Grounding resistance testing device
By adopting the dual-frequency point test method in the ground resistance test device and using the signal conversion circuit and the signal processing circuit, it is possible to accurately determine whether the ground circuit has a capacitive load, solving the problem of measurement inaccurate in the existing device and realizing a more accurate ground resistance test.
Patent Information
- Application Number
- CN202421814724.5
- Authority / Receiving Office
- CN · China
- Patent Type
- Utility models(China)
- Current Assignee / Owner
- Filing Date
- 2024-07-29
- Publication Date
- 2025-06-27
- Estimated Expiration
- 2034-07-29
AI Technical Summary
The existing ground resistance testing device cannot distinguish whether there is a capacitive load in the grounding system, resulting in inaccurate measurement results.
A ground resistance testing device is designed, and a dual-frequency point test method is used to generate a sine wave signal of the first frequency point and the second frequency point through the first frequency point signal conversion circuit and the second frequency point signal conversion circuit, respectively output to the ground circuit to be measured, and the induction signal is received and amplified through the signal processing circuit to determine whether the measurement results are the same to determine whether the loop has a capacitive load.
It is possible to verify whether there is a capacitive load in the loop under test while ensuring measurement accuracy, and solve the problem that existing devices cannot distinguish capacitive load.
Smart Images
Figure CN223038050U_ABST
Abstract
Description
Technical Field
[0001] This application relates to the technical field of grounding resistance measurement, and particularly relates to a grounding resistance test device. Background Art
[0002] With the continuous development of China's power system and the continuous expansion of the power grid scale, various microcomputers and monitoring devices have been widely used, and people's requirements for the grounding system have become increasingly higher. A safe and reliable grounding system is of extremely important significance for the safe operation of the power system and the prevention of accidents. Therefore, a grounding resistance test device is mainly used to detect the grounding system.
[0003] A grounding resistance test device known to the inventor cannot identify whether there is a capacitive load in the grounding system when conducting a test. Once there is a capacitive load in the grounding system, the measurement result of the grounding resistance test device will be inaccurate.
[0004] It should be noted that the information disclosed in the above background art section is only used to enhance the understanding of the background of the present disclosure, and therefore may include information that does not constitute the prior art known to those of ordinary skill in the art. Summary of the Utility Model
[0005] In view of at least one of the above technical problems, this application provides a grounding resistance test device, which solves the problem that it cannot identify whether there is a capacitive load in the grounding system when conducting a test.
[0006] An embodiment of this application provides a grounding resistance test device, including:
[0007] A main control circuit for providing a pulse signal with a fixed frequency, and the main control circuit is connected to a display module;
[0008] A signal generation circuit includes: a first frequency point signal conversion circuit and a second frequency point signal conversion circuit. The first frequency point signal conversion circuit is used to generate a first frequency point sine wave signal according to the pulse signal, and the second frequency point signal conversion circuit is used to generate a second frequency point sine wave signal according to the pulse signal;
[0009] A power amplification circuit for receiving the first frequency point sine wave signal and the second frequency point sine wave signal and respectively outputting them to the grounding loop to be measured;
[0010] A signal processing circuit for receiving a first induction signal and a second induction signal, amplifying and filtering the first induction signal and the second induction signal, and respectively outputting them to the main control circuit. Both the first induction signal and the second induction signal are obtained by sampling the induction current in the grounding loop to be measured;
[0011] The main control circuit is used to generate a first measurement result based on the amplified and filtered first induction signal; generate a second measurement result based on the amplified and filtered second induction signal; determine whether the first measurement result and the second measurement result are the same; if they are the same, the measured grounding loop has a pure resistive load; if they are different, the measured grounding loop has a capacitive load.
[0012] One of the technical solutions in the above technical solutions has at least the following advantages or beneficial effects: By setting the first frequency point signal conversion circuit and the second frequency point signal conversion circuit, the grounding resistance test device realizes dual-frequency point testing, which can not only ensure the measurement accuracy, but also detect whether there is a capacitive load in the measured loop.
[0013] In some possible implementation manners, the main control circuit can be selectively connected to the first frequency point signal conversion circuit and the second frequency point signal conversion circuit.
