The low-voltage distribution system consists of a distribution substation (usually reducing the transmission voltage of the grid to a distribution voltage), a high-voltage distribution line (that is, a voltage above 1 kV), a distribution transformer, and a low-voltage distribution line (1 kV The following voltage) and corresponding control and protection equipment.
1. Composition of low-voltage distribution system
1.1 Low-voltage circuit breaker: Low-voltage circuit breaker, also known as automatic switch, is an electrical appliance that not only has the function of manual switch, but also can automatically protect against voltage loss, undervoltage, overload, and short circuit. It can be used to distribute electric energy, start asynchronous motors infrequently, protect power lines and motors, etc., and can automatically cut off the circuit when they are seriously overloaded or short-circuited or under-voltage. Its function is equivalent to that of a fuse switch and The combination of overheating and underheating relays, etc., and generally does not need to change parts after breaking the fault current, has been widely used.
1.1.1 Circuit breaker accessories
1.1.2 Miniature circuit breaker: Miniature circuit breaker, referred to as MCB, is the most widely used terminal protection device in building electrical terminal power distribution devices
1.1.3 Molded case circuit breaker: The molded case circuit breaker can automatically cut off the current after the current exceeds the trip setting. Molded case refers to the use of plastic insulators as the casing of the device to isolate conductors and grounded metal parts. Molded case circuit breakers typically contain thermal magnetic trip units, while larger models have solid state trip sensors.
1.1.4 Frame type circuit breaker
1.1.5 Intelligent universal circuit breaker
1.2 Intelligent power distribution:
1.2.1 Low-voltage reactive power compensation complete set
1.2.2 Composite switch
1.2.3 Operating handle
1.2.4 Reactive Power Compensation Controller
1.3 Low-voltage distribution switch:
1.3.1 Load switch: Load switch, as the name implies, is a switch that can cut off the load current. It should be distinguished from high-voltage circuit breakers. The load switch has no arc extinguishing ability, cannot break the fault current, and can only break the load current under the normal operation of the system. , The load switch gets its name from this.
1.3.2 Isolating switch: The isolating switch is the most widely used electrical appliance in high-voltage switching appliances. Its working principle and structure are relatively simple, but due to the large amount of use and high requirements for working reliability, the design of substations and power plants , establishment and safe operation are all greatly affected. The main feature of the knife switch is that it has no arc extinguishing ability, and it can only open and close the circuit under the condition of no load current.
2. Classification of low-voltage distribution systems
2.1 I means that all live parts are insulated. T means that the neutral point is directly grounded.
2.2 T means that the equipment shell is directly grounded, and it has no direct relationship with any other grounding point in the system; N means that the load adopts zero connection protection.
2.3 C indicates that the working zero line and the protection line are integrated; S indicates that the working zero line and the protection line are strictly separated.
3. Precautions for low-voltage distribution system
3.1 The fire hazard of leakage and the factors causing leakage
After the insulation of electrical lines or equipment is damaged, under certain circumstances, electric leakage will occur to nearby substances (threaded metal pipes, metal casings of electrical devices, wet wood, etc.), which will make local substances electrified, causing serious or even fatal electric shocks to people , Sparks, arcs, overheating and high temperature, etc. may cause fires.
When electrical equipment leaks electricity and short-circuits the case, the current will form a closed loop with the equipment casing, the protective neutral wire (protective ground wire), and the neutral wire (earth). Usually, the leakage current will be very large, which will cause the fuse to operate and cut off the power supply. However, due to many reasons (such as the size of the fuse may be artificially increased several times or replaced by copper wire, the grounding device does not meet the requirements, resulting in a large grounding resistance, the grounding terminal of the grounding wire is not firmly connected, the protection device fails or the setting is unreasonable. etc.) will cause the overcurrent protection device to fail to protect the overcurrent, so that once the leakage occurs, it will continue to exist, resulting in electric shock or electrical fire accidents. Many leakage fire cases have also proved this point.
