Note: Descriptions are shown in the official language in which they were submitted.
CA 02265232 1999-03-11
Background of the Invention
The present invention relates generally to current
sensing devices for electrical systems, and more particularly to
resettable alternating current fault indicators.
Various types of self-powered fault indicators have
been constructed for detecting electrical faults in power
distribution systems, including clamp-on type fault indicators,
which clamp directly over cables in the systems and derive their
operating power from inductive coupling to the monitored
conductor, and test point type fault indicators, which are
mounted over test points on cables or associated connectors of
the systems and derive their operating power from capacitive
coupling to the monitored conductor. Such fault indicators may
be either of the manually reset type, wherein it is necessary
that the indicators be physically reset, or of the self-resetting
type, wherein the indicators are reset upon restoration of line
current. Examples of such fault indicators are found in products
manufactured by E.O. Schweitzer Manufacturing Company of
Mundelein, Ill., and in U.S. Pats. No. 3,676,740, 3,906,477,
4,063,171, 4,234,847, 4,375,617, 4,438,403, 4,456,873, 4,458,198,
4,495,489, 4, 4,974,329, and 5,677,678 of the present inventor.
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Detection of fault currents in fault indicators is
typically accomplished by means of magnetic switch means such as
a magnetic reed switch in close proximity to the conductor being
monitored. Upon occurrence of an abnormally high fault-
associated magnetic field around the conductor, the magnetic
switch actuates a trip circuit which produces current flow in a
trip winding to position an indicator flag visible from the
exterior of the indicator to a trip or fault indicating position.
Upon restoration of current in the conductor, a reset circuit is
l0 actuated to produce current flow in a reset winding to reposition
the target indicator to a reset or non-fault indicating position.
In certain applications, such as where the fault
indicator is installed in a dark or inaccessible location, the
need arises for a light indication in addition to the flag
indication. Repair crews can then more easily find the location
of the fault.
In certain of these applications the need also arises
for auxiliary contacts in the fault indicator for indicating or
recording the detection of a fault current at a location remote
from the fault indicator. For example, where fault indicators
are installed in each of multiple distribution circuits fed from
a common source, it may be desirable to monitor the fault
indicators at a central monitoring facility to enable a fault to
be quickly isolated. Repair crews can then be efficiently
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CA 02265232 1999-03-11
designated to the known location of the fault.
Because of the compact construction and limited power
available in self-powered fault indicators it is preferable that
the light indication be provided with minimal additional
circuitry and structure within the fault indicator while
providing reliable and extended operation following occurrence of
a fault. The present invention is directed to a novel fault
indicator light and auxiliary contact circuit which meets the
above requirements by utilizing a magnetic winding, such as the
actuator winding of the electro-mechanical indicator flag
assembly typically utilized in fault indicators, in conjunction
with a magnetic circuit to connect an internal battery upon
occurrence of a fault.
Accordingly, it is a general object of the present
invention to provide a new and improved fault indicator having a
light indication and contact closure indicative fault occurrence.
It is a more specific object of the present invention
to provide a new and improved self-powered fault indicator which
provides a light indication and contact closure for an extended
period of time following occurrence of a fault current in a
monitored conductor.
It is a still more specific object of the present
invention to provide a fault indicator wherein a light-indication
and contact closure are provided utilizing the electro-magnetic
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flag indicator assembly of the fault indicator in conjunction
with an internal battery.
