Abstract:
A measuring system for continuously monitoring a high-voltage bushing provides for multi-stage, secure protection. The system includes a measuring circuit, a connection plug, a connection cable, and a three-stage protective circuit. When overvoltages occur at the measurement connection, each protective stage can produce a short circuit at the measurement connection to limit the voltage. A first protective stage responds at a response voltage U 1 . A second protective stage connected between the first stage and the measurement connection responds at a higher response voltage U 2 . A low-pass filter in the first protective stage decouples the first and second protective stages. The first stage responds to overvoltages at operating frequency, the second stage responds to high-frequency overvoltages. A third protective stage is connected between the measurement connection and the first protective stage in parallel with the measurement connection and includes a mechanical switch for short-circuiting when a third response voltage is present.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a measuring system for continuously monitoring a high-voltage bushing in particular a housing bushing of a power transformer, including a measuring circuit for monitoring the high-voltage bushing, a connecting plug for connecting the measuring circuit to a measuring connection of the high-voltage bushing, a connecting cable for connecting the measuring circuit to the connecting plug, and a protection circuit for protecting the measuring connection from over-voltages. The protection circuit is designed to establish a short circuit between a high-voltage terminal and a ground terminal of the measuring connection if an overvoltage occurs. The protection circuit includes a first protection stage having a first response voltage and a second protection stage having a second response voltage connected in parallel with the first protection stage. The second protection stage is connected between the measuring connection and the first protection stage and the first protection stage includes a low-pass filter. 
     Transformers and their components are important parts of the power supply network. Their failure may result in the shutdown of power plants or network sections, which may entail considerable expense. The breakdown of high-voltage bushings is one of the most frequent causes of failure in transformers, in particular power transformers. Up to now, they have been inspected only during routine maintenance. This has been problematic due to the long intervals between the measurements, which did not always allow the detection of faults in a timely manner. Recently, however, there has been an increasing demand for measuring systems to enable the continuous monitoring (online monitoring) of high-voltage bushings. A brief overview may also be found in “Hochspannungstechnik: Grundlagen-Technologie-Anwendungen” (High-voltage technology: Basics-technology-applications) by Andreas Küchler (Chapter 6.4.8.2 of the 2009 edition, ISBN 978-3-540-78412-8). Additional descriptions of such measuring systems and of the requirements and options are in a plurality of technical articles, for example, in Liebschner et al. “Online-Monitoring of Capacitance and Dissipation Factor of High Voltage Bushings at Service Temperature” (15th ISH International Symposium on High Voltage Engineering, Ljubljana, 2007). 
     High-voltage bushings generally have a measuring connection for electrical measurements, which is connected to the outermost capacitive layer coating of the high-voltage bushing and which may be used for connecting a measuring system for measuring capacitance, the dissipation factor, insulation resistance, or polarization and depolarization currents. 
     The measuring connection itself has a ground terminal which is often formed by the grounded housing or a grounded flange, and a high-voltage terminal which is insulated and routed to the outside through the housing of the high-voltage bushing. If no measuring system is connected to the measuring connection, the ground terminal and the high-voltage terminal of the measuring connection must be short-circuited. Otherwise, the insulation of the measuring connection would be destroyed over time due to the high voltage between the high-voltage terminal and the ground terminal, subsequently resulting in the insulation in the interior of the bushing being destroyed, resulting in a breakdown of the bushing and thus the failure of the transformer. Considerable damage to switchgear or injury to persons in the vicinity may also occur. A voltage present at the measuring connection which results in destruction of the insulation of the measuring connection immediately or only after being present for a longer time is referred to below as overvoltage. 
     When the measuring system is connected, a measuring load in the measuring circuit of the measuring system limits the voltage present at the measuring connection, so that the insulation of the measuring connection is not damaged. However, in the event of the loss of the measuring load, for example, due to corrosion, aging, or a cable break, the above-described fault mechanisms would occur again. Therefore, a permanently connected measuring system must be considered to be an additional fault source for the failure of a high-voltage bushing. In addition, during the operation of such a measuring system, the measuring connection is susceptible to lightning strikes in the vicinity. These lightning strikes may also result in overvoltages at the measuring connection, thus damaging the measuring connection or the measuring system. Therefore, precautions must be taken to protect the measuring connection from overvoltage, whether through the loss of the measuring load or due to lightning strikes. 