[0014] In some possible implementation manners, the first frequency point signal conversion circuit includes: a forty-fourth resistor, a forty-fifth resistor, a forty-sixth resistor, a twenty-ninth capacitor, a thirtieth capacitor, and a thirty-first capacitor. The first end of the forty-fourth resistor is connected to the main control circuit, the first end of the forty-fifth resistor is connected to the second end of the forty-fourth resistor, the first end of the forty-fifth resistor is connected to the first end of the twenty-ninth capacitor, the first end of the forty-sixth resistor is connected to the second end of the forty-fifth resistor, the first end of the forty-sixth resistor is connected to the first end of the thirtieth capacitor, the second end of the forty-sixth resistor is connected to the first end of the thirty-first capacitor, the second end of the forty-sixth resistor is connected to the main control circuit, the second ends of the twenty-ninth capacitor, the thirtieth capacitor, and the thirty-first capacitor are commonly connected, and the second end of the twenty-ninth capacitor is grounded.
[0015] In some possible implementation manners, the forty-fourth resistor, the forty-fifth resistor, and the forty-sixth resistor have the same resistance value, and the twenty-ninth capacitor, the thirtieth capacitor, and the thirty-first capacitor have the same capacitance value.
[0016] In some possible implementation manners, the second frequency point signal conversion circuit includes: a sixtieth resistor, a sixty-first resistor, a sixty-second resistor, a thirty-third capacitor, a thirty-fourth capacitor, and a thirty-fifth capacitor. The first end of the sixtieth resistor is connected to the main control circuit, the first end of the sixty-first resistor is connected to the second end of the sixtieth resistor, the first end of the sixty-first resistor is connected to the first end of the thirty-third capacitor, the first end of the sixty-second resistor is connected to the second end of the sixty-first resistor, the second end of the sixty-second resistor is connected to the first end of the thirty-fifth capacitor, the second end of the sixty-second resistor is connected to the main control circuit, the second ends of the thirty-third capacitor, the thirty-fourth capacitor, and the thirty-fifth capacitor are commonly connected, and the second end of the thirty-third capacitor is grounded.
[0017] In some possible implementations, the resistances of the sixtieth resistor, the sixty-first resistor, and the sixty-second resistor are the same, and the capacitances of the thirty-third capacitor, the thirty-fourth capacitor, and the thirty-fifth capacitor are the same.
[0018] In some possible implementations, the signal processing circuit includes: a first-stage amplifier circuit, a second-stage amplifier circuit, and an MFB band-pass filter circuit connected in sequence. The first-stage amplifier circuit is configured to receive the first induction signal or the second induction signal and perform first-stage signal amplification. The second-stage amplifier circuit is configured to receive the signal output by the first-stage amplifier circuit and perform second-stage signal amplification. The MFB band-pass filter circuit is configured to receive the signal output by the second-stage amplifier circuit, perform filtering, and output the filtered signal to the main control circuit.
[0019] In some possible implementations, it further includes: a calibration circuit, which is used as a standard for calibration when the grounding resistance testing device is powered on.
[0020] The present utility model will be further described below in conjunction with the accompanying drawings and embodiments. Description of the Drawings
[0021] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings required for use in the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.
[0022] Figure 1 It is a schematic structural diagram of a grounding resistance meter provided by an embodiment of the present application;
[0023] Figure 2 It is a circuit diagram of a first frequency point signal conversion circuit provided by an embodiment of the present application;
[0024] Figure 3 It is a circuit diagram of a second frequency point signal conversion circuit provided by an embodiment of the present application;
[0025] Figure 4 It is a schematic structural diagram of a signal processing circuit provided by an embodiment of the present application;
[0026] Figure 5 It is a circuit diagram of an inspection circuit provided by an embodiment of the present application; Detailed Embodiments
[0027] To make the above objects, features, and advantages of the present application more apparent and understandable, the following provides a detailed description of specific embodiments of the present application in conjunction with the accompanying drawings. Many specific details are set forth in the following description to facilitate a thorough understanding of the present application. However, the present application can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without departing from the connotation of the present application. Therefore, the present application is not limited by the specific embodiments disclosed below.