There are many factors that cause leakage. In summary, there are mainly the following:
3.1 The installation of low-voltage distribution systems is mostly done by non-electrical professionals, with uneven quality and difficult to guarantee quality. It is manifested in: in a humid or acid-base corrosive environment, the wires are laid out, and the equipment is installed directly without protection: when wiring, Knives, pliers, hammers, etc. damage the insulation layer; the connection quality of wire joints and the quality of insulation wrapping and rolling do not meet the requirements and other non-standard phenomena:
3.1.1 Negligent inspection of electrical lines or equipment, insulation degradation due to overload or long service life;
3.1.2 Choose counterfeit or shoddy electrical products;
3.1.3 External factors: water immersion, extrusion, rat bites, etc.
3.2 Causes of fire caused by electric leakage
3.2.1 Leakage current causes fire. Leakage fault points are usually inaccurately contacted, seemingly connected but not connected, resulting in a large contact resistance, making it difficult for the overcurrent protection device to operate, and at the same time, an arc will be generated at the fault point. According to measurements, the arc temperature of only 0.5A current can exceed 2000°C, which is enough to ignite all combustibles.
3.2.2 Improper connections at the terminals of the protective neutral wire or the protective earth wire may cause a fire. If the connection terminals of the phase line and the neutral line are not connected properly, the equipment is not working normally, and it can be found and dealt with in time. However, the connection terminals of the protective zero line or ground line are not connected properly, the resistance is too large, the equipment works as usual, and the fault point is not easy to be found. Once leakage occurs, high resistance will appear due to loose or corroded joints at the fault point, resulting in local overheating, high temperature or arc at the connecting terminal, which can ignite surrounding combustible substances, or burn out electrical sockets, switches, etc., and ignite the wooden base. This is the more common form of fire leakage. In July 2000, there was a switchboard fire in a hotel in Hangzhou. Before the fire, the hotel was closed and the load was at a low point. When the fire broke out, the power consumption for lighting was still normal. After inspection, the main air switch in the distribution box was seriously carbonized, and the protective zero line terminal had metal concave melting marks, which was consistent with the leakage situation.
3.2.3 Fire caused by leakage voltage. After the leakage continues to occur, because the current cannot flow, and another circuit with a small resistance is connected to the ground, it will be conducted along the protective neutral line (ground line) so that the metal shells of all connected electrical devices have a voltage to the ground. At this time, it is possible to arc to the nearby low-potential water heating pipes, gas pipes and other metal components and become a fire source. Only a maintenance voltage of 20V can make the arc occur continuously, and it can also ignite the surrounding combustibles. If the arc flashes to the gas pipe, it may break through the pipe wall, causing gas leakage and causing a fire. It should be noted that due to the conduction of voltage, the leakage point is not necessarily consistent with the fire point.
3.2.4 If the wire diameter of the protective neutral wire or the protective ground wire is too small, when a large leakage current passes through, the temperature of the line will rise rapidly, which may also cause a fire.
3.3 Precautions against electric leakage fire
3.3.1 Strengthen the management of electrical practitioners
To establish and improve electrical operating procedures, all electrical practitioners must learn to master these operating procedures, and non-electrical professionals are not allowed to work. It is necessary to strengthen the training of electrical practitioners, hold regular training courses, improve the technical and safety awareness of electrical practitioners, and eliminate human factors that cause fires.
3.3.2 Take necessary technical protection measures
188.8.131.52 Install leakage protector. Measures such as protective zero connection and overcurrent protection devices installed in the current low-voltage distribution system cannot completely and effectively prevent the occurrence of leakage fires. Therefore, special leakage protectors for fire prevention should be installed at the main current incoming lines of buildings. In order to prevent large-scale power outages, leakage protectors should be installed in the main power distribution box and the user switch box respectively, and their rated operating current and rated operating time should be reasonably coordinated so that they have the function of hierarchical protection.
184.108.40.206 Reasonable selection of protective zero connection (protective grounding wire) and design of grounding resistance. The selection of the cross-sectional area of the protective zero connection and protective grounding wire must be determined through calculation and checked with the short-circuit current of the case. Its terminals must be reliably connected, no looseness is allowed, and its connection quality should be checked frequently. The protective grounding resistance value of electrical equipment should not exceed 4Ω. If the capacity of the electrical equipment is large and the melting current of the melt is also large, the cross section of the grounding wire should be increased or the grounding body should be connected in parallel to fully reduce the grounding resistance value and increase the leakage current. Short-circuit current, which is conducive to the action of the protection device.
220.127.116.11 Implement equipotential bonding. Leakage protectors only provide indirect contact protection for single-phase 220V lines. At the same time, there are various hidden dangers of operation failure due to factors such as wear and tear of parts, poor contact, unstable quality and short life span. It cannot be a reliable protection measure alone. Therefore, equipotential bonding should be implemented to effectively eliminate the generation of arcs and sparks between leakage electrical lines or equipment and low-potential metal components, that is, to eliminate the possibility of fire caused by leakage voltage. Equipotential bonding refers to the measure of connecting the protective zero bus with the building's main water pipe, main gas pipe, HVAC pipe and other metal pipes or devices with wires to achieve the purpose of equalizing the potential in the building, especially for flammable Explosive places have an irreplaceable role.
