All voltage withstand tests have a work instruction, no work instruction, you can find the corresponding equipment manual.
There is a very detailed description of the operating test method!
The so-called grounding, is a part of the equipment through the grounding device with the earth closely together. To date, grounding is still one of the most widely used and irreplaceable electrical safety measures. Whether it is electrical or electronic equipment, whether it is production equipment or living equipment, whether it is DC equipment or AC equipment, whether it is fixed equipment or mobile equipment, whether it is high-voltage equipment or low-voltage equipment, and whether it is a power plant or power users, are used in different ways, different purposes of the grounding measures to ensure the normal operation of the equipment or their safety.
A, the role of grounding
The role of grounding is to prevent people from suffering electric shock, equipment and lines are damaged, prevent fire and prevent lightning, prevent static damage and to protect the normal operation of the power system.
1. Prevent personal electric shock
Electrical equipment in normal operation of the non-electrified metal conductor part of the grounding electrode for a good metal connection, in order to protect the human body's safety, to prevent personal electric shock.
When the insulation of electrical equipment somewhere after the damage to the shell will be charged, due to the power neutral grounding, even if the equipment is not grounded, due to the line and the earth between the existence of capacitance, when the human body in contact with the shell of the equipment will also have a current flow through the human body; or line insulation is not good, if the human body touches this insulation damage to the shell of the electrical equipment, the current will be through the human body and become a pathway, thus making the human body suffer electric shock injury.
Electrical equipment with grounding, when the insulation is damaged, shell charged, grounding current will flow along the grounding pole and the human body two pathways, at this time, the human body and the grounding pole is a parallel relationship, the value of the current flowing through each pathway will be inversely proportional to the size of its resistance, the smaller the resistance of the grounding pole, the smaller the current flowing through the human body. Usually the resistance of the human body than the grounding electrode resistance hundreds of times, so the current flowing through the human body to offer the current flowing through the grounding electrode hundreds of times smaller. When the grounding resistance is very small, the current flowing through the human body is almost equal to zero, equivalent to the grounding pole will be the human body short, therefore, the human body can avoid the risk of electric shock.
So, regardless of construction or operation, in all seasons of the year, should ensure that the grounding resistance is not greater than the design or specification of the grounding resistance value to avoid electric shock injury.
2. Ensure the normal operation of the electrical system
Power system grounding is generally neutral grounding, neutral grounding resistance is very small, so the neutral point and the potential difference between the ground is close to zero. When the phase line touching the shell or ground, the other two phase-to-ground voltage, in the neutral insulation system will be raised to the root of the phase voltage of three times, and in the neutral grounding system is close to the phase voltage, so the neutral grounding will be conducive to the stable operation of the system, to prevent system oscillations, and the system's electrical equipment and lines only need to be considered according to the phase voltage of the insulation level, which reduces the manufacturing cost of electrical equipment and line This can reduce the manufacturing cost of electrical equipment and the construction cost of lines. Neutral point grounding system, but also to ensure the reliable operation of relay protection.
Communication systems generally use positive grounding, can prevent noise into and ensure the normal operation of communication equipment.
Electronic circuits need a stable reference point for normal operation, so they also need to be grounded.
3. Prevent the hazards of lightning and static electricity
Lightning will produce static induction and electromagnetic induction, materials in the production and transportation process due to friction caused by static electricity, may cause electric shock or fire hazards.
Directly under the influence of lightning, compared to the induction of lightning is greater, and the opportunity to occur more, so in order to prevent direct lightning, must be equipped with lightning protection devices.
All lightning protection devices and measures to prevent electrostatic hazards, the most important method is to set the grounding device.
Second, the classification of electrical grounding
1. According to the role or function of grounding to points
The so-called grounding, simply put, is a variety of equipment and the earth's electrical connection. Requirements for grounding a variety of equipment, such as power equipment, communications equipment, electronic equipment, lightning protection devices. The purpose of grounding is for the normal and safe operation of the equipment, as well as creating conditions for the safety of buildings and people. Commonly used grounding methods according to the role or function can be divided into the following categories.