summary of the Invention
The invention is directed to a fault indicator for
indicating the occurrence of a fault current in an electrical
conductor. The fault indicator comprises a housing, a battery, a
lamp operable from the battery and viewable from the exterior of
the housing, a first magnetic circuit including a magnetic pole
piece, a magnetically actuated switch and a bias magnet, the bias
magnet having a magnetic polarity which opposes a magnetic field
in the magnetic pole piece in one direction, and reenforces a
magnetic field in the magnetic pole piece in the other direction,
whereby the magnetically actuated switch is conditioned to open
in response to a magnetic field in the one direction and closed
in response to a magnetic field in the other direction, a
magnetic circuit including a magnetic pole piece, a second
magnetic actuated switch and a bias magnet, the bias magnet
having a magnetic polarity which opposes a magnetic field in the
magnetic pole piece in one direction, and reenforces a magnetic
field_in the magnetic pole piece in the other direction, means
including a magnetic winding in magnetic communication with the
magnetic pole pieces and responsive to the current in the
monitored conductor for developing magnetic fields in the
magnetic pole pieces in the one direction to condition the
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CA 02265232 1999-03-11
switches open during normal current flow in the monitored
conductor, and for developing magnetic fields in the magnetic
pole pieces in the opposite direction to condition the switches
closed upon occurrence of a fault current in the conductor, the
first magnetically actuated switch connecting the battery to the
lamp whereby the lamp lights in the fault indicating state, and
circuit means associated with the second magnetically actuated
switch whereby the switch provides a contact closure for use in
external signaling or control.
brief Descrjeption 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
objects 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 of an electric field
powered clamp-on fault indicator constructed in accordance with
the invention installed on a cable within a power distribution
system.
Figure 2 is a front view of the fault indicator of
Figure 1 showing the indicator flag and indicator light thereof.
Figure 3 is a cross-sectional view of the fault
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indicator of Figures 1 and 2 taken along line 3-3 of Figure 2.
Figure 4 is a cross-sectional view of the fault
indicator of Figures 1-3 taken along line 4-4 of Figure 3.
Figure 5 is a cross-sectional view of the fault
indicator of Figures 1-3 taken along line 5-5 of Figure 3.
Figure 6 is a cross-sectional view of the fault
indicator of Figures 1-3 taken along line 6-6 of Figure 3.
Figure 7 is a cross-sectional view of the fault
indicator of Figures 1-3 taken along line 7-7 of Figure 3.
Figure 8 is an exploded perspective view showing the
principal components of the magnetic actuator of indicator flag
assembly utilized in the fault indicator of Figures 1-4.
Figure 9 is a perspective view of the~assembled
magnetic actuator of the indicator flag assembly.
Figure 10 is an enlarged cross-sectional view of the
indicator flag assembly taken along line 10-10 of Figure 9.
Figure 11 is a cross-sectional view of the indicator
flag assembly taken along line 11-11 of Figure 10.
Figure 12 is an electrical schematic diagram of the
circuitry of the fault indicator shown in Figures 1-7.
Figures 13A and 13B are diagrammatic views of the
principal components of the indicator flag assembly of the fault
indicator in a reset indicating position.
Figures 14A and 14B are diagrammatic views similar to
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Figures 13A and 13B, respectively, showing the principal
components of the indicator flag assembly in transition between a
reset indicating position and a fault indicating position.
Figures 15A and 15B are diagrammatic views similar to
Figures 13A and 13B, respectively, showing the principal
components of the indicator flag assembly in a fault indicating
position.
Figure 16A and 16B are diagrammatic views similar to
Figures 13A and 13B, respectively, showing the principal
components of the indicator flag assembly in transition between a
fault indicating position and a reset indicating position.
Figure 17 shows a flasher circuit for use in the fault
indicator of Figure 1-16.
~escripof the Preferred Embodiment
Referring to the Figures, and particularly to Figure 1,
a clamp-on electric field powered current-reset fault indicator
constructed in accordance with the invention for indicating
fault currents in an electrical feeder or distribution cable 21
is seen to include a circuit module 22. In accordance with
20 conventional practice, the circuit module is attached to the
outer surface of cable 21, which may include a central conductor
25, a concentric insulating layer 26, and an electrically-
grounded rubber outer sheath 27.
