# Patent

## Title
Ground circuit impedance measurement apparatus and method

## Abstract

Novel ground circuit impedance measurement apparatus and method accurately measure ground circuit impedance. A ground circuit and electrical current source of 25 amperes or less simulate a ground fault. Ground impedance is determined by dividing voltage drop by amperage to determine impedance directly. In one aspect, impedance is determined and displayed automatically. Power wires transmit voltage and current data for the circuit being measured. The cable itself serves as a current-carrying and voltage-transmitting conduit to determine directly the circuit impedance. The novel ground circuit impedance device and method measure impedance in underground mine power systems connecting to an outside electrical power substation outside the mine and using portable transformers in the mine. In one aspect, ground circuit impedance is measured automatically in cables over 1 mile long.

## Background
This application is a Provisional of U.S. patent application U.S. Ser. No. 60/403,825 filed Aug. 15, 2002.


BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates to impedance measurement apparatus and method. In one aspect, this invention relates to the design, construction, and installation of ground circuit impedance measurement apparatus in an underground coal mine. In one aspect, this invention relates to the design, construction, and installation of ground circuit impedance measurement apparatus in other industrial applications where accurate measurement of ground circuit impedance is required.
2. Background
In underground coal mines, as well as other industrial applications, medium voltage and high voltage electrical power equipment is commonly used. As an example, in coal mines, the portable, high voltage electrical power equipment is connected to an outside electrical power substation by lengths of high voltage cable with high voltage connectors to couple the cables together as well as to portable transformers.
In order to insure the safety of mine personnel, federal laws require that all high voltage underground equipment be installed with adequate grounding means to insure that the voltage on such equipment not exceed 100 volts in the event of a ground fault (30 CFR 75.001).
INTRODUCTION TO THE INVENTION
No existing products are available which measure ground impedance on certain type circuits. The few companies that do attempt to estimate the circuit impedance do so by means of calculation which involves wire resistance tables when the wire size and length is known. What these calculations fail to take into consideration is the resistance of numerous connections such as high voltage couplers and bolted connections.
The specific problems to be overcome include providing a means of measuring impedance on a circuit which extends as much as several miles in length.
In underground mining applications, the ground fault current is limited to 25 amperes by a neutral grounding resistor. The total ground circuit impedance must not exceed 4 ohms in order to stay under the 100 volts allowed on the equipment frame. Ohm's Law shows the relationship of voltage to current and impedance:

V=IZ

V=(25 amps) (4 ohms)

Although the Federal Coal Mine Health and Safety Act of 1969 mandated the grounding requirement years ago, there remains to this day no practical means of accurately determining ground circuit impedance. The circuit impedance can be estimated by means of calculations which involve wire resistance tables when the wire size and wire length are known. These calculations fail to take into consideration the resistance of numerous connections such as high voltage couplers and bolted connections.
A product is needed to measure directly, without calculations, total circuit impedance including connection impedance.
SUMMARY OF THE INVENTION
The present invention provides novel apparatus and method for measuring ground circuit impedance including a ground circuit having an electrical current source, disconnecting electrical power by opening a main circuit breaker and a visible disconnect switch, a means of connecting power conductors, to the ground conductor at the end of the circuit, connecting an electrical alternating current source which passes current through the ground circuit, accurately measuring both the current introduced into the circuit and the resulting voltage drop produced in the ground circuit, and automatically calculating the ground impedance.
In one aspect, alligator clips are used to connect a current source to one phase wire and the ground wire. Another alligator clip is used to connect the voltage sensing device to another phase lead, which measures the voltage of that phase lead with respect to the ground wire connection. Since all phase leads are connected together at the far end of the circuit to be measured, the measures voltage is that which results from the current through the ground circuit. In applications where cable couplers are used, a special coupler may be used to conveniently connect all power conductors to the ground conductor. Power wires in the circuit to be measured serve as a means for carrying current and transmitting voltage data to determine directly the ground circuit impedance.
In underground mining applications, the novel ground circuit impedance device measures impedance in an underground mine where underground portable transformers are connected to the outside electrical power substation.
In applications which use cable couplers, inserting a special grounding coupler to provide a connection to ground off all power conductors may also be used to provide safety during inspection or troubleshooting.
Ground circuit impedance is measured automatically in cables of any length, including several miles in length.
Further embodiments of the present invention will become apparent from the detailed description which follows in conjunction with the figures of the drawings.



