Patent Document

FIELD OF THE INVENTION 
   The present invention is directed to a method and apparatus for testing grounding systems, and more particularly to a method and apparatus for testing grounding systems in communication towers by isolating the local ground from the utility neutral while simultaneously providing an effective safety ground. 
   BACKGROUND OF THE INVENTION 
   Communication towers include signal processing equipment that is particularly susceptible to noise generated from within the power system and grounding system for the tower. Improper or poor electrical grounding can create personnel safety problems as well as interference with signal processing equipment. Communication towers are generally equipped with power supplies for signal communications, controls, lighting, air conditioning and auxiliary equipment that may introduce transient voltages into the communications signals, particularly if not properly grounded. Furthermore, tall towers attract lightning. The high current of a lightning strike can cause extreme voltage changes if the grounding system does not have a low resistance to earth ground. 
   The National Electrical Code® 2005 Edition (NFPA 70) (Code) defines the terms “grounding electrode”, “grounding electrode conductor” and “grounding conductor” as parts of the tightly connected system referred to here simply as the “local ground”. The Code also defines “grounded conductor” as the line referred to here as the “utility neutral”. The local ground and utility neutral should be electrically connected at a single point, defined by the Code as the “main bonding jumper”. This jumper ideally is a single screw that can be removed to isolate the utility neutral from the local ground.) 
   Generally, as a safety measure, or in order to eliminate sources of interference, it is necessary to test the grounding system of a communication tower periodically to ensure that there is still a low resistance between the communication tower and earth ground. In order to ensure the accuracy of the ground resistance measurements, it is necessary to eliminate possible alternate ground paths or circuits that may falsely indicate good grounding system measurements. For example, the utility AC power system neutral is normally connected to earth ground at multiple sites across its distribution network. If the ground system for the utility distribution system provides a resistance comparable to or lower than that of the local ground, a ground current measuring device will measure the lower resistance of the two systems in parallel, rather than the actual resistance of the local ground. It is therefore necessary to isolate the local grounding system from the utility neutral grounding system during testing. 
   The resistance of local grounding systems is commonly tested using a ground resistance meter and the “fall of potential” method. Two auxiliary electrodes are driven into the soil at significant distances along a straight line away from the ground being tested. During a normal test of a ground rod (the simplest type of local ground) the meter supplies a specific pulsating current between the ground rod under test and the most remote auxiliary electrode. A series of measurements of the voltage drops between the ground rod under test and the intermediate electrode are made by moving the intermediate electrode in steps away from the ground rod under test. The most common equipment used in the fall of potential method generates a 128 Hz pulsating current between the local ground rod and the distant auxiliary electrode. Attempts to use bandpass filters to permit 60 Hz current to flow through the grounding circuit while blocking 128 Hz current feedback through the electrical utility have been unable to filter out all of the test frequency signal. During the fall of potential test the utility neutral must disconnected from the ground rod, as the current flow on the neutral will cause an inaccurate reading. 
   Currently, the methods employed to measure the resistance of local grounding systems at communication towers require that the utility neutral be completely disconnected from the local ground for the communication tower and associated equipment. This eliminates the redundancy of independent grounding systems, and creates an unacceptable risk for communications companies. This lack of redundancy creates a potential danger to personnel and equipment due to the possibility of potential differences between the two grounding systems. Possible high voltage transients present a danger of electrical shock to personnel, who may inadvertently come into contact between the neutral bus and ground during a grounding system test, forming a replacement connection for the open ground to neutral connection. Similarly, sensitive communications equipment power supplies are at risk of exposure to voltage and current in excess of their rated voltage and current due to a floating neutral voltage. 
   Therefore there is a need for a ground testing method and device that provides effective electrical ground isolation from the utility neutral for the ground resistance meter&#39;s signals, and simultaneously provides a grounding path capable of carrying enough current to trip circuit breakers in the event of a “hot” wire shorting to a floating neutral. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to a ground-current isolating circuit for providing a low voltage barrier between a utility neutral conductor and an associated local grounding system. The circuit comprises a first circuit arrangement to permit current in a first direction and a second circuit arrangement to permit current in a second direction. The first and second circuit arrangements have a characteristic threshold voltage for biasing at least one unidirectional element in each circuit arrangement to a conductive state. Current in a forward biased direction is blocked by the at least one unidirectional element until an applied forward biased voltage exceeds the threshold voltage. The circuit optionally includes a first terminal for connecting the utility neutral conductor to the ground isolating circuit, and a second terminal for connecting the associated local grounding system to the ground isolating circuit. In one embodiment, the circuit first and second circuit arrangements each comprise at least one diode. 
