Specifications of connecting screws
The requirements for electrical grounding in GB4706.1-2005 "Safety of Household and Similar Electrical Appliances Part 1: General Requirements" are: In the event of insulation failure, accessible metal parts of the appliance that may be live should be permanently and reliably connected to the equipment Internal ground terminal. or the ground contact of the appliance input socket.
Grounding is only required for equipment with metal enclosures. Equipment with non-metallic enclosures is protected by a plastic enclosure with reinforced insulation, so such equipment does not require protective earthing. When the household appliance is in a normal state, the metal shell is insulated from the internal live parts, and the metal shell is not charged. When the insulation of household appliances is damaged and the internal live parts contact the shell, the metal shell will bring a certain voltage to the ground. When the voltage exceeds the safe voltage, people will get an electric shock when they touch the casing, endangering personal safety. When the metal casing passes through the ground wire and the ground wire in the building and is connected to the ground, the current will not pass through the human body, thus protecting the safety of the user, so "grounding" is a safety technical protection measure. 1 The actual "ground" depends on the grid. There are two ways to actually "ground": protective neutral and protective ground. Protective grounding is to closely connect the metal casing of household appliances with the neutral wire of the power grid; protective grounding is to closely connect the metal casing of household appliances to the ground wire of the power grid. Their roles vary. In my country, household appliances generally use 220V AC power. There are two kinds of power supply grids: transformer low-voltage neutral point grounding and ungrounded 380/220V three-phase four-wire grid. The former is also known as a directly grounded grid, and the latter is also known as a grounded insulated grid. The protective grounding is suitable for the directly grounded power grid, and the protective grounding is suitable for the grounding of the insulated power grid. If the two are misused or mixed in the same power grid, it will not play a role in safety protection, but may cause other dangers. 2 Using zero-protection appliances, repeated grounding is essential. When a fault occurs and the internal dangerous live parts contact the metal casing, the short-circuit current forms a loop through the phase wire and the neutral wire, and the casing-to-ground voltage is equal to the neutral point. The sum of the voltage drop due to the ground voltage and the neutral short-circuit current. Therefore, when the neutral line is open or the impedance is large, the household appliance is prone to electric shock accidents. In order to overcome the above shortcomings, in the protection zero connection system, one or more places of the zero line are often grounded again through the grounding body. This grounding is called repeated grounding.
The effect of repeated grounding is: (1) Reduce the voltage of household appliances to ground. (2) Reduce the risk of zero line disconnection. (3) Reduce the risk of reverse connection between the phase line and the neutral line. (4) Increase the short-circuit current and shorten the fault duration. (5) Reduce the voltage drop loss on the line. (6) Improve the lightning protection performance and other advantages. However, despite being grounded for many times, the situation where the neutral line is disconnected should be avoided as much as possible. 3 "Grounding" status of household appliances At present, most of the indoor power lines of houses are phase lines, commonly known as live lines, and one is neutral lines, commonly known as neutral lines. Power cords for household appliances (such as washing machines, refrigerators, etc.). ) are three cores, which are used as phase line, neutral line and "ground line" respectively. The general wiring is that the phase wire is red or brown plastic shell, and the neutral wire is blue or black shell wire by L, which is marked as N, and the ground wire is a yellow-green two-color wire, which is marked by G, where the "ground wire" is connected to Electrical metal enclosure. The three wires are respectively connected to the power plug according to the regulations, and the corresponding sockets must be single-phase three-hole sockets, which are stipulated as "left zero and right fire on the ground". Due to the difference in the number of cores between the power cord and the plug of household appliances, the "grounding" conditions of household appliances are as follows:
Most users use household appliances only when the left end of the socket is connected to the neutral wire and the right end is connected to the phase wire (there are also a few users who reverse the neutral and phase wires of the socket). The grounding end (upper end) of the socket is useless and has no effect on the normal operation of household appliances, but there are no safety technical protection measures. Electric shock can occur when live parts inside the appliance come into contact with the metal casing. (2) Receptacle ground termination left
This method uses the working neutral as the protective neutral. When the live parts inside the appliance touch the metal casing, the short-circuit current reaches the ground terminal of the socket through the "ground wire", and then forms a loop through the neutral wire. When the neutral line is broken or the neutral line and the phase line are reversely connected, it will be very dangerous and cannot play a protective role. Therefore, this wiring method should not be promoted. (3) Connect the ground wire to the ground terminal of the socket
One is that the ground wire is connected to the water pipe, and the other is that the ground wire is connected to the ground through the grounding body. In the former case, if the grounding resistance meets the requirements, it is the same as the latter case and belongs to protective grounding. 4 Requirements for grounding in the standard GB4706.1-2005 clearly specifies the requirements for grounding resistance: the connection between grounding terminals or grounding contacts and grounding metal parts. should have low resistance values.
