Abstract:
A fused disconnect switch including a plurality of maintenance and installation enhancing features is disclosed. For example, a bipolar LED is provided as an alarm indicator so that the fused disconnect switch will function in power distribution systems which operate on both positive and negative voltages. Additionally, a manual test tab is provided for checking the alarm circuitry and a viewing window is provided so that the presence or absence of a fuse in the fused disconnect device can be ascertained without removing the fuse holder.

Description:
BACKGROUND 
     The present invention relates to low voltage, high current electrical power distribution systems and, more particularly, to fuse protected electrical disconnect devices for use in power distribution systems. 
     A common problem in various industries, for example the telecommunications industry, is the distribution of relatively low voltage, relatively high current electrical power to the various devices and equipment which require such power. Telecommunications system plants, such as telephone switching offices, are typically centralized and are, therefore, constructed on a large scale to serve many customers. Since telephone systems must be highly reliable, such plants usually have a plurality of high-capacity storage batteries to provide electrical power for operating equipment whenever power is not available from conventional sources. Often, the batteries are connected in parallel with suitable power conversion equipment which receives electrical power (typically high-voltage AC) from the commercial electrical utility and supplies converted power (low-voltage DC) for operating all equipment and for maintaining the batteries in a charge condition. Whenever power from these conventional sources becomes unavailable, power is immediately and automatically supplied from the batteries. 
     In such systems, several batteries and power converters are usually connected together so that large amounts of equipment are supplied with power from a large collection of power sources. It is desirable in such systems to be capable of isolating individual pieces of equipment, or small groups thereof, from the power supply to perform maintenance and installation activities. It is also desirable to provide overload protection for equipment on an individual basis. Accordingly, power is conventionally distributed to equipment from distribution panels having a plurality of individual fused disconnect devices. Each disconnect device controls power to a relatively small load--for example, a cabinet containing subscriber loop interface circuits for 100 subscribers and drawing 10-50 A in normal operation. 
     In the past, several fused disconnect devices have been developed to allow manual control of each load circuit and to provide overload and fault protection of each load circuit. Such devices have typically included a line side terminal for a wired connection to a power supply bus, a load side terminal for a wired connection to a load device, a housing and a removable fuse-containing holder which, when installed in the housing, provides an electrical connection between the line side terminal and the load side terminal. 
     Such conventional fused disconnect devices suffered from a plethora of drawbacks and limitations. For example, some of these disconnect devices suffered from an inability to test the alarm circuit while the fused disconnect device was in its operative position, i.e., when the fuse holder was inserted into the housing and the device was connected to both the load and line buses. Moreover, as more and more of these disconnect devices are used in remote sites which are not easily accessible and do not have proper lighting, it would be desirable to make the results of such an alarm test more readily ascertainable in a low-light environment. 
     Yet another problem with conventional fused disconnect devices is the inability to determine whether or not a fuse is actually provided in the removable fuse holder. In typical telecommunications power distribution systems, the fused disconnect devices are assembled in rows of approximately 10-20 units. However, not all of these units will necessarily be operational at any given time and units which are not operational will not have a fuse in the fuse holder. Since the removal of a fused disconnect switch from a functioning distribution unit would temporarily interrupt service, e.g., telephone service to a customer, it would be advantageous to provide a method for determining whether or not a fuse was provided in the disconnect switch without removing the fuse holder or the disconnect switch itself. 
     Additionally, fused disconnect devices, and other types of circuit breakers generally, have been plagued with difficulties in attachment to their intended line and load devices and the internal attachment between the housing or base unit and the removable fuse holder. Many such switch devices require, for example, adapters in order to be able to connect the switch device to the intended bus structure. Such adapters are both expensive, time consuming to use and add an additional connection to the circuit which reduces the overall system reliability. Accordingly, it would be desirable to provide a fused disconnect device which did not require any adapters and which could be readily connected to an intended bus structure with a minimum of installation effort. Further, it would be desirable to provide a removable fuse holder with an uncomplicated electrical and physical connection structure that nonetheless provided a secure connection with the base unit. 
     Another drawback associated with conventional telecommunication power distribution systems is the tendency to distribute power via individual bus bars and using individual load circuit protection devices. Individualizing all of these components is not cost effective and requires additional connection points which increases the number of potential points for system failure. 
     SUMMARY 
     These and other drawbacks, problems and limitations of conventional fused disconnect devices are overcome according to exemplary embodiments of the present invention which provide, among other features, an LED to give a visual indication of an alarm signal for a blown fuse in the disconnect switch which is readily visible even in a low-light environment. Since telecommunication switch equipment operates with both positive and negative DC voltages, a bipolar LED can be used to consolidate manufacturing into one product rather than having two products each with an LED of different polarity. 
     Moreover, since alarm circuitry needs to be tested to assure that all parts of the alarm circuit are functional, exemplary embodiments of the present invention provide a small opening on the top surface of the disconnect switch which can be probed manually during field operation. A metal tab located behind the opening deflects when the probe pushes against it and makes contact with the alarm circuitry to activate the LED and send an alarm signal to the monitoring station. 
