JP5518657B2 - Ground resistance meter - Google Patents

Description

  The present invention relates to a ground resistance meter that measures ground resistance based on a two-electrode method.

  As this type of ground resistance meter, a ground resistance meter disclosed in the following patent document is known. This ground resistance meter is composed of a transformer, a variable resistor connected in parallel with the secondary winding of the transformer, an oscillation circuit that oscillates a constant alternating voltage, a synchronous rectifier circuit that operates in synchronization with the oscillation circuit, A galvanometer, thermistor, and three measurement terminals (E measurement terminal, P measurement terminal, C measurement terminal) are provided. When measuring ground resistance based on the two-electrode method, connect the P measurement terminal and the C measurement terminal. The C measurement terminal is connected to the ground line (neutral line) side of the commercial power line. In addition, the E measurement terminal is connected to the measurement object for measuring the ground resistance.

  In this ground resistance meter, when an oscillation circuit is operated to generate an alternating voltage, a measurement current is measured at a C measurement terminal, a ground line of a commercial power line, a ground, a measurement object, and a primary winding of a transformer. After passing through the measurement loop that returns to the oscillation circuit. This measurement current causes a voltage drop between the E measurement terminal and the P measurement terminal due to the combined resistance (Rx + Ro) of the ground resistance Rx and the ground resistance Ro of the ground line and the measurement current. In addition, since a current is induced in the secondary winding of the transformer, a voltage drop also occurs between the E measurement terminal and the variable resistor variable terminal. The voltage generated by the two voltage drops is detected by a synchronous rectifier circuit synchronized with the oscillation of the oscillation circuit. Therefore, the combined resistance (Rx + Ro) can be measured by balancing the detector and reading the dial mark of the variable resistor at that time.

  Also, there is a case where the C measurement terminal is accidentally connected to the live wire instead of the ground wire side of the commercial power supply line. In this ground resistance meter, the thermistor is arranged in the measurement loop. Therefore, when the current flowing through the measurement loop increases due to the connection to the live wire, the thermistor generates heat and increases its resistance value. Is protected from damage.

Japanese Patent Laid-Open No. 11-118852 (page 2-4, FIGS. 1 and 3)

  However, this conventional ground resistance meter has the following problems to be improved. That is, in this earth resistance meter, as described above, when the measurement terminal is mistakenly connected to the live wire, the thermistor is activated and the current flowing through the measurement loop is narrowed to protect the circuit of the ground resistance meter. Yes. However, since a large leakage current (leakage current greater than the sensitivity current) flows from the live wire to the ground before the thermistor is activated and the current flowing through the measurement loop is reduced, the leakage current installed in the wiring path of the commercial power supply Since the breaker trips the open / close mechanism to open the open / close contact (the open / close contact interposed in the commercial power line), there is a problem of causing a power failure.

  The present invention has been made to improve such a problem, and a main object of the present invention is to provide a ground resistance meter capable of avoiding generation of a large leakage current even if a measurement terminal is mistakenly connected to a live wire. .

  In order to achieve the above object, a ground resistance meter according to the present invention includes a first measurement terminal connected to a neutral line of a commercial power supply line and a second measurement terminal connected to a measured ground electrode grounded to the ground. A current supply unit for supplying a measurement current; a voltage measurement unit for measuring a voltage between terminals generated between the first measurement terminal and the second measurement terminal due to the supply of the measurement current; and the measurement A processing unit that executes resistance calculation processing for calculating a combined resistance of a ground resistance of the ground electrode to be measured and a ground resistance of the neutral wire based on a current value of the current and a voltage value of the voltage between the terminals; A ground resistance meter disposed in a current path of the measurement current including the current supply unit, the first measurement terminal, and the second measurement terminal, wherein the first measurement terminal is the commercial power line. When connected to a live wire And a protective resistor that limits the current value of the current flowing to a predetermined current value or less, and is connected in parallel to the protective resistor, and is normally in an off state and shifted to an on state by control of the processing unit A switch for short-circuiting the protective resistor, and a first detection unit for detecting connection of the first measurement terminal to the neutral wire based on the voltage between the terminals when the switch is in the normal state, When the connection of the first measurement terminal to the neutral wire is detected by the first detection unit, the processing unit controls the switch to shift to an on state and executes the resistance calculation process .

  The ground ohm meter according to claim 2 is provided with a second detector for detecting connection of the first measurement terminal to the live electric wire based on the voltage between the terminals in the ground ohm meter according to claim 1. When the connection of the first measurement terminal to the live wire is detected by the second detection unit, the processing unit outputs a message to that effect to the output unit.

  The ground ohm meter according to claim 3 is the ground ohm meter according to claim 1 or 2, wherein at least one of the first measurement terminal and the second measurement terminal is not connected when the switch is on. A third detection unit configured to detect a connection state based on the voltage between the terminals, and the processing unit controls the switch when the at least one unconnected state is detected by the third detection unit; Transition to the off state.

  The ground resistance meter according to claim 1, wherein the first measurement terminal is connected to a live wire of a commercial power supply line, disposed in a current path of a measurement current including a current supply unit, a first measurement terminal, and a second measurement terminal. Is connected in parallel to the protective resistance, and is normally in an off state and controlled by the processing unit. A switch that shifts to the ON state and short-circuits the protective resistor, and a first detector that detects connection to the neutral line of the first measurement terminal based on the voltage between the terminals when the switch is in the OFF state. When the connection to the neutral wire of the first measurement terminal is detected by the first detection unit, the unit controls the switch to shift to the on state and executes a resistance calculation process.

