capability, greater ability to interrupt
against high rates o rise of transient recovery voltage and
to withstand the normal frequency recovery voltage indef-
initely without arcing restrikes.
Oil expulsion fuses have long been used in distri-
bution circuits to protect electrical equipment from the
deleterious eEfects of fault currents. Such fuses typically
comprise a tubular fuse cartridge having conductive terminal
caps at opposed ends, and adapted to receive a single
expendable fuse link comprising an elongate fusible element
contained within one arc tube. When an overcurrent or
fault current i.s experienced in -the circuit containing the
fuse, the fusible element is caused to melt, whereupon
arcing occurs within the arc tube between the severed seg-
ments of the fusible element. Heat produced by the arcvaporizes oil filling the tube producing pressurized
deionizing gas therefrom which vents at opposite ends of
the fuse cartridge. As the oil derived venting gases flow
past the arc, they serve to cool and deionize the latter
such that the arc is effectively extinguished. The disabled
fuse may subsequently be returned to service by simply
replacing the expendable fuse link contained within the
fuse cartridge.
~ 35
`~
, ~ , , .
3~J~
1 While oil expulsion fuses have heretofore generally
proven satisfactory for use in distribution circuits, it
has been found that there are some difficulties associated
with their use in high voltage applications. ~ith single
bore oil expulsion fuses, problems have been encountered in -
atternpting to in~errupt a low fault current against a high
rate of rise recovery voltage. This difficulty occurs over
a wide range of application voltages. In this regard, it
has been discovered that when conventional oil expulsion
fuses are constructed to provide the required current capacity
for use in the higher distribution voltage circuits, the
fuses do not reliably interrupt against higher transient
recovery voltage rates associated with the higher distribu-
tion voltages. Of course, this is a highly undesirable
characteristic since it represents a failure to clear the
overcurrent and could result in serious damage to electrical
apparatus relying upon the fuse for current protection.
While it has not been conclusively determined why larger
conventional oil expulsion fuses are not adapted for use in
high voltage distribution circuits, one theory is that the
larger internal diameter of the arcing tube required to
accomodate the desired ampacity fuse element, precludes
development of deionizing gas flow sufficient to adequately
extinguish the arc against the higher transient recovery
voltages. In any event, there simply is not commercially
available a high ampacity refusible oil expulsion fuse
capable of reliably interrupting a wide range of fault
- currents or harmful overcurrents. It has now been dis-
covered that these interruption and restrike problems can
be avoided by using a multiple fusible element design with
each fusible component received within a separate relatively
small diameter fuse bore and wherein gases generated during
:-~ arcing are controlled to assure interruption without
~estrike.
. , , ,, : ~ :
'.
t ' ~ 3~V ~,
1 While certain types of multiple fusible elemen~
expulsion fuse units have been proposed, such as those shown
in U.S. Letters Patent 2,156,058 to Lohausen and 2,291,341
to Lincks, these have of necessity been restricted to use
in air. These air environment devices have no practical
application in connection with fusing in oil because they
are incapable of allowing connective flow of oil therethrough.
Both oE the patents identified show an expulsion fuse having
a plurality of fusible elements, each element being provided
with its own arcing chamber. While these fuses may exhibit
limited increased current-carrying capacity for air applica-
tions, it is manifest that neither can be used in an oil
system. The apparent difficulty in Lohausen's fuse of
clearing all of the tubes of residual metal even though
arcing occurred in only one or a limited number of bores
upon interruption was recognized by Linck who sought to
solve the prohlem with mechanical contrivances in the nature
of ejector springs for the fusible elements of each bore.
~ven this device though would have a limited unfavorably
retarded response time with the arc being retained for an
undesirable period especially during low current faults.
Summary
The instant invention presents an oil expulsion
fuse suitable for use in high voltage distribution circuits
by virtue of the provision of multiple fuse wires each dis-
posed within a discrete, rupture-resistant bore such that a
selected fuse has the required ampacity as well as exhibits
arc-extinguishing capability sufficient to overcome the
transient recovery voltage rates associated with the inter-
ruption of high voltage circuits for that particular applica-
tion. More specifically, the fuse includes a synthetic resin,
~~ rod-like insert having three elongate, cylindrical bores
- ., . - ,
. . ~ ~`,'i :
,, ~ ~. ,; .
13V3~
. . ,
1 formed therein, each bore being sufficiently small in dia-
meter -to establish a flow rate of deionized gas ade~uate to
extinguish arcs formed within the bore upon melting of fuse
wire carried therewithin.