     Such a measuring system which measures the voltage present at the high-voltage bushing is described in the company publication “ABB, Bushing Potential Device, Type PBA2.” 
     There, a spark gap protects the measuring connection from overvoltages. The measuring connection may also be short-circuited manually using a grounding switch. 
     U.S. Pat. No. 4,757,263 A describes a device for monitoring the insulation of high-voltage facilities. Here, a measuring circuit is protected from overvoltage by two parallel varistors. 
     CN 201 654071 U describes a measuring connection including a protection circuit made up of a pair of diodes connected back-to-back and a spark gap which is connected in parallel with them. 
     The known related art is disadvantageous in that these protection circuits are not sufficient to meet the requirements of the operators of transformers having such high-voltage bushings for the failsafe long-term protection of the measuring connection. 
     BRIEF SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a measuring system which ensures a high level of protection of the measuring connection from overvoltages. 
     This object is achieved via the means of the present invention as claimed. 
     For this purpose, a measuring system is provided which is suitable for continuously monitoring a high-voltage bushing, in particular a housing bushing of a power transformer, and which includes at least the following components:
         a measuring circuit for monitoring the high-voltage bushing,   a connecting plug for connecting the measuring circuit to a measuring connection of the high-voltage bushing,   a connecting cable for connecting the measuring circuit to the connecting plug, and   a protection circuit for protecting the measuring connection from overvoltages, which is designed to establish a short circuit between a high-voltage terminal and a ground terminal of the measuring connection if an overvoltage occurs, wherein the protection circuit includes a first protection stage having a first response voltage and a second protection stage having a second response voltage, which is electrically connected in parallel with the first protection stage.       

     Within the context of the present invention, a short circuit is to be understood as meaning the high-voltage terminal and the ground terminal being electrically connected together in such a way that a voltage is present at the measuring connection which is non-critical for its insulation. 
     According to the present invention, it is provided that the second response voltage is greater than the first response voltage, that the second protection stage is connected between the measuring connection and the first protection stage and that the first protection stage includes a low-pass filter. The first and the second protection stages thus preferably respond to different events triggering an overvoltage and are used simultaneously as a mutual fallback level. This ensures a particularly high degree of protection of the measuring connection from overvoltages. In addition, a third protection stage is connected between the measuring connection and the first protection stage in parallel with the measuring connection and includes a mechanical switch via which the short circuit may be established if a third response voltage is present. Since mechanical switches are considered to be robust and failsafe, even better protection is ensured as a result. 
     Preferably, the first protection stage in a circuit branch connecting the high-voltage terminal to the ground terminal includes a varistor or a suppressor diode. These components switch rapidly and are able to discharge an overvoltage safely to ground and are also economical. 
     In an additional preferred embodiment, the second protection stage in a circuit branch connecting the high-voltage terminal to the ground terminal includes a spark gap. Spark gaps have a particularly high current-carrying capacity and thus provide reliable protection even at high currents. 
     In one advantageous embodiment of the present invention, it is provided that the third protection stage includes a series circuit made up of a spark gap, a rectifier, and a coil, and the mechanical switch is switchable via a current flowing through the coil. The switch and the coil thus form a reed relay. As a result, the switch is closed when the response voltage is present and the resulting current flows through the coil, thus establishing the short circuit of the measuring connection safely and without requiring manual intervention, thereby achieving further improved protection. 
     In addition, it is preferred that the reed relay is bistable. Once the switch has been closed, it therefore remains closed even if current is no longer flowing through the coil. If the third protection stage responds, which is preferably the case in the event of a failure of the first and second protection stages and the presence of an overvoltage which is greater than the response voltage U 3 , the short circuit is therefore established in a particularly safe and permanent manner. 
     It is also advantageous that at least one of the protection stages is designed to be redundant. Preferably, the first and/or the second protection stages are designed to be redundant and thus form additional failure protection. 
