PV modules connected in series should pay attention to:
When the photovoltaic system is connected to the grid for power generation, the photovoltaic array must realize the whole power point tracking control to get the total power output under any current sunshine continuously. Therefore, when designing the number of PV modules in series, the following issues should be noted:
1) The specifications, types, number of series, and installation angles of PV modules connected to the same inverter should be consistent.
2) The temperature coefficient of the optimal working voltage (Vmp) and open-circuit voltage (Voc) of the photovoltaic modules should be considered. The Vmp of the series-connected photovoltaic array should be within the inverter MPPT range, and the Voc should be lower than the inverter input voltage. Maximum value.
Generally, the DC input voltage range of the inverter is specific. The photovoltaic grid-connected inverter's recommended maximum DC input voltage is 1100V, and the MPPT range is 200V~1000V. When selecting the number of modules in a series, two aspects need to be considered: one is the open-circuit voltage. The high limit must be less than the maximum withstand voltage of the inverter; the second is that the low limit of the rated working voltage is not less than the minimum value of the inverter MPPT range. Combining the above conditions, we choose the maximum number of series connections for photovoltaic modules is not more than 21 as a series. At room temperature of 25℃, the open-circuit voltage is 39.8V×20 strings=796V, and the total power working voltage is 32.1V×20=642V, which meets the requirements of the machine.
System reliability and safety
1. The inverter has good reliability and safety
1) Synchronous closed-loop control function: real-time sampling and comparison of external power grid voltage, phase, frequency, and other signals, and always keep the inverter output synchronized with the external power grid, the power quality is stable and reliable, does not pollute the power grid, and has good safety performance.
2) It has the function of automatic shutdown and operation: the inverter detects the voltage, phase, frequency, DC input, AC output voltage, current, and other signals of the external power grid in real-time. When abnormal conditions occur, it will automatically protect and disconnect the AC output; when the cause of the fault disappears and the power grid returns to normal, the inverter will detect and delay for a certain period, then restore the AC output and automatically connect to the grid, with good reliability.
3) Protection function: It has protection functions such as overvoltage, loss of voltage, frequency detection, and protection, overload and overcurrent, leakage, lightning protection, grounding short circuit, and automatic isolation of the power grid.
2. System safety performance
Because the entire photovoltaic power generation system is equipped with a safe and reliable lightning protection device, the selected inverter has protections such as overvoltage, under-voltage, overload, and overcurrent, short-circuit grounding, leakage, etc., so the entire system has these protection functions to ensure that the design and The equipment usually operate to ensure the safety of electricity consumption of the whole system.
In the photovoltaic power station system, grounding is a crucial part of the electrical design, which is related to the safety of the equipment and personnel of the power station. A good grounding design can ensure that the power station is in a safe operating environment for a long time, reduce the fault frequency of the power station, and improve the overall operational efficiency of the power station. So what are the common grounding types in photovoltaic power plants?
1. What is grounding
Earthing refers to connecting the neutral point of the power system and electrical devices, the exposed conductive parts of electrical equipment, and the conductive parts outside the device to the ground through conductors. It can be divided into working grounding, lightning protection grounding, and protective grounding.
2.role of grounding
We often only know that grounding can prevent personal shocks. But, in fact, in addition to this function, grounding can also prevent equipment and lines from being damaged, prevent fires, prevent lightning strikes, prevent electrostatic damage, and ensure the regular operation of power systems.
01 Protection against electric shock
The impedance of the human body has a great relationship with the conditions of the environment. Therefore, grounding is an effective way to prevent electric shock. After the electrical equipment is grounded through the grounding device, the potential of the electrical equipment is close to the ground potential. Due to the grounding resistance, the electrical equipment to the ground potential always exists. The bigger it is, the more dangerous it is when someone touches it. However, suppose the grounding device is not provided. In that case, the voltage of the faulty equipment casing will be the same as the phase-to-ground voltage, which is still much higher than the grounding voltage, so the danger will also increase accordingly.
