JPH11118852A - Earth-resistance meter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To inform the erroneous connection with a commercial power source to an user in the measurement of the earth-resistance by a two-electrode method, by giving the alarm when the commercial power source is detected by a voltage detecting means which detects the voltage to be applied to a third measuring terminal. SOLUTION: In the case where the earth-resistance is measured by the two-electrode method, the measuring terminals P, C are short-circuited by a switch 13, and the measuring terminals E, C are respectively connected to an earthing body to be measured, and with an earthing line of the commercial power source, to measure the earth-resistance Rx. When the measurement terminal C is connected to a hot line for the commercial power source by an error, the electric current flowing in a measuring loop reaching the measuring terminal C through the measuring terminal C, a thermistor 6, an oscillating circuit 3, a transformer 1, the measuring terminal E and the earth-resistances Rx, Ro, is increased. Whereby the thermistor 6 generates the heat, the resistance value is increased, the electric current of the measuring loop is reduced, and the oscillating circuit 3 or the like is protected from AC 100 V. Further the AC 100 V is divided 8, 9, a FET 11 is switched ON by the voltage of a partial voltage point B, and an alarming sound is output from a buzzer 12.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ground resistance meter,
More specifically, the present invention relates to a grounding resistance meter of an AC potentiometer type used in a class A (first type) grounding work or the like.

[0002]

2. Description of the Related Art Grounding work performed on electrical equipment is indispensable for security to human bodies and maintenance of equipment. At the time of grounding work, a grounding resistance meter is used to measure the magnitude of the grounding resistance of the grounding body buried in the ground. This ground resistance meter is based on an AC potential difference method.

[0003] The outline of this ground resistance meter will be described with reference to FIG. This earth resistance meter has transformer 1,
A variable resistor 2 connected in parallel to the secondary winding 1B of the transformer 1, an oscillation circuit 3 oscillating a constant AC voltage, a synchronous rectification circuit 4 operating in synchronization with the oscillation circuit 3, A flow meter 5 and three measurement terminals E, P, and C are provided.

When measuring the ground resistance, a grounded body to be measured (not shown), which is a grounded body embedded in the ground, is connected to the measurement terminal E of the ground resistance meter, and is connected to the measurement terminals P and C. Are connected to auxiliary grounding electrodes (not shown) embedded in the ground at different locations. That is, the measurement method described hereafter is a three-electrode method using two auxiliary ground electrodes.

The ground resistance of the grounded object 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. Also,
The ground resistance of the auxiliary ground electrode connected to the current measuring terminal C is the auxiliary ground resistance Rc. When the measured grounding 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 ground.

When the oscillating circuit 3 is operated to oscillate an AC voltage, the measured current is measured by the measuring terminal C, the auxiliary resistor Rc,
The measurement loop returns to the oscillation circuit 3 via the ground resistance Rx, the measurement terminal E, and the primary winding 1A of the transformer 1.
As a result, a voltage drop due to the ground resistance Rx and the measurement current occurs between the measurement terminals EP.

At the same time, a current is induced in the secondary winding 1B of the transformer 1 by the measurement current, so that a voltage drop occurs between the measurement terminal E and the variable terminal 2A of the variable resistor 2. These two voltages are detected by a synchronous rectifier circuit 4 synchronized with the oscillation of the oscillation circuit 1.

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 can be obtained by the three-electrode method. That is, the voltage between the measurement terminal E and the measurement terminal P is Ex, and the measurement terminal E and the variable terminal 2 of the variable resistor 2 are
A is Rs, and the voltage drop due to Rs is E.
s, the winding ratio of the transformer 1 is 1: n, and the current flowing through the measurement loop is I, and when the galvanometer 5 is balanced,
The following equation holds. Ex = I · Rx Es = I · Rs / n Ex = Es

From these three equations, the equation of Rx = Rs / n is established, and from this equation, the ground resistance Rx can be obtained.

When such a three-electrode method is used, it is necessary to embed two auxiliary ground electrodes in the ground. However, in an urban area or the like, the ground may be paved and two auxiliary grounding electrodes may not be embedded. In such a case, the ground resistance Rx is measured by the two-electrode method.

