Note: Descriptions are shown in the official language in which they were submitted.
CA 02251542 1998-10-27
PARALLEL TERMINATION APPARATUS
AND METHOD FOR GROUND-CHECK MONITORS
Field of the Invention
This invention relates to apparatus and a method
utilizing a ground-check monitor for detecting a discontinuity
in the ground conductor of the leg or either arm of Y-
configured trailing cables supplying power to two loads i.e
one load at the end of each arm of the Y.
Background of the Invention
In the mining industry, portable power cables, referred
to as trailing cables, are used to supply power to equipment
such as pumps, drills, shovels, and mining machines. A
trailing cable includes three phase conductors, a ground
conductor (also referred to as a ground wire), and an
insulated ground-check wire (sometimes referred to as a pilot
wire) which is used to monitor continuity of the ground wire.
In most jurisdictions, continuity of the ground conductor in
a trailing cable must be monitored in order to ensure that,
in the event of a ground fault, a predictable ground-fault
current will flow and thereby allow coordinated ground-fault
protection to be reliably achieved.
At the junction box of the equipment receiving power from
a trailing cable, a termination device is connected between
the ground-check wire and the ground wire. At the other end
of the trailing cable (i.e. the location of the power source)
the ground-check and ground wires are connected to terminals
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of a ground-check monitor which measures and monitors the
electrical characteristics between these terminals. The
termination device used with a given ground-check monitor is
matched, by its circuit characteristics, to that particular
monitor type. In operation, the ground-check monitor permits
the trailing cable to be energized (i.e. powered) only if the
circuit characteristics associated with the termination
device are detected: otherwise, if either the ground wire or
the ground-check wire is broken, if the ground-check wire is
shorted to ground, or if the termination device is removed,
the ground-check monitor will not detect such characteristics
and will cause the cable to be de-energized. Typically the
termination device is a diode or zener diode.
In some applications, power is supplied to two pieces of
equipment from one remote power supply. It is desirable to
use one long trailing cable to feed a splitter box which feeds
two separate cables to supply power to the two pieces of
equipment (loads). This configuration of the trailing cable
is sometimes referred to as a Y-configuration (or T
configuration). However, if a termination device associated
with the ground-check monitor is connected across the ground-
check and ground wires at the end of each cable connecting the
loads to the power source (i.e. so as to monitor the ground
wire of each arm) a problem will be created. In such a
configuration, the electrical characteristics produced by
each of the parallel termination devices corresponds to the
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normal characteristics of the termination device which the
monitor is designed to detect and, therefore, the ground-check
monitor would detect such characteristic and fail to cause
power to be removed if either termination device is removed.
The problem of how to monitor the ground wires of all
sections of such a branched trailing cable has to date been
addressed by expensive and complicated circuit installations.
Such installations utilize either a second ground-check
monitor at the splitter box to monitor the ground-check wire
in the circuit of the second load or a multiplexer at the
splitter box to switch the monitor between the ground-check
wires of each of the two load circuits. Therefore, there is
a need in the industry for a solution to this problem which
is economical yet reliable from a safety perspective.
Summary of the Invention
In accordance with the invention there is provided a
combination of two termination devices for connection in
parallel between the ground-check and ground conductors of the
two arms of a Y-configured trailing cable supplying power to
two loads . Also in accordance with the invention there is
provided a method of detecting a discontinuity in the ground-
check loop of the leg or either arm of a Y-configured trailing
cable supplying power to first and second loads ( wherein the
ground-check loop of each arm includes the ground-check
conductor, the termination device and the ground conductor).
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A discontinuity in the ground conductor of the cable, which
may be due to any of a number of conditions (i.e. if either
the ground wire 6r the ground-check wire is broken, if the
ground-check wire is shorted to ground, or if the termination
device is removed), is detected by a monitor at the power
source whenever the monitor fails to detect a predetermined
electrical characteristic between the ground-check and ground
conductors. A first termination device and a second
termination device are provided. The configuration of the
devices is such that, individually, each produces an
electrical characteristic across the ground-check and ground
conductors which does not satisfy the predetermined electrical
characteristic. However, the first and second termination
devices together, connected in parallel, produce an electrical
characteristic across the ground-check and ground conductors
which does satisfy the predetermined electrical
characteristic.
The predetermined electrical characteristic may be a
zener-diode characteristic. Similarly, each of the first
and second termination devices may, individually, produce a
zener-diode characteristic with one of the zener-diode
characteristics being switched on and off at a predetermined
frequency.
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Description of the Drawings
The present invention is described in detail below with
reference to the'following drawings in which like reference
numerals refer throughout to like elements.