02 Ensure the regular operation of the power system
The grounding of the power system, also known as the working grounding, is generally grounded at the neutral point of the substation or substation. The grounding resistance requirement for working grounding is minimal, and a grounding grid is required for large-scale substations to ensure that the grounding resistance is small and reliable. The purpose of the working ground is to make the potential between the neutral point of the grid and the ground close to zero. The low-voltage power distribution system cannot avoid the phase line touching the shell or the ground after the phase line is broken. If the neutral point is insulated from the ground, the voltage to the bottom of the other two phases will rise to three times the phase voltage, which may cause the Electrical work equipment with a voltage of 220 to burn out. For the neutral point grounded system, even if one phase is short-circuited to the ground, the other two phases can still be close to the phase voltage, so the electrical equipment connected to the two different phases will not be damaged. In addition, it can prevent the system from oscillating, and the insulation level of electrical equipment and lines only needs to be considered according to the phase voltage.
03 Protection against lightning strikes and static electricity hazards
When lightning occurs, in addition to direct lightning, induction lightning is also produced, and induction lightning is divided into static blunt induction lightning and electromagnetic induction lightning. The most important method of all lightning protection measures is grounding.
3. Types of grounding
Common grounding types are as follows: working grounding, lightning protection grounding, protective grounding, shielding grounding, anti-static grounding, etc.
01 Lightning protection grounding
Lightning protection grounding is a grounding system to prevent damage when struck by lightning (direct strike, induction, or line introduction).
As a part of lightning protection measures, lightning protection grounding introduces lightning current into the earth. The lightning protection of buildings and electrical equipment mainly uses one end of the arrester (including the lightning rod, lightning protection belt, lightning protection net, lightning suppression device, etc.) to connect with the protected equipment. The other end is connected to the ground device. As a result, the lightning is directed towards itself, and the lightning current enters the earth through its down-conductor and grounding device. In addition, due to the side effect of electrostatic induction caused by lightning, to prevent indirect damage, such as house fire or electric shock, it is usually necessary to ground the building's metal equipment, metal pipes, and steel structures.
02 AC work grounding
AC work grounding is to connect a certain point in the power system directly or through special equipment to the earth for metal connection. Working grounding mainly refers to grounding the neutral end of the transformer or the neutral line (N line). The N wire must be insulated with a copper core. There are auxiliary equipotential bonding terminals in the power distribution, and the equipotential bonding terminals are generally in the cabinet. It must be noted that this terminal cannot be exposed; it cannot be mixed with other grounding systems, such as DC grounding, shielding grounding, anti-static grounding, etc.; nor can it be connected with PE wires.
03 Safety protection grounding
Safety grounding makes a good metal connection between the non-charged metal parts of electrical equipment and the grounding body. In a photovoltaic power station, there are mainly inverters, components, and distribution boxes that need to be grounded for safety protection.
▲Inverter shell grounding
▲Photovoltaic module grounding
04 Shield ground
To prevent the interference of external electromagnetic fields, the grounding of the outer casing of the electronic equipment and the shielded wires inside and outside the equipment or the metal pipes that pass through it is called shielding grounding. This grounding method is usually used to ground the shielding layer of the RS485 communication line in the photovoltaic power station, which can effectively prevent the electromagnetic field from interfering with the communication when multiple inverters perform 485 serial communication.
▲The shielding layer of the 485 communication line is grounded
05 Anti-static grounding
For some particular inverter installation environments, such as installation in a dry computer room, the grounding to prevent the interference of the electrostatic inverter generated by the arid climate of the computer room is called anti-static grounding. The anti-static grounding device can be shared with the safety grounding device of the inverter.
The standard grounding resistance specification requirements are shown in the following table:
Summarize
As a set of long-term operation systems, photovoltaic power plants need to be grounded during design and construction to reduce unnecessary operation and maintenance in the later stage to ensure the system's long-term stable, safe, and efficient operation.
With the wide application of photovoltaic power generation, the connection between photovoltaic modules and module strings, the DC terminal connection of combiner boxes, inverters, and other equipment are widely used in international standard MC4/H4 connectors, as shown in Figure 1 and Figure 1. 2 shown.
▲Figure 1
▲Figure 2
1. Performance requirements of photovoltaic connectors
So what are the performance requirements of photovoltaic connectors?
First, the photovoltaic connector should have good conductivity, and the contact resistance should not be greater than 0.35 milliohms.
Second, it needs to have good safety performance to ensure the safety performance of solar cell modules. Third, the environment and climate in which solar energy equipment is used are sometimes in terrible weather and environment. Therefore, it must have waterproof, have high temperature, corrosion resistance, high insulation, and other properties, and the protection level should reach IP68.