     In addition, one advantageous embodiment of the present invention provides that the protection circuit is integrated into the connecting plug. Preferably, the first, the second, and the third protection stages of the protection circuit are integrated into the connecting plug. A connecting cable from the connecting plug to the protection circuit is therefore superfluous, thus eliminating a potential cause of failure. The connecting plug may also be sealed in such a way that it protects the measuring connection from the penetration of air or moisture, and/or may have a mechanical cover protection which prevents the connecting plug from being able to be removed unintentionally or due to vibration of the measuring connection. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       The present invention is described in greater detail based on the drawings. They depict: 
         FIG. 1  a schematic representation of a measuring system connected to a bushing of a transformer; 
         FIG. 2  an equivalent circuit diagram of the arrangement according to  FIG. 1 ; 
         FIG. 3  a circuit diagram of a measuring system according to the present invention; and 
         FIG. 4  a diagram of one embodiment of a measuring system according to the present invention. 
     
    
    
     Corresponding parts are provided with the same reference numerals in all figures. 
     DESCRIPTION OF THE INVENTION 
       FIG. 1  depicts a measuring system  1  according to the present invention which is connected to the measuring connection  4  of the high-voltage bushing  2  of a power transformer  30 . A conductor  31  carrying a high voltage is routed out of a transformer  30 . A high-voltage bushing  2 , here having a capacitive field control, is used for insulating the conductor  31  from the grounded housing of the transformer  30 . Conductive cylindrical control coatings  32 ,  33  which are graduated in length are used for controlling the electric fields, in particular at the ends of the high-voltage bushing  2 . The outermost control coating  33  is also referred to as the ground coating, since it is closest to ground potential. Without a connected measuring system  1 , the ground coating  32  is generally grounded. A dielectric, which is not depicted, is present between the control coatings  32 ,  33 . Thus, adjacent control coatings  32 ,  33  act as capacitors. The space between the conductor  31  and the ground coating  33  is thus divided into a plurality of partial capacitances. To provide a better illustration, only a few control coatings  32  are shown here. 
     The bushing has an insulating housing  34 . An electrical connection is insulated and routed from the ground coating  33  through the bushing housing  34  to the outside and forms the high-voltage terminal  7  of a measuring connection  4 . The outer housing of the measuring connection  4  is generally grounded and is used as a ground terminal  8 . 
     A measuring system  1  for monitoring the high-voltage bushing  2  is connected to the measuring connection  4 . A connecting plug  3  establishes the mechanical and electrical contact with the measuring connection  4 . A connecting cable  6  leads from the connecting plug  3  to the measuring circuit  5 . The connecting cable  6  is, for example, designed as a coaxial cable, wherein the inner connector  42  is connected to the high-voltage terminal  7  and the outer conductor  41  is connected to the ground terminal  8 . A protection circuit  10  is connected to the connecting cable  6 . To provide a better illustration, the protection circuit  10  is shown here between the connecting plug  3  and the measuring circuit  5 . However, it may also be integrated into the measuring circuit or into the connecting plug. 
       FIG. 2  depicts an equivalent circuit diagram of the arrangement according to  FIG. 1 . The bushing  2  is depicted as a series connection of capacitors C 1  and C 2 . The capacitor C 2  corresponds to the capacitance made up of the ground coating  33  and ground  20 , and the capacitor C 1  corresponds to the capacitance of the interconnection of all other control coatings  33 . 
     A tap between the capacitors C 1  and C 2  forms the high-voltage terminal  7  of the measuring connection  4 . The high-voltage terminal  7  is thus electrically connected to the ground coating  33 . The measuring system  1  is inserted into a circuit between this tap and ground  20 . 
       FIG. 3  depicts a circuit diagram of a measuring system  1  according to the present invention, in particular the protection circuit  10 . Here, the high-voltage bushing is again depicted in the equivalent circuit diagram of the capacitors C 1  and C 2 . 
     A high voltage UH is present between the innermost layer coating, i.e., the one nearest to the conductor  31  carrying high voltage, and the ground  20 . A measuring circuit  5  as known from the related art is connected in parallel with the measuring connection  4 . The capacitance or ohmic resistance of the measuring circuit  5  is sized such that the voltage present at the measuring connection  4  is reduced to such an extent that it is able to be dissipated by the insulation  35  of the measuring connection  4  between the high-voltage terminal  7  and the ground terminal  8  without this insulation  35  possibly being damaged due to partial discharges. The level that this voltage may reach for this purpose depends on the sizing of the insulation  35 , but is typically several hundred volts. 
     Three protection stages  11 ,  12 ,  13  of a protection circuit  10  are each connected in parallel with the measuring connection  4  between the measuring connection  4  and the measuring circuit  5 . 