Basically, circuit module 22 includes a housing 30
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CA 02265232 1999-03-11
(Figure 2) within which circuitry for sensing fault currents is
contained, and a clamp assembly 31 for attaching the module to a
monitored conductor (such as cable 21) and for providing
sufficient electric field coupling to the conductor to power the
circuitry of the circuit module. An eye 36 on housing 30 may be
provided to allow use of a conventional hotstick during
installation or removal. A battery holder 28 within housing 30
includes a removable end cap 29 which provides~access to a
cylindrical battery compartment within which a battery 36 (Figure
5) is contained.
The indicator circuit module 22 also includes, in
accordance with conventional practice, a status-indicating flag
40 for indicating circuit status. The flag 40 may be viewed
through a window 41 at the front of the indicator module. In
operation, during normal current flow in conductor 21, indicator
flag 40 is positioned by circuitry in circuit module 22 so as to
be out of view. Upon the occurrence of a fault current in the
conductor, the indicator flag is repositioned by the circuitry so
as to present a red or fault-indicator surface for view through
windows on the front face of the module. The fault indicator
circuit includes an actuator winding 42 wound on bobbins 43 to
provide the required magnetic force.
Within housing 30 components of the fault indicator are
mounted a circuit board 44. These components include a magnetic
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reed switch 45, which is positioned with its axis perpendicular
to and spaced from the axis of conductor 21 so as to respond to
fault currents in the conductor in a manner well known to the
art.
To provide a first electrostatic point to the electric
field surrounding conductor 25 fault indicator 20 includes a flat
metallic plate 50 immediately behind circuit board 44.
To provide a second pickup point to the electric field,
fault indicator 20 includes on the inside surface of the
cylindrical housing 30 an electrically conductive band 51 formed
of a metallic material such as brass or steel. Where additional
magnetic shielding is required for the components of the
indicator steel is the preferred material for band 51, since it
provides a significant degree of magnetic shielding. The band,
which extends around a substantial portion of the inside
circumference of housing 30, is connected to the circuitry of the
fault indicator by a flexible conductor 52. Beneath band 51 the
inside surface of housing 30 may be provided with an
electrically-conductive coating which extends rearwardly of the
ring, and forwardly of the ring, to provide a greater degree of
electrostatic coupling.
The projecting end of housing 30 includes a transparent
section 53 through which an internal signal lamp 54 can be
viewed. Within housing 30 an integral partition 55 (Figure 3)
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serves as a mask and spacing element and a support for lamp 54,
and a transparent end cap sonically welded to the end of the
housing seals the interior against contamination while providing
the viewing windows 41 (Figure 1).
To provide an indication of the occurrence of a fault
current, the indicator module includes within the lower end of
housing 30 the indicator flag 40 mounted for rotation about a
pivot axis 57. As best seen in Figures 13-16;~target indicator
40 has a red face, which is only visible through window 41
following occurrence of a fault.
Secured to and pivotal with indicator flag 40 is a flag
actuator magnet 58 which is formed of a magnetic material having
a high coercive force, such as ceramic, and is magnetically
polarized to form four magnetic poles of opposite polarity, as
indicated in Figures 13-16, with like magnetic polarities along
diameters of the magnet.
A four pole pole piece 59, which is preferably formed
of a magnetic material having a relatively low coercive force,
such as chrome steel, is positioned in magnetic communication
with flag actuator magnet 58.
Energization of winding 42 by current in one direction
upon occurrence of a fault current in conductor 21, and
energization of winding 42 by current in the opposite direction
upon restoration of current in conductor 21, is accomplished by
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means of circuitry contained within circuit module 22.
Referring to Figure 12, the circuitry of fault
indicator 20 is seen to comprise a first rectifier network
comprising a pair of rectifier diodes 60 and 61 connected to the
capacitive pickup plate 50. A second rectifier network
comprising a pair of rectifier diodes 62 and 63 is connected to
the electrically conductive ring 51 and electrically conductive
coating provided on the inside surface of housing 30 to provide a
capacitive coupling to ground. Together, the two rectifier
networks provide high input impedance rectification of the
alternating current derived from the electric field surrounding
the monitored conductor 25 of cable 21 by the two radially-spaced
electrostatic pickup points to provide energization of the trip,
reset and trip inhibit circuits of fault indicator 20.