Third, the structure of the solar connector should be firm and reliable, and the connection force between the male and female connectors should not be less than 80N. For the MC4 connector connected to a four mm² cable, when carrying a current of 39A, the temperature should not exceed the upper limit temperature of 105 degrees. MC4/H4 connectors are single-core connectors with male and female headers and have many advantages such as good sealing, convenient connection, convenient maintenance, and maintenance.
2. Precautions for installation of photovoltaic connectors
The selection of the plug should pay attention to the product quality, including the size of the internal metal conductor, the material thickness, elasticity, and coating should meet the ability to carry a large current. Good contact, the plastic of the plug shell should ensure that the surface is smooth without cracks, and the interface is sealed well. When installing the component connector, avoid exposure to sunlight and rain to prevent aging of the connector, corrosion of the internal connector and cable, increase in contact resistance, or even spark, resulting in a decrease in system efficiency or a fire accident.
In installing photovoltaic connectors, the crimping link is the top priority, and professional crimping tools should be used. Before constructing the photovoltaic power station, the relevant engineering installers should be trained in crimping operations.
▲Figure 3
With the development of photovoltaic cell technology, the capacity of a single photovoltaic module is also increasing, and the string current is also gradually increasing. Although theoretically, the design carrying a draft of the MC4/H4 connector is sufficient to meet the requirements of these large-capacity modules, Due to various reasons, in recent years, many photovoltaic power stations have experienced more and more accidents in which the connectors are melted, burned, and even lead to the burning of combiner boxes and inverters. Figure 5, Figure 6, Figure 7.
▲Figure 5
▲Figure 6
▲Figure 7
As we all know, in a 100kWp photovoltaic power station, there are usually 600-1000 such connectors, and their working states, such as contact resistance, are critical to the regular operation of the photovoltaic power station. The poor working condition of the connector will affect the increase of the internal resistance of the DC side, which will lead to a decrease in the power generation efficiency of the power station. In the worst case, the poor contact will cause the connector to heat up or even burn the connector, which will lead to the burning of the combiner box and the inverter (Figure 7). And even more severe can lead to the occurrence of large-scale fires.
Summary: Component connectors, connector plug-ins connected to combiner boxes, and string inverters are where failures occur frequently. Although the connector is small, it is essential in the photovoltaic power generation system. Especially in the operation and maintenance process after the completion of the power station, it is necessary to pay attention to its operation status and regularly check the temperature rise of the connection plug to ensure that there is no abnormality and regular operation.
First of all, the indirect plug-ins of photovoltaic modules should be firmly connected, and the connection between the external cable and the connector should be tinned; after the photovoltaic module string is connected, the open-circuit voltage and short-circuit current of the photovoltaic module string should be tested; The drawings and specifications require reliable grounding.
During the installation of photovoltaic modules, special attention should be paid to the following precautions:
1) Only photovoltaic modules of the same size and specification can be connected in series;
2) It is strictly prohibited to install photovoltaic modules in rainy, snowy, or windy weather conditions;
3) It is strictly forbidden to connect the positive and negative quick plugs of the same piece of photovoltaic module connecting line;
4) The use of the photovoltaic module backplane (EVA) will be prohibited if it is damaged;
5) It is strictly forbidden to step on the battery board to avoid damage to components or personal injury;
6) It is strictly forbidden to squeeze or beat, collide or scratch the tempered glass of photovoltaic modules with sharp objects;
7) The unpacked solar panels at the construction site should be placed flat with the front-facing up, with wooden pallets or panel packaging at the bottom, and it is strictly forbidden to place them upright, obliquely, or suspended in the air, and it is strictly forbidden to expose the back of the modules to sunlight directly;
8) Two people should carry the modules at the same time during the handling process, and they should be handled with care to avoid significant vibrations to avoid cracking of the photovoltaic modules;
9) It is strictly forbidden to lift the module by pulling the junction box or connecting wire;
10) When installing the upper battery board, pay attention to the battery board frame scratching the installed battery board during transportation;
11) It is strictly forbidden for installation workers to use tools to touch the battery board at will, causing scratches;
12) It is strictly forbidden to touch the live metal parts of the photovoltaic module string;
13) For components whose open-circuit voltage exceeds 50V or whose maximum rated voltage exceeds 50V, there should be a conspicuous warning sign of electric shock danger near the component connection device.