  Therefore, according to this ground resistance meter, by setting the current value less than the sensitivity current of the earth leakage breaker to a predetermined current value, the first measurement terminal until the first measurement terminal is connected to the neutral wire. Since the switch is normally off even when it is accidentally brought into contact with the live wire, the current value of the current flowing in the current path can be limited to less than the sensitivity current by the protective resistance, A power failure caused by the operation of the breaker can be reliably prevented. In addition, in the state where the first measurement terminal is normally connected to the neutral wire (that is, the second measurement terminal is normally connected to the live wire), the protective resistance is automatically short-circuited by the switch and processed. Since the resistance calculation process is automatically executed by the unit, the combined resistance can be automatically measured.

  According to the ground resistance meter of claim 2, when the second detection unit detects the connection of the first measurement terminal to the live wire based on the voltage between the terminals, the first measurement terminal is connected to the live wire. Since the effect is output to the output unit by the processing unit, the operator confirms the connection state of the first measurement terminal and the second measurement terminal based on the output content, and sets the first measurement terminal to the neutral line. In addition, the second measurement terminal can be correctly connected to the ground electrode to be measured.

  The ground resistance meter according to claim 3, further comprising a third detection unit that detects an unconnected state of at least one of the first measurement terminal and the second measurement terminal based on the inter-terminal voltage when the switch is in the ON state. When at least one unconnected state is detected by the third detection unit, the processing unit shifts the switch to the off state.

  Therefore, according to this ground resistance meter, after the processing unit determines that the first measurement terminal and the second measurement terminal are in a normal connection state, and the protective resistance is short-circuited by the switch, the first measurement terminal and the second measurement terminal 2 When the measurement terminal reaches an unconnected state for some reason, the switch is shifted to the OFF state. Thereafter, when the operator connects each measurement terminal to the commercial power line, the first measurement terminal is mistakenly set. Even if it is connected to a live wire, the current flowing through the current path is limited by the protective resistance, so that it is possible to reliably prevent a power failure caused by the operation of the earth leakage breaker.

1 is a configuration diagram of a ground resistance meter 1. FIG. 5 is a flowchart for explaining resistance measurement processing by the ground resistance meter 1.

  Hereinafter, an embodiment of the ground resistance meter 1 will be described with reference to the accompanying drawings.

  First, the configuration of the ground resistance meter 1 will be described with reference to FIG.

  As shown in FIG. 1, the ground resistance meter 1 includes a first measurement terminal 2, a second measurement terminal 3, a current supply unit 4, a current limiting resistor 5, a capacitor 6, a protective resistor 7, a switch 8, a voltage dividing resistor 9, 10, amplifier 11, synchronous rectification unit 12, A / D conversion unit 13, rectification unit 14, reference voltage generation unit 15, first detection unit 16, second detection unit 17, third detection unit 18, processing unit 19 and output And a combined resistance (Ro + Rx) of the ground resistance Ro of the neutral wire 32 of the commercial power supply line 31 and the ground resistance Rx of the ground electrode 41 to be measured grounded to the ground G based on the two-electrode method. Can be measured as a grounding resistance.

  The first measurement terminal 2 is connected to the neutral wire 32 of the neutral wire 32 and the live wire 33 that constitute the commercial power line 31. The second measurement terminal 3 is connected to a measured ground electrode 41 grounded to the ground G.

  In the state where the first measurement terminal 2 is connected to the commercial power line 31 and the second measurement terminal 3 is connected to the measured ground electrode 41, the current supply unit 4 Current path A including the second measurement terminal 3 in FIG. 1 (in other words, the current supply unit 4, the first measurement terminal 2, the commercial power line 31, the ground G, the measured ground electrode 41, and the second measurement) A measurement current (alternating current) I1 having a predefined current value is supplied to a current path that returns to the current supply unit 4 via the terminal 3.

  Generally, the commercial power supply line 31 is provided with a leakage breaker (not shown), and the commercial power supply line is supplied when a certain leakage current (leakage current greater than the sensitivity current) flows from the commercial power supply line 31 to the ground G. The commercial power supply is shut off by switching an open / close contact (not shown) interposed in 31 from an on state to an off state. For this reason, a power failure occurs when the earth leakage breaker is activated. In this ground resistance meter 1, the current value of the measurement current I1 is set to a predetermined current value less than the sensitivity current of the leakage breaker so that the leakage breaker is not activated when the measurement current I1 is supplied. It is stipulated in. As an example, when the sensitivity current of the earth leakage breaker is AC 3 mA, the current value of the measurement current I 1 can be an arbitrary current value as long as it is less than AC 3 mA, but as an example, AC 2 mA or less AC 1 mA as an example. It is defined as a current value within the above current range. In this example, as an example, AC2 mA is defined as a current value defined in advance.

  In this example, in order to supply the measurement current I1 of this current value, the current supply unit 4 supplies the measurement current I1 of 2 mA as an example, and the upper limit output voltage is defined as AC20V as an example. It is composed of a constant current source (AC constant current source). In this example, the current supply unit 4 generates a synchronization pulse Sp synchronized with the measurement current I1 and outputs the synchronization pulse Sp to the synchronization rectification unit 12.

