JP2015042069A - Voltage detector and switch gear comprising the same - Google Patents

Abstract

【Task】
In addition to being able to accurately evaluate the main circuit voltage, it is possible to confirm its soundness regardless of the state of the main circuit.
[Solution]
In order to solve the above-described problem, the voltage detector 10 of the present invention is electrically connected to a main circuit, and when the main circuit is in an applied state, the phase voltage or line obtained by electrostatically dividing the main circuit voltage. It is possible to measure the inter-voltage, and when the main circuit is in a power failure state, a test terminal portion 13 is mounted which applies a voltage simulating the main circuit voltage.
[Selection] Figure 1

Description

  The present invention relates to a voltage detector and a switchgear equipped with the voltage detector, and more particularly to a voltage detector suitable for one installed to monitor the power application state of a main circuit and a switchgear equipped with the voltage detector.

  In general, a switch gear is a device used in a power receiving and distributing system, has functions such as turning on, shutting off, disconnecting, and grounding, and is an important device related to power supply. It is very important to grasp the state of the power distribution system safely and reliably in order to monitor the state of the system.

  The switchgear is grounded with the system for the purpose of ensuring safety by disconnecting a line from the system, or for the purpose of ensuring the safety of workers and inspectors when checking the soundness of the system. If a grounding operation is performed while the system is in a power-applied state, a ground fault will occur, and in some cases, the system will be completely blacked out, making it impossible to supply stable power.

  For this reason, the ground operation circuit is usually inserted with a condition that the power-supply state of the system is no voltage as a condition for establishing the ground operation circuit.

  For this purpose, a device for monitoring the power-supply state of the system is required. As a solution, there is one described in Patent Document 1, for example.

  In Patent Document 1, in order to determine whether or not the main circuit is charged, a voltage detecting capacitor and a voltage detecting coupling capacitor are electrically connected through a coaxial wiring, and the electrostatic partial pressure of the capacitor is determined. A voltage detector that compares the measured voltage with a preset threshold value using a comparator, and uses a lamp that is turned on and off by a relay contact that operates based on the output of the comparator to determine the power application state of the main circuit Is described.

JP 2009-164018 A

  However, since the voltage detector described in Patent Document 1 cannot determine the soundness of the voltage detector itself (whether the voltage detector operates) or not unless the main circuit is in an applied state. At the time of inspections that would stop all, the soundness of the voltage detector itself could not be confirmed. In addition, the voltage detector described in Patent Document 1 uses the electrostatic partial pressure of the voltage detection capacitor and the coupling capacitor, but considers the capacitance of the coaxial wiring and the voltage detector body. As a result, there was a possibility that the main circuit voltage could not be accurately evaluated.

  The present invention has been made in view of the above-mentioned points. The object of the present invention is not only that the main circuit voltage can be accurately evaluated, but also a voltage detector that can confirm its soundness regardless of the state of the main circuit, and The object is to provide a switchgear having the same.

  In order to achieve the above object, the voltage detector of the present invention is electrically connected to a main circuit, and when the main circuit is in an applied state, the main circuit voltage is divided into phase voltage or line-to-line. A voltage measurement is possible, and when the main circuit is in a power failure state, a test terminal portion is mounted for applying a voltage simulating the main circuit voltage.

  In order to achieve the above object, the switchgear of the present invention is a metal container having a vacuum inside and having at least a pair of main circuits composed of a fixed electrode and a movable electrode, and the movable electrode. Houses an operation mechanism for opening and closing the fixed electrode, a bus for supplying power to the main circuit, a cable for supplying power from the main circuit to the load side, and the metal container, operation mechanism, bus and cable A main body and a main circuit conductor fixed to the main body and electrically connected to the main circuit and the cable, and connected to the other end of the voltage detection capacitor via a coaxial wiring. A voltage detector for measuring a voltage of the circuit, wherein the voltage detector is a switchgear having the above-described configuration.