   The invention is also directed to a method of isolating a utility neutral conductor of an electrical utility service from an associated local grounding system during testing of a local grounding system associated with a structure. The method comprises connecting a voltage barrier circuit in parallel with a bonding jumper bonding the utility neutral conductor to the associated local grounding system, then disconnecting the bonding jumper after the voltage barrier circuit is in place; conducting a test of the local grounding system associated with the structure (measuring the electrical resistance between the local grounding system and earth); and reconnecting the bonding jumper between the utility neutral conductor and the associated local grounding system before removing the voltage barrier circuit. 
   An advantage of the present invention is improved safety due to the ability to test the local grounding system of a tower without having to disconnect it completely from the utility neutral conductor. 
   Another advantage is the ability to conduct sufficient current to trip a circuit breaker, in the event of failures that would make the utility neutral “hot”. 
   Another advantage is the ability to block ground currents that interfere with testing of structure&#39;s local grounding systems. 
   Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a simplified schematic diagram of one embodiment of the voltage barrier circuit of the present invention. 
       FIG. 2  is a graph showing the voltage-current characteristic of the voltage barrier circuit. 
       FIG. 3  is a block diagram showing the connections among the electric utility lines, the local ground and the communication tower with its associated communications equipment 
       FIG. 4  shows an alternate embodiment of the voltage barrier circuit using a conventional bridge rectifier. 
       FIG. 5  shows an alternate embodiment using a pair of bridge rectifiers. 
       FIG. 6  shows an embodiment having LED indicators and a test probe. 
       FIG. 7  shows an embodiment having a voltmeter. 
       FIG. 8  shows an embodiment having visual warning LEDs, a shorting relay and test terminals. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring first to  FIG. 1 , a basic circuit  10  applying the principle of the present invention is comprised of a pair of diodes  12 ,  14  connected in reverse parallel or antiparallel. When a voltage is applied across terminals  16  &amp;  18 , regardless of the polarity, current flows across the parallel diode path, provided that the voltage is equal to or greater than the forward bias voltage of the diodes. The forward bias voltage is an inherent property of all diodes. The forward bias voltage is defined as the voltage level required to forward bias the diode to a conductive state. Typically, the forward bias voltage of a single diode is very low, on the order of 0.7V, and may vary more or less according to the properties of the particular diode that is used. For the purpose of the present invention it is convenient to think of the forward bias voltage as a threshold voltage for conduction. Each single diode could be replaced by multiple diodes in series to increase threshold voltage and/or by multiple diodes in parallel to increase current-carrying capacity. 
     FIG. 2  is a graph showing the voltage-current profile of the present invention. V th  represents the voltage barrier or threshold of the circuit  10 . As the voltage increases from zero to V th , the current is approximately zero. When the voltage exceeds V th , the circuit will pass large currents with little further increase in voltage drop. As will be described in further detail below, various configurations of the voltage barrier circuits of the present invention provide a voltage barrier ranging from 0.7V to 2.8V, depending on the number of series-connected diodes in each leg of the circuit  10 . Voltage differences in this range present no danger of electrical shock to personnel. Therefore, low-voltage currents generated during the testing of the local ground are blocked from circulating through the voltage barrier circuit  10 , yet dangerous-voltage shorts cause enough current flow to trip circuit breakers. 
     FIG. 3  illustrates the connections among the electric utility lines, the local ground and the communication tower with its associated equipment. Utility power line  104  provides “hot” phases  26  and  27 , plus neutral line  20 . Within service panel  102  current passes through circuit breaker  106  and cable  28  to reach communications equipment  108  associated with tower  100 . The return path for current reaches utility neutral  20  by way of cable  28  and neutral bus  21 . The third wire in cable  28  is one of the safety ground wires  24 . All safety grounds are connected to ground bus  25  and hence to the local ground  23 , and should carry no current unless there is some type of failure or a lightning strike. Main bonding jumper  22  should be the only connection between neutral bus  21  and ground bus  25 . 
