Insulation resistance measuring device

Abstract

To provide an insulation resistance measuring device capable of increasing measurement precision of insulation resistance.SOLUTION: An insulation resistance measuring device comprises: a first terminal T1; a second terminal T2; a guard terminal G; a setting circuit for setting the input resistance of the guard terminal to a first value, and a second value that is smaller than the first value; a voltage determination section 13 for determining whether a first voltage value between the first terminal and the guard terminal is equal to or less than a first threshold; a voltage application circuit 11 for applying an application voltage between the first terminal and the second terminal; and a resistance measurement section 16 for measuring insulation resistance between the first terminal and the second terminal. When the voltage determination section determines that the first voltage value is equal to or less than the first threshold in a first state in which the setting circuit sets the input resistance to the first value, it is brought into a second state in which the setting circuit sets the input resistance to a second value and the voltage application circuit applies an application voltage, and the resistance measurement section measures the insulation resistance between the first terminal and the second terminal in the second state.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to an insulation resistance measuring device. [Background technology]

[0002] An insulation resistance measuring device that includes a ground terminal, a line terminal, and a guard terminal is known (for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 8-82501 Summary of the Invention [Problem to be solved by the invention]

[0004] The guard terminal is provided to measure insulation resistance without current flowing on the surface of the insulator. If a protection circuit is connected to the guard terminal to protect the insulation resistance measuring device when a high voltage is applied to the guard terminal, the accuracy of the insulation resistance measurement will decrease.

[0005] An object of the present disclosure is to provide an insulation resistance measuring device that can improve the accuracy of measuring insulation resistance. [Means for solving the problem]

[0006] According to an embodiment of the present disclosure, an insulation resistance measuring device includes a first terminal, a second terminal, a guard terminal, a setting circuit that sets the input resistance of the guard terminal to a first value and a second value lower than the first value, a voltage determination unit that determines whether a first voltage value between the first terminal and the guard terminal is equal to or lower than a first threshold value, a voltage application circuit that applies an application voltage between the first terminal and the second terminal, and a resistance measurement unit that measures the insulation resistance between the first terminal and the second terminal. In a first state in which the setting circuit sets the input resistance to the first value, when the voltage determination unit determines that the first voltage value is equal to or lower than the first threshold value, the device enters a second state in which the setting circuit sets the input resistance to the second value and the voltage application circuit applies an application voltage. In the second state, the resistance measurement unit measures the insulation resistance between the first terminal and the second terminal. In the first state, when the voltage determination unit determines that the first voltage value is higher than the first threshold value, the setting circuit maintains the first state, and the resistance measurement unit does not measure the insulation resistance. [Effects of the Invention]

[0007] According to the present disclosure, the accuracy of measuring insulation resistance can be improved. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a diagram showing a measurement method using the insulation resistance measuring device of the first embodiment. [Figure 2] FIG. 2 is a block diagram of the insulation resistance measuring device according to the first embodiment. [Figure 3] FIG. 3 is a block diagram of an insulation resistance measuring device according to Comparative Example 1. As shown in FIG. [Figure 4] FIG. 4 is a flowchart showing the operation of the insulation resistance measuring device according to the first embodiment. [Figure 5] FIG. 5 is a circuit diagram showing a voltage determination unit of the insulation resistance measuring device according to the second embodiment. [Figure 6] FIG. 6 is a flowchart showing the operation of the processor of the insulation resistance measuring device according to the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. The following embodiments are examples for embodying the technical ideas of the invention, and the present disclosure is not limited to the described configurations and numerical values. In each drawing, the same components are denoted by the same reference numerals, and duplicate explanations may be omitted as appropriate.

[0010] (First embodiment) Fig. 1 is a diagram showing a measurement method using the insulation resistance measuring device of the first embodiment. As shown in Fig. 1, a high-voltage cable 30 includes a core wire 31, an insulator 32, a shield layer 33, and an insulator 34. The core wire 31 is surrounded by the insulator 32, the insulator 32 is surrounded by the shield layer 33, and the insulator 34 surrounds the shield layer 33.