  In order to achieve the above object, each of the switchgears of the present invention is composed of a movable electrode and a fixed electrode, and the periphery of the movable electrode and the fixed electrode is covered with an insulating cylinder, and is closed, opened, and disconnected. A main circuit composed of two pairs of switch units operating at three positions, the main circuit comprising a vacuum switch formed inside a vacuum metal container, a movable electrode, and a fixed electrode; A vacuum grounding switch having a switch part in which the periphery of the movable electrode and the fixed electrode is covered with an insulating cylinder, the switch part being disposed in a vacuum vessel; and the vacuum switch and the vacuum grounding switch. An operation mechanism for opening and closing the movable electrode with the fixed electrode, a bus for supplying power to the main circuit, a cable for supplying power from the main circuit to a load side, the metal container, an operation mechanism, A housing for housing the bus and the cable; The voltage of the main circuit is measured by being connected to the other end of the voltage detection capacitor, which is fixed to the body and connected to the main circuit conductor electrically connected to the main circuit and the cable via the coaxial wiring. And the voltage detector is a switchgear having the above-described configuration.

  According to the present invention, the main circuit voltage can be accurately evaluated, and the soundness of the device can be confirmed regardless of the state of the main circuit.

It is sectional drawing which shows Example 1 of the switchgear of this invention. It is sectional drawing which shows the opening-and-closing part employ | adopted as Example 1 of the switchgear of this invention. It is a layout figure which shows an example of the voltage detector employ | adopted as Example 1 of the switchgear of this invention. It is a layout figure which shows the other example of the voltage detector employ | adopted as Example 1 of the switchgear of this invention. It is a layout figure which shows the further another example of the voltage detector employ | adopted as Example 1 of the switchgear of this invention. It is a flowchart for demonstrating operation | movement of the voltage detector employ | adopted as Example 1 of the switchgear of this invention. It is a figure for demonstrating the usage example of the test terminal part employ | adopted as the voltage detector of this invention. It is a figure for demonstrating the electrostatic partial pressure principle of the voltage detector of this invention. It is sectional drawing which shows Example 2 of the switchgear of this invention. It is a characteristic view which shows the correlation of the pressure which shows the discharge characteristic in a vacuum, and a discharge start voltage.

  Hereinafter, a voltage detector of the present invention and a switchgear equipped with the voltage detector will be described based on illustrated embodiments. In addition, in each Example, the same code | symbol is used for the same component.

  1 and 2 show a first embodiment of the switchgear of the present invention. As shown in the figure, the switchgear 1 of this embodiment includes an opening / closing part 100, an operating mechanism 3 provided on the upper part of the opening / closing part 100 for opening / closing an electrode of the opening / closing part 100, and the opening / closing part 100. A bus 4 connected to supply power to the opening / closing part 100, a cable 5 connected to the opening / closing part 100 and supplying power from the opening / closing part 100 to the load side, and a metal made to house them inside The main body 2 is schematically configured.

  The above-described switch 100 includes a vacuum valve 101 for a breaker and disconnect switch, a vacuum valve 102 for a ground switch, a bus-side conductor 112 that connects the vacuum valve 101 and the bus 4, and the bus-side conductor 112 and bus 4, a bushing 113 that connects the vacuum valve 101 and the cable 5, a bushing 117 that connects the main circuit conductor 116 and the cable 5, and one end connected to the main circuit conductor 116. A voltage detecting capacitor 152 to be connected to the casing 2 is fixed, one end is connected to the main circuit conductor 116, and the other end of the main circuit conductor 116 is connected via the coaxial wiring 11A to measure the voltage of the main circuit. The voltage detector 10 is generally configured.

  A vacuum valve 101 having a function of closing, opening, and disconnecting installed in the opening / closing unit 100 is housed in a metal container 12 having a floating potential inside the vacuum and the metal container 12, facing each other, and the main circuit Is constituted roughly by two pairs of fixed electrodes 110 and 114 and movable electrodes 120 and 121. Fixed electrode 110 is supported by one end of fixed conductor 111, and the other end of fixed conductor 111 is connected to bus-side conductor 112. On the other hand, the movable electrode 120 is supported by one end of the movable conductor 122, and the other end of the movable conductor 122 is connected to one side of the connecting conductor 124. The movable electrode 121 is supported by one end of the movable conductor 123, and the other end of the movable conductor 123 is connected to the other side of the connecting conductor 124. The fixed electrode 114 is supported by one end of the fixed conductor 115, and the other end of the fixed conductor 115 is connected to the main circuit conductor 116.