In the two-electrode method, as shown in FIG. 3, a measurement terminal P and a measurement terminal C are connected, and a ground line 1 of the commercial power supply 100 is connected.
The measurement terminal C is connected to the 01 side. After such a connection,
The galvanometer 5 is balanced by the variable resistor 2. Thus, if the ground resistance of the grounding body of the ground line 101 is Ro, the ground resistance Rx can be obtained as Rx + Ro = Rs / n.

[0012]

When the two-electrode method is used, prior to connecting the earth resistance meter to the commercial power supply 100, the ground wire 1 of the commercial power supply 100 is detected using an electric detector or the like.
01 and the live wire 102, and the measuring terminal C is connected to the ground wire 1
01 side.

As described above, even if the ground wire 101 is checked in advance, the measurement terminal C may be erroneously connected to the live wire 102 due to carelessness or the like. Then, the commercial power supply 100 is connected to the oscillation circuit 3 and the transformer 1 via the measurement terminal C.
, The oscillation circuit 3 and the transformer 1 may be damaged. Further, the voltage of the commercial power supply 100 may be induced in the transformer 1 and the synchronous rectifier circuit 4 and the galvanometer 5 may be damaged.

SUMMARY OF THE INVENTION The present invention has been made to solve such a problem, and an object of the present invention is to provide a grounding device which can notify a user of a connection error with a commercial power supply when measuring a grounding resistance by a two-electrode method. An object of the present invention is to provide an ohmmeter.

[0015]

In order to achieve the above object, the present invention provides a first measuring terminal connected to a grounded object to be measured and a second measuring terminal for a voltage connected to each of two auxiliary grounding electrodes. 2
A measuring terminal, a third measuring terminal for current, a primary winding connected between the first measuring terminal and the third measuring terminal, and a variable resistor on a secondary winding side thereof; Are connected in parallel, an oscillation circuit for supplying an AC measurement signal to a measurement current path including the primary winding, the first measurement terminal and the variable resistor in synchronization with the oscillation of the oscillation circuit. A synchronous rectifier circuit for detecting the voltage between the second measuring terminal and the voltage of the second measuring terminal; and a galvanometer connected to the synchronous rectifying circuit. A third resistance terminal is short-circuited to the third measurement terminal, and the third measurement terminal is connected to a ground line of a commercial power supply.
Voltage detection means for detecting a voltage applied to the measurement terminal,
And an alarming means for issuing an alarm when the commercial power supply is detected by the voltage detecting means.

According to this configuration, when the third measurement terminal is erroneously connected to the live power line of the commercial power supply, the alarm means operates, and the user immediately notices the error.

In the present invention, the voltage detecting means is preferably connected to a voltage dividing resistor connected between the first measuring terminal and the third measuring terminal, and connected to the voltage dividing point via a diode. And a semiconductor switch that is turned on by a threshold voltage.

Also, by providing overvoltage protection elements between the second measurement terminal and the synchronous rectification circuit and between the oscillation circuit and the third measurement terminal, these circuits can be protected in the event of incorrect connection. .

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, a preferred embodiment of the present invention will be described with reference to FIG. 1 in order to better understand the technical concept of the present invention. In this example,
Components that are the same as or are considered the same as the previously described ground resistance meter of FIG. 2 are given the same reference numerals.

The earth 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, and a buzzer. 12 and a switch 13. In this embodiment, the resistors 8 and 9 and the diode 1
0, a voltage detecting means is constituted by the field effect transistor 11, and a buzzer 12 is used as an alarming means.

The switch 13 is a switch for short-circuiting the voltage measuring terminal P (second measuring terminal) and the current measuring terminal C (third measuring terminal) when the earth resistance meter is used as a two-electrode method. It is.

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, for example, AC1
When a high power supply of 00 V is applied, the resistance value of the thermistor 6 increases with the generation of heat, so that the current flowing through the measurement current path is reduced.

Similarly, a positive temperature coefficient thermistor 7 as an overvoltage protection element is provided on the synchronous rectifier circuit 4 side, thereby protecting the synchronous rectifier circuit 4 and the galvanometer 5 from overvoltage.

In this embodiment, the voltage detecting means comprises voltage dividing resistors 8 and 9 connected between the measuring terminal E (first measuring terminal) and the measuring terminal C, and the voltage dividing means via the diode 10. A field effect transistor (FE) connected to point B
T) 11.