[0011] When measuring the volume resistance of the insulator 32, the shield layer 33 is exposed from the insulator 34, and the core wire 31 is exposed from the insulator 32. Terminals T1 and T2 of the insulation resistance measuring device 100 are connected to the core wire 31 and the shield layer 33, respectively. Terminals T1 and T2 are connected using, for example, alligator clips 35 and 37. The insulation resistance measuring device 100 applies a voltage between the shield layer 33 and the core wire 31 and measures the current flowing between the shield layer 33 and the core wire 31. However, if a current 38 A flows on the surface of the insulator 32, the measurement accuracy of the volume resistance of the insulator 32 decreases.

[0012] Therefore, bare conductor 36 is wound around the surface of insulator 32. Bare conductor 36 is connected to guard terminal G of insulation resistance measuring device 100. As a result, current 38B flowing on the surface of insulator 32 flows from bare conductor 36 to guard terminal G. This makes it possible to suppress a decrease in the accuracy of measuring volume resistance caused by the current flowing on the surface of insulator 32. The above is just one example, and guard terminal G is used when the current flowing through areas other than the one whose insulation resistance is to be measured is not to be involved in the insulation resistance measurement.

[0013] 2 is a block diagram of the insulation resistance measuring device according to the first embodiment. The insulation resistance measuring device 100 includes a voltage application circuit 11, a voltage measurement unit 12, a voltage determination unit 13, a switch 14, a current measurement unit 15, a resistance measurement unit 16, an input unit 18, an output unit 19, resistors R1 and R2, terminals T1 and T2, and a guard terminal G.

[0014] Terminal T1 (first terminal), terminal T2 (second terminal), and guard terminal G are connected to core wire 31, shield layer 33, and bare conductor 36, respectively (see FIG. 1). Voltage application circuit 11 applies voltage Va between terminals T1 and T2. Applied voltage Va is, for example, a DC voltage. The voltage of terminal T2 is, for example, positive relative to terminal T1. Voltage measurement unit 12 measures voltage value V2 of the voltage between terminals T1 and T2. For example, voltage measurement unit 12 has resistor Rm. The voltage value measured by resistive division of resistor Rm is output to resistance measurement unit 16. The resistance value of resistor Rm is, for example, 1 MΩ to 500 MΩ, and is, for example, 10 MΩ.

[0015] The resistor Rg includes resistors R1 and R2. The resistors R1 and R2 are connected in series between the guard terminal G and a node N1. The resistor Rg corresponds to the input resistance of the guard terminal G. The voltage determination unit 13 determines whether a voltage value V1 between the guard terminal G and the node N1 is equal to or less than a threshold value. The voltage determination unit 13 may also determine whether a voltage value between the guard terminal G and a terminal T1 is equal to or less than a threshold value. The voltage determination unit 13 turns on or off a switch 14 based on the determination result. The switch 14 is connected in parallel to the resistor R2 and in series to the resistor R1 between the guard terminal G and the node N1. The switch 14 (setting circuit) sets the input resistance of the guard terminal G to a high resistance value RH (first value) of the sum of the resistors R1 and R2 and a low resistance value RL (second value) of the resistor R1 that is lower than the high resistance value RH.

[0016] The current measurement unit 15 measures the current flowing between the node N1 and the terminal T1. For example, the current measurement unit 15 measures the current value I2 of the current flowing from the voltage application circuit 11 to the terminal T1. The current measurement unit 15 outputs the measured current value I2 to the resistance measurement unit 16. The resistance measurement unit 16 acquires the voltage value V2 measured by the voltage measurement unit 12 and the current value I2 measured by the current measurement unit 15. For example, the outputs of the voltage measurement unit 12 and the current measurement unit 15 are analog signals. The resistance measurement unit 16 converts the analog signals output by the voltage measurement unit 12 and the current measurement unit 15 into digital signals and acquires the converted signals as the voltage value V2 and the current value I2, respectively. The resistance measurement unit 16 calculates the resistance value of the insulation resistor based on the acquired voltage value V2 and current value I2. At least some of the functions of the voltage determination unit 13 and the resistance measurement unit 16 may be executed by a processor such as a CPU (Central Processing Unit) in cooperation with software. At least some of the functions of the voltage determination unit 13 and the resistance measurement unit 16 may be performed by dedicated hardware circuits.

