Note: Descriptions are shown in the official language in which they were submitted.
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SPECIFICATION
Background of the Invention
The present invention is generally directed to
aircuit monitoring devices, and more particularly, to an
~ ~-oved battery-powered fault indicator.
Various types o~ fault indicators have been
constructed for detecting electrical faults in power
distribu~ion systems, including clamp-on type fault
indicators, wnich clamp over a cable of the system and derive
their operating power from the ma~netic field surrounding the
cable, test-point mounted type fault indicators, which attach
to test point sockets provided on connectors or other system
c- ~onents and derive their operating power by means of a
capacltive coupling to the monitored conductor, and
battery~powered fault indicators, which derive their
operating power from an internal battery. These three types
of fault indicators may be either sel~-resetting, wherein the
fault indication is automatically reset following resumption
o~ current fluw ln the conductor, or manually reset, wherein
an operator must manually clear a fault indication.
In previous fault indicator~ detection and
indication of fault currents has typically been accomplished
by circuitry which included a reed switch positioned in closP
~LO' ~ ty to the monitored conductor, and an electromagnetic
indlcator actuated by the circuitry to either fault or reset
indicating positions. Upon occurrence of a ~ault current,
the reed switch was actuated and current flowed through a
2 ~
magnetic winding to magnetize a magnetic pole assembly, which
magnetically positioned a movable indicator, typically a
rotatably mounted flag member visible from the exterior of
the indicator housing, to a fault-indica~ing position. In
self-resetting indicators, a reset current typically
r~m~netized the magnetic pole assembly to reposition the
~lag member to a reset-indicating position.
The use of an electromagnetic indicators in such
prior fault indicators, while providing satisfactory
performance, undesirably complicated their manufacture by
requiring the fabrication, assembly and adjustment of a
number of precision parts of dif~erent materials. The
physical size o~ practical electromagnetic indicator
constructions prevented a desired reduction in the size of
pr~or fault indiaator housings. Moreover~ in battery-
powered fault indlcators, the use of limited battery power
for the relatively large power requirements o~ an
electromechanical indicator was undesirable.
The present invention overcomes these drawbacks by
providing a battery-powered fault indicator which
incorporates a high impedance non-mechanical electronic
indicator in ths ~orm of a liquid crystal display. This
résults in an improved fault indicator which is more compact
and easier to manufacture, and which provides in a preferred
construction an indication to the user as to whether the
monitored line is powered.
Accordingly, it is a general o~ject of the present
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invention to provide a new and improved battery-powered fault
indicator.
It is a more specific ob~ect of the present
invention to provide a battery-powered fault indicator which
avoids the use of electromechanical indicator mechanisms.
It is a still more specific object o~ the invention
to provide a battery powered fault indicator which is more
compact and easier to manufacture.
Summary of the Invention
The invention is directed to a fault indicator
operable from a battery for indicating the occurrence of a
fault current in a monitored electrical conductor. The fault
indicator includes a housing, and status indicatlng means
within the housing comprising a high impedance display device
having at least a pair o~ display electrodes disposed in
operative association with a layer of voltage-responsive
light controlling material, the display device providing on
the exterlor of the housing a first display condition in the
absence of an actuating signal applied to the display
electrodes, and a second display condition in the presence of
an actuating signal applied to the display electrodes. The
indicator further includes charge storage means, switch means
rendered conductive upon the occurrence of a fault current in
the monitored conductor for momentarily connscting the
battery to the charge storage means to develop a charge
therein following the ~ault current, and circuit means for
connecting the charge storage means to the display electrodes
whereby the high impedance display device is caused to
display the second condition following the fault current.
Brief Description of the Drawings
The features of the present invention which are
believed to be novel are set forth with particularity in the
appended claims. The invention, together with the further
ob~ects and advantages thereof, may best be understood by
reference to the following description taken in conjunction
with the accompanying drawings, in the several figures of
which like reference numerals identify like elements, and in
which:
Figure 1 is a perspective view, showing a
battery-powered fault indicator constructed in accordance
with the invention mounted on a power distribution cable.
Figure 2 is an enlarged cross-sectional view of the
cable-mounted fault indicator taken along line 2-2 of Figure
1 .