Figure 1 is a cross-sectional view of one example of a
trailing cable;
Figure 2 is a schematic diagram showing the installation
of ground-wire monitoring apparatus within a power-supply
centre and a trailing cable extending from the power-supply
centre to a single load and a matching termination device for
that ground-wire monitor;
Figure 3 is a functional block diagram of a portion of
the ground-wire monitor and termination device shown in Figure
2;
Figure 4 is a block diagram showing a portion of the
ground-wire monitor shown in Figure 2;
Figure 5 is a circuit diagram of the termination device
shown in Figure 2;
Figure 6 is a block diagram showing the ground-wire
monitor and power supply of Figure 2 and a trailing cable
extending from the supply to a splitter box and splitting into
a Y-configuration to supply power to two loads;
Figure 7 is a circuit diagram of a first termination
device in accordance with the invention for connection across
the ground-check and ground wires of the trailing-cable arm
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(A) which supplies power to the first load shown in Figure 6;
and,
Figure 8 is 'a circuit diagram of a second termination
device in accordance with the invention for connection across
the ground-check and ground wires of the trailing-cable arm
(B) which supplies power to the second load shown in Figure
6.
Detailed Description of a Preferred Embodiment
The inventors have developed parallel termination devices
l0 which are installed between the ground-check and ground wires
at the end of each of the trailing cables which are the arms
of a Y-configured trailing cable and which simulate, through
their parallel-circuit characteristics seen by a ground-check
monitor at the main power supply, the characteristics of the
single termination device which is designed for use with that
particular ground-check monitor.
The embodiment which has been developed by the inventors
is illustrated by Figures 6-8 and is designed for use with the
SE-134* (*trademark) ground-check monitor manufactured and
sold by the assignee of the subject matter claimed herein.
A copy of the specification sheet pertaining to the SE-134
monitor, which is available to the public, is appended hereto
as Appendix A. It is to be understood, however, that
alternative embodiments may be implemented in accordance with
the invention, if desired, for use with any other ground-check
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monitor which operates by looking for circuit characteristics
associated with a particular termination device.
Figure 1 of 'the drawings shows a cross-section of one
example of a trailing cable 10 having three phase conductors
30, two ground conductors (wires) 40 and an insulated ground
check wire 50. The ground conductors are connected together
at both ends and are collectively referred to herein as the
cable ground, ground conductor, or ground wire 40. Figure 2
schematically shows the installation of a ground-check monitor
20 and its termination device 80 used to monitor the
continuity of a ground wire 40 of a trailing cable 10
supplying power to a single load 90. Ground wire 40 of the
cable 10 is connected at the main power supply 60 to ground
terminal (G) 42 of the monitor 20 through the ground bus 70
and at the load to termination device 80 through the frame of
the load 90. The ground-check wire 50 of the cable 10 is
connected at the main power supply 60 to the ground-check
terminal (GC) 52 of the monitor 20 and at the load 90 to the
termination device 80.
Figure 3 illustrates, in a functional block diagram form,
a portion of the ground-wire monitor 20 and its termination
device 80. Figure 4 is a block diagram showing the logic and
drive functions of the ground-wire monitor 20 based on the
inputs V1 and V2 which are continuously measured and analyzed
when the termination device 80 is connected at the load 90
across the ground-check wire 50 and ground wire 40. A DC
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voltage source 100 of 27.5 Volts supplies a resistance 110
and a 24-Volt zener-diode-characteristic device 120 which are
connected in series. The device 120 which is preferred for
use in the monitor 20 is a circuit similar to that of Figure
8 herein but which produces a 24-Volt zener-diode
characteristic (rather than 12-Volt) that is more ideal than
that of an actual 24-Volt zener diode in that the circuit is
less sensitive to temperature and its voltages are more
precise.
The ground-check-wire terminal 52 of the monitor 20
connects to the cathode of the zener-diode-characteristic
device 120, and the ground-wire terminal 42 of the monitor 20
connects to the anode of the zener-diode-characteristic device
120. The trailing cable 10 extends from the monitor 20 and
main power supply 60 to the load 90. The ground-check wire
50 of the cable 10 is connected at the power-supply end to the
ground-check-wire terminal 52 of the monitor 20 and at the
load end to the cathode of the termination device 80 the
circuitry of which, as a whole, provides a 6-Volt zener-diode
characteristic (see Figure 5 showing the circuitry). The
ground wire 40 of the cable 10 is connected at the power-
supply end to the ground-wire terminal 42 of the monitor 20
and at the load end to the anode of termination device 80.
The resistors 150 and 160 represent the resistance of the
ground-check wire 50 and ground wire 40, respectively, and the
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values of these will vary depending on the trailing cable 10
and its length.
The voltages V1 and V2 (130,140) taken across the
resistor 110 and the zener-diode-characteristic device 120 are
measured by the monitor 20 and these parameters are input to
logic and computational circuitry 170 which determines whether
the required 6-Volt zener-diode characteristic of the matching
termination device 80 is present. If it is present,
continuity in wire 40 is determined by the monitor 20 and when
this occurs the monitor's circuitry activates output relay
drivers and this, in turn, causes an output relay 180 to be
activated and allows power to be supplied to the cable 10. The
monitor circuitry is satisfied if the voltage (V2) between the
ground-check and ground terminals remains in a 5-V to 9-V
window (the window is not symmetrical about 6 Volts because
the monitor must tolerate resistance in the ground-check
loop). The monitor 20 determines the current in the ground-
check loop from the value of V1, the voltage across resistor
110 when the value of V2 is less than 24 Volts. The design
of the monitor 20 is such that its logic circuitry will not
accept, as valid, a resistive termination device with a
resistance that could produce V2 = 6 Volts. (This is done by
initially testing for a zener completion.)