BRIEF DESCRIPTION THE DRAWINGS
FIG. 1 is a schematic diagram of the high voltage circuit to be tested by the present invention including the ground wire and numerous connections which connect six high voltage transformer frames to an earth ground bed via a high voltage cable.
FIG. 2 is schematic diagram of a high voltage coupler of the present invention.
FIG. 3 is a schematic diagram of an AC current source and monitoring gauges for the apparatus and method of the present invention.



DETAILED DESCRIPTION OF THE INVENTION
The apparatus and method of the present invention provide novel ground circuit impedance measuring in medium and high voltage electrical power systems.
Specific embodiments of the apparatus and method of the present invention include means for circulating current through a grounding circuit and measuring the voltage drop across the ground circuit to determine circuit impedance. The novel device of this invention uses one of the power wires to carry current, and another power wire as means to measure voltage.
In one aspect, the novel apparatus and method of the present invention provide a means of measuring circuit impedance on very long cables. Some uses of the present invention provide for direct measurement of either ground circuit impedance or power circuit impedance.
The apparatus and method of the high voltage coupler component, when used, of the present invention also include a preferred method of making the novel apparatus by inserting the coupler in the end of the cable coupler, a connection to ground of all three phase wires in order to provide safety during inspection or troubleshooting.
The present invention also includes a preferred method of installing and constructing the ground impedance measurement apparatus of the present invention.
One of the problems of attempting to measure the impedance of very long circuits is that the two end connections of the circuit to be measured are often miles apart.
The apparatus and method of the present invention provide for means and method for using the cable itself as a current-carrying and voltage-transmitting conduit in order to determine directly the circuit impedance.
To facilitate an understanding of the means and method of operation in accordance with the present invention, reference is made to the illustrations presented in the figures of the drawings.
FIG. 1 is a schematic diagram of the high voltage circuit to be tested by the present invention including the ground wire and numerous connections which connect six high voltage transformer frames to an earth ground bed via a high voltage cable.
Referring now to FIG. 1, an example is shown of a typical high voltage electrical power system, as would commonly be found in an underground mine. Substation 10 is associated with neutral grounding resistor 12 and earth ground bed 14. High voltage cable 16 carries current from substation 10 to high voltage coupler 18 and step down transformer 20, 22, 23, 24, 25 and, 26, which transform the distribution voltage to utilization voltage at difference locations.
FIG. 2 is schematic diagram of a high voltage coupler of the present invention.
Referring now to FIG. 2, a schematic diagram illustrates a high voltage coupler of the present invention. High voltage cable 28 is connected to high voltage complex 30 which connects to all three power wires 32, 34, and 36 to ground wire 38.
FIG. 3 is a schematic diagram of an AC current source and monitoring gauges for the apparatus and method of the present invention. Neutral grounding resistor is shown at 40. Voltmeter 50, ammeter 52, and AC current source 54 are shown connected to high voltage cable 60. Phase 1 is indicated at 62 and phase 2 is indicated at 64. Phase 3 is indicated at 66, and ground is indicated at 68.
Referring now to FIG. 3, a schematic diagram illustrates an AC current source and monitoring gauges for the apparatus and method of the present invention.
The present invention provides novel apparatus and method for accurately measuring the ground circuit impedance by completing the following simple steps. First, the power is disconnected by opening the main circuit breaker and visible disconnect switch. Next, a special high voltage coupler as illustrated in FIG. 2 is inserted into the high voltage coupler at the end of the circuit. Finally, the AC current source with ammeter and voltmeter as illustrated in FIG. 3 is connected via alligator clips to two of the phase wires and the ground wire.
The current is adjusted to 25 amperes or less, which simulates a ground fault by passing the current through the ground circuit. The voltmeter reads the total voltage drop across the ground circuit. The ground impedance, by Ohm's law, is determined by dividing the voltage reading by the current reading.
In respect to a current of 25 amperes, to allow this unit to be more compact, a smaller current is used, which consumes less power and provides for easier portability. The original concept does not change in that the ground impedance is determined by dividing the voltage by the current, but the calculation is made automatically by a microprocessor which allows the impedance to be read directly on a meter or digital display.
The apparatus and method of the present invention also are useful in locating poor connections in high voltage couplers used in underground and surface mines. Poor connections result in a higher than normal resistance circuit, which connections are detected by the apparatus and method of the present invention.