     A suppressor diode  21  is connected in parallel with the measuring connection  4  in a circuit branch  37 . Below a first response voltage U 1 , the resistance of the suppressor diode  21  is high, i.e., it has a high ohmic resistance which is significantly higher than that of the measuring circuit  5 . If the first response voltage U 1  is exceeded, for example, at 100 volts, it becomes conductive, its ohmic resistance decreasing by multiple orders of magnitude, and thus short-circuits the high-voltage terminal  7  and the ground terminal  8  of the measuring connection. The ohmic resistance of the suppressor diode  21  is then so low that a voltage in the range of the first response voltage U 1  arises at the measuring connection  4 , which is non-critical for its insulation  35 . Depending on the type of high-voltage bushing and the level of the high voltage, a current of several milliamperes up to 100 milliamperes flows through the suppressor diode. A second, identical suppressor diode  22  is connected in parallel with the suppressor diode  21  and thus forms a redundant branch of the first protection stage  11 . Instead of suppressor diodes  21 ,  22 , varistors or components having a current-voltage characteristic similar to the suppressor diode  21  may be used. A low-pass filter  23 , here, in the form of an inductor, is connected between the circuit branch  37  and the high-voltage terminal  7 . The low-pass filter  23  is sized such that frequencies which correspond to the operating frequency of the transformer  30 , i.e., approximately 50 to 60 Hertz, pass unhindered, whereas clearly higher frequencies are attenuated. The low-pass filter  23  and one or both suppressor diodes  21 ,  22  together form the first protection stage  11 . 
     A second protection stage  12  is connected between the first protection stage  11  and the measuring connection  4 . A circuit branch  38  connected in parallel with the measuring connection  4  includes a spark gap  24  which, for example, may be a protective spark gap or a gas discharge tube. Below a second response voltage U 2 , its resistance is high, i.e., it has a high ohmic resistance which is significantly higher than that of the measuring circuit  5 . If the second response voltage U 2  is exceeded, it becomes conductive, its ohmic resistance decreasing by multiple orders of magnitude, and establishes a short circuit between the high-voltage terminal  7  and the ground terminal  8 . The second response voltage U 2  is higher than the first response voltage U 1  and is, for example, 150 volts. A second, identical spark gap  25  is connected in parallel with the circuit branch  38  and thus forms a redundant branch of the second protection stage  12 . Spark gaps  24 ,  25  are available as components which already contain two separate spark gaps  24 ,  25 . A redundantly designed second protection stage  12  is thus highly compact and simple to implement. However, the redundant branches of the first and/or second protection stages  11 ,  12  may also be omitted in the case of lower safety requirements. 
     Overvoltages at the measuring connection  4  may have various causes. One is the loss of the measuring load. This happens if a defect occurs at the measuring circuit  5 , for example, caused by corrosion or aging of components, or in the case of a break in the connecting cable  6  between the measuring connection  4  and the measuring circuit  5  or protection circuit  10 . If the break in the connecting cable  6  occurs between the measuring connection  4  and the protection circuit  10 , the protection circuit  10  is no longer able to perform its function, i.e., of establishing a short circuit at the measuring connection  4 . Therefore, the protection circuit  10  must be situated as close as possible to the measuring connection  4 , preferably integrated into the connecting plug  3 . In the event of a break in the connecting cable  6  between the protection circuit  10  and the measuring circuit  5  or in the event of a defect in the measuring circuit  5 , an overvoltage may occur at the measuring connection  4 . This overvoltage is an AC voltage having the operating frequency of the transformer  30 . Such an event is therefore also referred to as an “operating-frequency” event. This operating-frequency AC voltage is able to pass the low-pass filter  23  virtually unhindered. This overvoltage is thus present at both the first protection stage  11  and at the second protection stage  12  of the protection circuit  10 . If the overvoltage exceeds the response voltage U 1  of the first protection stage  11 , at least one of the two suppressor diodes  21 ,  22  becomes conductive, thus short-circuiting the measuring connection  4  and limiting the voltage at the measuring connection  4  to a value which is harmless to the insulation  35  of the measuring connection  4 . The suppressor diode  21  or an equivalent component should be designed in such a way that it is able to carry the current now flowing over it for a longer period, for example, up to the next planned maintenance of the bushing  2 , without being destroyed. The defect in the connecting cable  6  or in the measuring circuit  5  may now be remedied during the next maintenance of the bushing  2 . Afterwards, the voltage at the measuring connection  4  will again be limited by measuring circuit  5 . 