Figure 5 is a circuit diagram of the termination device
80 the components of which are identified in Table 1 below.
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Table 1 - Matching Termination Device f80)
Reference no. Component Tune
190 ~ rectifier diode 1N3660R
192 metal oxide varistor 18ZA1
194 inductor 5uH
200 transistor MJ11033
210 resistor 182
220 capacitor 47nF
230 resistor 5622
240 resistor 15002
250 variable resistor 5KS2
2 60 resistor 33202
270 precision reference TL431
The foregoing Figures 1-5 demonstrate the prior art
installation of a ground-check monitor used for a single load
supplied through a trailing cable. Figures 6-8, described
below, illustrate a ground-check monitor installation for a
double load supplied through the arms of a Y-configured
trailing cable in accordance with the invention.
Figure 6 shows, in block form, the main power supply and
ground-check monitor 60,20, the trailing cable 10 extending
therefrom, a splitter box 280 and two cable arms A and B
feeding a first load 290 and a second load 310, respectively.
The ground-check and ground wires of the cable arms A and B
are connected together through first and second termination
devices 300, 320, respectively, connected at the first and
second loads between the ground-check and ground wires.
When connected as described, in parallel, the first and
second termination devices 300, 320 have a combined
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characteristic (i.e. as seen from the monitor 20) which
simulates termination device 80 such that a ground
discontinuity in 'either of the cable arms A or B causes the
combined characteristic of those devices 300, 320 to change
and differ from that of the matching device 80, thereby
causing the monitor 20 to deactivate the output relay 180 and
de-energize the complete cable 10 (including the arms).
Figures 7 and 8 are circuit diagrams of the first and
second termination devices 300 and 320, respectively, the
components of which are identified in Tables 2 and 3 below.
Devices 300 and 320 are both connected across the ground-check
and ground wires 50,40 of the trailing cable 10 and are,
therefore, electrically connected in parallel. Table 4 below
shows the voltage characteristic at the ground-check and
ground wires 50, 40 of the cable 10 for various conditions when
the termination devices 300,320 are connected in parallel as
described with respect to Figure 6.
Table 2- First Termination Device (300)
Reference no. Component Type
350 rectifier diode 1N3660R
360 transient ,voltage 1.5KE33CA
suppressor
370 inductor 5uH
380 transistor MJ11033
390 resistor 472
400 resistor 330052
410 precision reference TL431
420 resistor 27K~2 l~
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430 variable resistor 5KS2
440 resistor 3320f2
1~
450 diode 1N4005
460 ~ capacitor 47uF 63V
470 voltage regulator LM2936
480 capacitor lOUF 35V
490 undervoltage MC33064
sensing circuit
500 microcontroller PIC12C508
510 resistor 1KS2
520 transistor 2N3904
Table 3- Second Termination Devicef320)
Reference no. Component Type
525 inductor 5uH
530 rectifier diode 1N3660R
540 metal oxide 18ZA1
varistor
550 transistor MJ11033
560 resistor 47i~
570 resistor 15002 1~
580 resistor lOKt2 1~
590 precision reference TL431
600 variable resistor 5K~2
610 resistor 3320 ~
1$
Table 4 - Monitor Ground-check and Ground Wire
Voltages for Various Termination Conditions
Termination Tyl~e Voltage Between GC and C Monitor Output
Open Circuit ='~ ~°' Discontinuity
(No load)
w..............
Matching Termin- Continuity
ation Device 80 °"
w.__._...._....
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Termination ='" Discontinuity
Device 300
w "a
,
Termination Discontinuity
Device 320
w.............. "a
Termination ,~, Continuity
Devices 300
and 320 in w "a
Parallel
As can be seen from the second column of Table 4 the
voltage characteristic of the parallel termination devices 300
and 320 is equivalent to termination device 80, namely, a 6-
Volt-Zener-diode characteristic. However, individually the
characteristic of each of the termination devices 300 and 320
is not equivalent to that of termination device 80 and,
therefore, does not satisfy the monitor 20. If only one of
the devices 300 and 320 is connected the average value of the
voltage between the ground-check and ground wires is 12 Volts
DC. However, when both of these parallel devices are
connected the 12-Volt zener dominates the 24-Volt zener and
the action of a switch causes the average value of the voltage
between the ground-check and ground wires to be 6 Volts DC
(i.e. the average value of a 12-Volt square wave with a 50$
duty cycle is 6 Volts).
It is to be understood that the specific monitor and
parallel termination devices described herein are not intended
to limit the invention which, from the teachings provided
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herein, could be implemented and embodied in any number of
alternative combinations of monitor/parallel termination
devices by persons skilled in the art. Rather the invention
is defined by the appended claims.