   Referring to  FIGS. 1 and 3 , during testing of the local grounding system resistance, main bonding jumper  22  is replaced by circuit  10 , with terminal  16  attached to neutral bus  21  and terminal  18  attached to ground bus  25 . This effectively isolates the local grounding system  23 ,  24 ,  25  for testing purposes, yet still maintains sufficient current-carrying capacity to trip a circuit breaker  106  in case failures would otherwise put dangerous voltages on neutral bus  21 . 
   In the event of a power system failure, e.g., a fault or a lightning strike, the voltage barrier circuit  10  permits the flow of current between the neutral bus  21  and a grounding system  23 , effectively providing the safety of the solid connection  22 . If the power system failure applies a voltage in excess of V th  across the voltage barrier circuit  10 , it causes one or both of the diodes  12 ,  14  to conduct high currents through to ground. The current capacity of circuit  10  is not unlimited, and a lightning strike could destroy it; however, proper component selection, as practiced by those skilled in the art, will yield a circuit capable of withstanding failures in the ordinary utility power circuits. 
   Referring next to  FIG. 4 , a rectifier bridge  30  is used as the voltage barrier circuit  10 . (Catalog data indicates that conventional bridge rectifiers rated for 35 to 50 amps continuous current can handle brief surges of hundreds of amps, sufficient to trip circuit breakers.) A shorting leadwire  32  is connected between the positive terminal  34  and the negative terminal  36  of a conventional bridge rectifier. Diodes  12   a  and  12   b  are series connected in a first path  38 , and diodes  14   a  and  14   b  are series connected in the reverse direction of diodes  12   a  and  12   b  in the opposite path  40 . Both paths follow the same direction through shorting wire  32 . The voltage barrier V th  is approximately 1.4V, since each diode in a pair of series connected diodes requires approximately 0.7V of forward biasing voltage to become conductive, and the respective forward biasing voltage of the pair is cumulative. 
   Referring next to  FIG. 5 , in another embodiment of the present invention the voltage barrier circuit  10  uses a second conventional bridge rectifier  52  in place of the shorting wire  32  of  FIG. 4 . The voltage barrier circuit  10  shown in  FIG. 5  provides four-diode isolation, or in other words, V th  is approximately equal to 2.8 V. A greater threshold voltage V th  provides higher reliability of the ground measurements during testing because the likelihood of a potential difference of 2.8 volts in the ground testing path is less than a potential difference of 1.4V, and 1.4V difference is less likely than a 0.7V potential difference. This greater difference also allows the use of standard LEDs to indicate voltage differences approaching or exceeding V th , as indicated in  FIG. 6 . 
   Rectifier  30  has a negative terminal  36  connected to a positive terminal  56  of rectifier  52  by a first connector  58 . Rectifier  52  has negative terminal  60  connected to positive terminal  34  of rectifier  30  by a second connector  64 . Diodes  12   a ,  13   a - 13   d  and  12   b  form a first series path  38  and diodes  14   a ,  13   a - 13   d  and  14   b  form a second path  40  opposite first path  38 . Note that diodes  13   a  and  13   b  are connected in parallel with diodes  13   c  and  13   d . Both pairs conduct current in the same direction for both paths, just as shorting wire  32  did in  FIG. 4 . 
   The voltage barrier circuit  10  of  FIG. 5  can use off-the shelf components, i.e., bridge rectifiers  30 ,  52 , with a pair of external connectors  58 ,  64 , that may be easily friction-connected to the rectifiers  30 , 52 . Thus, it is unnecessary to modify the rectifier packages as, for example, in  FIG. 4 , where a shorting connector  32  is inserted in the rectifier  30 . 
   Referring next to  FIG. 6 , the voltage barrier circuit  10  is shown with a status indicator for visual signaling to the user of voltage differences between the neutral terminal  16  and the grounding terminal  18 . Two light-emitting diodes (LEDs)  70  and  72 , and the voltage barrier circuit  10  are connected in parallel. First LED  70  is connected in series with a resistor  74 . Preferably, resistor  74  is rated at  10  ohms. First LED  70  has a threshold voltage V LED  of approximately 1.7 V. LED  70  illuminates if the voltage across the neutral terminal  16  and grounding terminal  18  exceeds 1.7 V. Varieties of LEDs can have greater or lesser threshold voltage parameters, and may be substituted in order to give more or less sensitive indications. 