(1) system grounding in the power system will be a point with the earth for proper connection, known as the system grounding or work grounding. Such as transformer neutral point grounding, zero line repeated grounding.
(2) the protective grounding of equipment, various electrical equipment, metal casing, line metal pipe, cable metal protective layer, installation of electrical equipment, metal support, etc., due to the damage to the insulation of the conductor may be electrically charged, in order to prevent these non-electrically charged metal part of the excessive ground voltage endangering the safety of the face of the setting of the grounding, known as the protective grounding.
(3) lightning grounding in order to make the lightning current safely discharged to the earth, in order to protect the struck building or electrical equipment and take the grounding, known as lightning grounding.
(4) shielding grounding on the one hand, in order to prevent interference and intrusion of foreign electromagnetic waves, resulting in malfunction of electronic equipment or communication quality degradation, on the other hand, in order to prevent the high-frequency energy generated by the electronic equipment to the outside leakage, and the line filter, transformer electrostatic shielding, cable metal shielding, etc., grounding, known as shielding grounding. In order to reduce the vertical pipeline in the shaft of high-rise buildings by lightning current induction of induced electric potential, and the shaft concrete wall of the steel to be grounded, also belongs to the shielding grounding.
(5) anti-static grounding static electricity is due to friction and other reasons for the accumulation of charge, in order to prevent electrostatic accidents or affect the normal operation of electronic equipment, the need for static charges to the earth quickly discharged to the ground, this grounding is known as anti-static grounding.
(6) equipotential grounding hospitals, some special examination rooms, treatment rooms, operating rooms and wards, patients have access to the metal parts (such as bed frames, bed lamps, medical appliances, etc.), there should not be a dangerous potential difference exists, so these metal parts are connected to each other to become equipotential body and be grounded, this grounding method is called equipotential grounding. High-rise buildings in order to reduce the potential difference caused by lightning current, each layer of reinforcing steel mesh and large metal objects connected together and grounded, also isotropic grounding.
(7) electronic equipment signal grounding and power grounding electronic equipment signal grounding (or logic grounding) refers to the signal circuit amplifiers, mixers, scanning circuits, logic circuits, etc. There is a unified reference potential and grounding, grounding is the purpose of the signal does not cause errors. Power grounding is the unified grounding of all relays, motors, power supply units, high-current devices, indicators, and other circuits to ensure that the interference signal in these circuits is leaked into the ground without interfering with the normal operation of other sensitive signal circuits.
Comprehensive grounding classification is roughly as follows.
In accordance with the formation of grounding, can be divided into two categories of normal grounding and fault grounding. The former is in order to some kind of need for artificial grounding, the latter is naturally formed by a variety of external or self-factors grounding.
In accordance with the different roles of grounding, and normal grounding can be divided into two categories of work grounding and safety grounding.
Working grounding usually has the following three cases.
①Using the earth as the circuit grounding. This grounding in normal circumstances, there is also a current through the earth, such as DC work grounding, weak work grounding, "two wires and a ground" system of power supply in a phase of the grounding and so on.
② maintain the safe operation of the system grounding. Under normal circumstances, there is no current or only a small unbalanced current through the earth. Such as 11OkV above the neutral point of the system ground, low-voltage three-phase four-wire system of transformer neutral point ground.
3 in order to prevent lightning strikes and over-voltage hazards to equipment and people set up grounding, that is, over-voltage protection grounding, also known as lightning grounding.
Safety grounding mainly includes: to prevent damage to electrical facilities or electrical equipment insulation to endanger the safety of the face of the set of protective grounding; to eliminate the accumulation of static electricity generated in the production process, resulting in electrocution or explosion of the set of anti-static grounding; in order to prevent the electromagnetic effect of the metal casing of the equipment, shielding or shielding the outer skin of the line of the shielding of the grounding; in order to prevent the pipeline corrosion of electrochemical corrosion, the use of Cathodic protection or sacrificial anode of the electrical method of protective grounding.