[0017] The input unit 18 is a switch, button, or key, and a user operates the input unit 18 when measuring insulation resistance using the insulation resistance measuring device 100. The output unit 19 is an interface with a display device or external device, and outputs the resistance value of the measured insulation resistance and error information.

[0018] (Block diagram of a comparative example) Fig. 3 is a block diagram of an insulation resistance measuring device according to Comparative Example 1. As shown in Fig. 3, in an insulation resistance measuring device 110 of Comparative Example 1, a resistor Rg is connected between a guard terminal G and a node N1. The other configurations are the same as those of the first embodiment shown in Fig. 2.

[0019] A measurement object 39 is connected between terminals T1 and T2, and a guard terminal G is connected to the surface of the measurement object 39. Current 38B flowing from terminal T2 on the surface of the measurement object 39 flows from guard terminal G to node N1. Therefore, current 38B does not flow through the current measurement unit 15 and is not involved in insulation resistance measurement. Resistor Rg is a protection circuit that prevents damage to the insulation resistance measuring device 110 (e.g., current measurement unit 15) when a high voltage is applied to guard terminal G. Since it functions as a protection circuit, resistor Rg has a resistance value of, for example, 10 MΩ. If the resistance value of resistor Rg is high, part of current 38B will flow to terminal T1 as current 38C flowing on the surface of the measurement object 39. Current 38C will flow through the current measurement unit 15. This reduces the accuracy of the insulation resistance measurement.

[0020] On the other hand, if the resistance value of resistor Rg is reduced, current 38C is reduced and the measurement accuracy of the insulation resistance is improved. However, if a high voltage is applied to guard terminal G, there is a possibility that insulation resistance measuring device 110 (e.g., current measuring unit 15) may be damaged.

[0021] 4 is a flowchart showing the operation of the insulation resistance measuring device according to the first embodiment. As shown in FIG. 4, when the insulation resistance measuring device 100 is not measuring insulation resistance, the switch 14 sets the resistor Rg to a high resistance value RH (step S10). Specifically, the switch 14 remains off, and the resistor Rg maintains the high resistance value RH. Next, the voltage application circuit 11 maintains a state in which the applied voltage Va is not generated (step S11). At this time, the voltage application circuit 11 has a high impedance. The state in which the resistor Rg has the high resistance value RH and the applied voltage Va is not generated is the first state.

[0022] Next, the insulation resistance measuring apparatus 100 starts measurement (step S12). For example, the user connects terminals T1, T2, and guard terminal G to the core wire 31, shield layer 33, and bare conductor 36, respectively. After that, the user inputs a command to start measurement into the input unit 18. For example, the user presses the measurement key.

[0023] Next, in the first state, the voltage determination unit 13 determines whether the voltage value V1 between the guard terminal G and the node N1 is equal to or less than the threshold value Th1 (step S13). If the determination is Yes, the switch 14 sets the resistor Rg to a low resistance value RL (step S14). Specifically, the switch 14 is turned on, and the resistor Rg has the low resistance value RL. Next, the voltage application circuit 11 generates the application voltage Va (step S15). The state in which the resistor Rg has the low resistance value RL and the application voltage Va is generated is the second state.

[0024] Next, resistance measurement unit 16 acquires voltage value V2 and current value I2 from voltage measurement unit 12 and current measurement unit 15, respectively (step S16). Resistance measurement unit 16 calculates the resistance value of insulation resistance R from voltage value V2 and current value I2 (step S17). For example, resistance measurement unit 16 calculates V2 / I2 as the resistance value of insulation resistance R.

[0025] Next, the output unit 19 outputs the calculated resistance value of the insulation resistance R (step S18). Specifically, the output unit 19 is a display unit that displays the measured resistance value of the insulation resistance R. Alternatively, the output unit 19 is an interface with an external device that outputs the measured resistance value of the insulation resistance R to the external device.