  Around the fixed electrode 110 and the movable electrode 120, and around the fixed electrode 114 and the movable electrode 121, an arc shield 119 for preventing an arc discharged from the electrodes when the movable electrodes 120 and 121 are turned on and off, respectively, is disposed. The arc shield 119 is sandwiched between insulating ceramic cylinders 118 that cover the periphery of the electrodes.

  The connecting conductor 124 is connected to the operation rod 126 via the ceramic rod 125. Between the operation rod 126 and the metal container 12, a bellows 130 for operating the operation mechanism 3 while maintaining a vacuum state is provided. Further, the operation rod 126 projects to the outside of the vacuum valve 101, and is ceramic. An end located on the opposite side to the rod 125 is supported by an insulating rod 127 and connected to the operation mechanism 3.

  As shown in FIG. 2, each part of the opening / closing part 100 is integrally molded with an insulator 20 such as epoxy. Further, a conductive coating 21 is applied to the surface of the insulator 20, and the conductive coating 21 is at a ground potential.

  On the other hand, the vacuum valve 102 for opening and closing the ground is schematically constituted by an insulating ceramic cylinder 140, a fixed electrode 145 whose periphery is covered by end plates 141 and 142 located at both ends thereof, and a movable electrode 146 facing the fixed electrode 145. Has been. The fixed electrode 145 is supported by one end of the fixed conductor 143, and the other end of the fixed conductor 143 is connected to the main circuit conductor 116. The movable electrode 146 is supported by one end of the movable conductor 144 supported by the end plate 141 via the bellows 148, and one end of the movable conductor 144 protrudes from the ceramic cylinder 140 and the end plates 141 and 142. ing. The bellows 148 follows the movement of the movable conductor 144, and by providing the bellows 148, the gap between the movable conductor 144 and the end plate 141 can be sealed. The end of the movable conductor 144 opposite to the side on which the movable electrode 146 is supported is connected to a connecting conductor 149 that is installed, and the connecting conductor 149 is connected to the insulating rod 151 via the metal fitting 150. Connected with. The insulating rod 151 is connected to the operation mechanism 3 at the upper part thereof. The parts inside the vacuum valve 102 are joined by high-temperature brazing in a vacuum furnace.

  In the present embodiment, the voltage detector 10 described above is electrically connected to the main circuit, and when the main circuit is in an applied state, the phase voltage or line voltage obtained by electrostatically dividing the main circuit voltage. And a test terminal unit 13 for applying a voltage simulating the main circuit voltage when the main circuit is in a power failure state.

  As shown in FIG. 3, the test terminal unit 13 mounted on the voltage detector 10 in the present embodiment includes a phase voltage terminal 14 and a ground potential terminal 15, and between the phase voltage terminal 14 and the ground potential terminal 15. The phase voltage and the line voltage between the phase voltage terminals 14 are measured. The ground potential terminal 15 is arranged in parallel corresponding to each phase of the phase voltage terminal 14. In addition, a voltage presenting indicator 16 and a voltage not presenting indicator 17 are installed for each phase, and when the control power is applied to the voltage detector 10, the voltage presenting indicator 16 indicating the main circuit voltage state is lit. To do.

  The voltage display 16 and the voltage non-display 17 are different in color depending on the presence or absence of voltage, and can be distinguished spatially by changing the height of the position of the display. Here, the voltage presence state is red, and the voltage absence state is green. However, the combination of colors and the shape and type of the display are not limited to this. For example, in the example, a square LED is used, but a round LED may be used, and the color may be orange or blue. In addition, although a self-luminous LED is used as an example of the display, there is no limitation such as an LCD or a miniature bulb. Furthermore, the state of the main circuit can be confirmed by distinguishing not only a display device that can be visually recognized but also a pattern in which sound is generated by the presence or absence of a voltage, for example, even with sound.

  In FIG. 3, the ground potential terminal 15 is arranged in parallel corresponding to each phase of the phase voltage terminal 14. However, as shown in FIG. 4, the ground potential terminal 15 and the phase voltage terminal 14 are connected to each phase. While alternately arranged on a straight line, the voltage presenting display 16 and the voltage non-display 17 may be arranged alternately on each other in a straight line.

  Further, as shown in FIG. 5, by arranging the three phase voltage terminals 14 diagonally or laterally, the ground potential terminal 15 is used in common for each phase with respect to the three phase voltage terminals 14. A layout is also possible. Of course, the configuration of the phase voltage terminal 14 or the ground potential terminal 15 and the layout thereof are not limited to those shown here, but can be freely performed.