The pulsating direct current developed by diodes 60 and
61 during normal current flow in conductor 25 is applied to a
trip capacitor 66 and a trip inhibit capacitor 66 connected in
series across the output terminals of the network. A zener diode
67 connected across trip capacitor 65 limits the voltage
developed across that capacitor to the threshold voltage of the
zener diode, typically in the order of 50 volts, and a forward-
biased diode 68 connected across trip inhibit capacitor 66 limits
the voltage developed across that device during the discharge
cycle of the forward drop of the diode, typically in the order of
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.07 voltage. The pulsating direct current developed by diodes 62
and 63 is applied to a reset capacitor 70 connected across the
output of the second rectifier network, causing that capacitor to
also be charged during normal current flow.
To provide for periodic reset of the fault indicator
capacitor 70 is periodically discharged through the series-
connected windings 42 of the flag indicator assembly. To this
end, a silicon controlled rectifier 72 is periodically
conditioned into conduction by the discharge of a neon lamp 73 in
the gate circuit of the SCR upon the voltage across capacitor 70
exceeding a predetermined threshold level. The neon lamp,
because of its relatively high threshold, typically in the order
of 60 volts, is particularly attractive as a threshold device.
However, it will be appreciated that where desired other
avalanche type devices, such as four-layer diodes can be provided
for this purpose. A resistor 74 completes the gate circuit.
Following the discharge of reset capacitor 70 through
winding 42, the voltage across the capacitor drops, neon lamp 73
extinguishes, and SCR 92 ceases to conduct. Capacitor 70 then
begins_to recharge until the voltage the across reset capacitor
70 again reaches the threshold level of neon lamp 73 and
conduction through SCR 72 accomplishes another reset cycle. The
repetition rate of the reset cycle is dependent on the
capacitance of reset capacitor 60 and the effective impedance of
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the charging circuit. In practice, the reset cycle may occur
approximately every 2 minutes.
Upon occurrence of a fault current in conductor 25,
trip capacitor 65 is caused to discharge in a reverse direction
through winding 42 through a second silicon controlled rectifier
(SCR) 75. This results from closure of reed switch contacts 45,
which are positioned in close magnetic proximity to cable 21 and
connected to the gate electrode of SCR 75 through a gate circuit
comprising a series resistor 77 and a resistor 78.
Trip capacitor 65 continues to discharge until the
discharge current is no longer sufficient to maintain conduction
through SCR 75. Magnetic pole piece 59 of the flag indicator
assembly however remains in a magnetic polarity which maintains
the indicator flag 40 in a fault indicating position. Upon
restoration of normal current in conductor 25 it remains for the
reset circuit of reset capacitor 70 to remagnetize pole piece 59
so as to reposition flag indicator 40 to a reset-indicting
position.
To prevent false fault indications by fault indicator
20 as_a result of inrush current associated with initial power-up
of the monitored conductor 25, the fault indicator includes the
trip inhibit capacitor 66 and its associated inhibit circuitry
for discharging trip capacitor 65 upon such initial power-up. In
particular, the control electrodes of an enhanced FET-type
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transistor 71 are connected across trip inhibit capacitor 66
through a resistor 82. During normal operation the forward-bias
of diode 67 present across capacitor 86 constitutes a reverse
bias to transistor 81 which biases the transistor into cut-off.
