JP3652852B2 - Ground resistance meter - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a ground resistance meter, and more particularly, to an AC potentiometer type ground resistance meter used in A-type (first type) grounding work or the like.
[0002]
[Prior art]
Grounding work applied to electrical facilities is considered indispensable for the safety of human bodies and the maintenance of equipment. During grounding work, a grounding resistance meter is used to measure the grounding resistance of the grounding body buried in the ground. Some earth resistance meters are based on the AC potential difference method.
[0003]
The outline of this ground resistance meter will be described with reference to FIG. This ground resistance meter is synchronized with the transformer 1, the variable resistor 2 connected in parallel to the secondary winding 1B of the transformer 1, the oscillation circuit 3 for oscillating a constant voltage of alternating current, and the oscillation circuit 3. A synchronous rectifier circuit 4, a galvanometer 5 and three measurement terminals E, P, C.
[0004]
When measuring the ground resistance, a measurement ground E (not shown) which is a ground body embedded in the ground is connected to the measurement terminal E of the ground resistance meter, and the measurement terminal P and the measurement terminal C are separately connected. Auxiliary grounding poles (not shown) embedded in the ground at the locations are connected respectively. That is, the measurement method described below is a three-electrode method using two auxiliary grounding electrodes.
[0005]
The ground resistance of the measured ground connected to the measurement terminal E is Rx, and the ground resistance of the auxiliary ground electrode connected to the voltage measurement terminal P is the auxiliary ground resistance Rp. The ground resistance of the auxiliary ground electrode connected to the current measurement terminal C is the auxiliary ground resistance Rc. When the measured ground body and the two auxiliary grounding electrodes are embedded in the ground, the grounding resistance Rx and the auxiliary grounding resistances Rp and Rc are connected to each other on the ground.
[0006]
When the AC voltage is oscillated by operating the oscillation circuit 3, the measurement current passes through the measurement terminal C, the auxiliary resistance Rc, the ground resistance Rx, the measurement terminal E, and the primary winding 1 </ b> A of the transformer 1 to the oscillation circuit 3. Flow back through the measurement loop. As a result, a voltage drop due to the ground resistance Rx and the measurement current occurs between the measurement terminals EP.
[0007]
At the same time, since a current is induced in the secondary winding 1B of the transformer 1 by this measurement current, a voltage drop also occurs between the measurement terminal E and the variable terminal 2A of the variable resistor 2. These two voltages are detected by the synchronous rectifier circuit 4 synchronized with the oscillation of the oscillation circuit 1.
[0008]
Therefore, by operating the variable resistor 2 to balance the galvanometer 5 and reading the dial scale of the variable resistor 2 at that time, the ground resistance Rx is obtained by the three-electrode method. That is, the voltage between the measurement terminal E and the measurement terminal P is Ex, the resistance between the measurement terminal E and the variable terminal 2A of the variable resistor 2 is Rs, the voltage drop due to the resistance Rs is Es, and the winding of the transformer 1 When the ratio is 1: n and the current flowing through the measurement loop is I, the following formula is established when the galvanometer 5 is balanced.
Ex = I ・ Rx
Es = I · Rs / n
Ex = Es
[0009]
From these three equations, Rx = Rs / n
From this equation, the ground resistance Rx can be obtained.
[0010]
When such a three-electrode method is used, it is necessary to embed two auxiliary grounding electrodes in the ground. However, in the city center or the like, the ground is paved, and the two auxiliary grounding poles may not be embedded. In such a case, the ground resistance Rx is measured by the two-electrode method.
[0011]
In the two-electrode method, as shown in FIG. 3, the measurement terminal P and the measurement terminal C are connected, and the measurement terminal C is connected to the ground line 101 side of the commercial power supply 100. After such connection, the galvanometer 5 is balanced by the variable resistor 2. Thus, when the ground resistance of the grounding body of the ground wire 101 is Ro,
Rx + Ro = Rs / n
As described above, the grounding resistance Rx can be obtained.