  As shown in FIG. 1, the current limiting resistor 5 is disposed together with the capacitor 6 and the protective resistor 7 in a state of being connected in series between the current supply unit 4 and the first measurement terminal 2. The current limiting resistor 5 is in a normal connection state for measuring the combined resistance (Ro + Rx), that is, the protective resistor 7 is short-circuited by the switch 8, and the first measurement terminal 2 is neutral of the commercial power supply line 31. In the connection state in which the second measurement terminal 3 is connected to the ground electrode 41 to be measured and connected to the line 32, the output voltage of the current supply unit 4 is less than the upper limit output voltage (AC20V) (approximately half with a margin). The resistance value to be maintained is predetermined. Since the combined resistance (Ro + Rx) of the commercial power line is generally about 2 kΩ, in this example, the resistance value of the current limiting resistor 5 is defined as 2 kΩ as an example. With this configuration, in a normal connection state for measuring the combined resistance (Ro + Rx), the output voltage of the current supply unit 4 is that the measurement current I1 of AC2 mA flows through the current limiting resistor 5 and the combined resistance (Ro + Rx). , AC8V (= (2 kΩ + 2 kΩ) × 2 mA).

  The capacitor 6 removes the DC component when the DC current is included in the measurement current I1 output from the current supply unit 4, and a DC voltage is applied between the first measurement terminal 2 and the second measurement terminal 3. When this occurs, this DC voltage is prevented from being applied to the current supply unit 4 side.

  The protective resistor 7 is a current that flows through the current path A when the first measurement terminal 2 is mistakenly connected to the live wire 33, specifically, a current that flows between the commercial power line 31 and the ground G (an earth leakage breaker). Current) is limited to a predetermined current value or less. Here, the current value defined in advance is an arbitrary current value less than the sensitivity current (less than AC3 mA) of the leakage breaker. Therefore, the protective resistor 7 avoids a situation where the earth leakage breaker is activated due to an erroneous connection of the first measurement terminal 2 to the live wire 33. In this example, by defining the resistance value of the protective resistance 7 as 200 kΩ as an example, even if the voltage of the live wire 33 is AC100V to AC400V, the first measurement terminal 2 is erroneously connected to the live wire 33. In this case, the current value of the current flowing through the earth leakage breaker is limited to less than the sensitivity current, specifically, 2 mA (= 400 V / 200 kΩ) or less.

  As described above, the current value of the measurement current I1 is also defined as an arbitrary current less than the sensitivity current of the leakage breaker (less than AC3 mA). It is also possible to adopt a configuration with a predetermined current value.

  The switch 8 is connected in parallel with the protection resistor 7. Further, the switch 8 is normally in an off state (open state), and shifts to an on state (closed state) under the control of the processing unit 19 to short-circuit the protective resistor 7. In this example, as an example, the switch 8 is a self-returning relay circuit that is normally in an OFF state (a relay circuit that automatically returns from an ON state to a normal state (OFF state) by cutting off a control voltage to the relay circuit. ).

  The voltage dividing resistors 9 and 10 are connected in series with each other and are connected between the first measurement terminal 2 and the second measurement terminal 3. Further, the combined resistance value of the voltage dividing resistors 9 and 10 is defined to be a sufficiently large resistance value (1 MΩ as an example in this example) as compared with the resistance value (2 kΩ) of the combined resistance (Ro + Rx) for the commercial power line 31. Has been. Therefore, when the combined resistance (Ro + Rx) of the commercial power supply line is connected in parallel with the voltage dividing resistors 9 and 10, most of the current supplied from the current supply unit 4 flows through the combined resistance (Ro + Rx). Has been. The resistance values of the voltage dividing resistors 9 and 10 are defined as 900 kΩ and 100 kΩ as an example, and the inter-terminal voltage V1 generated between the first measurement terminal 2 and the second measurement terminal 3 is reduced to 1/10. Divide pressure. The amplifier 11 is configured as a buffer amplifier (amplifier with an amplification factor of 1) using an operational amplifier configured as a voltage follower circuit as an example, and an inter-terminal voltage between the first measurement terminal 2 and the second measurement terminal 3. A voltage Vdi (hereinafter also referred to as a “divided voltage Vdi”) output by dividing the voltage V1 by the voltage dividing resistors 9 and 10 is input with a high impedance, and the divided voltage Vdi is output with a low impedance as it is. . It is also possible to employ a configuration in which the amplifier 11 amplifies the input divided voltage Vdi with a predetermined amplification factor and outputs it as a new divided voltage Vdi.

  The synchronous rectification unit 12 receives the divided voltage Vdi output from the amplifier 11 and the synchronization pulse Sp output from the current supply unit 4, and rectifies the divided voltage Vdi in synchronization with the synchronization pulse Sp (synchronous rectification). As a result, the voltage is rectified in a state where the voltage drop is extremely small, converted into a DC voltage Vdc1, and output. The synchronous rectification unit 12 constitutes a voltage measurement unit VM together with the A / D conversion unit 13 and the processing unit 19, and the voltage measurement unit VM is an inter-terminal voltage between the first measurement terminal 2 and the second measurement terminal 3. V1 is measured. The A / D converter 13 receives the DC voltage Vdc1 and samples it at a predetermined sampling period, thereby converting it into voltage data Dv indicating the amplitude of the DC voltage Vdc1 and outputting it. The rectifier 14 performs full-wave rectification on the divided voltage Vdi output from the amplifier 11, converts it to a DC voltage Vdc2, and outputs it.

  The reference voltage generation unit 15 generates and outputs a first reference voltage Vr1, a second reference voltage Vr2, and a third reference voltage Vr3 in which each voltage value is maintained at a predetermined constant DC voltage value.

  Specifically, the first reference voltage Vr1 is a DC voltage that exceeds the DC voltage Vdc2 output from the rectifier 14 when the first measurement terminal 2 is connected to the neutral line 32 in the normal state (OFF state) of the switch 8. The value is specified in advance.