The method and procedure of the present invention used in measuring ground impedance are used for locating poor connections, but rather than connecting one of the alligator clip leads to the ground wire, all three of the leads from the AC current source are connected to the phase leads.
Several advantages are obtained through the use of the preferred apparatus and method of the present invention disclosed in the detailed description herein below. One advantage is an ability to determine accurately if the total ground circuit impedance rises to an unsafe level, where personnel may be exposed to unsafe voltage levels on equipment frames during ground fault conditions.
A further advantage of the apparatus and method of the present invention is an ability to detect poor connections in high voltage couplers which, if left undetected, could result in coupler failure due to overheating.
To facilitate an understanding of the characteristics of the invention, an underground coal mine application is used as an example.
Installation of the invention in an underground coal mine setting or elsewhere is performed by an MSHA-certified electrician who would insure that the circuit is positively disconnected before attaching the high voltage shorting coupler to the end of the cable.
The apparatus and method of the operation of the present invention provide the following differences over previously available impedance measurement devices.
Key benefits and features include the ability to accurately measure impedance directly on extremely long circuits by using the cable itself as a means of transmitting current and voltage data.
The measurement system of the present invention provides an effective and economical means to accurately determine the true impedance of a high voltage electrical power circuit or a high voltage ground circuit.
The high voltage coupler connects the three power conductors to ground at the end of the circuit being tested. This component has some noteworthy advantages over anything presently available for use in shorting power wires to ground for safety reasons during maintenance, trouble-shooting, or inspection.
Returning to the overall system, it has been found that no device is presently available which will perform the function this device does. The problem with measuring impedance on these high voltage circuits results from the very long distance which separates the two ends of the circuit. If the high voltage cable were “looped back” so that both ends of the cable were next to each other, measuring the ground circuit impedance by an ohmmeter connected to each end of the cable would provide an accurate measurement. When the cable ends are far apart, measurement with an ohmmeter requires one lead of the meter be as long as the cable which is impractical and the measurement includes the impedance of the meter lead.
The device of the present invention uses the cable itself as a meter lead to measure the voltage which results across the ground circuit when a current is applied through it. Using the novel device, the impedance of the cable used to measure, acting as a “meter lead” has no influence on the accuracy of the measurement.
In respect to the words “impedance” and “resistance,” the two words are synonymous in that they both describe an opposition to the flow of electrical current, but “impedance” is used in alternating current (AC) circuits, and “resistance” is used in direct current (DC) circuits. Since the novel device has an application in AC circuits only, the correct term is “impedance.”
The apparatus and method of the present invention provides specifically for measuring ground circuit impedance in high voltage underground mine power systems, but is also useful in many other applications. The apparatus and method of the present invention provide accurate means of determining the true impedance of the ground circuit.
The apparatus and method of the present invention is light-weight and portable, and powered by rechargeable battery. The apparatus and method of the present invention measure impedance not just resistance. In one aspect, one pushbutton operation provides a digital display. The apparatus and method of the present invention has been found to be accurate to 0.1 ohm between 0 ohm and 10 ohms.
In applications such as resistance grounded systems where regulations permit a maximum voltage on equipment frames under ground fault conditions, it is essential the ground circuit impedance be available to operating personnel. The apparatus and method of the present invention accurately measures ground circuit impedance using novel apparatus and method which is not affected by pilot wire resistance, contact resistance, or other factors affecting conventional methods.
The present invention has been found to be useful in locating poor connections in high voltage couplers used in underground and surface mines. Poor connections result in higher than normal resistance circuit, which can be easily detected by this device.
The same procedure used in measuring ground impedance is used, but rather than connecting one of the alligator clip leads to the ground wire, all three of the leads from the AC current source would be connected to the phase leads.
The apparatus and method of the present invention as shown and illustrated in the detailed description of the specification and the accompanying figures of the drawings are not intended to be limited to the specific examples shown and described, but the apparatus and method of the present invention are intended to include the novel developments encompassed by the claims which follow and equivalents thereof.

