1. Grounding is required according to the working requirements of the circuit;
Second, in order to protect the safety of people and equipment, grounding or zeroing is required.
According to its role, it can be divided into four types:
a. working grounding; b. protective grounding; c. protective zeroing; d. repeated grounding.
First, work groundIn the low-voltage power system using 380/220V, four wires are generally drawn from the power transformer, that is, three phase wires and one neutral wire, and these four are used for power and lighting. The power uses three phase lines, and the lighting uses a phase line and a neutral line. In such a low-voltage system, it is considered that in the case of normal or faulty, the electrical equipment can be reliably operated, and the safety of the human body and the equipment is favorable. Generally, the neutral point of the system is directly grounded, that is, the working ground. The neutral line is also called the neutral line, which is called the neutral point.
Working grounding
There are two functions of working grounding. One is to reduce the risk of one-phase grounding; to stabilize the potential of the system, to limit the voltage not to exceed a certain range, and to reduce the risk of high voltage entering the low voltage.
Second, the protection groundThe protective grounding is to prevent the grounding of the metal casing of the electrical device, the structure of the power distribution device, and the wiring tower and the like to endanger the safety of the person and equipment. The so-called protective grounding is a kind of electrical metal part that is normally not charged, and the metal part of the electrical appliance that may be charged after the insulation material is damaged or otherwise (ie, the metal structural part insulated from the charged part) is reliably connected with the grounding body by the wire. Protect the wiring method.
Then the application range of protective grounding: protective grounding is suitable for ungrounded power grids. In this kind of power grid, regardless of the environment, any metal parts that may present dangerous voltage due to insulation damage or other reasons shall, unless otherwise stipulated, take protective grounding measures, including:
(1) Metal casings, bases and transmissions connected to motors, transformers, switchgear, lighting fixtures and other electrical equipment; (2) Metal frame or reinforced concrete frame of indoor and outdoor power distribution devices, and metal close to live parts (3) metal frame or casing of power distribution screen, console, protective screen and power distribution cabinet (box); (4) metal casing of cable connector box, metal sheath of cable and wiring steel pipe;
In addition, some metal towers and reinforced concrete towers of overhead power lines, secondary coils of transformers, etc. should also be grounded.
Third, the protection is connected to zeroProtection and zero connection is the way in which the metal part of the equipment is not charged under normal conditions, and the wire is directly connected to the system. Take the protection and zero connection method to ensure personal safety and prevent electric shock accidents.
The principle of protecting the zero working principle is to connect the metal casing of the electrical equipment to the neutral line of the power grid to protect the personal safety. In a zero-connected power grid with a voltage lower than 1000 volts, if the electrical equipment is charged due to insulation damage or accidental conditions, a single-phase short-circuit of the phase-to-neutral line is formed, and the protection device on the line (automatic switch or The fuses act quickly and cut off the power supply, so that the metal part of the device does not have dangerous voltage for a long time, which ensures personal safety. In a multi-phase AC power system, the neutral point of the star-connected winding is directly grounded to the same potential as the earth, that is, zero potential. A wire drawn from a neutral point of the ground is called a neutral line. On the same power supply electrical equipment, some equipment is not allowed to be protected to zero, and the other part is protected ground (see grounding). Because when the protective earthing device is energized, if the grounding resistance r'D is large and the fault current ID is insufficient to operate the protection device, the voltage U0=IDrD always exists on the neutral line due to the existence of the working resistance rD. At this time, there is a dangerous voltage U0 on the outer casing of the protection zero device for a long time, which endangers personal safety.
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