  In this case, a combined resistance (Ro + Rx) of a commercial power supply line is connected to the voltage dividing resistors 9 and 10 in parallel. In addition, since the protection resistor 7 is included in the current path A, the output voltage of the current supply unit 4 reaches the upper limit output voltage (AC 20 V). Therefore, the divided voltage Vdi output from the voltage dividing resistors 9 and 10 is approximately AC 0.02 V (= (20 V × 2 kΩ / (200 kΩ + 2 kΩ + 2 kΩ)) × (1/10)), and is output from the rectifying unit 14. The DC voltage Vdc2 is also about DC 0.02V. Thereby, in this example, the 1st reference voltage Vr1 is prescribed as DC0.1V which is slightly over this DC0.02V as an example.

  The second reference voltage Vr2 exceeds the DC voltage Vdc2 output from the rectifier 14 when the first measurement terminal 2 is not connected to the live wire 33 in the normal state (off state) of the switch 8, and the normal state of the switch 8 When the first measurement terminal 2 is connected to the live wire 33 in the OFF state, the DC voltage value is defined in advance to be lower than the DC voltage Vdc2 output from the rectifying unit 14.

  In this case, the maximum value of the DC voltage Vdc2 when the first measurement terminal 2 is not connected to the live wire 33 in the normal state (off state) of the switch 8 is at least one of the first measurement terminal 2 and the second measurement terminal 3. Is the voltage when not connected (or disconnected). At this time, since the output voltage of the current supply unit 4 becomes the upper limit output voltage (AC 20 V), the divided voltage Vdi becomes about AC 1.7 V (= 20 V × 100 kΩ / (200 kΩ + 1 MΩ)) and is output from the rectifying unit 14. The DC voltage Vdc2 is also about DC1.7V. In order to facilitate understanding of the invention, a voltage drop caused by rectification in the rectification unit 14 is ignored. On the other hand, when the first measurement terminal 2 is connected to the live wire 33 in the normal state (off state) of the switch 8, the voltage of the live wire 33 is in the range of AC100V to AC400V. At least AC 100 V is applied to. Therefore, the divided voltage Vdi is AC 10 V (= 100 V × 100 kΩ / 1 MΩ), and the DC voltage Vdc2 output from the rectifier 14 is also DC 10 V. Thus, in this example, the second reference voltage Vr2 is defined as DC8V slightly lower than DC10V as an example of a voltage that is higher than DC1.7V and lower than DC10V.

  The third reference voltage Vr3 is such that at least one of the first measurement terminal 2 and the second measurement terminal 3 is not connected when the switch 8 is on (the first measurement terminal 2 is the neutral wire 32 and the live wire 33). The DC voltage value that is lower than the DC voltage Vdc2 output from the rectifier 14 when the second measurement terminal 3 is not connected to the ground electrode 41 to be measured) Has been.

  In this case, the measurement current I1 output from the current supply unit 4 flows through a path reaching the current supply unit 4 via the current limiting resistor 5, the switch 8, and the voltage dividing resistors 9 and 10. For this reason, the output voltage of the current supply unit 4 has reached the upper limit output voltage (AC 20 V). As a result, the divided voltage Vdi output from the voltage dividing resistors 9 and 10 is approximately AC2V (= (20V × 100 kΩ / (1 MΩ + 2 kΩ))), and the DC voltage Vdc2 output from the rectifier 14 is also approximately DC2V. . Thereby, in this example, the 3rd reference voltage Vr3 is prescribed as DC1V which is slightly less than this DC2V as an example.

  As an example, the first detection unit 16 includes a comparator in which the first reference voltage Vr1 is input to the non-inverting input terminal and the DC voltage Vdc2 is input to the inverting input terminal. As described above, the first reference voltage Vr1 is higher than the DC voltage Vdc2 output from the rectifier 14 when the first measurement terminal 2 is connected to the neutral line 32 in the normal state (off state) of the switch 8. The voltage value is defined in advance. That is, the DC voltage Vdc2 output from the rectifier 14 is less than the first reference voltage Vr1 only when the first measurement terminal 2 is connected to the neutral line 32 in the normal state (off state) of the switch 8. Yes.

  Therefore, with this configuration, the first detection unit 16 detects the connection to the neutral wire 32 of the first measurement terminal 2 based on the inter-terminal voltage V1 when the switch 8 is in the OFF state. Specifically, the first detection unit 16 compares the first reference voltage Vr1 and the DC voltage Vdc2, thereby allowing the first measurement terminal 2 and the second measurement terminal 3 in the normal state (off state) of the switch 8. It is detected whether or not the inter-terminal voltage V1 is a voltage when the first measurement terminal 2 is connected to the neutral wire 32, and this inter-terminal voltage V1 makes the first measurement terminal 2 to the neutral wire 32. A detection signal S1 whose level changes only when the voltage is the voltage at the time of connection is generated and output to the processing unit 19. In this example, as described above, since the first reference voltage Vr1 is input to the non-inverting input terminal and the DC voltage Vdc2 is input to the inverting input terminal, the first detector 16 detects the inter-terminal voltage V1. The detection signal S1 is generated and outputted only when the voltage is the voltage when the first measurement terminal 2 is connected to the neutral line 32, and the level becomes the high level in other cases.

  As an example, the second detection unit 17 includes a comparator in which the second reference voltage Vr2 is input to the non-inverting input terminal and the DC voltage Vdc2 is input to the inverting input terminal. As described above, the second reference voltage Vr2 exceeds the DC voltage Vdc2 output from the rectifying unit 14 when the first measurement terminal 2 is not connected to the live wire 33 in the normal state (off state) of the switch 8. In addition, when the switch 8 is in a normal state (off state), the DC voltage value is defined in advance to be lower than the DC voltage Vdc2 output from the rectifying unit 14 when the first measurement terminal 2 is connected to the live wire 33. That is, only when the first measurement terminal 2 is connected to the live wire 33, the DC voltage Vdc2 output from the rectifying unit 14 exceeds the second reference voltage Vr2.