[0026] Next, insulation resistance measuring apparatus 100 ends the measurement, for example, when a certain time has passed since measuring the insulation resistance or when the user operates input unit 18 (step S19). Specifically, voltage application circuit 11 ends generation of applied voltage Va. Switch 14 turns off. The user checks the resistance value. The user may check the resistance value before the measurement is completed, or after the measurement is completed as long as the measured value is held on the display unit after the measurement is completed.

[0027] If the answer is No in step S13, the insulation resistance measuring apparatus 100 ends the measurement (step S20). Specifically, the switch 14 remains off, and the voltage application circuit 11 remains in a state in which the applied voltage Va is not generated (i.e., the first state). Next, the output unit 19 outputs error information (step S21). Specifically, if the output unit 19 is a display unit, the display unit displays a message indicating that an error has occurred. Alternatively, if the output unit 19 is an interface with an external device, error information about the external device is output. Then, the process ends.

[0028] According to the first embodiment, in the first state in which the switch 14 sets the resistance value of Rg to the high resistance value RH (as in step S13, Yes), when the voltage evaluation unit 13 determines that the voltage value V1 (first voltage value) is equal to or less than the threshold value Th1 (first threshold value), the switch 14 sets the resistance value of the resistor Rg (i.e., the input resistance of the guard terminal G) to the low resistance value RL (as in step S14), and the voltage application circuit 11 generates the applied voltage Va. In the second state, as in steps S16 and S17, the resistance measurement unit 16 measures the insulation resistance between the terminals T1 and T2. When the voltage evaluation unit 13 determines that the voltage value V1 is greater than the threshold value Th1 (as in step S13, No), the switch 14 remains in the first state, and the resistance measurement unit 16 does not measure the insulation resistance.

[0029] As a result, when a high voltage is applied to the guard terminal G, the resistor Rg has a high resistance value RH and functions as a protection circuit. When a high voltage is not applied to the guard terminal G, the resistor Rg has a low resistance value RL, improving measurement accuracy.

[0030] As in step S16, when measuring the insulation resistance, the resistance measuring unit 16 acquires the voltage value V2 (second voltage value) and the current value I2 from the voltage measuring unit 12 and the current measuring unit 15, respectively. The resistance measuring unit 16 calculates the insulation resistance R based on the voltage value V2 and the current value I2. In this way, the insulation resistance R can be calculated.

[0031] 2, the current measuring unit 15 is connected in series with the voltage measuring unit 12 between terminals T1 and T2, and is connected in series with the resistor Rg between terminal T1 and the guard terminal G. This prevents the current flowing through the guard terminal G from flowing through the current measuring unit 15, allowing the insulation resistance R to be measured with high accuracy.

[0032] As in step S21, the output unit 19 outputs error information, which allows the user to recognize that a voltage is being supplied to the guard terminal G. The user can confirm that a high voltage is being applied to the guard terminal G and the reason for this.

[0033] From the viewpoint of allowing resistor Rg to function as a protection circuit when switch 14 is off and lowering the resistance value of resistor Rg when switch 14 is on, the resistance value of resistor R2 is preferably 10,000 times or more, and more preferably 100,000 times or more, the resistance value of resistor R1. The resistance value of resistor R1 is, for example, 0Ω to 10Ω, and is, for example, 5Ω. The resistance value of resistor R2 is, for example, 1MΩ to 100MΩ, and is, for example, 10MΩ.

[0034] From the viewpoint of preventing damage to the insulation resistance measuring device 100, the threshold value Th1 is preferably, for example, 80 V or less, and more preferably 50 V or less. If the threshold value Th1 is too small, the insulation resistance cannot be measured even if a voltage of a negligible level is applied to the guard terminal G. From this viewpoint, the threshold value Th1 is preferably, for example, 10 V or more. One example of the threshold value Th1 is 48 V.

[0035] (Second embodiment) Fig. 5 is a circuit diagram showing a voltage determination unit of an insulation resistance measuring device according to a second embodiment. As shown in Fig. 5, the voltage determination unit 13 includes a comparator 40, N-channel field effect transistors (FETs) 41 and 42, an NPN bipolar transistor 43, a photovoltaic coupler 44, a processor 45, and resistors R11 to R14. The resistors R1 and R2 are connected in series between the guard terminal G and ground (e.g., node N1). The switch 14 is connected in parallel with the resistor R2. The switch 14 is an N-channel FET.