Though each of the fuse wires is substantially
identical in size and material to others in the fuse, in-
heren~ material and physical differences assure sequential
' fusing of the wires and arcing in the chambers such that theover current is cleared within the last chamber to arc.
Expansion chambers common to all bores are located
at both ends of the fuse and communicating with the surrounding
oil environment. One of these expansion chambers has a more
restricted flow relief area which causes it to vent to ambient
- more slowly and therefore develop a.higher pressure than the
lS opposite expansion chamber. This pressure difference causes
a flow of oil and gases down the bores containing the first
elements to melt, which did not experience sufficient arcing to
clean themselves of melted element metal.
The synthetic resin insert is adapted to be comple-
- 20 mentally received within a rigid tubular fuse cartridge such
that the latter provides additional strength to protect
against rupturing of the fuse wire chambers. The chambers
at opposite ends of the bores are in part defined by
threaded fittings which no~ only permit oil flow into the
elongate bores and gas venting therefrom, but also at the same
~ time preclude expulsion of the insert when overcurrents are
; encountered.
Convective heat transfer from the fuse wires to the
surrounding oil in the respective chambers is responsible for
an increase i.n the current-carrying capacity per unit cross-
n - section of the present invention over single element oil
- expulsion fuses. Thus, the multiple wire design provides. desired ampacity while also permitting the provision of a
number of wholly separate fuse chambers each of a sufficiently
~; 35
':' .. i
~ ` ,
,
. .. .
- ~
.
. .
-- 5
~3~3~i0
1 small diameter to assure proper clearing of fault currents
notwithstanding the higher transient voltage recovery rates
associated with high voltage distribution circuits.
Detailed Description Of The Drawing
~ Fig. 1 is a perspective view of a pad mounted
double fused vacuum switchgear of the type described in
U.S. patent 4,083,Q28 issued to Haubèin et al. on April 4,
.
1978 and assigned to the presen~ applicant.
Fig. 2 is a longitudinal cross-sectional view of a
high continuous current capacity -oil expulsion fuse con-
structed in accordance with the principles of the present
invention;
Fig. 3 is an elevational view, with portions shown
in cross-section, of the expulsion fuse link which forms a
part of the fuse illustrated in Fig. 2;
Fig. 4 is an enlarged bottom end view of the link
shown in Fig. 3;
Fig. 5 is an enlarged cross-sectional view taken
- along line 5-5 of Fig. 3;
Fig. 6 is a top end view of the link shown in
Fig. 3; and
Fig. 7 is a fragmentary, vertical cross-sectional
view of the upper end of the fuse assembly as depicted in
Fig. 2, but illustrating a modified form of the invention
with more severely restricted flow of oil and gases from the
- upper end of the fuse during the interruption made thereof.
io - Detailed Description
There is shown in Fig. 2 an oil expulsion fuse 10
comprising a rigid, nonconductive, tubular fuse cartridge 12;
a pair of conductive end caps 14 enclosing opposite ends
~ ' .
,. . .
:
..
,~
~ - 6 -
1 of cartridge 12; and an expulsion fuse link 16 disposed
within the cartridge 12 intermediate the caps 14.
The fuse link 16 injcludes a cylindrical rod-like ~-
insert 18 constructed of a synthetic resin material such as
a fluorocarbon polymer, preferably polytetrafluoroethylene
(Teflon*being a specific example), and dimensioned to be
complementally received by the cartridge 12. Three cylindrical
bores are formed in the insert 18, extending parallel to and
symmetrically arranged around the longitudinal axis thereof.
The bores 20 are of relatively small diameter in comparison
with the diameter of the insert 18; in the preferred embodi-
ment, bores 20 are .133 inches in diameter whereas the insert
18 is .432 inches in diameter. The insert 18 has secured on
opposite ends respectively a flanged metal contact 22 and an
opposed slotted contact 24.
As shown in Fig. 5, for example, the bores 20 are
positioned within the insert 18 in a manner to maximize the
minimum wall thickness of the bores 20. This construction,
combined with the inherent strength of the base material
for insert 18, renders the chambers 20 substantially non-
burstible under the influence of deionizing gas build-up
in the respective bores 20 generated upon operation of the
fuse 10. Hence, the discrete nature of the bores 20 is at
all times maintained, significantly contributing to the
ability of the fuse 10 to reliably clear fault currents in
high voltage distribution circuits as described hereinbelow.