  Therefore, with this configuration, the second detection unit 17 detects the connection of the first measurement terminal 2 to the live wire 33 based on the inter-terminal voltage V1. Specifically, the second detection unit 17 compares the second reference voltage Vr2 with the DC voltage Vdc2, thereby determining the inter-terminal voltage V1 between the first measurement terminal 2 and the second measurement terminal 3 in the first measurement. It is detected whether or not the voltage is the voltage when the terminal 2 is connected to the live wire 33, and the level is only when this inter-terminal voltage V1 is the voltage when the first measurement terminal 2 is connected to the live wire 33. A changing detection signal S2 is generated and output to the processing unit 19. In this example, as described above, since the second reference voltage Vr2 is input to the non-inverting input terminal and the DC voltage Vdc2 is input to the inverting input terminal, the first detection unit 16 includes the first measurement terminal 2. Detection is performed only when the inter-terminal voltage V1 between the second measurement terminals 3 is a voltage when the first measurement terminal 2 is connected to the live wire 33, and the level becomes a low level in other cases. A signal S2 is generated and output.

  As an example, the third detection unit 18 includes a comparator in which the third reference voltage Vr3 is input to the non-inverting input terminal and the DC voltage Vdc2 is input to the inverting input terminal. As described above, the third reference voltage Vr3 is supplied from the rectifier 14 when at least one of the first measurement terminal 2 and the second measurement terminal 3 is not connected in the ON state of the switch 8. A DC voltage value lower than the output DC voltage Vdc2 is defined in advance. That is, the DC voltage Vdc2 output from the rectifying unit 14 is the third reference only when at least one of the first measurement terminal 2 and the second measurement terminal 3 is not connected when the switch 8 is on. The configuration exceeds the voltage Vr3.

  Therefore, with this configuration, the third detection unit 18 detects the unconnected state of at least one of the first measurement terminal 2 and the second measurement terminal 3 based on the inter-terminal voltage V1 when the switch 8 is in the on state. Specifically, the third detection unit 18 compares the third reference voltage Vr3 with the DC voltage Vdc2 to generate between the first measurement terminal 2 and the second measurement terminal 3 when the switch 8 is on. Detecting whether or not the inter-terminal voltage V1 is a voltage when at least one of the first measurement terminal 2 and the second measurement terminal 3 is not connected (or disconnected); A detection signal S3 whose level changes only when the voltage is in the unconnected state is generated and output to the processing unit 19. In this example, as described above, since the third reference voltage Vr3 is input to the non-inverting input terminal and the DC voltage Vdc2 is input to the inverting input terminal, the third detection unit 18 includes the first measurement terminal 2. Only when the second measurement terminal 3 is in an unconnected state, the detection signal S3 becomes a low level, and otherwise the detection signal S3 becomes a high level.

  The processing unit 19 includes a CPU and a memory (both not shown), and executes the resistance measurement process shown in FIG. As shown in the figure, in this resistance measurement process, the processing unit 19 includes a first measurement terminal 2 and a second measurement terminal 3 based on the detection signals S1, S2, and S3 output from the detection units 16, 17, and 18. Connection state determination processing, resistance calculation processing of the combined resistance (Ro + Rx) based on the voltage data Dv output from the A / D conversion unit 13 and the current value (2 mA) of the measurement current I1 output from the current supply unit 4 The control process for the switch 8 and the output process to the output unit 20 are executed. Further, the current value (2 mA) of the measurement current I1 supplied from the current supply unit 4 is stored in the memory of the processing unit 19 in advance. The output unit 20 is configured by a display device including a display device such as an LCD (Liquid Crystal Display) as an example, and displays the result of each process executed by the processing unit 19 on the screen.

  Next, the measurement operation of the combined resistance (Ro + Rx) by the ground resistance meter 1 will be described with reference to FIGS. As shown in FIG. 1, it is assumed that the measured ground electrode 41 is grounded to the ground G in advance.

  First, as a pretreatment for measurement using the ground resistance meter 1, the first measurement terminal 2 is connected to the neutral wire 32, and the second measurement terminal 3 is connected to the measured ground electrode 41.

  Next, measurement using the ground resistance meter 1 is started. In this case, in the ground resistance meter 1, the current supply unit 4 supplies the measurement current I1 and outputs the synchronization pulse Sp. The reference voltage generation unit 15 generates and outputs the reference voltages Vr1, Vr2, and Vr3. Further, the switch 8 is in a normal state (off state) because the control by the processing unit 19 is not performed.

  In this state, the processing unit 19 executes a resistance measurement process 50 shown in FIG. In the resistance measurement process 50, the processing unit 19 first executes a connection state determination process (first connection state determination process) for determining the connection state of the measurement terminals 2 and 3 in the normal state of the switch 8 (step 1). 51).

  In this connection state determination process, the processing unit 19 activates the first measurement terminal 2 in the normal state (off state) of the switch 8 based on the levels of the detection signals S1 and S2 output from the detection units 16 and 17. In a state where it is connected to the electric wire 33, a state where the first measurement terminal 2 is connected to the neutral wire 32, or a state where either the first measurement terminal 2 or the second measurement terminal 3 is not connected Determine if it exists.