2. According to the form of grounding to classify
Grounding electrode (body) can be divided into two types of grounding electrode and loop-type grounding electrode according to its arrangement. If the shape of the grounding pole to points, there are several basic forms of tubular, band and ring. If the structure of the grounding pole to points, there are natural grounding pole and artificial grounding pole points. Available as a natural grounding pole: water and sewage metal piping; with the earth has a reliable connection between the buildings and structures of the metal structure; laid in the ground and the number of not less than two cable metal skin and laid in the ground in a variety of metal pipelines, but combustible liquids and combustible or explosive gas pipeline except. Available as artificial grounding pole, generally steel pipe, angle steel, flat steel and round steel and other steel. If in the chemical corrosive soil, can be used to galvanize the above kinds of steel or copper grounding electrode. The schematic of the grounding device is shown in Figure 4-1.
Figure 4-1 grounding device schematic
The arrangement of the grounding device if the use of a single grounding electrode or external lead-type grounding electrode, due to the potential distribution is not uniform, the human body is still at risk of electric shock. In addition, the reliability of the single grounding electrode or external leaded grounding electrode is relatively poor. As can be seen from Figure 4-1, the external leaded grounding electrode and indoor grounding trunk connected to only two trunks. If these two trunks are faulty, the entire indoor grounding trunk and outdoor grounding network disconnect, this time the indoor equipment is equivalent to no grounding. Of course, the two trunks at the same time the possibility of failure and disconnection is unlikely.
In order to eliminate the shortcomings of a single grounding electrode or external lead to the grounding electrode, you can use the loop type grounding electrode, as shown in Figure 4-2. Loop type grounding electrode potential distribution is more uniform, human contact voltage UC and step voltage Ub is relatively small. However, the potential distribution outside the grounding electrode is still not uniform, and its step voltage is still very high. In order to avoid this shortcoming, can be buried in the loop type grounding electrode outside some with the grounding electrode does not have electrical connection of flat steel. In this way, the potential distribution outside the grounding electrode, as shown in Figure 4-3, falls flat. Therefore, in general, the use of loop-type grounding electrode should be preferred. Only when it is difficult or expensive to use the loop type grounding electrode, only then consider the use of external leaded grounding electrode.
Figure 4-2 loop type grounding pole arrangement
Figure 4-3 loop type grounding pole buried near the flat steel potential changes
Three, the scope of the electrical grounding
1. AC system
(1) less than 50V AC line less than 50V AC line is usually not grounded, but one of the following conditions must be grounded.
1) When the power is supplied by a transformer and the neutral point of the transformer is ungrounded.
②Powered by the transformer, and the transformer ground voltage exceeds 150V.
③It should not be grounded when an isolation transformer is taken, but the core must be grounded.
④ Overhead lines installed outside the building.
(2) 50 ~ lkV AC system 50 ~ lkV AC system may not be grounded if the following conditions are met.
①Industrial speed control drive system for the power supply rectifier separate drive system.
② for melting, refining, heating or similar to the industrial furnace power supply electrical system.
3 separate drive system powered by the transformer; the primary side of the transformer rated voltage less than 1kV dedicated control system, the control power supply with power supply continuity, the control system is equipped with a ground detector, and to ensure that only a full-time staff can monitor and maintain the situation.
(3) 1 ~ 1OkV AC system l ~ lOkV AC system can be grounded by the arc-canceling coil or resistor as needed, but for mobile equipment use l ~ 1OkV AC system should be grounded.
2. DC system
(1) two-wire DC system DC two-wire distribution system should be grounded.
The following conditions may not be grounded.
①Systems equipped with a ground detector and supplying power to industrial equipment only to a limited extent.
②Systems with line-to-line voltages ≤50V or >300V and insulated to ground.
③ DC systems that are rectified and supplied from a grounded AC system.
④ DC fire signaling lines with maximum current ≤0.03A.
(2)Three-wire DC systemThe neutral wire of the three-wire DC system is suitable for direct grounding operation.
3. Electrical equipment
(1) electrical equipment exposed conductive part of the grounding of electrical equipment is the following conditions of the exposed conductive part should be grounded.
①Motor, transformer, general electrical appliances, portable and mobile electrical appliances and other metal base and shell.