In this case, the diode 10 is a rectifier diode for preventing a negative voltage from being applied to the field effect transistor 11. Field effect transistor 1
Reference numeral 1 denotes an enhancement type MOS FET, which is turned on when the gate voltage increases in the positive direction. As a result, conduction is established between the drain and the source, and the buzzer 12
Sounds.

The respective 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, the gate voltage which turns on the field effect transistor 11 at the voltage dividing point B is applied. Is set to appear.

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 grounded object to be measured, and the measurement terminal C is connected to the ground wire 101 (see FIG. 3) of the commercial power supply. Of the ground resistance Rx is measured.

At this time, at the input point A of the voltage detecting means,
The AC voltage of the oscillation circuit 3 is applied, but since the value is lower than the voltage of the commercial power supply, the voltage at the voltage dividing point B does not become a value enough to turn on the field effect transistor 11. As a result, the field effect transistor 11 is kept off, and the buzzer 12 does not emit an alarm sound.

By the way, when the measuring terminal C is erroneously connected to the live electric wire 102 (100 VAC, see FIG. 3) of the commercial power supply, the measuring terminal C, thermistor 6, the oscillation circuit 3, the transformer 1, the measuring terminal E, the grounding resistor Rx and The current flowing through the measurement loop reaching the measurement terminal C via the ground resistance Ro increases.

As a result, the resistance of the thermistor 6 increases due to the generation of heat, so that the current flowing through the measurement loop is reduced, and the oscillation circuit 3 and the transformer 1
Protected from 0V. Similarly, the thermistor 7 protects the synchronous rectifier circuit 4 from AC100V.

The voltage of 100 VAC is divided by the resistors 8 and 9 of the voltage detecting means, and the voltage at the 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.

As described above, when the ground resistance is measured by the two-electrode method, even if the measuring terminal C is erroneously connected to the live line of the commercial power supply, the transformer 1, the oscillation circuit 3, the synchronous rectification circuit 4, and the galvanometer Since the total 5 is protected from the overvoltage and the buzzer 12 emits an alarm sound, the user immediately notices the erroneous connection.

In this embodiment, a buzzer is used as the alarm means. However, a lamp may be used together or only a lamp may be used instead of the buzzer. Although the enhancement type is used for the field effect transistor 11,
If the connection direction of the diode 10 is changed, a depletion type may be used.

In the above embodiment, an example of measurement 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 prior art.

[0035]

As described above, according to the present invention,
In the measurement by the two-electrode method, even if the measuring terminal of the auxiliary grounding electrode is connected to the live power line side of the commercial power supply, the internal components are protected from the overvoltage, and an alarm is issued by the alarm means. The user can immediately notice the connection mistake.

[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 earth 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 rectifier circuit 5 Galvanometer 6,7 Thermistor 8,9 Resistance 10 Diode 11 Field effect transistor 12 Buzzer

Claims (3)

[Claims] 1. A first measuring terminal connected to a measured grounding body, and a second voltage connecting terminal connected to each of two auxiliary grounding poles.
A measuring terminal, a third measuring terminal for current, a primary winding connected between the first measuring terminal and the third measuring terminal, and a variable resistor on a secondary winding side thereof; Are connected in parallel, an oscillation circuit for supplying an AC measurement signal to a measurement current path including the primary winding, the first measurement terminal and the variable resistor in synchronization with the oscillation of the oscillation circuit. A synchronous rectifier circuit for detecting the voltage between the second measuring terminal and the voltage of the second measuring terminal; and a galvanometer connected to the synchronous rectifying circuit. A third resistance terminal is short-circuited to the third measurement terminal, and the third measurement terminal is connected to a ground line of a commercial power supply.
Voltage detection means for detecting a voltage applied to the measurement terminal,
An earth resistance meter, comprising: an alarm unit that issues an alarm when the commercial power supply is detected by the voltage detection unit. 2. The voltage detecting means according to claim 1, wherein said voltage detecting means is connected between said first measuring terminal and said third measuring terminal, and a voltage dividing point is connected via a diode to a predetermined threshold. The earth resistance meter according to claim 1, comprising a semiconductor switch that is turned on at a value voltage. 3. An overvoltage protection element is provided between the second measurement terminal and the synchronous rectifier circuit and between the oscillation circuit and the third measurement terminal. 2. The earth resistance meter according to 1.