  Specifically, as described above, the second detection unit 17 is configured such that the voltage V1 between the first measurement terminal 2 and the second measurement terminal 3 is a voltage when the first measurement terminal 2 is connected to the live wire 33. The detection signal S2 having a low level is generated only when the signal is low, and is at a high level otherwise. In addition, since the direct-current voltage Vdc2 exceeds the first reference voltage Vr1 when the inter-terminal voltage V1 is a voltage when the first measurement terminal 2 is connected to the live wire 33, the first detection unit 16 has a low level. Detection signal S2 is output. For this reason, when the level of the input detection signal S2 is low, the processing unit 19 determines that the first measurement terminal 2 is connected to the live wire 33 and outputs to that effect to the output unit 20 Processing is executed (step 52). In this example, since the output unit 20 is configured by the display device as described above, the output processing displays that the first measurement terminal 2 is connected to the live wire 33 on the screen of the display device. Is done. Since the operator of the ground resistance meter 1 displays this fact on the output unit 20, the operator can confirm the connection state of the first measurement terminal 2 and correctly connect the first measurement terminal 2 to the neutral wire 32. It is possible. Even when the first measurement terminal 2 is connected to the live wire 33, as described above, the current flowing between the commercial power supply line 31 and the ground G (current flowing through the earth leakage breaker) is the current. A protective resistor 7, which is defined as a resistance value that can be limited to a current value (2 mA) or less of the measurement current I 1 supplied from the supply unit 4, that is, less than the sensitivity current of the leakage breaker (less than 3 mA AC), is arranged in the current path A. Therefore, power failure due to malfunction of earth leakage breaker is prevented.

  On the other hand, in step 51, when at least one of the first measurement terminal 2 and the second measurement terminal 3 is not connected (or disconnected), as described above, the DC voltage Vdc2 output from the rectifier 14 is DC1. Therefore, the level of the detection signal S2 output from the second detection unit 17 becomes High level, and the level of the detection signal S1 output from the first detection unit 16 becomes Low level. Therefore, in this state, the processing unit 19 determines that at least one of the first measurement terminal 2 and the second measurement terminal 3 is in an unconnected (or disconnected) state, and notifies the output unit 20 to that effect. The output process to be output to is executed (step 53). Therefore, the fact that at least one of the measurement terminals 2 and 3 is not connected is displayed on the screen of the display device constituting the output unit 20. Since the operator of the ground resistance meter 1 displays this on the output unit 20, the operator confirms the connection state of the first measurement terminal 2 and the second measurement terminal 3, and connects the first measurement terminal 2 to the neutral line 32. In addition, the second measurement terminal 3 can be correctly connected to the measured ground electrode 41.

  The processing unit 19 repeatedly executes the above steps 51, 52, and 53 until the first measurement terminal 2 is correctly connected to the neutral wire 32 and the second measurement terminal 3 is correctly connected to the measured ground electrode 41. . In this case, the first measurement terminal 2 is correctly connected to the neutral wire 32 from the beginning, and the second measurement terminal 3 is correctly connected to the ground electrode 41 to be measured from the beginning, or the above steps 52 and 53. When the operator correctly confirms the content displayed on the output unit 20 by the output process and reconnects the first measurement terminal 2 and the second measurement terminal 3 correctly, the first detection unit 16 determines that the level is High level. A detection signal S1 is generated and output. For this reason, in step 51, since the level of the input detection signal S1 is the high level, the processing unit 19 normally connects the first measurement terminal 2 to the neutral line 32, and the second measurement terminal 3 It is determined that it is normally connected to the measured ground electrode 41, and a control process for controlling the switch 8 in the normal state to the ON state is executed (step 54). By this control process, the protective resistor 7 is short-circuited by the switch 8 that is turned on.

  Subsequently, the processing unit 19 executes a connection state determination process (second connection state determination process) for determining a connection state for each of the measurement terminals 2 and 3 when the switch 8 is on (step 55).

  In this connection state determination process, the processing unit 19 determines that the first measurement terminal 2 and the first measurement terminal 2 determined to be normally connected in step 51 based on the level of the detection signal S3 output from the third detection unit 18. 2 It is determined whether or not the measurement terminal 3 is not connected (or disconnected) for some reason. In this case, as described above, in the third detector 18, when the switch 8 is in the on state, at least one of the first measurement terminal 2 and the second measurement terminal 3 is not connected (or disconnected). The detection signal S3 is generated and output only when the signal level is Low level, otherwise it is High level. For this reason, when the level of the input detection signal S3 is the Low level, the processing unit 19 has reached at least one of the first measurement terminal 2 and the second measurement terminal 3 in an unconnected state (or a disconnected state). And control processing for controlling the switch 8 in the on state to the off state is executed (step 56), and then at least one of the first measurement terminal 2 and the second measurement terminal 3 is not connected. (Or a disconnection state) The output process which outputs that it has reached to the output part 20 is performed (step 57). As a result of this output processing, the fact that it is not connected is displayed on the screen of the display device that constitutes the output unit 20. Since the operator of the earth resistance meter 1 displays this on the output unit 20, the operator confirms the connection state of the measurement terminals 2 and 3 and sets the first measurement terminal 2 to the neutral wire 32 and the second measurement. The terminal 3 can be correctly connected to the ground electrode 41 to be measured. In addition, even when the first measurement terminal 2 is mistakenly connected to the live electric wire 33 during this connection work, the switch 8 is in an OFF state and the protective resistor 7 is disposed in the current path A. The current that flows between the commercial power line 31 and the ground G (current that flows through the earth leakage breaker) is limited to less than the sensitivity current of the earth leakage breaker (less than AC 3 mA). Therefore, the power failure resulting from the malfunction of the earth leakage breaker is prevented.