When the photovoltaic system is connected to the grid for power generation, the photovoltaic array must realize the whole power point tracking control to get the total power output under any current sunshine continuously. Therefore, when designing the number of PV modules in series, the following issues should be noted:
1) The specifications, types, number of series, and installation angles of PV modules connected to the same inverter should be consistent.
2) The temperature coefficient of the optimal working voltage (Vmp) and open-circuit voltage (Voc) of the photovoltaic modules should be considered. The Vmp of the series-connected photovoltaic array should be within the inverter MPPT range, and the Voc should be lower than the inverter input voltage. Maximum value.
Generally, the DC input voltage range of the inverter is specific. The photovoltaic grid-connected inverter's recommended maximum DC input voltage is 1100V, and the MPPT range is 200V~1000V. When selecting the number of modules in a series, two aspects need to be considered: one is the open-circuit voltage. The high limit must be less than the maximum withstand voltage of the inverter; the second is that the low limit of the rated working voltage is not less than the minimum value of the inverter MPPT range. Combining the above conditions, we choose the maximum number of series connections for photovoltaic modules is not more than 21 as a series. At room temperature of 25℃, the open-circuit voltage is 39.8V×20 strings=796V, and the total power working voltage is 32.1V×20=642V, which meets the requirements of the machine.
System reliability and safety
1. The inverter has good reliability and safety
1) Synchronous closed-loop control function: real-time sampling and comparison of external power grid voltage, phase, frequency, and other signals, and always keep the inverter output synchronized with the external power grid, the power quality is stable and reliable, does not pollute the power grid, and has good safety performance.
2) It has the function of automatic shutdown and operation: the inverter detects the voltage, phase, frequency, DC input, AC output voltage, current, and other signals of the external power grid in real-time. When abnormal conditions occur, it will automatically protect and disconnect the AC output; when the cause of the fault disappears and the power grid returns to normal, the inverter will detect and delay for a certain period, then restore the AC output and automatically connect to the grid, with good reliability.
3) Protection function: It has protection functions such as overvoltage, loss of voltage, frequency detection, and protection, overload and overcurrent, leakage, lightning protection, grounding short circuit, and automatic isolation of the power grid.
2. System safety performance
Because the entire photovoltaic power generation system is equipped with a safe and reliable lightning protection device, the selected inverter has protections such as overvoltage, under-voltage, overload, and overcurrent, short-circuit grounding, leakage, etc., so the entire system has these protection functions to ensure that the design and The equipment usually operate to ensure the safety of electricity consumption of the whole system.
In the photovoltaic power station system, grounding is a crucial part of the electrical design, which is related to the safety of the equipment and personnel of the power station. A good grounding design can ensure that the power station is in a safe operating environment for a long time, reduce the fault frequency of the power station, and improve the overall operational efficiency of the power station. So what are the common grounding types in photovoltaic power plants?
1. What is grounding
Earthing refers to connecting the neutral point of the power system and electrical devices, the exposed conductive parts of electrical equipment, and the conductive parts outside the device to the ground through conductors. It can be divided into working grounding, lightning protection grounding, and protective grounding.
2.role of grounding
We often only know that grounding can prevent personal shocks. But, in fact, in addition to this function, grounding can also prevent equipment and lines from being damaged, prevent fires, prevent lightning strikes, prevent electrostatic damage, and ensure the regular operation of power systems.
01 Protection against electric shock
The impedance of the human body has a great relationship with the conditions of the environment. Therefore, grounding is an effective way to prevent electric shock. After the electrical equipment is grounded through the grounding device, the potential of the electrical equipment is close to the ground potential. Due to the grounding resistance, the electrical equipment to the ground potential always exists. The bigger it is, the more dangerous it is when someone touches it. However, suppose the grounding device is not provided. In that case, the voltage of the faulty equipment casing will be the same as the phase-to-ground voltage, which is still much higher than the grounding voltage, so the danger will also increase accordingly.