Respective fuse wires 26 extend through each bores
20 and are secured at opposite ends to the contacts 22, 24
by soldering or other suitable means. In the preferred
embodiment, the fuse wires 26 are .075 inches in diameter
are are comprised of eutectic solid wire solder having a
composition of 49.8 percent tin, 32 percent lead, and 18.2
percent cadmium. The fuse wires 26, in combination with
their respective chambers 20, define separate current-
interrupting links which operate in response to a current of
-
* a trademark
;~,
',
- 7 --
~3~3
predetermined magnitude.
The end caps 14 are securely coupled with the cartrid~e
12 by way known in the art. Each
cap ~14 has a central threaded opening 28 extending there-
through including a shoulder 30 adapted to engage a respective
contact 22, 24 of the fuse link 16. In this connection, when
the link 16 is positioned within the cartridge 12, ~he flange
on contact 22 seats against the shoulder 30 on one of the
caps 14 and the slotted contact 24 is deformed in such a man-
ner as to seat against the shoulder 30 of the opposite-contact
14. Thus, the fuse link 16 is firmly secured within the
cartridge 12 and positive electrical contact is established
between the contacts 22, 24 and respective end caps 14.
Each threaded opening on the end caps 14 is adapted
to receive a removable, threaded contact plug 32a or 32b as
the case may be whereby the fuse link 16 is positively
locked within the cartridge 12. The upper plug 32a viewing
Fig. 2 has an elongated passage 33 partially therethrough
along with four opposed vent parts 34 communicating therewith.
The diameter of passage 33 is greater than that of parts 34,
The lower plug 32b on the other hand has a central passage
35 extending completely through the same and communicating
with four vents 37 which are of the same diameter as vents
34, and thus of smaller diameter than passage 35.
It is thus apparent that the passages of the plugs
32, end caps 14 and opposed extremities of insert 18 define
; 30 respective chambers 39 and 41 which are common to corresponding
ends of the bores 20.
Vents 34 and 37 communicating with chambers 39 and
41 allow oil to flow through the bores 20 of insert 18 during
the normal continuous current carrying mode of fuse 10.
`
.
, . : . : .
- ~ ~- . .- , : . .
' 1~L3V35
1 However, chamber 41 has more openings to the surrounding oil
media than chamber 39 because of the outlet 35a of passage
35.
Referring now to ~ig. 1, there is shown a preferred
environment of use for the oil expulsion fuse 10 of the present
invention. ~ pad mounted, double fused switchgear 36 has a
numb~r oE removable fuse assemblies 38 each including an
oil expulsion fuse 10 in series combination with a conventional
current-limiting fuse 40. The fuse assembly 38 is coupled
in series between a switch shown schematically in the drawing
and broadly designated ~2, and a tap line 44, in a manner
well known in this art. Though not shown, the switchgear
36 is provided with a switch 42 and fuse assembly 38 for
each of the six tap lines 44 illustrated in Fig. 1.
Typically, the housing of the switchgear 36 contains a large
reservoir of dielectric liquid, such as mineral transformer
oil, the assemblies 38 being submersed in the liquid.
Under normal operating conditions, electrical
loads are carriecl and switched through the switchgear 36,
and the distribution current is conducted through the fuse
assembly 38. Should a fault current be experienced on the
load side of the switchgear 36, the fuse assembly 38 functions
to protect both the source and load sides from the overcurrent.
In this regard, the current-limiting fuse 40 actuates in
response to high level fault currents to quickly clear the
fault before damage occurs, while the expulsion fuse 10
clears low range faults by melting of the fuse links 26.
When a low level fault current is experienced in
the distribution circuit, the temperature of the fuse wires
26 rises as a result of the increased current. By virtue of
the inherent varying cross-sectional thickness of the
.~ individual fusible wires, one of the same will tend to melt
before the others. When that occurs, although a transitory
arc may be produced in that associated bore, the current is
then carried by the remaining wires because of the parallel
.~ . .
-
,:
.
. ~ :
3~3
- 9 -
1 electrical connection. Melting of the second fusible
element again results in only minor arcing if any and
further concentration of the current in the remaining un-
melted fuse wire. ~hen this! last fusible element melts, an
arc of appreciable extent and time duration is produced
thereby producing sufficient gas from ionization of the oil
to (l) clean that bore of melted fuse element metal
, and interru~t the Eault current against the transient re-
covery voltage and (2) equally as important, clean the re-
maining bores as well oE element metal even though they did
not significantly arc, and thus preventing an arc restrike
which if did occur might not be interrupted because of a
deficiency of oil in the bores following current interruption.