3.3.3 Strengthen the review of electrical design. For the low voltage distribution system in the building, all relevant departments should strictly check the electrical design according to the relevant technology. For the interior decoration project of the building, the "Code for Fire Protection Design of the Interior Decoration of Buildings" should be strictly implemented, and flammable and combustible Materials, especially when there are electrical lines passing combustibles, they should be protected by metal pipes or flame-retardant hard plastic pipes. Due to the good insulation performance of plastics, it can better prevent leakage. When using metal pipes for wiring, it is necessary to prevent the insulating layer be damaged. Power distribution devices (switches, sockets, distribution boxes, etc.) and electrical equipment should keep a sufficient safe distance from combustibles. If they are really inseparable, thermal insulation protection measures should be taken. After the completion of the construction project, its electrical installation should be put into use after passing the inspection of a special inspection agency, and try to prevent the occurrence of leakage fires from the source.
4. National standards for low-voltage distribution systems
GB/T 18216.1-2000 Testing, measuring or monitoring equipment for electrical safety protection detection of low-voltage distribution systems below AC 1 000 V and DC 1 500 V Part 1: General requirements Implemented on 2001-05-01
GB/T 18216.2-2002 Testing, measuring or monitoring equipment for electrical safety protection detection of low-voltage distribution systems up to AC 1000V and DC 1500V-Part 2: Insulation resistance 2003-05-01 Implementation
GB/T 18216.3-2007 Test, measurement or monitoring equipment for electrical safety protection measures for low-voltage distribution systems below AC 1000V and DC 1500V Part 3: Loop impedance 2007-12-01 Implementation
GB/T 18216.4-2007 Testing, measuring or monitoring equipment for electrical safety protection measures for low-voltage distribution systems below AC 1000V and DC 1500V-Part 4: Grounding resistance and equipotential grounding resistance 2007-12-01 Implementation
GB/T 18216.5-2007 Test, measurement or monitoring equipment for electrical safety protection measures for low-voltage distribution systems up to AC 1000V and DC 1500V Part 5: Resistance to ground Implemented on 1 December 2007
GB/T 18216.12-2010 Test, measurement or monitoring equipment for electrical safety protection measures for low-voltage distribution systems below AC 1000V and DC 1500V Part 12: Performance measurement and monitoring device (PMD) 2011-05-01 Implementation
GB/T 18268.22-2010 Electromagnetic Compatibility Requirements for Electrical Equipment for Measurement, Control and Laboratory Use Part 22: Special Requirements Test Configuration, Working Conditions and Performance Criteria for Portable Test, Measurement and Monitoring Equipment Used in Low-Voltage Distribution Systems 2011- 05-01 Implementation
GB 18802.1-2011 Low-voltage surge protectors (SPD) Part 1: Performance requirements and test methods for surge protectors for low-voltage distribution systems
GB/T 18802.12-2006 Surge Protective Devices (SPD) for Low-Voltage Distribution Systems Part 12: Guidelines for Selection and Use 2006-06-01 Implementation
GB/T 18216.1-2012 Test, measurement or monitoring equipment for electrical safety protection measures for low-voltage distribution systems below AC 1000V and DC 1500V Part 1: General requirements Announcement No. 28 of 2012
GB/T 18216.2-2012 Test, measurement or monitoring equipment for electrical safety protection measures for low-voltage distribution systems below AC 1000V and DC 1500V Part 2: Insulation resistance Announcement No. 28 of 2012
GB/T 18216.4-2012 Test, measurement or monitoring equipment for electrical safety protection measures for low-voltage distribution systems below AC 1000V and DC 1500V Part 4: Grounding resistance and equipotential grounding resistance Announcement No. 28 of 2012
GB/T 18216.5-2012 Test, measurement or monitoring equipment for electrical safety protection measures for low-voltage distribution systems below AC 1000V and DC 1500V Part 5: Ground impedance Announcement No. 28 of 2012
GB/T 18216.3-2012 Test, measurement or monitoring equipment for electrical safety protection measures for low-voltage distribution systems below AC 1000V and DC 1500V Part 3: Loop impedance Announcement No. 28 of 2012