  The processing unit 19 repeatedly executes steps 55 to 57 until the first measurement terminal 2 is correctly connected to the neutral wire 32 and the second measurement terminal 3 is correctly connected to the measured ground electrode 41. In this case, when the operator confirms the contents displayed on the output unit 20 by the output process in step 57 and correctly connects the first measurement terminal 2 and the second measurement terminal 3, the third detection unit 18 generates and outputs a detection signal S3 having a high level. For this reason, in step 55, since the level of the input detection signal S3 is the high level, the processing unit 19 normally connects the first measurement terminal 2 to the neutral line 32, and the second measurement terminal 3 It is determined that it is normally connected to the measured ground electrode 41, and resistance calculation processing is executed (step 58).

  As described above, when the first measurement terminal 2 is normally connected to the neutral wire 32 and the second measurement terminal 3 is normally connected to the measured ground electrode 41, the current is output from the current supply unit 4. The measurement current (AC current) I1 passes through the current path A (that is, the current supply unit 4, the first measurement terminal 2, the commercial power supply line 31, the ground G, the measured ground electrode 41, and the second measurement terminal 3). Therefore, the divided voltage Vdi is supplied to the combined resistance (Rx + Ro) of the ground resistance Ro of the neutral wire 32 and the ground resistance Rx of the ground electrode 41 to be measured. This indicates a voltage generated due to the flow of current.

  Therefore, in this resistance calculation process, the processing unit 19 is first based on voltage data Dv indicating the amplitude of the DC voltage Vdc1 output from the synchronous rectification unit 12 (also voltage data indicating the amplitude of the divided voltage Vdi). Then, the voltage value of the AC voltage between the first measurement terminal 2 and the second measurement terminal 3 (as an example, the effective value of this AC voltage) is calculated, and then the calculated voltage value of the AC voltage is stored in the memory. The combined resistance (Rx + Ro) is calculated based on the measured current value I1 (2 mA in this example).

  Finally, the processing unit 19 executes output processing (step 59), and displays the calculated combined resistance (Rx + Ro) on the screen of the display device configuring the output unit 20. Thereby, the resistance measurement process 50 is completed.

  As described above, in the ground resistance meter 1, the first measurement terminal 2 is disposed in the current path A of the measurement current I 1 including the current supply unit 4, the first measurement terminal 2, and the second measurement terminal 3. A protective resistor 7 for limiting the current value of the current flowing through the current path A when connected to the live wire 33 of the power line 31 to a predetermined current value (current value less than the sensitivity current of the leakage breaker) or less, and protection A switch 8 that is connected in parallel to the resistor 7, is normally in an off state, and is turned on by control from the processing unit 19 to short-circuit the protective resistor 7, and a neutral line of the first measurement terminal 2 The first detection unit 16 that detects connection to the terminal 32 based on the voltage V1 between the terminals, and the processing unit 19 is a voltage when the voltage V1 between the terminals connects the first measurement terminal 2 to the neutral wire 32. When the first detector 16 detects that A) when the first detection unit 16 detects a connection to the first measurement terminal 2 of the neutral line 32, it executes the resistance calculation process.

  Therefore, according to the ground resistance meter 1, even when the first measurement terminal 2 is accidentally brought into contact with the live wire 33 until the first measurement terminal 2 is connected to the neutral wire 32, Since the switch 8 is normally in an OFF state, the current value of the current flowing through the current path A is limited by the protective resistor 7 to be less than the sensitivity current of the leakage breaker, so that power failure caused by the operation of the leakage breaker is surely prevented. can do. Further, when the first measurement terminal 2 is normally connected to the neutral wire 32 (that is, the second measurement terminal 3 is also normally connected to the live wire 33), the protective resistor 7 is automatically switched by the switch 8. Since the resistance calculation process is automatically executed by the processing unit 19, the combined resistance (Ro + Rx) can be automatically measured.

  Moreover, according to this earth resistance meter 1, when the 2nd detection part 17 detects the connection to the live electric wire 33 of the 1st measurement terminal 2 based on the voltage V1 between terminals, the 1st measurement terminal 2 is the live wire 33. Is connected to the output unit 20 by the processing unit 19, the operator confirms the connection state of the first measurement terminal 2 and the second measurement terminal 3 based on the output content, The first measurement terminal 2 can be correctly connected to the neutral wire 32, and the second measurement terminal 3 can be correctly connected to the measured ground electrode 41.

  Further, in the ground resistance meter 1, when the voltage V1 between the first measurement terminal 2 and the second measurement terminal 3 is in the ON state of the protective resistor 7, the first measurement terminal 2 and the second measurement terminal 3 When the third detection unit 18 detects whether or not the voltage when at least one is in the unconnected state and detects that the voltage is when the third detection unit 18 is in the unconnected state, The processing unit 19 shifts the switch 8 from the on state to the off state.

  Therefore, according to the ground resistance meter 1, after the processing unit 19 determines that the first measurement terminal 2 and the second measurement terminal 3 are in a normal connection state, and the protective resistor 7 is short-circuited by the switch 8, When the first measurement terminal 2 and the second measurement terminal 3 reach an unconnected state (or a disconnected state) for some reason, the switch 8 is shifted to an off state. , 3 is connected to the commercial power line, even if the first measurement terminal 2 is mistakenly connected to the live wire 33, the current flowing through the current path A is limited by the protective resistor 7, so that the leakage breaker is activated. The resulting power failure can be reliably prevented.