Upon loss of excitation trip capacitor 65 is caused to partially
discharge through a resistor 73 into trip inhibit capacitor 66,
causing the voltage across that device to reverse polarity and
progressively increase in the reverse direction as the device is
charged. Eventually the threshold voltage of transistor 81 is
reached and the transistor is rendered conductive by the applied
forward bias from capacitor 66, causing trip capacitor 65 to
rapidly discharge through a resistor 104 and therefore be
unavailable for providing trip current to winding 42 upon
conduction by SCR 75. Thus, after loss of voltage in conductor
25 the fault indicator is non- responsive to a fault current
which occurs following the restoration of power in conductor 25,
and does not become operative for this purpose until capacitor 65
has again been charged. This may in practice require several
minutes.
_ The time required for the fault indicator to respond to
a voltage loss depends on the relative capacitances of capacitors
65 and 66, the resistance of the resistor 73, and the threshold
voltage level of transistor 81, which may typically be in the
order of 3.5 volts. Typically, a response time of 0.1 second is
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obtained, corresponding to approximately 6 alternating current
cycles in a standard 60 hertz system.
The operation of the indicator flag assembly is
illustrated in Figure 13-16. The indicator, which may be
identical in construction and operation to that described in U.S.
Patent No. 4,495,489 of the present inventor, is seen to include
the indicator flag 40, actuator magnet 58, pole piece 59 and
winding 42. The indicator flag includes two indicator segments
on either side of the axis of rotation which preferably each
extend less than 90° around the axis of rotation.
When aligned as shown in Figures 13A-13B the flag
segments are masked and are not visible to the observer through
windows 41. However, upon occurrence of a fault current, flag
member 40 rotates 90° such that the indicator segments are
positioned as shown in Figures 15A and 15B and are visible
through the windows. The indicator segments are preferably
colored red, o r another highly visible color, to clearly
indicate the occurrence of a fault current when viewed through
the windows.
_ Actuation of flag member 40 between reset and fault
indicating positions is accomplished by flag actuator magnet 58
which is rotatably coupled to the flag member by a shaft. The
shaft is maintained in alignment with the axis of indicator
housing 30 by means of bearing surfaces in divider wall 55, which
CA 02265232 1999-03-11
also provides a reset-indicating surface viewable through windows
41, when the indicator flag is in its reset position. This
surface is preferably colored white, or some other color
contrasting with the color of the indicator flag segments, to
clearly indicate a reset condition when viewed through the
windows.
Actuator magnet 58, which may be formed of a magnetic
material having a high coercive force, such as ceramic, is formed
to provide four magnetic poles of opposite polarity, with
opposite polarities at 90° about the circumference of the magnet.
Actuator magnet 58 and hence indicator flag 40, are biased to the
position and magnetic polarities shown in Figures 13A and 13B
when the fault indicator 20 is in a non-trip or reset condition
by means of a generally cross-shaped magnetic pole piece 59
formed of a magnetic material having a relatively low coercive
force, such as chrome steel. The pole piece includes four
magnetic poles in magnetic communication with flag actuator 58.
Upon loss of voltage in conductor 25 pole piece 68 is
remagnetized to the magnetic polarities shown in Figures 14A-14B
and 15A-15B by momentary energization of magnetic winding 42. As
a result, the poles of flag actuator magnet 58 are repelled by
adjacent like-polarity poles of the pole piece and the indicator
flag is caused to rotate 90° to the indicating position shown in
Figures 15A-15B. In this position, the red indicator segments of
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CA 02265232 1999-03-11
the indicator flag 40 are visible through windows 41 and a
lineman viewing the fault indicator is advised that a fault
current has occurred in conductor 25.
The indicator flag 40 remains in the fault-indicating
position until the poles of pole piece 59 are subsequently
remagnetized to the magnetic polarity shown in Figures 16A-16B by
momentary application of a reset current to winding 42. This
causes flag actuator magnet 58 to again be repelled by the
adjacent poles of pole piece 59 so as to rotate indicator flag 40
to a vertical position, as shown in Figures 13A-13B.
By reason of the highly effective electrostatic
coupling provided to the electric field surrounding conductor 21
by the combination of plate 44 and conductive coating 567, and
the high input impedance of the fault indicator circuitry,
sufficient voltage is derived from the potential gradient around
the conductor to obviate the need for external grounding
connections or electrically conductive ground plane members
projecting from the housing.