[0036] Resistors R11 and R12 are connected in series between the guard terminal G and ground in parallel with the resistors R1 and R2. A node N11 is a node between the resistors R11 and R12. A reference voltage Vref is input to the positive input terminal of the comparator 40, and the node N11 is electrically connected to the negative input terminal. The reference voltage Vref is a voltage equivalent to the threshold value Th2 when converted into the voltage of the guard terminal G. A resistor R13 and an N-channel FET 41 are connected in series between the power supply Vs and ground. A node N12 is a node between the resistor R13 and the N-channel FET 41. The output signal of the comparator 40 is input to the gate of the N-channel FET 41.

[0037] Node N13 is electrically connected to node N12. Node N13 is electrically connected to ground via resistor R14. Node N13 is electrically connected to the input terminal of processor 45, and a signal at node N13 is input to processor 45 as input signal IN. An output terminal of processor 45 is electrically connected to node N14, and an output signal OUT of processor 45 is output to node N14. An N-channel FET 42 is connected between node N14 and ground. Node N13 is electrically connected to the gate of N-channel FET 42.

[0038] A photovoltaic coupler and an NPN bipolar transistor 42 are connected in series between a power supply Vs and ground. A node N14 is electrically connected to the base of an NPN bipolar transistor 43. The output terminal of the photovoltaic coupler 44 is electrically connected to the gate of the switch 14.

[0039] When a high voltage higher than the threshold value Th2 is not applied to the guard terminal G, the voltage at node N11 becomes lower than the reference voltage Vref, and the comparator 40 outputs a high level. The N-channel FET 41 turns on. The levels of nodes N12 and N13 become low. The input signal IN to the processor 45 becomes low, and the processor 45 can recognize that no high voltage is being applied to the guard terminal G. The N-channel FET 42 is off. Therefore, the output signal OUT of the processor 45 becomes the level of node N14.

[0040] When a high voltage is applied to the guard terminal G, the voltage at node N11 becomes higher than the reference voltage Vref, causing the comparator 40 to output a low level. The N-channel FET 41 turns off. Nodes N12 and N13 become high level. The processor 45 can recognize that a high voltage is being applied to the guard terminal G. Because node N13 is at a high level, the N-channel FET 42 is on. Therefore, the level of node N14 becomes low level regardless of the output signal OUT of the processor 45.

[0041] When the level of node N14 is low, the NPN bipolar transistor 43 is turned off. The photovoltaic coupler 44 is disconnected from the power supply and ground, and the photovoltaic coupler outputs a low level to the gate of the switch 14. This turns off the switch 14, and the resistor Rg has a high resistance value RH.

[0042] When node N14 is at a high level, NPN bipolar transistor 43 turns on. Photovoltaic coupler 44 is connected between the power supply and ground, and outputs a high level to the gate of switch 14. This turns on switch 14, and resistor Rg becomes a low resistance value RL.

[0043] 6 is a flowchart showing the operation of the processor of the insulation resistance measuring device according to the second embodiment. The processor 45 is, for example, a CPU, and works in conjunction with a program to perform part of the operation of the voltage determination unit 13. The processor 45 may be a processor common to the processor that executes at least part of the functions of the resistance measurement unit 16, or may be a separate processor.

[0044] As shown in Fig. 6, the processor 45 determines whether or not insulation resistance measurement is in progress (step S30). For example, in Fig. 4 of the first embodiment, if the process is after step S14 and before step S19, the processor 45 determines "Yes." If the insulation resistance measuring device 100 is not measuring insulation resistance, the processor 45 determines "No." If the determination is "No," the processor 45 sets the output signal OUT to a low level L (step S35). Then, the process ends and returns to step S30. As a result, regardless of whether a high voltage is applied to the guard terminal G, the resistor Rg has a high resistance value RH, and the resistor Rg functions as a protection circuit.