Figure 3 is a cross-sectional view of the cable-
mounted fault indicator taken along line 3-3 of Figure 2.
Figure 4 is an enlarged exploded perspective view
showing the principal elements of the liqu~d crystal display
component utilized in the fault indicator of Figure~ 1-3.
E'igure 5 is an electrical schematic diagram of the
fault indicator of Figures 1-3.
Figure 6 is a cross-sectional view of a battery-
powered sel~-resetting fault indicator installed on a power
distribution cable.
Figure 7 is an electrical schematic diagram of the
fault indicator of Figures 5 and 6.
Description of the Pre~erred Embodiment
Referring to the drawings, and particularly to
Figures 1-3, a fault indicator module 20 constructed in
acaordance with the invention for detecting faults in the
conductor 16 of a flexible cable 17 of the type commonly
utillzed in power distribution systems is seen to include a
generally cylindrical housin~ 22, formed of an insulating
plastic or similar material and attached by flexible arms 18
to the cable, within which the electrical components of the
fault indicator are contained. In particular, within housing
22 the fault indicator module includes a disc-shaped
insulator board 23 positioned perpendicularly to the axis of
' 15 the housing at a location intermediate the ends thereof, and
a layer 24 (Figure 4) of high electrical resistance potting
compound at the inside end of the housing for securing the
insulator board in place. Additional electrical components
included in housing 22 include, in accordance with the
29 invention, a battery 25, a reed ~witch 26 aligned tangentally
to the monitored conductor for sensing the occurrence of a
fault current in the conductor, a high impedance electronic
: display component 27 ~or visually indicating the occurrence
of a fault curxent, and an optional second reed switch 28
aligned parallel to the monitored conductor for manually
resetting a fault condition.
As ~hown in Figures 2 and 3, w~thin housing 22 the
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liquid crystal display component 27 is positioned against a
transparent window 30 such that the indicator face of the
component is readily viewable from the exterior of the
housing. A mask formed o~ a thin sheet of opaque material
may be provided on the inside surface of window 30 so that
only the indicator face can be seen. The liquid crystal
di~play component 27 is capable of producing a display 33 in
the form of an "F". Electrical connections are made with the
compo~ent by means of two electrically conductive terminals
35 and 36 arranged along the top edge of the component.
Internally, as shown in Figure 4, the liquid
crystal di~play component 27 includes, generally, a
transparent ~ace plate 40, a front polarizer 42, a glass
plate 43 on which a single transparent backplane electrode 44
is provided, a perimeter seal 46 containing a layer 47 of
twisted nematic liquid crystal mater~ 21, electrically
con~uctive edge contacts 48, a ylass plate 4~ on which a
si~gle transparent indicator segment electrode 50 is
cont~ned, a rear polarizer 52 aligned at right angles to
front polarizer 42, a reflector 53 and a reax supporting
plate 54.
Display component 27 is conventional in
construction and operation. The "F" display is formed by the
nematic liquid crystal layer 47, which in the absence of an
applied electric field has the property of xotating the
po~arizatisn of light as it passes through the molecular
layers of the layer. In particular, as randomly polarized
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light enters the display component through ~ace plate 40,
only vertically polarized light is able to pass through front
polarizer 42. In the absence of an applied electric field,
the polarization of this polarized light is rotated 90~ as it
passes through liquid crystal layer 47 and reaches the rear
polarizer S0. Since the polarization of,the light has been
rotated the light is able to pass through this polarizer onto
the re~lective layer 51, wherein it is reflected back through
polarizer 50, rotated by liquid crystal layer 47 and passed
through polarizer 42 to front plate 40 and window 30. Thus,
in the absence of an applied electric field light èntering
face plate 40 is passed through the device and reflected back
out, presenting a blank or white colored appearance to the
observer.
By applying an electric field between the indicator
electrodes 44 a~d S0 the liquid crystal layer, in the
intervening portion, is caused to pass incoming light without
rotating it~ polarization, thereby selectively blocking the
transmission and re~lection o~ light to the viewing window 30
: 20 in ~he associated "F" pattern. The result is that the
selected character is displayed to the viewer as a darkened
letter.
