Abstract:
A digital voltage indicator includes a probe having a first electrode including a hollow portion and a probe tip projecting from the hollow portion, a second electrode extending into the hollow portion and spaced therefrom, and dielectric material disposed between the second electrode and the hollow portion. The indicator may include a reference conductor connected to the second electrode and detection circuitry connected between the probe and the reference conductor for detecting a voltage difference therebetween, and an annunciator connected to the detection circuitry for displaying the measured voltage.

Description:
BACKGROUND 
   This application relates to voltage indicators and, in particular, voltage indicators for indicating high voltages, such as AC transmission line voltages in the kilovolt range. The application relates in particular to probes for such voltage indicators. 
   A voltage indicator, in its simplest form, indicates the presence of a voltage near an energized power line or equipment by means of an annunciator, such as a buzzer, light or the like. More sophisticated voltage indicators may be calibrated or adjusted to indicate the presence of voltage only above a predetermined threshold and to give no indication of voltages below that threshold. Still more sophisticated indicators may give an actual indication of the magnitude of the voltage detected, by use of an analog meter display or a digital display. 
   Voltage indicators are, in general, different from voltmeters in that they are single-point measurement devices. In other words, they contact only one line or only one point and do not measure voltage with respect to a fixed reference, such as ground or other line of different phase. Voltage indicators are also generally not able to measure DC voltages, since DC measurements typically require a direct connection to both the energized conductor and a reference conductor, such as ground. 
   Without a direct connection to a reference conductor, a voltage indicator must determine the voltage on an energized conductor based on the strength of the electric field surrounding the conductor which, for a given geometry, is directly proportional to the voltage on the conductor. Thus, voltage indicators can give an estimation of the actual applied voltage by measuring the strength of the electric field surrounding the conductor. However, for a conductor at a given voltage, the strength of the electric field can very significantly, depending upon the geometry of the conductor, the distance between the energized conductor and other conductors or ground, and the placement of the voltage indicator on the conductor. For this reason, voltage indicators that display actual voltage have very large published measurement tolerances, usually +/−25%. The actual measurement error can approach +/−50% for measurements made near ground or a grounded electrical conductor. 
   The reason for this may be explained by reference to  FIGS. 1–3 .  FIG. 1  shows an overhead high voltage electric line  10  supported on poles  11 . The line generates an electric field  15  that surrounds the conductor. Electric fields can be pictured as a series of concentric circles  16  surrounding the conductor, called equipotential lines, because the electric potential or voltage is equal everywhere along the given line. Each of these field lines  16  represents a percentage of the actual line voltage. The high voltage line  10  itself is at 100% voltage, the innermost field line may represent 90% voltage, the next may represent 80%, and so on, out to the outermost circle, which may represent 10% voltage. The spacing between the equipotential lines indicates the strength of the electric field, with closer spacing corresponding to higher field strength. 
   The voltage indicator  20  in  FIG. 1  includes a housing  21  which houses the electronic circuitry of the indicator and is mounted at the end of an elongated hot stick  23 . The housing  21  carries a probe hook  25 , which is connected to the circuitry in the housing  21 . The voltage indicator  20  measures the strength of the electric field by measuring the voltage difference between the conductor  10  that the hook probe  25  is touching and the electric field a distance way from the conductor  10 . In practice, the voltage indicator  20  is actually measuring a voltage between two electrodes, one being the hook probe  25  and the second being a conductive coating on the inside of the housing  21  to shield the electronic circuitry from the strong electric fields. Thus, this shielding coating serves as a reference electrode for measuring the strength of the electric field. It is known from calculation and testing that, for the geometry of a typical single overhead high-voltage conductor, the voltage indicator housing  21  will be located at about the 80% equipotential line when the hook probe  25  is placed over the conductor  10 . Thus, the voltage indicator  20  will measure the difference between the conductor at 100% and the reference electrode at 80%, the difference between the two being 20% of the voltage. The voltage indicator circuitry is calibrated to measure and display 20% of voltage as the actual line voltage, in kilovolts. 