## Claims

1. A method for measuring ground circuit impedance, comprising:
a) providing a ground circuit having an electrical AC current source for supplying electrical power to the ground circuit;
b) disconnecting the electrical power by opening a main circuit breaker and a visible disconnect switch;
c) inserting a high voltage coupler at the end of the circuit;
d) connecting the electrical AC current source with an ammeter and a voltmeter;
e) providing and adjusting AC current to 25 amperes or less, thereby simulating a ground fault by passing the electrical AC current through the ground circuit;
f) measuring total AC voltage drop across the ground circuit; and
g) determining ground impedance by dividing the total AC voltage drop by the electrical AC current to determine impedance directly.

2. The method for measuring ground circuit impedance of claim 1, wherein the ammeter and the voltmeter are connected by alligator clips to two phase wires and to a ground wire.

3. The method for measuring ground circuit impedance of claim 1, wherein the impedance is determined and displayed automatically.

4. The method for measuring ground circuit impedance of claim 1, wherein a high voltage coupler is used to connect the power wires to the ground wire.

5. The method for measuring ground circuit impedance of claim 1, wherein the power wires of the circuit being measured serve as a means for transmitting voltage and current data.

6. A method for measuring a ground circuit impedance of claim 1, comprising measuring impedance in an underground mine.

7. A method for measuring ground circuit impedance of claim 1, wherein said electrical current source comprises an outside electrical power substation outside an underground mine.

8. A method for measuring ground circuit impedance of claim 1, comprising inserting the coupler in the end of the cable coupler to provide a connection to ground of all three phase wires to provide safety during inspection or troubleshooting.

9. A method for measuring ground circuit impedance of claim 1, comprising using the cable itself as a current-carrying and voltage-transmitting conduit to determine directly the circuit impedance.

10. A method for measuring ground circuit impedance of claim 1, comprising using a portable transformer.

11. A method for measuring ground circuit impedance of claim 1, comprising measuring impedance in a cable over 1 mile long.

12. A ground circuit impedance measurement device, comprising:
means for circulating AC current through a ground circuit and means for measuring an AC voltage drop across the ground circuit to determine circuit impedance, further comprising:
a ground circuit having an electrical AC current source for providing electrical power to the around circuit;
a main circuit breaker and a visible disconnect switch;
a high voltage coupler at the end of the circuit;
an ammeter and voltmeter;
means for adjusting AC current to 25 amperes or less, thereby simulating a around fault by passing the AC current through the ground circuit;
means for measuring total AC voltage drop across the ground circuit; and
means for determining around impedance by dividing the AC voltage drop by the AC current reading to determine impedance directly.

13. A ground circuit impedance measurement device as set forth in claim 12, comprising one power wire to carry current and a second power wire to measure voltage.

14. A ground circuit impedance measurement device as set forth in claim 12, comprising means for measuring circuit impedance on cables longer than one mile.

15. A ground circuit impedance measurement device as set forth in claim 12, comprising means for measuring circuit impedance on cables longer than two miles.

16. A ground circuit impedance measurement device as set forth in claim 12, comprising means for measuring circuit impedance on cables longer than three miles.

17. A ground circuit impedance measurement device as set forth in claim 12, comprising cables longer than two miles wherein the ammeter and voltmeter are connected by alligator clips to two phase wires and to a ground wire.

18. A ground circuit impedance measurement device as set forth in claim 12, comprising cables longer than three miles wherein the impedance is calculated and displayed automatically.

19. A method of measuring ground impedance by dividing an indicated AC voltage by an indicated AC current on cables longer than one mile, comprising:
a) providing a ground circuit in an underground coal mine having an electrical AC current source for providing electrical power to the ground circuit;
b) disconnecting power by opening a main circuit breaker and a visible disconnect switch;
c) inserting a high voltage coupler at the end of the circuit;
d) connecting the electrical AC current source with an ammeter and voltmeter by alligator clips to two phase wires and to a ground wire;
e) providing and adjusting electrical AC current to 25 amperes or less, thereby simulating a ground fault by passing the electrical AC current through the ground circuit;
f) measuring total AC voltage drop across the ground circuit wherein the power wires of the circuit being measured serve as a means of transmitting voltage and current data; and
g) determining ground impedance by dividing the AC voltage drop by the AC current reading to determine impedance directly and displayed automatically.