② generator neutral cabinet shell and its outlet cabinet shell.
③ Voltage transformer and current transformer secondary winding.
④Metallic conductive part of the transmission device of the electrical equipment.
5 power distribution, control, protection of the screen (cabinet, box) and the metal frame and base of the operating table, etc., the metal shell of the fully enclosed combination of electrical parts.
6 metal architecture of indoor and outdoor power distribution units and metal architecture of reinforced concrete foundations, as well as metal shelters and metal gates near electrically charged parts of substations, power plants and other places.
⑦Exposed conductive parts of lighting fixtures.
8 AC and DC power cable junction box, terminal box and the expansion of the metal casing and cable metal protective layer, accessible threading steel pipe, laying cable metal trunking, cable bridges.
9 installed in the power line tower switchgear, capacitors and other electrical devices on the exposed conductive part and its metal support part.
⑩ in non-asphalt ground in residential areas, ungrounded system or by the arcing coil grounding or by the resistor grounding system in the yuan lightning line overhead power lines of metal towers and reinforced concrete towers, equipped with lightning lines of overhead lines of the tower.
○11 capped control cable metal protective layer, non-capped or non-metallic sheathed cable within the unused 1 to 2 core wires.
○12 Metal enclosure for closed busbar.
○13 metal box enclosure for box-type substation.
(2) Electrical equipment exposed conductive part of the ungrounded electrical equipment belonging to the following cases of the exposed conductive part can not be grounded.
①In non-conductive places (such as wood, asphalt and other poor conductive ground and insulated walls) used in the electrical equipment.
②Installed in the distribution panels, control panels and electrical installations on the electrical measuring instruments, relays, and other low-voltage appliances and other metal enclosures.
③ Metal bases of insulators that do not cause dangerous voltages on the support when insulation damage occurs.
④Exposed conductive parts of electrical equipment or electrical devices with AC rated voltage <50V and DC rated voltage <120V in dry places (except places with explosion hazard).
⑤Equipment installed on a metal structure that has been grounded and has good electrical contact with the structure, such as casing bases, etc. (except for places with explosion hazards).
⑥Exposed conductive parts of electric motors and appliances that have reliable electrical contact with the chassis that has been grounded (except for places where there is a danger of explosion)
⑦Metallic supports in the room of storage batteries with a rated voltage of ≤220V.
(3) external conductive part of the external conductive part of the application may produce a shock hazard should be grounded. Usually need to be grounded part of the following cases.
①In order to improve the safety of large areas of potentially electrically charged metal structures in or on the building, when they may come into contact with a person, then they should be grounded.
②Metal frames of elevators equipped with cables.
③Rails and trusses of cranes.
④Metal hoisting ropes or cables of the elevator car may not be grounded if they have been connected to the body of the elevator to form an electrical path.
⑤ Exposed metal parts in the mobile home or coach, including the metal structure of the mobile home and the metal frame of the coach, shall be grounded.
6 metal enclosures such as metal spacers and metal shelters around electrical equipment with line-to-line voltage exceeding 750V outside the substation or transformer room.
4. Mobile and vehicle-mounted generators
(1) Mobile generator When this mobile generator supplies power only to the equipment mounted on top of the generator or the socket on the generator supplies power only to the equipment to which the flexible cord and plug are connected, and the exposed conductive portion of the equipment and the grounding electrode on the socket have been connected to the generator frame, the mobile generator's frame may be left ungrounded, and the frame may be used as the grounding for the generator power supply system.
(2) A vehicle-mounted generator may use the frame of the vehicle used for the generator power supply system mounted on the vehicle as the grounding electrode for the system when all of the following conditions are met.
(1) The frame grounding portion of the generator is connected to the frame of the vehicle.
②The generator supplies power only to equipment mounted on the vehicle or to flexible cord and plug connected equipment only through receptacles mounted on the vehicle or on the generator.
(iii) The exposed conductive parts of the equipment and the earth electrode on the socket are connected to the generator frame.
(3) Connection of Neutral Wire If the generator is a stand-alone system, the neutral wire should be connected to the generator frame.