[0028] Please refer to Figure 1 、 Figure 2 、 Figure 3 , Figure 1 which is a schematic structural diagram of the grounding resistance meter provided by an embodiment of the present application, Figure 2 which is a circuit diagram of the first frequency point signal conversion circuit provided by an embodiment of the present application, Figure 3 which is a circuit diagram of the second frequency point signal conversion circuit provided by an embodiment of the present application. The grounding resistance test device includes: a main control circuit, a signal generation circuit, a power amplification circuit, a signal processing circuit, and an inspection circuit. Among them, the grounding resistance test device is a clamp meter, having an excitation clamp head and a sampling clamp head. The power amplification circuit is connected to the excitation clamp head, the sampling clamp head is connected to the signal processing circuit, and the measured grounding loop is clamped by the excitation clamp head and the sampling clamp head for measurement in this way.
[0029] The following provides a specific introduction to the grounding resistance test device.
[0030] The main control circuit is used to provide a pulse signal with a fixed frequency. The signal generation circuit includes: a first frequency point signal conversion circuit and a second frequency point signal conversion circuit. The first frequency point signal conversion circuit is used to generate a first frequency point sine wave signal according to the pulse signal, and the second frequency point signal conversion circuit is used to generate a second frequency point sine wave signal according to the pulse signal; the power amplification circuit is used to receive the first frequency point sine wave signal and the second frequency point sine wave signal and output them to the measured grounding loop respectively; the signal processing circuit is used to receive the first induction signal and the second induction signal, amplify and filter the first induction signal and the second induction signal, and output them to the main control circuit respectively. Both the first induction signal and the second induction signal are obtained by sampling the induction current in the measured grounding loop; the main control circuit is used to generate a first measurement result based on the amplified and filtered first induction signal; generate a second measurement result based on the amplified and filtered second induction signal; determine whether the first measurement result and the second measurement result are the same; if they are the same, the measured grounding loop has a pure resistive load; if they are different, the measured grounding loop has a capacitive load.
[0031] By setting the first frequency point signal conversion circuit and the second frequency point signal conversion circuit, the present grounding resistance test device realizes dual-frequency point testing, which can not only ensure the measurement accuracy but also detect whether there is a capacitive load in the measured loop.
[0032] In this embodiment, the first frequency point refers to a frequency of 1.50 KHz, and the second frequency point refers to a frequency of 2.00 KHz.
[0033] The power amplification circuit is used to amplify the sine wave signal of the first frequency point or the sine wave signal of the second frequency point, and then drive the excitation clamp head. The power amplification circuit is not specifically limited. Specifically, the power amplification circuit is a power amplification chip with the model OPA547. This power amplification chip supports dual power supply, can output a sustainable current of up to 500 mA at most, and has a current limiting resistor to adjust the output current.
[0034] During actual operation, the grounding resistance testing device clamps the grounding loop under test, the grounding resistance testing device is powered on, and the grounding resistance testing device defaults to output a pulse signal with a fixed frequency to the first frequency point signal conversion circuit. The first frequency point signal conversion circuit converts the pulse signal into a sine wave signal of the first frequency point and inputs it into the main control circuit. The main control circuit outputs the sine wave signal of the first frequency point to the power amplification circuit. The power amplification circuit amplifies the sine wave signal of the first frequency point, and then drives the excitation clamp head to make the excitation clamp head release a signal. The grounding loop under test senses the excitation signal and generates an induced current. The sampling clamp head senses the first induced current in the grounding loop under test and generates a first induced signal, which is output to the signal processing circuit for amplification and filtering. The signal processing circuit outputs the processed first induced signal to the main control circuit and generates a first measurement result based on the amplified and filtered first induced signal. Then, the first frequency point signal conversion circuit is switched to the second frequency point conversion circuit. The grounding resistance testing device outputs a pulse signal with a fixed frequency to the second frequency point signal conversion circuit. The second frequency point signal conversion circuit converts the pulse signal into a sine wave signal of the second frequency point and inputs it into the main control circuit. The main control circuit outputs the sine wave signal of the second frequency point to the power amplification circuit. The power amplification circuit amplifies the sine wave signal of the second frequency point, and then drives the excitation clamp head to make the excitation clamp head release a signal. The grounding loop under test senses the excitation signal and generates an induced current. The sampling clamp head senses the second induced current in the grounding loop under test and generates a second induced signal, which is output to the signal processing circuit for amplification and filtering. The signal processing circuit outputs the processed second induced signal to the main control circuit and generates a second measurement result based on the amplified and filtered second induced signal. It is judged whether the first measurement result and the second measurement result are the same; if they are the same, the grounding loop under test has a pure resistive load; if they are different, the grounding loop under test has a capacitive load.