02 Ensure the regular operation of the power system
The grounding of the power system, also known as the working grounding, is generally grounded at the neutral point of the substation or substation. The grounding resistance requirement for working grounding is minimal, and a grounding grid is required for large-scale substations to ensure that the grounding resistance is small and reliable. The purpose of the working ground is to make the potential between the neutral point of the grid and the ground close to zero. The low-voltage power distribution system cannot avoid the phase line touching the shell or the ground after the phase line is broken. If the neutral point is insulated from the ground, the voltage to the bottom of the other two phases will rise to three times the phase voltage, which may cause the Electrical work equipment with a voltage of 220 to burn out. For the neutral point grounded system, even if one phase is short-circuited to the ground, the other two phases can still be close to the phase voltage, so the electrical equipment connected to the two different phases will not be damaged. In addition, it can prevent the system from oscillating, and the insulation level of electrical equipment and lines only needs to be considered according to the phase voltage.
03 Protection against lightning strikes and static electricity hazards
When lightning occurs, in addition to direct lightning, induction lightning is also produced, and induction lightning is divided into static blunt induction lightning and electromagnetic induction lightning. The most important method of all lightning protection measures is grounding.
3. Types of grounding
Common grounding types are as follows: working grounding, lightning protection grounding, protective grounding, shielding grounding, anti-static grounding, etc.
01 Lightning protection grounding
Lightning protection grounding is a grounding system to prevent damage when struck by lightning (direct strike, induction, or line introduction).
As a part of lightning protection measures, lightning protection grounding introduces lightning current into the earth. The lightning protection of buildings and electrical equipment mainly uses one end of the arrester (including the lightning rod, lightning protection belt, lightning protection net, lightning suppression device, etc.) to connect with the protected equipment. The other end is connected to the ground device. As a result, the lightning is directed towards itself, and the lightning current enters the earth through its down-conductor and grounding device. In addition, due to the side effect of electrostatic induction caused by lightning, to prevent indirect damage, such as house fire or electric shock, it is usually necessary to ground the building's metal equipment, metal pipes, and steel structures.
02 AC work grounding
AC work grounding is to connect a certain point in the power system directly or through special equipment to the earth for metal connection. Working grounding mainly refers to grounding the neutral end of the transformer or the neutral line (N line). The N wire must be insulated with a copper core. There are auxiliary equipotential bonding terminals in the power distribution, and the equipotential bonding terminals are generally in the cabinet. It must be noted that this terminal cannot be exposed; it cannot be mixed with other grounding systems, such as DC grounding, shielding grounding, anti-static grounding, etc.; nor can it be connected with PE wires.
03 Safety protection grounding
Safety grounding makes a good metal connection between the non-charged metal parts of electrical equipment and the grounding body. In a photovoltaic power station, there are mainly inverters, components, and distribution boxes that need to be grounded for safety protection.
▲Inverter shell grounding
▲Photovoltaic module grounding
04 Shield ground
To prevent the interference of external electromagnetic fields, the grounding of the outer casing of the electronic equipment and the shielded wires inside and outside the equipment or the metal pipes that pass through it is called shielding grounding. This grounding method is usually used to ground the shielding layer of the RS485 communication line in the photovoltaic power station, which can effectively prevent the electromagnetic field from interfering with the communication when multiple inverters perform 485 serial communication.
▲The shielding layer of the 485 communication line is grounded
05 Anti-static grounding
For some particular inverter installation environments, such as installation in a dry computer room, the grounding to prevent the interference of the electrostatic inverter generated by the arid climate of the computer room is called anti-static grounding. The anti-static grounding device can be shared with the safety grounding device of the inverter.
The standard grounding resistance specification requirements are shown in the following table:
Summarize
As a set of long-term operation systems, photovoltaic power plants need to be grounded during design and construction to reduce unnecessary operation and maintenance in the later stage to ensure the system's long-term stable, safe, and efficient operation.
With the wide application of photovoltaic power generation, the connection between photovoltaic modules and module strings, the DC terminal connection of combiner boxes, inverters, and other equipment are widely used in international standard MC4/H4 connectors, as shown in Figure 1 and Figure 1. 2 shown.
▲Figure 1
▲Figure 2
1. Performance requirements of photovoltaic connectors
So what are the performance requirements of photovoltaic connectors?
First, the photovoltaic connector should have good conductivity, and the contact resistance should not be greater than 0.35 milliohms.
Second, it needs to have good safety performance to ensure the safety performance of solar cell modules. Third, the environment and climate in which solar energy equipment is used are sometimes in terrible weather and environment. Therefore, it must have waterproof, have high temperature, corrosion resistance, high insulation, and other properties, and the protection level should reach IP68.