  As shown in FIG. 7, the test terminal unit 13 is electrically connected at the same potential as the detection voltage V electrostatically divided from the main circuit, and in a state where the main circuit voltage is not applied, By applying the AC voltage directly from the unit 13, it is possible to simulate a state in which the detection voltage V is applied to the voltage detector 10. This function makes it possible to confirm the soundness of the voltage detector 10 regardless of the state of the main circuit voltage.

As shown in FIG. 8, when the capacitance of the voltage detection capacitor 152 is C ₀ , the capacitance of the coaxial wiring 11A is C _C , and the capacitance of the voltage detector 10 is C _V , the main circuit Assuming that the voltage applied to the conductor 116 is U ₀ , the detection voltage V represented by the equation 1 is applied to the inside of the voltage detector 10 and the test terminal unit 13.

[Equation 1]
V = C ₀ / (C ₀ + C _C + C _V ) · U ₀
If the electrostatic capacity of the voltage measuring device (for example, a tester) is C _m , it is synonymous with measuring the voltage U ₀ applied to the main circuit conductor 116 from Equation 2 by measuring the detection voltage V. The main circuit voltage can be easily measured from the test terminal unit 13.

[Equation 2]
V = C ₀ / (C ₀ + C _C + C _V + C _m ) · U ₀
FIG. 6 shows a flowchart for explaining the operation of the voltage detector 10. First, when the main circuit is charged, the detection voltage V electrostatically divided (S2) from the main circuit and the voltage (S3) from the test terminal unit 13 are applied to the voltage detector 10 (S1). At this time, the reference voltage provided in the detection circuit is compared with the detection voltage by a comparator (mounted in the voltage detector 10) (S4), and when the detection voltage is larger than the reference voltage, a voltage presence indication is displayed. (S5), it is determined that there is a voltage (S7). At this time, the signal output from the comparator is received by a contacted device such as an auxiliary relay or a contactless device such as a transistor or FET (Field Effect Transistor) (S6), thereby amplifying the signal.

  For this contact output, it is also possible to detect the presence or absence of voltage for each phase and provide an output for each phase, because the comparator compares so that it becomes a batch output when any phase is detected The reference voltage is a device that uses a constant voltage generating device such as a regulator or a constant voltage source, and this reference voltage is a device that determines the operating threshold of the voltage detector. There is a merit that the operation threshold can be changed by arbitrarily changing the reference voltage.

  On the other hand, the reference voltage provided in the detection circuit is compared with the detection voltage by a comparator (S4). If the detection voltage is not larger than the reference voltage, a display indicating no voltage is displayed (S9), and it is determined that there is no voltage (S10 ). At this time, the contacted device such as the auxiliary relay does not operate the signal output from the comparator (S8).

By the way, it can be seen from the above-described equation 1 that the detection voltage can be arbitrarily set by a combination of capacitances. However, in equation 1, since the voltage detection capacitor 152 is built in the switching unit 100, (1) (2) The coaxial wiring 11A is not easy to use if it is too short or too long. Therefore, it is reasonable to fix the length and to manage the fixed capacitance. In consideration of this, the capacitance _CV of the voltage detector 10 may be arbitrarily set.

Therefore, in this embodiment, in order to adjust the electrostatic capacitance C _V of the voltage detector 10 optionally, a C _V1 of the capacitance 18 of the adjusting capacitor assumed to be replaced, a voltage detector specific it is possible to cope with be divided into a fixed capacitance C _V2 of the capacitance 19.

  Accordingly, the detection voltage can be arbitrarily set by electrostatic partial pressure by replacing only the adjustment capacitor without changing the circuit of the voltage detector 10.

  According to the present embodiment, the main circuit voltage can be accurately evaluated, and the soundness of the apparatus can be confirmed regardless of the state of the main circuit.

  FIG. 9 shows a second embodiment of the switchgear of the present invention. This embodiment shown in the figure is an example of a switchgear that can be used not only for voltage detection in Embodiment 1 but also for vacuum monitoring of a vacuum valve.

  The switchgear of the present embodiment shown in FIG. 9 is almost the same in configuration as the switchgear 1 of the first embodiment shown in FIG. A terminal 50 for monitoring the voltage induced from the main circuit is provided, and is fixed to the housing 2 and connected to the terminal 50 via the coaxial wiring 11B to monitor the degree of vacuum in the metal container 12 The voltage detector 10A is provided.