     An additional possible cause of an overvoltage at the measuring connection  4  may be a lightning strike in or in the vicinity of the bushing  2 . Such a lightning strike is a brief, transient event, which may result in very high overvoltages at the measuring connection  4 . However, in contrast to the previously described event, frequencies occur which are considerably above the operating frequency of the transformer  30 . Therefore, such an event is termed “high-frequency.” The low-pass filter  23  attenuates these frequencies, so that the overvoltage occurring at the suppressor diode(s)  21 ,  22  of the first protection stage  11  has a lower amplitude and/or a time delay. However, at the second protection stage  12 , this overvoltage is undelayed and at full amplitude. Therefore, despite the higher response voltage U 2 , one or both of the spark gaps  24 ,  25  becomes conductive and short-circuits the measuring connection  4 . Without the low-pass filter  23 , in the case of such a high-frequency event, the first protection stage  11  would switch instead of the second protection stage  12 , since, on the one hand, it has a lower response voltage U 1 , and on the other hand, the suppressor diodes  21 ,  22  generally have a more rapid response behavior, i.e., a shorter switchover time from high resistance to conductive, than the spark gaps  24 ,  25 . Since much higher voltages may occur in a high-frequency event than in an operational-frequency event, and due to the frequency dependency of the capacitance C 1 , the currents flowing via the short circuit are also higher there and may amount to several hundred amperes. Such high-frequency events are connected by the circuit arrangement of the first and second protection stages  11 ,  12  by means of the spark gaps  24 ,  25 , which are able to carry considerably higher currents than the suppressor diodes  21 ,  22 . The first protection stage  11  thus preferably switches in the case of operating-frequency overvoltages, and the second protection stage  12  switches in the case of high-frequency events. If the first protection stage  11  should fail, the second protection stage  12  also switches in the case of an operating-frequency event, but only in the case of the higher response voltage U 2 . If the second protection stage  12  fails, the first protection stage  11  also switches in the case of a high-frequency event, but is time-delayed by the low-pass filter  23 . 
     If the event triggering the overvoltage has ended, which may be the case in the event of a lightning strike after just a few fractions of a second, and in the case of a repair, for example, of a break in the connecting cable  6 , may even take months, the voltage present at the first or second protection stage  11 ,  12  again falls below the response voltages U 1 , U 2 . The resistance of the spark gaps  24 ,  25  or the suppressor diodes  21 ,  22  again becomes high, and the short circuit of the measuring connection is again opened. The short circuit established by the first or second protection stage  11 ,  12  is thus reversible and is opened if the event triggering the overvoltage has ended. 
     The first and the second protection stages  11 ,  12  thus respond to different events; however, they act as a fallback level for each other if one of the protection stages  11 ,  12  fails. 
     If a lightning strike occurs after a failure of the second protection stage  12 , the first protection stage  11  may be destroyed by the high currents occurring and also fail. Since, in addition, little is currently known about the long-term behavior of the components used in the first or second protection stage  11 ,  12 , in particular if they are operated over a longer period in the connected, i.e., conductive, state, it cannot be ruled out that both protection stages  11 ,  12  fail. For this case, a third protection level  13  is provided which, however, may also be omitted for lower safety requirements. 