[0012]
[Problems to be solved by the invention]
When the two-electrode method is used, before connecting the ground resistance meter to the commercial power source 100, the ground wire 101 and the live wire 102 of the commercial power source 100 are examined using a voltage detector or the like, and the measurement terminal C is connected to the ground wire 101. Try to connect to the side.
[0013]
As described above, even if the ground wire 101 side is examined in advance, the measurement terminal C may be mistakenly connected to the live wire 102 side due to carelessness. Then, the voltage of the commercial power supply 100 is applied to the oscillation circuit 3 and the transformer 1 via the measurement terminal C, and there is a possibility that the oscillation circuit 3 and the transformer 1 are damaged. In addition, the voltage of the commercial power supply 100 may be induced by the transformer 1 and the synchronous rectifier circuit 4 and the galvanometer 5 may be damaged.
[0014]
The present invention has been made to solve such a problem, and its purpose is to provide a ground resistance meter capable of notifying a user of a connection error with a commercial power source when measuring the ground resistance by the two-electrode method. It is to provide.
[0015]
[Means for Solving the Problems]
In order to achieve the above object, the invention according to claim 1 is a first measurement terminal connected to the measured ground body, a second measurement terminal for voltage connected to each of the two auxiliary ground electrodes, and A third measuring terminal for current, and a primary winding connected between the first measuring terminal and the third measuring terminal, and a variable resistor is arranged in parallel on the secondary winding side. A transformer connected, an oscillation circuit for supplying an AC measurement signal to a measurement current path including the primary winding, and the first measurement terminal and the variable resistor in synchronization with the oscillation of the oscillation circuit A synchronous rectifier circuit for detecting a voltage between the second measurement terminal and the voltage of the second measurement terminal, and a galvanometer connected to the synchronous rectification circuit, the second measurement terminal and the third measurement terminal at the time of two-electrode method measurement Are short-circuited, and the third measurement terminal is connected to the ground line of the commercial power supply. In includes a voltage detecting means for detecting a voltage applied to the third measuring terminal, and a warning means to alert when the commercial power is detected at the same voltage detecting means, said voltage detecting means, A voltage dividing resistor connected between the first measurement terminal and the third measurement terminal, and a semiconductor switch connected to the voltage dividing point via a diode and turned on at a predetermined threshold voltage. It is characterized by.
[0016]
According to this configuration, when the third measurement terminal is mistakenly connected to the live power line side of the commercial power supply, the alarm means operates, and the user immediately notices the error.
[0017]
According to a second aspect of the present invention, a first measurement terminal connected to the measured ground body, a second measurement terminal for voltage connected to each of the two auxiliary ground electrodes, and a third measurement for current A transformer having a primary winding connected between the terminal, the first measurement terminal and the third measurement terminal, and a variable resistor connected in parallel to the secondary winding side; An oscillation circuit for supplying an AC measurement signal to a measurement current path including the primary winding, a voltage between the first measurement terminal and the variable resistor in synchronization with oscillation of the oscillation circuit, and the first A synchronous rectifier circuit for detecting the voltage of the two measurement terminals, and a galvanometer connected to the synchronous rectifier circuit, the second measurement terminal and the third measurement terminal are short-circuited during the two-electrode method measurement, In the ground resistance meter in which the third measurement terminal is connected to the ground line of the commercial power supply, A voltage detecting means for detecting a voltage applied to the constant terminal; and an alarm means for issuing an alarm when the commercial power is detected by the voltage detecting means. An overvoltage protection element is provided between the rectifier circuit and the oscillation circuit to the third measurement terminal.
[0018]
As described above, by providing the overvoltage protection element between the second measurement terminal and the synchronous rectification circuit and between the oscillation circuit and the third measurement terminal, it is possible to protect these circuits at the time of erroneous connection.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
Next, in order to better understand the technical idea of the present invention, a preferred embodiment will be described with reference to FIG. In this embodiment, components that are the same as or the same as those of the ground resistance meter of FIG. 2 described above are given the same reference numerals.