     Yet another feature of exemplary embodiments of the present invention is the provision of a viewing window by way of which the presence or absence of a fuse in the fuse holder can be visually determined without removing the fuse holder or the disconnect switch. In this way, power distribution will not be interrupted by a maintenance worker&#39;s field check of the disconnect switch fuse. According to some exemplary embodiments, a light pipe can be provided to improve the viewing angle of the fuse through the viewing window and to assist in low-light environments. 
     According to another exemplary embodiment of the present invention, a removable fuse holder includes a line side connection terminal which comprises a blade which is generally parallel to a plane which can include, for example, a side of the fuse holder. Among other benefits attributable to this line side connection configuration, the removable fuse holder can be more easily disconnected from the base unit. 
     Moreover, this configuration also lends itself to the provision of plural removable fuse holders which are removably connected with a base unit and which share a common line bus and alarm bus. By sharing line and alarm connections, exemplary embodiments of the present invention reduce the number of connections and expense associated with fused disconnect devices. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing, and other, objects, features and advantages of the present invention will be more readily understood upon reading the following detailed description in conjunction with the drawings in which: 
     FIG. 1 illustrates a side view of a removable fuse holder and a base unit into which the removable fuse holder can be inserted according to an exemplary embodiment of the present invention; 
     FIG. 2 depicts a different side view of the removable fuse holder of FIG. 1; 
     FIG. 3 is a cutaway view of a fused disconnect switch according to an exemplary embodiment of the present invention; 
     FIG. 4 is a cut away view of a removable fuse holder according to an exemplary embodiment of the present invention illustrating aspects of an alarm circuit; 
     FIG. 5 is a top view of the fused disconnect switch of FIG. 3; 
     FIG. 6 is an exemplary base unit according to another embodiment of the present invention; 
     FIG. 7 is a cutaway view of the base unit of FIG. 6 with a removable fuse holder inserted therein; and 
     FIG. 8 is a side view of the base unit of FIG. 6 with four removable fuse holders inserted therein; and 
     FIG. 9 is a bottom view of the base unit of FIG. 6 showing four line side bus bar contacts. 
    
    
     DETAILED DESCRIPTION 
     A description of an exemplary fused disconnect switch according to the present invention begins with FIG. 1, which illustrates an exemplary removable fuse holder 10. The removable fuse holder 10 has fuseclips 34 and 42 for holding a fuse 14 in place and conducting current therethrough. The presence or absence of the fuse 14 can be readily determined using viewing window 16 (shown in hidden lines in FIG. 1) even when the removable fuse holder 10 is inserted into the base unit as shown and described below. 
     The removable fuse holder 10 has a handle portion 18 with areas 20 and 22 which are designed for gripping. This permits easy removal of the fuse holder 10 from the base unit in removal direction D. Removable fuse holder 10 also has a load contact 24 and blade line contact 26 which can be inserted into mating contacts in the base unit as described below. As can be seen in FIG. 1, a lengthwise dimension of blade contact 26 is roughly parallel to removal direction D. 
     FIG. 2 illustrates the removable fuse holder 10 in a side view. This view provides a perspective on the relative size and shape of blade contact 26 according to this exemplary embodiment. Moreover, it can be seen that a plane defined by the lengthwise edge 27 and widthwise edge 29 of the blade contact 26 is generally parallel to a plane defined by the removal direction arrow D and lies and a line B which is perpendicular to arrow D in the direction of a thickness of the fuse holder 10. It will be noted that, in this exemplary embodiment, the plane defined by the blade contact 26 is also generally parallel to the side 25 of the fuse holder 10, although those skilled in the art will appreciate that the configuration of the fuse holder 10 could be modified such that the side 25 is not parallel to the plane defined by blade contact 26. 
     FIG. 3 illustrates the entire fused disconnect device, i.e., base unit 30 and removable fuse holder 10 in its inserted position, in a sectional view in order to describe elements which are covered by the exterior housing 32. A fuse 14 is held in the removable fuse holder 10 by fuseclips 34 and 42 and retaining springs 12. Although only one fuse is illustrated in FIG. 1, those skilled in the art will readily appreciate that the base unit/fuse holder structure could be modified so that plural fuses could be provided and that this exemplary embodiment can be readily adapted to plural circuits within one base unit. As can be seen in FIG. 3, when the removable fuse holder 10 is inserted into the base unit 30, the fuse 14 is completely obscured from view making the viewing window 16 (shown in FIG. 1 and 5) useful in determining the presence or absence of fuse 14. A light pipe 17 (shown in FIG. 5) can be provided in the aperture between the fuse 14 and the top of the removable fuse holder 10 to maximize the amount of light which is reflected from the top (silvered) surface of the fuse and transmitted back to the aperture for low-light environments. 