A contact closure for actuation of indicator lamp 54 is
obtained in fault indicator 20 upon occurrence of a fault current
in monitored conductor 21 by providing a first auxiliary magnetic
circuit. In particular, and referring to Figures 3-9, the first
magnetic circuit is formed by a first U-shaped magnetic pole
piece 84, a reed switch 85 and a bias magnet 86. Pole piece 84,
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CA 02265232 1999-03-11
like pole piece 59, is preferably formed of a magnetic material
having a relatively low coercive force, such as chrome steel.
Winding 42 wraps around both pole piece 59 and pole piece 84, so
that the direction of the magnetic field induced in both pole
pieces is dependent on the direction of current in the winding.
The lead wires of reed switch 85 are attached to the ends of pole
piece 84 by metallic caps 87 (Figure 11), to complete the
magnetic circuit. To avoid a short circuit across the switch the
lead wires are electrically isolated from the pole pieces by
means of sleeves 88 formed of a vinyl or other non-electrically
conductive material.
In operation, when fault indicator 20 is in a reset
state with indicator flag 40 positioned as shown in Figure 13A,
and the magnetic circuit through reed switch 85 is as shown in
Figure 13B. In the absence of bias magnet 86 the magnetic field
between the poles of pole piece 84 would cause the contacts of
reed switch 85 to close. However, bias magnet 86 is polarized to
oppose the magnetic poles as now polarized so that the field
between the poles is sufficiently weakened so that the reed
switch contacts do not close and no fault is signaled.
Upon occurrence of a fault, the polarity of the
magnetic poles of pole piece 84 changes, as shown in Figures 14B
and 15B. Magnet 86 now works to strengthen the magnetic field
applied to the reed switch contacts, and the contacts close.
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To prevent undesired actuation of reed switch 85 from
the external magnetic field associated with conductor 25 the
switch is preferably aligned with its axis generally parallel to
the axis of the monitored conductor. With this alignment, to
avoid actuation of the switch by the stray magnetic field of
winding 42, the reed switch 85 may be contained within a
cylindrical sleeve 89 of magnetically conductive material, such
as copper, with bias magnetic 86 may be positioned on the outside
surface of the sleeve with its axis parallel-spaced to the axis
of the reed switch. However, where the monitored conductor is
sufficiently spaced from the reed switch that the magnetic field
of the conductor is not a factor, the reed switch can be aligned
with its axis perpendicular to the axis of the actuator winding
42 as shown in Figure 3 to minimize the effect of winding 42 on
the reed switch. In this case the magnetic shield 89 may not be
required.
The leads of reed switch 85 can be magnetically coupled
to and electrically isolated from the magnetic poles of pole
piece 110 by soldering or otherwise attaching the switch leads to
the metallic caps 87 which are fitted over the sleeves 88.
A light indication of fault occurrence is obtained by
connecting battery 36 through switch contacts 85 to a flasher
circuit 90, which provides a flashing signal to signal lamp 54.
Flasher circuit 90 is preferably a commercially available
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CA 02265232 1999-03-11
component adapted to power lamp 54, which is preferably a light
emitting diode (LED).
With LED 54 positioned as shown behind flag 40, the
light is viewable from the front of fault indicator 22, and from
the sides of the fault indicator through the transparent end
portion 53 of housing 30.
Battery 36 is preferably a thionyl chloride lithium
battery, such as type TL-593-S manufactured by TADIRAN, Ltd. of
Israel, which provides a constant 3.6 volt output to depletion.
Flasher circuit 90 and LED 54 are preferably as shown in Figure
17.
