Electricity: electrothermally or thermally actuated switches – Electrothermally actuated switches – Fusible element actuated
Reexamination Certificate
1999-05-18
2001-02-27
Picard, Leo P. (Department: 2835)
Electricity: electrothermally or thermally actuated switches
Electrothermally actuated switches
Fusible element actuated
C337S228000, C337S231000, C337S233000, C337S260000, C337S293000, C337S295000
Reexamination Certificate
active
06194989
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to fuses in general, and particularly to current-limiting, time-delay fuses.
A current-limiting time delay fuse
10
employs a built-in delay that allows temporary and harmless inrush currents to pass without the fuse being opened, but which is designed to open in response to a sustained overload and short circuit currents. Such a dual-element fuse is used in circuits subjected to temporary inrush current transients, such as motor starting currents, to provide both high performance short-circuit current protection and time-delay overload current protection.
One conventional type of such a fuse
10
, depicted in
FIG. 1
, comprises a body which includes an electrically insulative tube
12
formed for example of glass reinforced polyester, a pair of copper knife blade terminals
14
connected to respective brass end plates
16
, and a pair of steel end caps or ferrules
18
. The end caps
18
are attached to the tube
12
by screws
20
(or rivets) to close the ends of the tube and retain the end plates
16
. Each terminal
14
projects through a slit
24
formed in a radial portion
15
of a respective end cap
18
, and is supported or attached to the tube
12
by a flat pin or roll pin (not shown) extending through the terminal.
Alternatively, as shown in
FIGS. 2 and 3
, the terminals
14
A could be brazed to thick end bells
16
A which are inserted into respective ends of the tube
12
A such that radial holes
26
A formed in each end bell
16
A become aligned with respective radial holes
28
A formed in the tube
12
A. Cylindrical drive pins
30
A would be force-fit through respective pairs of holes
26
A,
28
A to secure the end bells to the tube.
Disposed within a cavity
32
formed by the tube
12
are fuse elements. Preferably, two types of fuse elements
34
,
36
are provided, namely, an overcurrent trigger mechanism
34
and a short circuit interrupting fusible element
36
. There is at least one of each type of fuse element. The cavity
32
is filled with an arc-quenching filler material
33
such as quartz sand.
Each overcurrent trigger mechanism
34
includes an alloy solder
38
for series-connecting the mechanism
34
to one of the fuse elements
36
, a trigger
40
, a coil compression spring
42
surrounding the trigger
40
, an absorber
44
surrounding the spring
42
, a heater element
46
, and an insulator
48
. The trigger mechanism
34
utilizes stored energy of the spring
42
to break the current in the event of low level overcurrents or overloads, and will hold an overload that is five times greater than the ampere rating of the fuse for a minimum time, e.g., about ten seconds.
Each short circuit fuse element
36
comprises a strip
50
of fusible metal, such as silver, copper, copper alloy, etc., having parallel rows
52
of perforations. Adjacently disposed perforations define therebetween current-carrying weak spots of substantially reduced cross-section designed to break in response to a short circuit overload current.
Although such fuses have performed acceptably, certain shortcomings exist. For instance, in the short circuit fuse elements
36
, the strips
50
are supported only by their weak spots which provide very little strength for the fuse element while being handled during the fuse-manufacturing process. Consequently, the fuse elements
36
are susceptible to mechanical fatigue and breakage due to normal handling during manufacture, as well as due to mechanical and thermal fatigue caused by steady state and transient current load current cycling.
Heretofore, the fatigue problem due to handling has been solved by the use of special equipment, tool fixturing and procedures designed to reduce the amount of worker handling. Those measures, however, increase capital expenditures and slow the production rate.
Another shortcoming relating to a time delay current-limiting fuse, or to fuses in general, which are filled with an arc-quenching filler involves the need to plug a hole in which the filler has been introduced. In that regard, the filler is typically introduced through a hole which must be plugged or sealed, in order to retain the filler. A variety of methods of sealing or plugging have been used, such as metal drive plugs, set screws, steel balls, and metal cups, as well as adhesives and glues such as epoxy, but all suffer from various limitations. For example, drive plugs require costly fabrication machinery, set screws are also costly in that they require that the filler hole be machined to form a screw thread; balls and cups are held in place by an interference-fit and are less costly, but the interference-fit is not always reliable, whereby the balls or cups may become dislodged; adhesives are messy to apply and hard to control.
Additional shortcomings may result from the ability to provide the tubes of fuses with shorter lengths. If a fuse manufacturer is to incorporate shorter fuse tube lengths, then certain spacing requirements must be satisfied to ensure that a user can safely grip a fuse without simultaneously touching parts of the fuse which will produce an electrical shock. These spacing requirements are spelled out in the Underwriters Laboratory standards for electrical equipment that use these fuses in a covered device (i.e., disconnect switch). The spacing requirements specifically pertain to what is known as phase-to-phase and phase-to-ground distances between live and/or dead metal parts. A live metal part means a metal conductor at some voltage potential with respect to ground. A dead metal part means a metal conductor at no voltage potential with respect to ground.
In that regard, a common problem involving the application of shorter fuse tube lengths to a typical fuse design is that the longitudinal space between the live metal end caps is so short as to create spacing violations for phase-to-phase and phase-to-ground distances in existing equipment designed to specific Underwriters Laboratory standards. To overcome this spacing violation, several design approaches have been considered. One approach involved the use of heat shrink plastic wrap over the metal end caps, and another approach employed plastic end caps (e.g., see Swain U.S. Pat. No. 2,863,967). Both of those approaches proved either too expensive or impractical due to strength issues.
Yet another shortcoming involving the manufacture of shorter fuses is that in order to make the fuse body shorter the fuse blades must become longer to continue satisfying the dimensional requirements of the fuse. By making the fuse blades longer, a greater mechanical moment may be imposed during installation of the fuse. To accommodate this greater mechanical moment, a stronger mechanical system must be provided. The typical knife blade fuse depicted in
FIG. 1
does not provide the necessary mechanical system to support the force exerted on the longer blade of a short-body fuse. The fuse depicted in
FIGS. 2 and 3
, however, will support this force because of the added strength from the pinned mechanical system to the high strength tube. However, the cost of the pinned mechanical system is too high in cost to implement for all types of fuses, because it uses a very expensive tube material (e.g., glass melamine) and the fuse must be assembled on a C-shaped metal frame which is very labor intensive.
Therefore, it would be desirable to provide a fuse of the type containing an arc-quenching filler with a more effective fill-hole plugging arrangement.
It would also be desirable to provide a short-circuit fuse element which is less susceptible to mechanical and thermal fatigue due to handling as well as due to steady state and transient load current cycling.
It would also be desirable to provide a fuse which provides for strong reinforcement and closure of the ends of the fuse tube while ensuring that ample phase-to-phase and phase-to-ground distances are created.
SUMMARY
In accordance with the present invention, a fuse element comprises a body of metallic material including at least first and second parallel, superimposed s
Armstrong Teasdale LLP
Cooper Technologies Company
Picard Leo P.
Vortman Anatoly
LandOfFree
Fuse element having parallel strips does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Fuse element having parallel strips, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Fuse element having parallel strips will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2572344