Pressure in the chambers 39 and 41 rises rapidly
lS with formation of the deionizing gas such that the latter
- seeks to vént through the open ends of the chambers, The
pressurized deionizing gas flows out of each end of the
bore in which the arc has occurred to eject metal and other
arc maintaining matter thus effecting extremely rapid
suppression and extinguishment of the arc. It is to be
- noted that the relatively small diameter of each of the
bores 20 assures that a suffieient gas flow is established
to extinguish virtually any arc formed therein against the
higher transient recovery voltage expected at the higher
distribution voltages.
Since chamber 41 is able to vent gas more rapidly
than chamber 39, a part of the gas directed into chamber
39 is forced to flow through bores 20 toward chamber 41
thus assisting to clean out of the non-arcing bores to
effectively eliminate the possibility of arc generation or
restrike in these bores when the recovery voltage occurs.
Fig. 7 illustrated a modified form of the invention
wherein only two vents 34' are provided in terminal means 32
communicating chamber 39' with the surrounding oil
medium to even more severely limit outflow of
.. . . . .
~ ,.
.
31~3~
-
1 from chamber 39'. In this case, the vents 34' are also
preferably no more than aboutl one fourth of the area of
respective vents 34.
The disabled fuse 10 may subsequently be returned
to service after replacing the spent fuse link 16. In
this regard, assembly 38 is simply remvoed from the housing
of switchgear 36, and the contact plugs 3~ are subsequently
unscrewe~ to provide access into the interior of the fuse
cartridge 12. Slotted contact 24 is then restraightened
such that the spent fuse link 16 may be removed from the
cartridge 12 and a new fuse link 16 substituted thereor.
During normal service, the fuse 10 is capable of
conducting high continuous currents without melting of the
fuse wires 26 such that the fuse assembly 38 is suited for
use in heavy service, high voltage distribution circuits
Further in this regard, it is noted that the oil within the
housing of switchgear 36 is permitted to flow into the
bores 20 through the vent ports 3~ and 37. Hence, each of
the fuse wires 26 transfers heat by convection transversely
or radially to the oil thereby increasing the ampacity of
the wires 26. This heat transfer accounts for the fact that
the three fuse wires 26 have a greater combined ampacity
than a single fuse wire presenting the same cross-sectional
area. In other words, the multiple fuse wire design of
the present invention results in an increase in ampacity
per unit cross-sectional area of the fuse wire over con-
ventional single wire designs.
The increase in ampaclty due to the provision of
multiple fuse wires, while only negligible in air expulsion
fuses, is dramatic in oil expulsion fuses. In actual tests
using a .075 inch diameter single fuse wire and two .050
_ inch diameter multiple fuse wires, ampacity was shown to
increase by only 5 percent in an air expulsion fuse whereas
a 30 percent increase in ampacity was realized in an oil
,, ,
. , ,
:
., . ~ ,
3~
l expulsion fuse. This startli.ng difference, heretofore
unrecognized, may possibly be explained by the different
means by which heat is transferred from air fuses as
compared with heat transfer from oil fuses. More speci-
fically, heat loss in an air fuse is primarily by axialconduction to the relatively large terminals on opposite
ends`of the ~use, ~hereas heat ioss in an oil fuse is
primarily by transverse or radial convection to the sur-
rounding oil medium. It has been found that the ampacity
per unit of fuse element cross-sectional area increases
as the fuse element diameter decreases. Thus, the plurality
of small diameter fuse elements results in significantly
greater ampacity than a single fuse element of similar
total cross-sectional area.
- 15 It is to be understood that the fuse 10 may be ~-
tailored to meet particular service demands by eliminating
one or more of the fuse wires 26. In this regard, the
ampacity of the tailored Euse 10 is directly proportional
to the number of fuse wires 26 utilized (even though the
insert 18 may still contain three chambers 20).
From the foregoing, it is clear that the present
invention offers a unique expulsion fuse suitable for
service in high voltage distribution circuits. The multi-
bore design results in increased ampacity as well as
provides the fuse with the ability to effectively clear
fault currents even against the high transient recovery --
rates experienced in interrupting high voltage distribution
circuits without attendant arcing restrikes.
. ,. ~ .
: " ~- . , .' ' ~
,