[0020]
This ground resistance meter includes a transformer 1, a variable resistor 2, an oscillation circuit 3, a synchronous rectifier circuit 4, a galvanometer 5, thermistors 6, 7, resistors 8, 9, a diode 10, a field effect transistor 11, a buzzer 12, and a switch. 13 is provided. In this embodiment, the resistors 8 and 9, the diode 10, and the field effect transistor 11 constitute voltage detecting means, and the buzzer 12 is used as alarm means.
[0021]
The switch 13 is a switch for short-circuiting the voltage measurement terminal P (second measurement terminal) and the current measurement terminal C (third measurement terminal) when the ground resistance meter is used as the two-electrode method.
[0022]
A positive temperature coefficient thermistor 6 as an overvoltage protection element is provided between the measurement terminal C and the oscillation circuit 3. That is, when a high power supply of, for example, AC 100 V is applied from the commercial power supply to the measurement terminal C, the resistance value of the thermistor 6 increases with heat generation, and thereby the current flowing through the measurement current path is reduced.
[0023]
Similarly, a positive temperature coefficient thermistor 7 as an overvoltage protection element is also provided on the side of the synchronous rectifier circuit 4, thereby protecting the synchronous rectifier circuit 4 and the galvanometer 5 from overvoltage.
[0024]
In this embodiment, the voltage detecting means is connected to the voltage dividing point B through the voltage dividing resistors 8 and 9 connected between the measuring terminal E (first measuring terminal) and the measuring terminal C and the diode 10. And a connected field effect transistor (FET) 11.
[0025]
In this case, the diode 10 is a rectifier diode for preventing a negative voltage from being applied to the field effect transistor 11. The field effect transistor 11 is an enhancement type MOS • FET and is turned on when the gate voltage increases in the positive direction. As a result, the drain-source is conducted, and the buzzer 12 sounds.
[0026]
The resistance values of the resistors 8 and 9 are such that when, for example, 100 V AC is applied to the input point A of the voltage detecting means, a gate voltage appears to turn on the field effect transistor 11 at the voltage dividing point B. Is set to
[0027]
Next, the operation of this embodiment will be described. First, when the ground resistance is measured by the two-electrode method, the measurement terminal P and the measurement terminal C are short-circuited by the switch 13. Then, in the same manner as described above, the measurement terminal E is connected to the measured grounding body, and the measurement terminal C is connected to the ground line 101 (see FIG. 3) of the commercial power source, so that the measured grounding body is connected. Measure the ground resistance Rx.
[0028]
At this time, the AC voltage of the oscillation circuit 3 is applied to the input point A of the voltage detection means. Since the value is lower than the voltage of the commercial power supply, the voltage at the voltage dividing point B turns on the field effect transistor 11. Not enough to do. As a result, the field effect transistor 11 is kept off, and the buzzer 12 does not sound an alarm sound.
[0029]
By the way, if the measurement terminal C is mistakenly connected to the live power line 102 (AC100V, see FIG. 3) of the commercial power supply, the measurement terminal C, the thermistor 6, the oscillation circuit 3, the transformer 1, the measurement terminal E, the ground resistance Rx, and the ground resistance Ro. And the current flowing through the measurement loop leading to the measurement terminal C increases.
[0030]
Thereby, since the resistance value of the thermistor 6 increases with heat generation, the current flowing through the measurement loop is narrowed down, and the oscillation circuit 3 and the transformer 1 are protected from AC 100V. Similarly, the synchronous rectifier circuit 4 is also protected from AC 100 V by the thermistor 7.
[0031]
Further, AC100V is divided by the resistors 8 and 9 of the voltage detecting means, and the voltage at the voltage dividing point B is applied to the gate of the field effect transistor 11 via the diode 10. As a result, the field effect transistor 11 is turned on, a current flows through the buzzer 12, and an alarm sound is output.