  That is, in the present embodiment, the object electrically connected by the coaxial wiring 11B is not the voltage detection capacitor 152 but the terminal 50. This terminal 50 is connected to the vacuum valve 101 built in the opening / closing part 100. The induced voltage is monitored.

  In principle, it is the same as the voltage detector 10, but in the case of vacuum monitoring, when the degree of vacuum in the metal container 12 is normal, the vacuum degree in the metal container 12 is abnormal without operating. What is necessary is just to set an operation | movement threshold voltage and an electrostatic partial pressure ratio so that it may operate | move occasionally.

  According to Patent Document 1, in general, a vacuum switchgear performs an electrode opening / closing operation inside a vacuum vessel, so that the pressure inside the vacuum vessel (vacuum pressure) affects the withstand voltage performance and shut-off performance of the device. FIG. 10 is a so-called Paschen curve showing the discharge characteristics in vacuum, and shows the correlation between the pressure and the discharge start voltage. As shown in the figure, the discharge start voltage depends on the vacuum pressure, and from this it can be seen that the withstand voltage performance and the cutoff performance in the vacuum vessel depend on the vacuum pressure.

  When a vacuum abnormality occurs in the metal container 12, the pressure resistance deteriorates as the degree of vacuum of the vacuum valve 101 deteriorates. In a state where insulation is maintained, a voltage induced from the main circuit is applied to the terminal 50. However, when the withstand voltage performance deteriorates, a voltage is directly applied to the terminal 50 without maintaining insulation. .

  By setting the operation threshold and the electrostatic partial pressure so as to reliably detect such a voltage change, the vacuum monitoring voltage detector 10A can be used as a vacuum monitoring application.

  In addition, when the vacuum abnormality occurs only in one phase, the current can be interrupted by another healthy phase, but when the vacuum abnormality occurs in two or more phases, the electric circuit cannot be interrupted. . Even if the switching operation of the switchgear is performed in a situation where the electric circuit cannot be interrupted, the current may not be interrupted and the vacuum valve 101 may burst.

  In order to avoid such a situation, a detection contact is provided for each phase when detecting an increase in the monitoring voltage of the terminal 50 due to a vacuum abnormality, and if only one phase is a vacuum abnormality, an alarm contact output, two or more phases When a vacuum abnormality occurs, it can be dealt with by installing a function for providing a trip lock contact output of the switchgear in the vacuum monitoring voltage detector 10A. A similar interlock can also be configured as a switch gear.

  Even with this configuration of the present embodiment, the same effects as those of the first embodiment can be obtained.

  In the second embodiment described above, the vacuum monitoring is realized by monitoring the change in the voltage applied to the terminal 50 when the vacuum in the metal container 12 is abnormal. In this embodiment, the voltage detector 10 is Consider using the device as a device that senses a voltage increase in a healthy phase during a fault (a ground fault in one phase).

Since the voltage detector 10 described above monitors the phase voltage, if the rated voltage is U, the value of U ₀ = U / √3 is monitored.

  However, when a one-line ground fault occurs, the phase of the ground fault becomes 0, but the phase voltage of other healthy phases jumps up to √3 times. The voltage detector 10 can detect the occurrence of a one-line ground fault by setting a threshold value or electrostatic voltage division ratio using this principle.

  In addition, this invention is not limited to an above-described Example, Various modifications are included. For example, the above-described embodiments have been described in detail for easy understanding of the present invention, and are not necessarily limited to those having all the configurations described. Further, a part of the configuration of one embodiment can be replaced with the configuration of another embodiment, and the configuration of another embodiment can be added to the configuration of one embodiment. Further, it is possible to add, delete, and replace other configurations for a part of the configuration of each embodiment.