[0035] In some embodiments, the main control circuit can be selectively connected to the first frequency point signal conversion circuit and the second frequency point signal conversion circuit.
[0036] Specifically, the main control circuit may be connected to a frequency point switching switch. By pressing the frequency point switching switch, the switching between the first frequency point signal conversion circuit and the second frequency point signal conversion circuit is realized.
[0037] In some embodiments, as Figures 1 to 3 shown, the first frequency point signal conversion circuit includes: a forty-fourth resistor, a forty-fifth resistor, a forty-sixth resistor, a twenty-ninth capacitor, a thirtieth capacitor, and a thirty-first capacitor. The first end of the forty-fourth resistor is connected to the main control circuit. The first end of the forty-fifth resistor is connected to the second end of the forty-fourth resistor. The first end of the forty-fifth resistor is connected to the first end of the twenty-ninth capacitor. The first end of the forty-sixth resistor is connected to the second end of the forty-fifth resistor. The first end of the forty-sixth resistor is connected to the first end of the thirtieth capacitor. The second end of the forty-sixth resistor is connected to the first end of the thirty-first capacitor. The second ends of the twenty-ninth capacitor, the thirtieth capacitor, and the thirty-first capacitor are commonly connected, and the second end of the twenty-ninth capacitor is grounded.
[0038] Specifically, the forty-fourth resistor, the forty-fifth resistor, and the forty-sixth resistor have the same resistance value, which is 22 KΩ. The twenty-ninth capacitor, the thirtieth capacitor, and the thirty-first capacitor have the same capacitance value, which is 4.7 nF.
[0039] Among them, the forty-fourth resistor and the twenty-ninth capacitor form a first-order circuit. The forty-fifth resistor and the thirtieth capacitor form a second-order circuit. The forty-sixth resistor and the thirty-first capacitor form a third-order circuit. The first-order circuit converts the pulse signal into a parabolic exponential waveform. The second-order circuit converts the parabolic exponential waveform into a triangular wave. The third-order circuit converts the triangular wave into a first frequency point sine wave signal.
[0040] In some embodiments, as Figures 1 to 3 shown, the second frequency point signal conversion circuit includes: a sixtieth resistor, a sixty-first resistor, a sixty-second resistor, a thirty-third capacitor, a thirty-fourth capacitor, and a thirty-fifth capacitor. The first end of the sixtieth resistor is connected to the main control circuit. The first end of the sixty-first resistor is connected to the second end of the sixtieth resistor. The first end of the sixty-first resistor is connected to the first end of the thirty-third capacitor. The first end of the sixty-second resistor is connected to the second end of the sixty-first resistor. The second end of the sixty-second resistor is connected to the first end of the thirty-fifth capacitor. The second ends of the thirty-third capacitor, the thirty-fourth capacitor, and the thirty-fifth capacitor are commonly connected.
[0041] In some embodiments, as Figure 1 shown, the sixtieth resistor, the sixty-first resistor, and the sixty-second resistor have the same resistance value, which is 16 KΩ. The thirty-third capacitor, the thirty-fourth capacitor, and the thirty-fifth capacitor have the same capacitance value, which is 4.7 nF.
[0042] Among them, the sixtieth resistor and the thirty-third capacitor form a first-order circuit, the sixty-first resistor and the thirty-fourth capacitor form a second-order circuit, and the sixty-second resistor and the thirty-fifth capacitor form a third-order circuit. The first-order circuit converts a pulse signal into a parabolic exponential waveform, the second-order circuit converts the parabolic exponential waveform into a triangular wave, and the third-order circuit converts the triangular wave into a sine wave signal at the second frequency point.
[0043] Please refer to Figure 4 , Figure 4 which is a schematic structural diagram of the signal processing circuit provided by an embodiment of the present application.