Third, the structure of the solar connector should be firm and reliable, and the connection force between the male and female connectors should not be less than 80N. For the MC4 connector connected to a four mm² cable, when carrying a current of 39A, the temperature should not exceed the upper limit temperature of 105 degrees. MC4/H4 connectors are single-core connectors with male and female headers and have many advantages such as good sealing, convenient connection, convenient maintenance, and maintenance.
2. Precautions for installation of photovoltaic connectors
The selection of the plug should pay attention to the product quality, including the size of the internal metal conductor, the material thickness, elasticity, and coating should meet the ability to carry a large current. Good contact, the plastic of the plug shell should ensure that the surface is smooth without cracks, and the interface is sealed well. When installing the component connector, avoid exposure to sunlight and rain to prevent aging of the connector, corrosion of the internal connector and cable, increase in contact resistance, or even spark, resulting in a decrease in system efficiency or a fire accident.
In installing photovoltaic connectors, the crimping link is the top priority, and professional crimping tools should be used. Before constructing the photovoltaic power station, the relevant engineering installers should be trained in crimping operations.
▲Figure 3
With the development of photovoltaic cell technology, the capacity of a single photovoltaic module is also increasing, and the string current is also gradually increasing. Although theoretically, the design carrying a draft of the MC4/H4 connector is sufficient to meet the requirements of these large-capacity modules, Due to various reasons, in recent years, many photovoltaic power stations have experienced more and more accidents in which the connectors are melted, burned, and even lead to the burning of combiner boxes and inverters. Figure 5, Figure 6, Figure 7.
▲Figure 5
▲Figure 6
▲Figure 7
As we all know, in a 100kWp photovoltaic power station, there are usually 600-1000 such connectors, and their working states, such as contact resistance, are critical to the regular operation of the photovoltaic power station. The poor working condition of the connector will affect the increase of the internal resistance of the DC side, which will lead to a decrease in the power generation efficiency of the power station. In the worst case, the poor contact will cause the connector to heat up or even burn the connector, which will lead to the burning of the combiner box and the inverter (Figure 7). And even more severe can lead to the occurrence of large-scale fires.
Summary: Component connectors, connector plug-ins connected to combiner boxes, and string inverters are where failures occur frequently. Although the connector is small, it is essential in the photovoltaic power generation system. Especially in the operation and maintenance process after the completion of the power station, it is necessary to pay attention to its operation status and regularly check the temperature rise of the connection plug to ensure that there is no abnormality and regular operation.
First of all, the indirect plug-ins of photovoltaic modules should be firmly connected, and the connection between the external cable and the connector should be tinned; after the photovoltaic module string is connected, the open-circuit voltage and short-circuit current of the photovoltaic module string should be tested; The drawings and specifications require reliable grounding.
During the installation of photovoltaic modules, special attention should be paid to the following precautions:
1) Only photovoltaic modules of the same size and specification can be connected in series;
2) It is strictly prohibited to install photovoltaic modules in rainy, snowy, or windy weather conditions;
3) It is strictly forbidden to connect the positive and negative quick plugs of the same piece of photovoltaic module connecting line;
4) The use of the photovoltaic module backplane (EVA) will be prohibited if it is damaged;
5) It is strictly forbidden to step on the battery board to avoid damage to components or personal injury;
6) It is strictly forbidden to squeeze or beat, collide or scratch the tempered glass of photovoltaic modules with sharp objects;
7) The unpacked solar panels at the construction site should be placed flat with the front-facing up, with wooden pallets or panel packaging at the bottom, and it is strictly forbidden to place them upright, obliquely, or suspended in the air, and it is strictly forbidden to expose the back of the modules to sunlight directly;
8) Two people should carry the modules at the same time during the handling process, and they should be handled with care to avoid significant vibrations to avoid cracking of the photovoltaic modules;
9) It is strictly forbidden to lift the module by pulling the junction box or connecting wire;
10) When installing the upper battery board, pay attention to the battery board frame scratching the installed battery board during transportation;
11) It is strictly forbidden for installation workers to use tools to touch the battery board at will, causing scratches;
12) It is strictly forbidden to touch the live metal parts of the photovoltaic module string;
13) For components whose open-circuit voltage exceeds 50V or whose maximum rated voltage exceeds 50V, there should be a conspicuous warning sign of electric shock danger near the component connection device.