In accordance with the invention, a second auxiliary
contact closure is obtained in fault indicator 20 for external
signaling or contact purposes upon occurrence of a fault current
in monitored conductor 25 by providing a second magnetic circuit
in association with the indicator assembly. In particular, and
referring again to Figures 3-9, the second magnetic circuit is
formed by a second U-shaped magnetic pole piece 91, a reed switch
92 and a bias magnet 93. Pole piece 91, like pole pieces 59 and
84, is preferably formed of a magnetic material having a
relatively low coercive force, such as chrome steel. Winding 42
wraps around both pole pieces 59 and 84 and pole piece 91, so
that the direction of the magnetic field induced in all three
pole pieces is dependent on the direction of current in the
CA 02265232 1999-03-11
winding. The lead wires of reed switch 92 are attached to the
ends of pole piece 91 by metal caps 87 and insulating sleeve 88
to complete the magnetic circuit without developing a short
circuit across the switch. The caps are in turn connected by
lead wires 94 and 95 of a cable 96 to an external location for
signaling and/or control purposes.
In operation, when fault indicator 20 is in a reset
state with indicator flag 40 positioned as shown in Figure 13A,
and the magnetic circuit through reed switch 92 is as shown in
Figure 13B. In the absence of bias magnet 93 the magnetic field
between the poles of pole piece 91 would cause the contacts of
reed switch 92 to close. However, bias magnet 93, like bias
magnet 86, is polarized to oppose the magnetic poles as now
polarized so that the field between the poles is sufficiently
weakened so that the contacts of reed switch 92 do not close and
no fault is signaled.
Upon occurrence of a fault, the polarity of the
magnetic poles of pole piece 91 changes, as shown in Figures 14B
and 15B, and magnet 93 works to strengthen the magnetic field
applied to the reed switch contacts. The contacts now close,
signaling a fault.
To prevent undesired actuation of reed switch 91 from
the external magnetic field associated with conductor 25, the
switch, like reed switch 85, is preferably aligned with its axis
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CA 02265232 1999-03-11
generally parallel to the axis of the monitored conductor. In
this case, to avoid actuation of the switch by the stray magnetic
field of winding 42, the reed switch 92 is preferably contained
within a cylindrical sleeve 97 of magnetically conductive
material, such as copper, with bias magnetic 93 being positioned
on the outside surface of the sleeve with its axis parallel-
spaced to the axis of the reed switch. However, where the
monitored conductor is sufficiently spaced from the reed switch
that the magnetic field of the conductor is not a factor, the
reed switch can be aligned with its axis perpendicular to the
axis of the actuator winding 42 as shown in Figure 3 to minimize
the effect of winding 42 on the reed switch. In this case the
cylindrical magnetic shield 97 may not be required.
Referring to Figure 17, LED 54 is caused to flash by
flasher circuit 90, which includes a bistable multivibrator 100,
such as a type 555 chip and a switching transistor 101. The
multivibrator, which may be a type 555, switches states at a
repetition rate dependent on capacitor and the series combination
of resistors 103 and 104. The output of the multivibrator is
connected to the base electrode of transistor 101 by a resistor,
causing the transistor to alternate between conductive and non-
conductive states, thereby flashing LED 54.
A photocell 106 may be optionally connected across
capacitor 102 to suspend flashing during daylight hours, thereby
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CA 02265232 1999-03-11
increasing the life of battery 36.
It will be appreciated that while the auxiliary contact
arrangement of the invention has been shown incorporated in a
capacitively coupled electric field powered fault indicator, the
inventive arrangement finds equal utility in inductively coupled
current powered fault indicators.
Thus, a compact externally-powered fault indicator has
been described which upon sensing of a fault current provides
dual contact closures for lighting an internal lamp and for
external signaling and control purposes. By utilizing the
existing single electro-mechanical indicator flag assembly, a
minimal number of additional components are required, making the
device especially well suited for economically upgrading existing
fault monitoring systems.
While a particular embodiment of the invention has been
shown and described, it will be obvious to those skilled in the
art that changes and modifications may be made therein without
departing from the invention in its broader aspects, 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.
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