Ground faults are different from normal electrical short faults. It is a short-circuit fault of the charged conductor through the metal material and the earth. Because the ground fault is relatively concealed, it is often the cause of multiple fires, and often accompanied by ground faults, electric shock personal injury accidents occur. Therefore, in order to ensure the safety of residential residents, it is necessary to strengthen the prevention of ground faults. 400W Carbon Fiber Heater,Carbon Fibre Patio Heater,Carbon Fibre Room Heater,Carbon Fibre Radiant Heater Foshan Shunde Josintech Electrical Appliance Technology Co.,Ltd , https://www.josintech.com
1 You cannot change the ground system by yourself. If the TN-C-S system (the transformer above 100KVA, the neutral point grounding resistance is 4 ohms, and the PE line grounding resistance is 10 ohms) is changed to the TT system. When a single-phase grounding short circuit occurs in the metal casing of the electrical equipment, since the PE wire is not grounded according to the grounding resistance of the TT system, the metal casing of the equipment must be brought with a higher voltage (the theoretical calculation reaches 157V), thereby causing an indirect electric shock accident.
2 Do not connect PE wire to N wire. If the PE line is connected to the N line, it becomes a TN-C system, and its adverse consequences have been discussed previously.
3 Each residential main power incoming circuit breaker shall have leakage protection. In addition to the air conditioning power supply, other power outlets should be provided with leakage protection devices. It protects against electrical fire and electric shock by two-pole protection. The household appliances in the house are usually powered by the socket, and a quick leakage protector with a rated current of not more than 30MA should be installed on the power socket to prevent personal injury caused by electric shock. When an arc-shaped ground fault occurs, the arc has a large impedance, which limits the ground fault current, so that the circuit breaker and the fuse cannot cut off the power in time, causing a fire. Therefore, it is necessary to install a leakage protector with a rated operating current of 300MA at the power inlet of the house. The well has a delay of 0.15S to avoid the occurrence of electrical fires (the arc energy below 500MA is not enough to ignite the fire).
4 for equipotential and local equipotential bonding. Equipotential bonding is an important safety measure to prevent the risk of electric shock. It can greatly reduce the contact voltage that people experience when grounded. It can be seen from the analysis results that it is much better than reducing the contact voltage amplitude by repeated grounding. In this way, even in the case of failure of the ground fault protection, it is possible to eliminate the electric shock and casualty accident within a large range.
(1) Total equipotential bonding (NEB). The function is to reduce the contact voltage of indirect contact with electric shock in the building and the potential difference between different metal parts, and to eliminate the hazard of dangerous voltage failure from the outside of the building through the electrical circuit and various metal pipes. Through the total equipotential bonding terminal board (grounding busbar) near the wiring box, the conductive parts of the queue are connected to each other: the PE busbar of the incoming wiring box; the metal pipes of the utility (such as L, sewage, heat, gas) Pipes); metal structures of buildings; grounding leads of artificial grounding. Each power supply incoming line of the building shall be connected to the total potential, and the terminals of each total equipotential bonding plate shall be connected to each other.
After the total equipotential bonding, the grounding electrode of the repeated grounding in the TN system is already outside the total equipotential bonding zone, and the specified grounding resistance value is meaningless. Because the residential users are all in the total equipotential bonding zone, the contact shock voltage caused by the ground fault is determined by the voltage drop of the PE line, and has nothing to do with the presence or absence of repeated grounding. The steel bars of the building and the metal pipes of various utilities are themselves very good grounding bodies (grounding resistance is less than 1 ohm) and have played a better role than repeated grounding. Through analysis, after the equipotential bonding, the voltage drop on the PE line is only 1/3~l/4 of the repeated grounding.
(2) Local equipotential bonding. The bathroom is the place where the electric shock is the biggest because of its special humidity. The purpose of the local equipotential bonding of the bathroom is to make the connection of different conductive parts of the dangerous voltage difference at the same time in the local range, so that the voltage drop caused by the fault current is smaller and the contact voltage is low. The pressure limit value of the contact. The local equipotential bonding of the bathroom connects the following conductive parts to each other: PE busbars or PE trunks; metal pipes of public facilities; steel mesh and metal structures of buildings.
The occurrence of an electric shock personal injury event is due to the dangerous indirect contact voltage. The purpose of building grounding is to reduce the contact voltage. In modern residential building construction, equipotential bonding is required according to the specifications, so that a relatively low contact resistance is obtained, which greatly reduces the contact voltage. After the equipotential bonding, the dangerous fault voltage entering the PE line outside the building is eliminated, the electromagnetic field interference is eliminated, the harm caused by the unreliable protection of the protector and the lightning protection are reduced. In addition, under normal circumstances, the bathroom is the most dangerous place for electric shock. Because of the local equipotential, any conductive part that the body may come into contact is equipotentially connected, so that the voltage drop caused by the fault current is smaller. Moreover, it is clearly stated in the "Code for Electrical Design of Civil Buildings" that the splash-proof socket should be changed in the bathroom; in the shower area (within the shower area and the deep basin along 0.6 meters), it is strictly forbidden to set the socket; the ventilation fan junction box should be more than 2.25 meters from the ground. To make residential users safer to use. The new "House Design Code" implements the "people-oriented" thinking, and in the general rule, the article "Residential design should be human-centered" is added, and the safety of the house is fully considered.