     The third protection stage  13  is connected in parallel with the measuring connection  4 , between it and the second protection stage  12 , and includes a series circuit made up of a spark gap  26 , a rectifier  27 , here, a diode, and a coil  28 . The spark gap  26  is connected to the high-voltage terminal  7 , and the coil  28  is connected to the ground terminal  8 . A mechanical switch  29  is connected in parallel with this series circuit between the high-voltage terminal  7  and the ground terminal  8 . The switch  29  and the coil  28  form a reed relay. If no current flows through the coil  28 , the switch  29  is open. If a direct current or a rectified alternating current flows through the coil  28 , the switch  29  is closed by the magnetic field of the coil  28  being generated as a result, and thus establishes a short circuit of the measuring connection  4 . The spark gap  26  has a response voltage U 3  at which it becomes conductive. The current which then flows through the spark gap  26  is rectified by the rectifier  27  and generates a magnetic field in the coil  28 . The coil  28 , in particular its number of windings, is sized in such a way that the current which then flows causes the switch  29  to be closed. The current now flows via the switch  29 , whereby the voltage at the measuring connection  4  falls below the response voltage U 3 , and the spark gap  26  is extinguished and its resistance again becomes high. Current then no longer flows through the coil  28 , and the magnetic field collapses. The switch  29  is bistable and thus remains in the closed state, even if the magnetic field has collapsed. The switch  29  may again be opened via a magnetic field which is opposite to the magnetic field required to close the switch  29 . This may be achieved using a permanent magnet which is kept at a suitable location in the vicinity of the switch  29 . If the protection circuit  10  or even just the protection stage  13  is integrated into the connecting plug  3 , a marking may be provided on the connecting plug  3  which indicates where and in which orientation a permanent magnet must be positioned for opening the switch  29 . The switch  29  may alternatively be opened by applying a countervoltage to the coil  28  which is opposite to the response voltage U 3 , for example, using an additional voltage source, or by providing an additional coil through which, for example, a current flows in response to a switching pulse from the control room, and as a result, a magnetic field is generated which is suitable for opening the switch  29 . 
     The response voltage U 3  of the spark gap  26  is also the response voltage of the third protection stage  13  and is sized in such a way that it is greater than the response voltage U 2  of the second protection stage  12 . However, the third protection stage  13  could also act as the first failure level, and the first and second protection stages  11 ,  12  could act as its fallback levels. The response voltage U 3  would then have to be sized smaller than the response voltage U 1 . 
     Should an overvoltage occur if both the first and the second protection levels  11 ,  12  have failed, or if an overvoltage results in a successive failure of the first and second protection levels  11 ,  12 , the third protection level  13  would short-circuit the measuring connection  4  if the overvoltage exceeds the response voltage U 3 . Since mechanical switches are able to carry high currents and are considered to be robust and failsafe, such a third protection stage  13  constitutes a high level of protection of the measuring connection  4  from overvoltages. However, unlike in the first or second protection stage  11 ,  12 , here, a manual intervention for opening the short circuit is necessary; therefore, the third protection stage  13  is preferably used as failure protection for the first and second protection stages  11 ,  12 . 
       FIG. 4  depicts a diagram of a measuring system  1  according to the present invention including a protection circuit  10  which is integrated into the housing  40  of the connecting plug  3 . The protection circuit  10  may be designed as shown in  FIG. 3 . An electrical connection runs from the ground coating  33  of the high-voltage bushing  2  to the high-voltage terminal  7  of the measuring connection  4 . A grounded flange  36  on the bushing housing  34  forms the outer housing of the measuring connection  4  and thus its ground terminal  8 . An insulator  35  is present between the high-voltage terminal  7  and the outer housing which insulates the high-voltage terminal  7  from ground  20  and simultaneously seals the measuring connection  4 , so that no air or moisture is able to reach the high-voltage bushing  2 . The connecting plug  3  is plugged into the measuring connection  4 . A socket  39  in the connecting plug  3  establishes the electrical connection to the high-voltage terminal  7 . The housing  40  of the connecting plug  3  is electrically conductive and establishes the connection to ground  20  via the flange  36 . The protection circuit  10  is situated in the interior of the housing  40  of the connecting plug  3 . A protection circuit  10  as described above is made up of components which are highly compact, some being less than one millimeter in size. As a result, a protection circuit  10  having two or three protection stages  11 ,  12 ,  13  is able to be accommodated in a connecting plug  3  whose dimensions are limited by an often small amount of space in the area of the measuring connection  4 . The protection circuit  10  is electrically connected in the interior of the connecting plug  3  to the socket  39  and to ground  20  via the housing  40  of the connecting plug  3 . The connecting cable  6  is situated on the side of the connecting plug  3  facing away from the measuring connection  4 . This cable is designed as a coaxial cable. The inner conductor  42  is electrically connected to the high-voltage terminal  7 , and the outer conductor  41  is connected to ground  20  via the housing of the connecting plug  3 . The connection may be designed to be fixed or detachable, for example, having a bayonet connector. The other end of the connecting cable  6  is connected to the measuring circuit  5 . This connection may also be designed to be fixed or detachable.