[0044] In some embodiments, as Figure 4 shown, the signal processing circuit includes: a first-stage amplifier circuit, a second-stage amplifier circuit, and an MFB band-pass filter circuit connected in sequence. The first-stage amplifier circuit is used to receive the first induction signal or the second induction signal and perform first-stage signal amplification. The second-stage amplifier circuit is used to receive the signal output by the first-stage amplifier circuit and perform second-stage signal amplification. The MFB band-pass filter circuit is used to receive the signal output by the second-stage amplifier circuit for filtering and output the filtered signal to the main control circuit.
[0045] Please refer to Figure 5 , Figure 5 which is a circuit diagram of the test circuit provided by an embodiment of the present application.
[0046] In some embodiments, as Figure 5 shown, it further includes: a calibration circuit, which is used as a standard for calibration when the grounding resistance testing device is powered on. This test circuit is connected to the main control circuit.
[0047] The calibration circuit may include a twentieth resistor, a twenty-first resistor, a thirtieth resistor, a thirty-seventh resistor, a fifth triode, and a tenth field effect transistor. The first end of the twentieth resistor is connected to the REFA terminal. The first end of the twenty-first resistor is connected to the second end of the twentieth resistor. The first end of the tenth field effect transistor is connected to the twentieth resistor. The second end of the tenth field effect transistor is grounded. The first end of the fifth triode is connected to the 3.3V power supply terminal. The second end of the fifth triode is connected to the third end of the tenth field effect transistor and the first end of the thirty-seventh resistor. The second end of the thirty-seventh resistor is grounded. The third end of the fifth triode is connected to the main control circuit through the thirtieth resistor.
[0048] Among them, the resistance values of the twentieth resistor and the twenty-first resistor are known. During calibration, the main control circuit controls the fifth triode to turn on the tenth field effect transistor.
[0049] The calibration circuit is used when the grounding resistance test device is powered on. During operation, a lead wire passes through the clamp head, and both ends of the lead wire are respectively connected to the REFA terminal and the second terminal of the tenth field effect transistor. When the main control circuit outputs a low level, the fifth triode conducts, and the third terminal of the tenth field effect transistor changes from a low level to a high level, thereby realizing the conduction of the tenth field effect transistor. At this time, the lead wire, the tenth field effect transistor, the twentieth resistor, and the twenty-first resistor form a closed loop. The resistance value of this loop is composed of the on-resistance of the tenth field effect transistor, the twentieth resistor, and the twenty-first resistor. Since this resistance is known, the resistance value at this time can be calibrated as a standard.
[0050] The test method implemented by the grounding resistance test device in the embodiments of the present application includes: steps S100 to S800;
[0051] Step S100, output a sine wave signal of the first frequency point to the grounding loop to be measured;
[0052] Step S200, receive the first induction signal, and the first induction signal is obtained by sampling the first induction current in the grounding loop to be measured;
[0053] Step S300, generate a first measurement result based on the first induction signal;
[0054] Step S400, output a sine wave signal of the second frequency point to the grounding loop to be measured;
[0055] Step S500, receive the second induction signal, and the second induction signal is obtained by sampling the second induction current in the grounding loop to be measured;
[0056] Step S600, generate a second measurement result based on the second induction signal;
[0057] Step S700, determine whether the first measurement result is the same as the second measurement result;
[0058] Step S800, if they are different, the grounding loop to be measured has a capacitive load. If they are the same, the grounding loop to be measured is a pure resistive load.
[0059] In some embodiments, it includes: a signal generation circuit, and the signal generation circuit includes: a first frequency point signal conversion circuit and a second frequency point signal conversion circuit;
[0060] The method further includes:
[0061] Monitor the state of the frequency point switching switch;
[0062] Based on the state of the frequency point switching switch, output a pulse signal with a fixed frequency to the first frequency point signal conversion circuit or the second frequency point signal conversion circuit.
[0063] The above are only the preferred embodiments of the present application, and do not impose any form of limitation on the present application. Any person skilled in the art can make many possible changes and modifications to the technical solution of the present application, or modify it into an equivalent embodiment with equivalent changes, without departing from the scope of the technical solution of the present application. Therefore, all equivalent changes made according to the shape, structure and principle of the present application without departing from the content of the technical solution of the present application shall be covered by the protection scope of the present application.