   This arrangement works reasonably well for overhead line conductors as they are typically arranged in power distributions systems. However, in a typical power distribution system, a high voltage line may travel some distance overhead on poles and then be connected to an underground cable to provide power to a residential or commercial subdivision. Commercially available voltage indicators typically have accessory probes available that are specialized for underground applications. The disadvantage of using voltage indicators at or near ground level is that they are generally calibrated for electric fields typical of overhead line geometry, and the geometry of equipment at or near ground levels is very different from overhead.  FIG. 2  shows the electric field  15  from an overhead line  10 , which is connected to underground equipment  17 , and also shows the electric field  15 A surrounding a terminal  18  on the equipment  17  mounted near ground at the same voltage. Electric fields are always distributed completely between an energized conductor and a grounded conductor or ground. If the ground is far way, as it typically is for overhead conductors, the electric field extends radially away from the conductor uniformly in all directions and spreads out over a great distance. If a grounded conductor or ground is close to an energized line or terminal, then the electric field is compressed into the shorter distance between the energized line or terminal and ground and the field strength is higher, because electric field strength is expressed as volts per distance. As can be seen in  FIG. 3 , the energized terminal  18  on the equipment  17  mounted on the ground is very close to both the grounded metal services of the equipment enclosure and the ground itself. When the voltage indicator probe hook  25  is placed on this terminal  18 , the housing  21  is now disposed at about the 20% equipotential line. Thus, the voltage indicator  20  will measure the distance between the conductor at 100% and the reference electrode at 20%, the difference between the two being 80% of the voltage. Because the voltage indicator circuitry has been calibrated to measure and display only 20% of the voltage as the actual line voltage, it will now indicate a voltage magnitude that is substantially higher than the actual voltage on the equipment terminal  17 . 
   Most commercially available voltage indicators have an optional accessory probe for voltage measurements on equipment at or near ground level. These probes are usually called “underground” probes, not necessarily because they are used underground, but because they are used on equipment that is connected to underground power cables. These probes typically provide only for making an electrical connection between the voltage indicator and the different types of terminals. These probes do not address the issue of inaccuracy of measurements made on this equipment. 
   SUMMARY 
   The improvement described herein is for a digital voltage indicator with an accessory probe that can improve the accuracy of measurements made near ground to at least as good as that for measurements on overhead lines. More particularly, there is described an accessory probe which compensates for the close proximity to ground and the resultant higher field strength by providing a substantial impedance between the voltage indicator and the energized terminal at which voltage is being measured. This substantial impedance acts to reduce the voltage measured by the voltage indicator between the accessory probe and its reference electrode. 
   In an embodiment a probe for a high-voltage indicator includes a first electrode including a hollow portion and a probe tip projecting from the hollow portion, a second electrode extending into the hollow portion of the first electrode and spaced therefrom, dielectric material disposed between the second electrode and the hollow portion of the first electrode, and a connector connected to the second electrode. 
   In an embodiment, a high-voltage indicator for use with an AC electric field source near ground comprises a probe including a probe tip for coupling to the source, a reference conductor, detection circuitry connected between the probe and the reference conductor for detecting the voltage difference therebetween, the detection circuitry being calibrated for electric fields typical of sources remote from ground, an annunciator connected to the detection circuitry for indicating the voltage of the source, and a substantial impedance connected between the probe tip and the detection circuitry sufficient to compensate for the greater field strength of the source near ground so as to indicate the source voltage with an accuracy comparable to that achievable when the detection circuitry is used with sources remote from ground. 
   There is also disclosed a method of measuring a high voltage of an AC electric field source near ground comprising providing detection circuitry coupled between a probe tip and a reference conductor and calibrated for electric fields typical of sources remote from ground, exposing the probe tip to an AC electric field source near ground, compensating for the greater field strength of the source near ground by connecting a substantial impedance between the probe tip and the detection circuitry, and measuring the voltage difference between the compensated probe tip and the reference conductor. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For the purpose of facilitating an understanding of the subject matter sought to be protected, there are illustrated in the accompanying drawings embodiments thereof, from an inspection of which, when considered in connection with the following description, the subject matter sought to be protected, its construction and operation, and many of its advantages should be readily understood and appreciated. 
       FIG. 1  is a diagrammatic illustration of a prior-art voltage indicator used for indicating the voltage on an overhead power transmission line; 
       FIG. 2  is a view similar to  FIG. 1  and illustrating underground equipment to which the overhead line is connected; 
       FIG. 3  illustrates the use of the prior-art voltage indicator for measuring the voltage at a terminal of the underground equipment of  FIG. 2 ; 
       FIG. 4  is a perspective view of an embodiment of a digital voltage indicator; 
       FIG. 5  is a top plan view of the housing of the voltage indicator of  FIG. 4 ; 
       FIG. 6  is an enlarged, cross-sectional view of an underground probe for use with the voltage indicator of  FIG. 4 ; and 
       FIG. 7  is a schematic circuit diagram of the electronic circuitry of the voltage indicator of  FIG. 4 . 