     The fuse 14 is connected to the line side circuit through fuseclip 34. Fuseclip 34 is integrally formed with lead 36 which extends through the removable fuse holder 10, the end portion of which constitutes blade contact 26. The blade contact 26 fits snugly into contact 38 which is formed in the base unit 30, when the removable fuse holder 10 is inserted into the housing. A lead from contact 38 extends to the line contact 39 which is connectable to the line side bus. 
     Similarly, the fuse 14 is electrically connected to load terminal 40 through fuseclip 42. Fuseclip 42 extends through the removable fuse holder where it becomes contact 24 described above. A male connection piece 44 disposed in base unit 30 mates with the contact 24 when the removable fuse holder 10 is inserted into the base unit 30. The connection piece 44 then extends to load terminal 40, thereby completing the electrical connection. 
     The operation of the alarm circuit will now be described with respect to FIGS. 3 and 4. When the fuse 14 becomes nonconductive, i.e., the filament 100 burns out and this causes filament 101 to open. Filament 101 holds pin 45 in place against spring pressure created by spring 102. When filament 101 opens, pin 45 is pushed up toward conductive plate 103 by unconstrained spring 102. Wire 104 is electrically connected to fuse cap 105 and pin 45 such that when pin 45 contacts electrical plate 103, an electrical connection is made to fuse cap 105 through wire 104. Since fuse cap 105 is also held by line fuse clip 34, electric power from fuse clip 34 is connected to light LED 48 by pin contact 45. 
     The provision of a bipolar LED 48 allows, among other advantages, for fused disconnects according to the present invention to be used regardless of the polarity of the line current. Another feature of the present invention, illustrated very generally in FIG. 4, is the provision of a parallel combination of the bipolar LED and a relatively large resistor 56. In this way, should bipolar LED 48 burn out, the alarm circuit will not be broken and current will flow through the resistor 56 to produce the alarm signal at alarm output 50. 
     Exemplary embodiments of the present invention also provide a method for testing the alarm circuit manually as will now be described with respect to FIGS. 3, 4 and 5. FIG. 5 illustrates a top view of the base unit 30 with the removable fuse unit 10 inserted therein. Also visible from this perspective are the viewing window 16 and bipolar LED 48. An aperture 60 is provided in the top of the removable fuse holder 10 for probing with an elongated tool (not shown). 
     Looking back to FIG. 3, within the aperture 60 is a metal tab 62 which can be deflected by inserting the elongated tool therein. When deflected, the metal tab 62 contacts pin 64 which is connected to the line side and is, therefore, always hot when the fused disconnect is attached to its respective buses. When metal tab 62 contacts pin 64, the alarm circuit is completed, the LED 48 lights and an alarm signal is sent out over alarm output 50. In this way, the alarm circuit can be tested without removal of the fused disconnect and without interruption of power service to the equipment serviced by the fused disconnect. 
     According to another exemplary embodiment of the present invention, elements of the power distribution panel for a telecommunication DC power frame and elements of a fused disconnect switch can be provided in a single package. This exemplary embodiment of the present invention is illustrated in FIGS. 6 through 9. In FIG. 6, a housing 70 can hold a plurality of removable fuse holders 10 (only one of which is shown in FIG. 6 in its inserted position). The housing is attached to a base unit 72 in which a unit line side bus bar/line connection terminal 74 distributes power to several load side terminals through the fuse holders 10 via connections 78 as may be understood by reference to FIG. 9. Although four load side terminals 80, 82, 84 and 86 and four line side contacts 90, 92, 94 and 96,are shown in the exemplary embodiment illustrated in FIGS. 6 through 9, those skilled in the art will readily appreciate that any number of terminals and corresponding disconnect devices 10 can be provided, e.g., eight terminals. 
     Each load side terminal contains fused circuit protection, in this exemplary embodiment, by way of the removable fuse holder 10 which is described in more detail above. The alarm contact 50 of each removable fuse holder 10 provides an alarm signal as described in the previous exemplary embodiment, but is now attached to a unit alarm bus 76 which provides an alarm indication in the event that a load side fault occurs. 
     As best seen in the cut-away view of FIG. 7, the removable fuse holders 10 can be readily removed and inserted into the housing without adapters in direct connection with the load and line buses. This ease-of-use feature of the present invention for this exemplary embodiment having multiple removable fuse holders is facilitated by, for example, the shape and positioning of the blade contact 26 relative to the removable fuse holder 10. As illustrated in FIG. 7, each blade contact 26 can be inserted into a corresponding contact 38, with a plurality of these contacts extending in a line across the housing 70. 
     By way of the present invention, the provision of common line and alarm buses eliminates additional parts which are conventionally required to carry current in telecommunication systems where power is distributed via individual bus bars and individual load circuit protection devices. This is both cost-effective and, by reducing the number of connections involved, provides a reduction in the number of potential failure points in the system. This type of modular construction also saves material and labor at an installer&#39;s site. 
     The above-described exemplary embodiments are intended to be illustrative in all respects, rather than restrictive, of the present invention. Thus the present invention is capable of many variations in detailed implementation that can be derived from the description contained herein by a person skilled in the art. All such variations and modifications are considered to be within the scope and spirit of the present invention as defined by the following claims.