Referring to Figure 7, the necessary signal for
actuating the "F" display 33 is provided by circuitry within
housing 22. In particular, upon occurrence of a fault
current, battery 25 is momentarily connected by reed switch
26 to a capacitor 63, which is charged by the battery during
the switch closure. A series-connected resistor 64 serves to
control the rate at which the capacitor is charged. The
voltage developed across capacitor 63 by the charge is
applied to the "F" display electrodes of,display component 27
through a resistor 65, causing the display component to
display the desired "F" character. This display continues
until the charge on capacitor 63 and its associated voltage
level depletes to a level insufficient to maintain the
alignment of the molecules in the nematic liquid crystal
layer. In practice, capacitor 63 will have sufficient charge
to maintain the fault indication for up to 36 hours, the
actual display duration belng dependent primarily on the net
charge on the capacitor and the internal lea~age current of
the capacitor. ~ lesser time duration may be provided if
desired by increasing the current drain from the capacitor.
Once the fault indication has occurred, should it
be desired to cancel the "F" indication, as for example when
re-powering the af~ected circuit after a transient fault, it
i8 only necessary to discharge capacitor 63 by actuating the
manual reset reed switch 28 by momentarily applying a
magnetic ~ield at the exterior of the housing. To this end,
the reed switch is preferably located at an accessible
loca~ion near the front face plate of the housing, with its
axis aligned generally parallel to the axis o~ conductor 16
so that the switch will not be actuated by fault currents in
the con~uGtor.
In many applications it is desirable that following
a fault occurrence a fault indicator be self-resetting, i.e.,
automatically return to a reset state upon restoration of
voltage to the monitored conductor. As shown in Figure 7,
this may be accomplished by providing an optional reset
circùit 68 which includes a field ef~ect transistor (FET) 69
having principal source and drain electrodes connected
through ~ resistor to capacitor 63, and a high impedance gate
electrode coupled thxough a series anti-resonance resistor 70
to the monitored AC conductor by means of a capacitive
coupling plate 71.
Upon restoration of voltage in the conductor, FET
69 is caused to conduct and dischar~e capacitor 63. A fixed
series resistance 72 reduces the rate of discharge through
the pri~cipal electrodes of FE~ 69 to preclude capacitor 63
~rom being discharged durin~ a fault, prior to interruption
of voltage on the monitored, conductor, as well as preventing
the discharge curr~nt from exceeding the limits of the FET.
An additional resistor 73 connected between the drain and
gate electrodes biases the device to a cut-o~f state in the
absence of a s1gnal on the gate electrode.
; While the use o~ other types o~ high impedance
voltage-actuated light controlling devices is possible,
liquid crystal display (LCD) components are particularly well
~ adapted to the invention. The high input impedance of these
;25 devices, typically in excess of 10 x 10 15 ohms, results in a
very low current drain from the display capacitor. This
allows the capacitor to retain sufficient charge for
operating the display over an extended period, typically in
excess of 36 hours, limited primarily by the internal leakage
current of the capacitor. In practice, this is more than
sufficient time for a repair crew to locate and repair a
fault, even in a large power distribution system. A lesser
reset time may in fact be preferred in some applications.
Furthermore, depending on the particular liquid
crystal matPrial used, LCD components exibit a well defined
threshold voltage below which the display does not respond to
an applied si~nal. In one successful embodiment of the
invention using a Type 7 LCD manufactured by Hamlin, Inc. of
L ke Mills, Wisconsin, U.S.A., this threshold is 2.0 volts,
which is suf~iciently high to prevent the display from
falsely responding to residual voltages, but not no high as
~15 to require an undesirably large charge on the display
c~p~citor.
While an "F" shaped display has been shown, it will
be appreciated that other display characters, or symbols, can
be used instead. For example, the "F" can be replaced with
an "S" (for "short-circuit") or a downwardly pointing arrow
symbol.
While particular embodiments of the invention have
been shown and described, it will be obvious to those
ski~led in the art that changes and modifications may be made
therein without departing from the lnvention in its broader
aspe~ts, and, therefore, the aim in the appended claims is
to cover all such changes and modifications as fall within
the true spirit and scope of the invention.