[0045] If the result of step S30 is Yes, the processor 45 acquires the input signal IN (step S31). Next, the processor 45 determines whether the input signal IN is at high level H or low level L (step S32). If the input signal IN is determined to be at high level H, a high voltage is applied to the guard terminal G. Therefore, the output signal OUT is set to low level L (step S35). Then, the process ends and returns to step S30. As a result, if a voltage is applied to the guard terminal G during measurement of the insulation resistance, the resistor Rg can be set to a high resistance value RH and used as a protection circuit.

[0046] When the input signal IN is determined to be low level L in step S32, the processor 45 determines whether the input signal IN alternates between high level H and low level L within a predetermined period (step S33). If the result is Yes, the voltage applied to the guard terminal G is AC. In this case, even if the voltage applied to the guard terminal G is low, an AC voltage is being applied to the guard terminal G. Therefore, the output signal OUT is set to low level L (step S35). Then, the process ends and returns to step S30. As a result, when an AC voltage is being applied to the guard terminal G, even if the voltage applied to the guard terminal G drops momentarily, the resistor Rg can be used as a protection circuit by setting the resistor Rg to a high resistance value RH.

[0047] If the result of step S33 is No, the processor 45 sets the output signal OUT to a high level H (step S34), and then ends the process and returns to step S30.

[0048] To summarize the above, when the voltage value V1 of the guard terminal G is higher than the threshold value Th2 determined by the reference voltage Vref, the node N13 becomes high level, the switch 14 turns on regardless of the output signal OUT of the processor 45, and the resistor Rg is set to the high resistance value RH.

[0049] When the voltage value V1 at the guard terminal G is equal to or lower than the threshold value Th2, the node N13 goes low, and the switch 14 is turned on or off by the output signal OUT of the processor 45. When the insulation resistance measuring apparatus 100 is not in a mode for measuring insulation resistance, the output signal OUT of the processor 45 is low L, and the resistor Rg has a high resistance value RH. Even when the insulation resistance measuring apparatus 100 is in a mode for measuring insulation resistance, if the voltage value V1 is an AC voltage value, the output signal OUT of the processor 45 is low L, and the resistor Rg has a high resistance value RH. When the insulation resistance measuring apparatus 100 is in a mode for measuring insulation resistance and the voltage value V1 is a DC voltage value, and if the voltage value V1 is equal to or lower than the threshold value Th2, the output signal OUT of the processor 45 is high H, and the resistor Rg has a low resistance value RL.

[0050] According to the second embodiment, the voltage evaluation unit 13 includes a processor 45 and a dedicated circuit. The processor 45 cooperates with a program to cause the switch 14 to change the resistance Rg from the low resistance value RL to the high resistance value RH when the voltage value V1 at the guard terminal G exceeds a threshold value Th2 (second threshold) in the second state. The dedicated circuit causes the switch 14 to change the resistance Rg from the low resistance value RL to the high resistance value RH without an instruction from the processor 45 when the voltage value V1 exceeds the threshold value Th2 in the second state. Controlling the switch 14 using the processor 45 requires time for calculations by the processor 45. Therefore, a dedicated analog circuit that does not use a program is provided separately from the processor 45. This allows the resistance Rg to be quickly changed to the high resistance value RH when a high voltage is applied to the guard terminal G during insulation resistance measurement.

[0051] Even if voltage value V1 becomes equal to or less than threshold value Th2, if a certain condition is satisfied, such as No in step S30, processor 45 causes switch 14 to set resistor Rg to high resistance value RH without relying on instructions from a dedicated circuit. This allows resistor Rg to function as a protection circuit, for example, when insulation resistance measuring apparatus 100 is not in a mode for measuring insulation resistance, for example, in standby mode, by setting resistor Rg to high resistance value RH.

[0052] Even if the voltage value V1 becomes equal to or less than the threshold value Th2, when the processor 45 determines that the voltage value V1 is an AC voltage value, as in step S33, the processor 45 causes the switch 14 to set the resistor Rg to a high resistance value RH without relying on instructions from the dedicated circuit. When the voltage value V1 becomes equal to or less than the threshold value Th2, the dedicated circuit attempts to set the resistor Rg to a low resistance value RL. However, if an AC voltage is applied to the guard terminal G, setting the resistor Rg to a low resistance value RL may damage the current measurement unit 15 and other components. Therefore, the processor 45 causes the resistor Rg to be set to a high resistance value RH without relying on instructions from the dedicated circuit. This prevents the current measurement unit 15 and other components from being damaged.