   

   DETAILED DESCRIPTION 
   Referring to  FIG. 4 , there is illustrated a voltage indicator, generally indicated by the numeral  30 , having a housing  31 , which may be formed of a suitable plastic material. Referring also to  FIG. 5 , the housing  31  has a generally box-like configuration with an open front, having a top wall  32 , a bottom wall  33 , opposed end walls  34  and  35  and a rear wall  36 . Mounted in the rear wall  36  is a probe coupler  37  and mounted on the end wall  35  is a hot stick coupler  38  for coupling a hot stick  23  (see  FIGS. 1 and 2 ), in a known manner. The open front of the housing  31  may be closed by a suitable bezel  39 , 
   Mounted within the housing  31  is electronic circuitry  40  schematically illustrated in  FIG. 7 . The circuitry  40  includes a terminal  41  connected to the probe coupler  37  and a terminal  42  connected to an enclosure shield  42   a , which may be an electrically conductive coating formed on the inner surfaces of the housing  31 , in a known manner. The terminals  41  and  42  are connected to a voltage divider  43 , which may include resistors  44  and  45  connected across the terminals  41  and  42 . The junction between the resistors  44  and  45  is connected to a rectifier  46 , which may include diodes  47  and  48 , the diode  48  being connected in parallel with the resistor  45  and having its anode connected to the terminal  42 , while the diode  47  has its anode connected to the cathode of the diode  48 . The cathode of the diode  47  is connected to one input terminal of an analog-to-digital converter (“ADC”  50 ), the other input terminal of which is connected to the terminal  42 , a capacitor  49  being connected across the input terminals of the ADC  50 . The output terminals of the ADC  50  are connected to a digital display  51 . The voltage indicator  30  may include an ON button  53  ( FIG. 4 ), which may be in the nature of a biased-open pushbutton switch, which could be located at any desirable point in the circuitry  40 . 
   The voltage indicator  30  is provided with a hook probe  55 , which may be substantially the same as the probe  25  shown in  FIGS. 1 and 3 , having a straight shank  56  and a threaded end  57  which may be adapted to be threadedly engaged with the probe coupler  37 , in a known manner. 
   Referring also to  FIG. 6 , there is additionally provided an underground probe  60 , which may be substituted for the hook probe  55  for use in underground applications. The probe  60  includes a first electrode  61  having a hollow cylindrical wall  62  closed at one end by a circular end wall  63  from which coaxially extends an elongated probe tip  64 . A second solid cylindrical electrode  65  is disposed coaxially within the cylindrical wall  62 , being spaced therefrom and from the end wall  63 , the electrode  65  being provided at its proximal end with a threaded connector  66  adapted to be threadedly engaged with the probe coupler  37 . The space between the electrodes  61  and  65  is filled with a solid dielectric material  67 , which serves to support the electrodes in coaxial spaced-apart relationship and provide electrical insulation therebetween, so that the electrode  65  cooperates with the cylindrical wall  62  to define a capacitor. The dielectric material  67  may be a suitable plastic material which forms an encapsulating sheath  68  which may completely encapsulate the electrode  65  except for the proximal end face thereof, as well as the electrode  61 , except for the distal end portion of the probe tip  64 . The dielectric material  67  also defines a spacing filler  68  which occupies the space between the electrodes. 
   The space between the two electrodes  61  and  65  and the properties of the dielectric insulating material  67  determine the amount of capacitance in the capacitor defined by the probe  60 . The amount of capacitance required to compensate for the higher electric field strength in voltage measurements near ground has been experimentally determined to be about 10 picofarads. 
   As can be seen from  FIG. 7 , in operation, the two input terminals  41  and  42  of the voltage indicator  30  are, respectively, connected to one of the probes and to the shield  42   a  of the housing  31 . The voltage divider  43  reduces the high incoming voltage, which may be up to 100 volts, to a lower voltage, which may be approximately 2 volts, required by the display  51 . The rectifier  46  converts the AC signal to DC and the capacitor  49  acts to provide a steady-state DC voltage required for stable measurement. The ADC  50  converts the incoming signal to a digital output to selectively turn on the appropriate segments of the two digits of the digital display  51  to display the measured voltage in kilovolts. 
   When the hook probe  55  is installed on the voltage indicator  30 , the two resistors  44  and  45  alone determine the ratio of the voltage divider  43  and the proportion of the input voltage that will be measured and displayed. When the hook probe  55  is replaced with the underground probe  60 , the impedance of the probe resulting from its capacitance changes the ratio of the voltage divider  43 , so as to significantly reduce the proportion of the voltage that will be measured and displayed. Thus, the underground probe  60  serves to compensate for the close proximity to ground and the resulting high field strength by providing a substantial impedance between the voltage indicator and the energized terminal where voltage is being measured. This substantial impedance acts to reduce the voltage measured by the voltage indicator  30  between the accessory probe and the reference electrode formed by the shield  42   a . While a capacitive impedance is disclosed, the principles of the invention would be applicable to other types of impedance, such as resistance. 
   From the foregoing, it can be seen that there has been provided an improved voltage indicator and underground probe therefor, which compensates for the close proximity to ground of “underground” voltage terminals and provides for measurements with accuracy comparable to that achieved with measurement of voltages on overhead wires. 
   The matter set forth in the foregoing description and accompanying drawings is offered by way of illustration only and not as a limitation. While particular embodiments have been shown and described, it will be apparent to those skilled in the art that changes and modifications may be made without departing from the broader aspects of applicants&#39; contribution. The actual scope of the protection sought is intended to be defined in the following claims when viewed in their proper perspective based on the prior art.