  Note that the ground resistance meter 1 includes the third detector 18, and once it is determined that the connection state of the first measurement terminal 2 and the second measurement terminal 3 is normal, the protective resistor 7 is short-circuited by the switch 8. Even when the first measurement terminal 2 and the second measurement terminal 3 are not connected (or disconnected) for some reason, the processing unit 19 controls the switch 8 to be in an OFF state, Although the preferable structure which prevents the power failure resulting from the incorrect connection to the live electric wire 33 of the 1st measurement terminal 2 in the subsequent connection operation | work is employ | adopted, the 1st measurement terminal 2 and the 2nd measurement terminal 3 are once normal once. A configuration in which the third detection unit 18 is omitted may be employed when there is a low possibility of being in an unconnected state after entering the connected state. In this configuration, the reference voltage generation unit 15 generates two types of voltages, the first reference voltage Vr1 and the second reference voltage Vr2.

  Further, the ground resistance meter 1 includes the second detection unit 17, and when the connection of the first measurement terminal 2 to the live wire 33 is detected by the second detection unit 17, the processing unit 19 indicates that. However, a configuration in which the first detection unit 16 is provided and the second detection unit 17 is not provided may be employed. Even in this configuration, the switch 8 is normally in an OFF state, and the processing unit 19 is not detected until the first detection unit 16 detects that the first measurement terminal 2 is normally connected to the neutral wire 32. Since the switch 8 is controlled to be in the ON state by this, even if the first measurement terminal 2 is connected to the live wire 33 before the switch 8 is switched to the ON state, the current flowing through the current path A is It can limit to less than the sensitivity current of the earth leakage breaker, and can prevent the occurrence of power failure.

  Further, in the ground resistance meter 1 described above, the current supply unit 4 is configured by a constant current source, but may be configured by an AC constant voltage source. In this configuration, when the switch 8 is in the on state and the first measurement terminal 2 and the second measurement terminal 3 are in the normal connection state, the output voltage of the current supply unit 4 configured by the AC constant voltage source is The measurement current I1 flowing in the path A is defined as a constant voltage having a current value less than the sensitivity current. As an example, when the current limiting resistor 5 is 2 kΩ and the combined resistance (Rx + Ro) is 2 kΩ and 2 mA less than the sensitivity current is supplied as the measurement current I1 as in the above example, this current supply unit 4 is constituted by an AC constant voltage source of AC8V (= (2 kΩ + 2 kΩ) × 2 mA). In this case, when the voltage dividing ratio of the voltage dividing resistors 9 and 10 is set to 1/10, which is the same as the above example, the first reference voltage among the reference voltages Vr1, Vr2, and Vr3 generated by the reference voltage generating unit 15 is used. The same voltage may be used for Vr1, but for the second reference voltage Vr2 and the third reference voltage Vr3, the upper limit output voltage of the current supply unit 4 composed of the above-described constant current source is AC20V, whereas Since the output voltage of the current supply unit 4 composed of the voltage source is AC8V and is a lower voltage, it is regulated to a lower voltage.

  With this configuration, even in a ground resistance meter in which the current supply unit 4 is configured with a constant voltage source, the same effect as the ground resistance meter 1 in which the current supply unit 4 is configured with a constant current source can be achieved.

  Further, in the ground resistance meter 1 described above, the switch 8 is configured using a self-reset relay circuit. However, the latch-type relay circuit is used to control from the on state to the off state and from the off state to the on state. It is also possible to adopt a configuration in which the processing unit 19 controls both of these controls. The ground resistance meter 1 employs a configuration in which the processing unit 19 controls the switch 8 so that the combined resistance (Rx + Ro) can be automatically measured. The switch 8 is manually operated. It is also possible to adopt a configuration for shifting from the normal state to the on state by operation.

DESCRIPTION OF SYMBOLS 1 Ground resistance meter 2 1st measurement terminal 3 2nd measurement terminal 4 Current supply part 7 Protection resistance 8 Switch 16 1st detection part 17 2nd detection part 18 3rd detection part 19 Processing part 20 Output part 32 Neutral wire 33 Active Wire 41 Ground to be measured A Current path VM Voltage measurement unit

Claims (3)

A current supply unit for supplying a measurement current between a first measurement terminal connected to the neutral line of the commercial power supply line and a second measurement terminal connected to the ground electrode to be measured grounded to the ground;
A voltage measuring unit for measuring a terminal voltage generated between the first measurement terminal and the second measurement terminal due to the supply of the measurement current;
A processing unit that executes resistance calculation processing for calculating a combined resistance of the ground resistance of the ground electrode to be measured and the ground resistance of the neutral wire based on the current value of the measurement current and the voltage value of the terminal voltage A ground resistance meter comprising:
When the first measurement terminal is connected to a live power line of the commercial power line, disposed in the current path of the measurement current including the current supply unit, the first measurement terminal, and the second measurement terminal. A protective resistor that limits the current value of the current flowing through the current path to a predetermined current value or less;
A switch that is connected in parallel to the protective resistor, is in an off state in a normal state, and shifts to an on state by the control of the processing unit to short-circuit the protective resistor,
A first detector that detects connection of the first measurement terminal to the neutral wire based on the voltage between the terminals when the switch is in the normal state;
When the connection of the first measurement terminal to the neutral wire is detected by the first detection unit, the processing unit controls the switch to shift to an on state and executes the resistance calculation process Ground resistance meter. A second detector for detecting connection of the first measurement terminal to the live wire based on the voltage between the terminals;
The ground resistance meter according to claim 1, wherein when the connection of the first measurement terminal to the live electric wire is detected by the second detection unit, the processing unit outputs that fact to the output unit. A third detector that detects an unconnected state of at least one of the first measurement terminal and the second measurement terminal based on the voltage between the terminals when the switch is on;
3. The ground resistance meter according to claim 1, wherein when the at least one unconnected state is detected by the third detection unit, the processing unit controls the switch to shift to an off state. 4.

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