   Second LED  72  is connected in series with a diode  76 , the diode  76  and LED  72  being connected in parallel with both the LED  70  and the voltage barrier circuit  10 . The connection of the diode  76  in series with LED  72  forms a combined threshold of approximately 2.4V for illuminating the LED  72 , since the forward-bias voltage of diode  76  is approximately 0.7 V and the threshold voltage of the LED  72  is approximately 1.7 V. 
   The configuration of indicating LEDs  70 ,  72  described above may also optionally include a test probe  80 , for testing the operation of the LEDs before the neutral connection  16  is made. The probe line  82  includes a diode  84 , and a resistor  86  connected in series with the diode  84  and the probe  80 . The diode  84  is preferably capable of withstanding at least 200 volts of reverse bias potential. Resistor  86  preferably is rated at 2 K-ohms and is capable of dissipating at least 3.5 watts of power. 
   Referring next to  FIG. 7 , another embodiment of the present invention includes a measurement of voltage difference between the service neutral  20  and grounding system  24 . An AC voltmeter  90  is connected across the neutral connection  16  and the grounding connection  18 , in parallel with the voltage barrier circuit  10 . This provides a continuous reading of voltage difference instead of the two-step LED level indicators, but requires battery power or power derived from the AC line itself. 
   The voltage barrier circuit  10  of the present invention may be used to facilitate testing of a communication tower grounding system, but is not limited to such an application. Ground system testing generally is not a continuous process, but is done periodically, as required, to maintain the integrity of the communications or other grounding systems. It is therefore contemplated that the present invention may be utilized as a temporary or permanent installation, to bypass the solid bonding jumper  22 . In a temporary application of the voltage barrier circuit  10 , clamping devices (not shown) may be attached to the neutral bus  21  and to the ground bus  25  of the electrical utility service  102 , to shunt around the solid neutral-to-ground connection  22 , before removing the solid neutral-to-ground connection  22 . The test of the tower  100  grounding system can then be conducted, the neutral-to-ground connection  22  restored upon completion of the ground testing, and the clamping devices removed. 
   Alternatively, the voltage barrier circuit  10  may be installed as a hard wired component of an electrical service panel. In a permanently installed embodiment, referring to  FIG. 8 , a neon bulb  98  may be connected in parallel with the voltage barrier circuit  10  and in series with a resistor  126 . The neon bulb  98  illuminates when the neutral line voltage becomes elevated to line voltage, signaling a hazardous condition. A heavy-duty shorting switch  128  also is connected in parallel with voltage barrier circuit  10 . This switch normally is closed, and is physically configured so that it cannot be accidentally left open. It is opened only during testing, and must be closed before the door to the electrical panel  102  can be closed. If the test measurements of the grounding system test are not measurably affected by opening and closing switch  128 , an improper ground connection is indicated. 
   Also, a pair of opposed LEDs  110  and  112  is provided to indicate a significant DC or AC voltage difference between the neutral connection  16  and the ground connection  18 . If only one of the LEDs  110 ,  112  is illuminated, a DC voltage difference is indicated. If both LEDs  110  and  112  are illuminated, an AC voltage difference is indicated. Finally, battery test terminals  114  and  116  may optionally be connected to LEDs  110  and  112 , respectively. Battery test terminals  114  and  116  provide a means for testing the LEDs  110 ,  112 , for operability. A 9 V battery may be connected across terminals  114  and  116 , to test both LEDs simultaneously. Diodes  120  prevent reverse-biasing LEDs  110 ,  112 , and resistors  122  limit the current. 
   It is noted that the above configurations of the voltage barrier circuit  10  are examples and the invention is not limited to the embodiments shown, as will be readily apparent to those skilled in the art. Other embodiments of the present invention may be configured, for example, by cascading combinations of the above circuits. By selectively cascading two or more configurations, various barrier voltages may be tailored to suit particular test conditions confronted in the field. 
   Furthermore, the invention may be practiced using devices other than diodes, such as vacuum tubes, gas discharge tubes and other power thyristors. 
   While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.

Technology Category: 3