[0053] The threshold value Th3 (third threshold value) by which the dedicated circuit determines the voltage value V1 may be different from the threshold value Th2 (second threshold value) by which the processor 45 determines the voltage value V1.

[0054] The switch 14 is an N-channel FET that is turned off when a low level is input to its gate. In other words, the switch 14 is normally off. As a result, the switch 14 sets the resistor Rg to a high resistance value RH when no power is supplied. Therefore, since the resistor Rg normally has a high resistance value RH, damage to the insulation resistance measuring device 100 can be suppressed even if a high voltage is applied to the guard terminal G.

[0055] Although the embodiments have been described above, they are presented as examples and the present invention is not limited to the above embodiments. The above embodiments can be implemented in various other forms, and various combinations, omissions, substitutions, modifications, etc. can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, and are also included in the scope of the invention and its equivalents as set forth in the claims. [Explanation of symbols]

[0056] 11: Voltage application circuit 12: Voltage measurement section 13: Voltage judgment unit 14: Switch 15: Current measurement section 16: Resistance measurement section 18: Input section 19: Output section

Claims

1. A first terminal; A second terminal; a guard terminal; a setting circuit that sets the input resistance of the guard terminal to a first value and a second value that is lower than the first value; a voltage determination unit that determines whether a first voltage value between the first terminal and the guard terminal is equal to or less than a first threshold; a voltage application circuit that applies a voltage between the first terminal and the second terminal; a resistance measuring unit that measures an insulation resistance between the first terminal and the second terminal; Equipped with In a first state in which the setting circuit has set the input resistance to the first value, when the voltage determination unit determines that the first voltage value is equal to or less than the first threshold value, the setting circuit sets the input resistance to the second value and the voltage application circuit applies an application voltage, and the state is changed to a second state. In the second state, the resistance measurement unit measures an insulation resistance between the first terminal and the second terminal; In the first state, when the voltage determination unit determines that the first voltage value is greater than the first threshold value, the setting circuit maintains the first state, and the resistance measurement unit does not measure the insulation resistance. Insulation resistance measuring device.

2. a voltage measurement unit that measures a voltage between the first terminal and the second terminal; a current measuring unit that measures a current flowing between the first terminal and the second terminal; Equipped with When measuring the insulation resistance, the resistance measurement unit acquires a second voltage value and a current value from the voltage measurement unit and the current measurement unit, respectively, and calculates the insulation resistance based on the second voltage value and the current value.

2. The insulation resistance measuring device according to claim 1.

3. 3. The insulation resistance measuring device according to claim 2, wherein the current measuring unit is connected in series with the voltage measuring unit between the first terminal and the second terminal, and the guard terminal is connected to a node between the voltage measuring unit and the current measuring unit.

4. The voltage determination unit a processor, operating in cooperation with a program, causing the setting circuit to set the input resistance from the second value to the first value when the first voltage value becomes greater than a second threshold in the second state; a dedicated circuit that causes the setting circuit to change the input resistance from the second value to the first value without an instruction from the processor when the first voltage value becomes greater than a third threshold value in the second state; The insulation resistance measuring device according to claim 1 , further comprising:

5. 5. The insulation resistance measuring device according to claim 4, wherein the processor causes the setting circuit to set the input resistance to the first value without relying on an instruction from the dedicated circuit when a condition is satisfied even if the first voltage value becomes equal to or less than the second threshold value.

6. 5. The insulation resistance measuring device according to claim 4, wherein when the processor determines that the first voltage value is an AC voltage value even if the first voltage value is equal to or less than the second threshold value, the processor causes the setting circuit to set the input resistance to the first value without relying on an instruction from the dedicated circuit.

7. 4. The insulation resistance measuring device according to claim 1, wherein the setting circuit sets the input resistance to a first value when power is not supplied.

8. 4. The insulation resistance measuring device according to claim 1, further comprising an output unit that outputs error information when, in the first state, the voltage determination unit determines that the first voltage value is greater than the first threshold value.

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