[0032]
In this way, when the ground resistance is measured by the two-electrode method, even if the measurement terminal C is mistakenly connected to the live wire of the commercial power source, the transformer 1, the oscillation circuit 3, the synchronous rectifier circuit 4, and the galvanometer 5 are While being protected from the overvoltage, an alarm sound is emitted from the buzzer 12, so that the user immediately notices the erroneous connection.
[0033]
In this embodiment, a buzzer is used as an alarm means. However, a lamp display may be used together or in place of the buzzer. Further, although the enhancement type is used for the field effect transistor 11, a depletion type may be used as long as the connection direction of the diode 10 is changed.
[0034]
In the above-described embodiment, the measurement example by the two-electrode method has been described. However, it is needless to say that the measurement by the three-electrode method can be performed by switching the switch 13 as in the conventional case.
[0035]
【The invention's effect】
As described above, according to the present invention, when the measurement by the two-electrode method is performed, even if the measurement terminal of the auxiliary grounding electrode is connected to the live power line side of the commercial power source, the internal components are protected from the overvoltage. In addition, since the alarm is issued by the alarm means, the user can immediately notice the connection error.
[Brief description of the drawings]
FIG. 1 is a block diagram showing an embodiment of the present invention.
FIG. 2 is a block diagram showing a conventional ground resistance meter.
FIG. 3 is a block diagram showing a measurement example of a two-electrode method using a conventional ground resistance meter.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Transformer 2 Variable resistor 3 Oscillation circuit 4 Synchronous rectification circuit 5 Galvanometer 6, 7 Thermistor 8, 9 Resistance 10 Diode 11 Field effect transistor 12 Buzzer

Claims (2)

A first measurement terminal connected to the ground to be measured, a second measurement terminal for voltage and a third measurement terminal for current connected to each of the two auxiliary ground electrodes, the first measurement terminal, and the first measurement terminal A transformer having a primary winding connected between the three measurement terminals, a variable resistor connected in parallel on the secondary winding side, and a measurement current path including the primary winding An oscillation circuit that supplies an AC measurement signal to the circuit, and a synchronous rectification circuit that detects the voltage between the first measurement terminal and the variable resistor and the voltage at the second measurement terminal in synchronization with the oscillation of the oscillation circuit And a galvanometer connected to the synchronous rectifier circuit, the second measurement terminal and the third measurement terminal are short-circuited when the two-electrode method is measured, and the third measurement terminal is connected to the ground line of the commercial power source. In the connected earth resistance meter,
Voltage detection means for detecting a voltage applied to the third measurement terminal; and alarm means for issuing an alarm when the commercial power supply is detected by the voltage detection means, and the voltage detection means A voltage dividing resistor connected between one measuring terminal and the third measuring terminal, and a semiconductor switch connected to the voltage dividing point via a diode and turned on at a predetermined threshold voltage. Ground resistance meter. A first measurement terminal connected to the ground to be measured, a second measurement terminal for voltage and a third measurement terminal for current connected to each of the two auxiliary ground electrodes, the first measurement terminal, and the first measurement terminal A transformer having a primary winding connected between the three measurement terminals, a variable resistor connected in parallel on the secondary winding side, and a measurement current path including the primary winding An oscillation circuit that supplies an AC measurement signal to the circuit, and a synchronous rectification circuit that detects the voltage between the first measurement terminal and the variable resistor and the voltage at the second measurement terminal in synchronization with the oscillation of the oscillation circuit And a galvanometer connected to the synchronous rectifier circuit, the second measurement terminal and the third measurement terminal are short-circuited when the two-electrode method is measured, and the third measurement terminal is connected to the ground line of the commercial power source. In the connected earth resistance meter,
A voltage detection means for detecting a voltage applied to the third measurement terminal; and an alarm means for issuing an alarm when the commercial power supply is detected by the voltage detection means. A ground resistance meter , wherein an overvoltage protection element is provided between the first and second synchronous rectifier circuits and between the oscillation circuit and the third measurement terminal .

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