  DESCRIPTION OF SYMBOLS 1 ... Switch gear, 2 ... Housing, 3 ... Operation mechanism, 4 ... Busbar, 5 ... Cable, 10 ... Voltage detector, 10A ... Vacuum detector voltage detector, 11A, 11B ... Coaxial wiring, 12 ... Metal container, 13 ... Test terminal section, 14 ... Phase voltage terminal, 15 ... Ground potential terminal, 16 ... Display with voltage, 17 ... No voltage display, 18 ... Capacitance of adjustment capacitor, 19 ... Static capacitance inherent to voltage detector Capacitance 20 ... insulator, 21 ... conductive coating, 50 ... terminal, 100 ... opening / closing part, 101,102 ... vacuum valve, 110, 114,145 ... fixed electrode, 111,115,143 ... fixed conductor, 112 ... bus side conductor 113, 117 ... bushing, 116 ... main circuit conductor, 118, 140 ... ceramic cylinder, 119 ... arc shield, 120, 121, 146 ... movable electrode, 122, 123, 144 ... movable conductor, 124 149 ... connection conductor, 125 ... ceramic rod, 126 ... operating rod, 127,151 ... insulating rods, 130,148 ... bellows, 141,142 ... end plate, 150 ... fitting, 152 ... voltage detection capacitor.

Claims (11)

  When the main circuit is electrically connected and the main circuit is in an applied state, it is possible to measure the phase voltage or the line voltage obtained by electrostatically dividing the main circuit voltage, and the main circuit is in a power failure state. In this case, the voltage detector is mounted with a test terminal portion for applying a voltage simulating the main circuit voltage. The voltage detector of claim 1, wherein
The test terminal unit includes a phase voltage terminal and a ground potential terminal, and measures a phase voltage between the phase voltage terminal and the ground potential terminal and a line voltage between the phase voltage terminals. . The voltage detector according to claim 2, wherein
The ground potential terminal is installed corresponding to each phase of the phase voltage terminal. The voltage detector according to claim 2, wherein
The ground potential terminal is provided with one common to each phase of the phase voltage terminal. The voltage detector according to any one of claims 1 to 4,
A voltage detector, wherein an adjustment capacitor for adjusting a capacitance of the voltage detector is mounted. The voltage detector according to any one of claims 1 to 4,
The voltage detector is mounted with a display for displaying the main circuit voltage in an applied state or a power failure state. The voltage detector according to claim 6.
The voltage detector comprises a voltage presenting indicator for displaying the presence of voltage for each phase and a voltage non-displaying indicator for indicating no voltage for each phase. The voltage detector according to claim 6 or 7,
The voltage detector, wherein the display is displayed using a voltage that is electrostatically divided from the main circuit. A metal container having at least a pair of main circuits each composed of a fixed electrode and a movable electrode inside the vacuum, an operating mechanism for opening and closing the fixed electrode of the movable electrode, and power to the main circuit A bus for supplying electric power from the main circuit to the load side, a housing for housing the metal container, the operation mechanism, the bus and the cable, and the main circuit and the cable fixed to the housing. A voltage detector for measuring the voltage of the main circuit connected to the other end of the capacitor for voltage detection, one end of which is connected to the electrically connected main circuit conductor;
The switch gear according to any one of claims 1 to 8, wherein the voltage detector is the voltage detector according to any one of claims 1 to 8. Each consists of a movable electrode and a fixed electrode, and the periphery of the movable electrode and the fixed electrode is covered with an insulating cylinder, and has a main circuit composed of two pairs of switch units that operate at three positions: closed, open, and disconnected. The main circuit is composed of a vacuum switch having an inside disposed in a vacuum metal container, a movable electrode and a fixed electrode, and a switch unit in which the periphery of the movable electrode and the fixed electrode is covered with an insulating cylinder. A vacuum grounding switch in which the opening / closing part is disposed in a vacuum vessel, and an operation mechanism for performing an opening / closing operation of the movable electrode in the vacuum switch and the vacuum grounding switch, A bus for supplying power to the main circuit; a cable for supplying power from the main circuit to a load; a housing for housing the metal container, the operating mechanism, the bus and the cable; Electrically connected to the circuit and the cable Are connected via the other end coaxial wiring of the voltage detecting capacitor having one end connected to the main circuit conductor and a voltage detector for measuring the voltage of the main circuit,
The switch gear according to any one of claims 1 to 8, wherein the voltage detector is the voltage detector according to any one of claims 1 to 8. The switchgear according to claim 9 or 10,
The metal container is provided with a terminal for monitoring a voltage induced from the main circuit in the metal container, and is fixed to the casing and connected to the terminal via a coaxial wiring. A switchgear comprising a vacuum monitoring voltage detector for monitoring the degree of vacuum.

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