Claims
1. A ground resistance test device, characterized in that: include: A main control circuit, used for providing a pulse signal of a fixed frequency, wherein the main control circuit is connected to a display module; The signal generating circuit comprises: a first frequency signal conversion circuit and a second frequency signal conversion circuit, wherein the first frequency signal conversion circuit is used to generate a first frequency sine wave signal according to the pulse signal, and the second frequency signal conversion circuit is used to generate a second frequency sine wave signal according to the pulse signal; A power amplifier circuit, used for receiving the first frequency point sine wave signal and the second frequency point sine wave signal, and outputting them to the ground loop under test respectively; A signal processing circuit is used to receive a first sensing signal and a second sensing signal, amplify and filter the first sensing signal and the second sensing signal, and output them to a main control circuit respectively, wherein the first sensing signal and the second sensing signal are both obtained by sampling the induced current in the ground loop under test; The main control circuit is used to generate a first measurement result based on the amplified and filtered first sensing signal; A second measurement result is generated based on the amplified and filtered second induction signal; and it is determined whether the first measurement result and the second measurement result are the same; if they are the same, the ground loop under test has a pure resistive load; if they are different, the ground loop under test has a capacitive load.
2. The ground resistance testing device according to claim 1, characterized in that: The main control circuit can be selectively connected to the first frequency point signal conversion circuit and the second frequency point signal conversion circuit.
3. The ground resistance testing device according to claim 1, characterized in that: The first frequency signal conversion circuit includes: a forty-fourth resistor, a forty-fifth resistor, a forty-sixth resistor, a twenty-ninth capacitor, a thirtieth capacitor and a thirty-first capacitor. The first end of the forty-fourth resistor is connected to the main control circuit, the first end of the forty-fifth resistor is connected to the second end of the forty-fourth resistor, the first end of the forty-fifth resistor is connected to the first end of the twenty-ninth capacitor, the first end of the forty-sixth resistor is connected to the second end of the forty-fifth resistor, the first end of the forty-sixth resistor is connected to the first end of the thirtieth capacitor, the second end of the forty-sixth resistor is connected to the first end of the thirty-first capacitor, the second end of the forty-sixth resistor is connected to the main control circuit, the second ends of the twenty-ninth capacitor, the second end of the thirtieth capacitor and the second end of the thirty-first capacitor are connected in common, and the second end of the twenty-ninth capacitor is grounded.
4. The ground resistance testing device according to claim 3, characterized in that: The resistance values of the forty-fourth resistor, the forty-fifth resistor and the forty-sixth resistor are the same, and the capacitances of the twenty-ninth capacitor, the thirtieth capacitor and the thirty-first capacitor are the same.
5. The ground resistance testing device according to claim 1, characterized in that: The second frequency signal conversion circuit includes: a 60th resistor, a 61st resistor, a 62nd resistor, a 33rd capacitor, a 34th capacitor and a 35th capacitor. The first end of the 60th resistor is connected to the main control circuit, the first end of the 61st resistor is connected to the second end of the 60th resistor, the first end of the 61st resistor is connected to the first end of the 33rd capacitor, the first end of the 62nd resistor is connected to the second end of the 61st resistor, the second end of the 62nd resistor is connected to the first end of the 35th capacitor, the second end of the 62nd resistor is connected to the main control circuit, the second end of the 33rd capacitor, the second end of the 34th capacitor and the second end of the 35th capacitor are connected in common, and the second end of the 33rd capacitor is grounded.
6. The ground resistance testing device according to claim 5, characterized in that: The resistance values of the 60th resistor, the 61st resistor, and the 62nd resistor are the same, and the capacitances of the 33rd capacitor, the 34th capacitor, and the 35th capacitor are the same.
7. The ground resistance testing device according to claim 1, characterized in that: The signal processing circuit includes: a first-stage amplifier circuit, a second-stage amplifier circuit and an MFB band-pass filter circuit connected in sequence, the first-stage amplifier circuit is used to receive the first sensing signal or the second sensing signal and perform primary signal amplification, the second-stage amplifier circuit is used to receive the signal output by the first-stage amplifier circuit and perform secondary signal amplification, and the MFB band-pass filter circuit is used to receive the signal output by the second-stage amplifier circuit for filtering, and output the filtered signal to the main control circuit.
8. The ground resistance testing device according to claim 7, characterized in that: Also includes: The verification circuit is used as a standard for calibration when the ground resistance test device is turned on.