There are two purposes, one is to be grounded according to the working requirements of the circuit, and the other is to ensure the safety of people and equipment. According to its function, it can be divided into four types. a. working grounding, b. protective grounding, c. protection zeroing, d. repeated grounding.
1. Work grounding
In the low-voltage power system using 380/220V, four wires are generally drawn from the power transformer, namely three phase wires and one neutral wire, which are used for both power and lighting. Three phase wires are used for power, one phase wire and a neutral wire are used for lighting. In such a low-voltage system, considering that when the normal or faulty situation, the electrical equipment can be operated reliably, and is conducive to the safety of people and equipment, and generally the neutral point of the system is directly grounded, that is, the working grounding. The three coils of the transformer are also called the neutral wire, that is, the neutral line, and the point is called the neutral point.
1.1 The role of working grounding
The function of working grounding has two points, one is to reduce the danger of one-phase grounding, stabilize the potential of the system, limit the voltage not to exceed a certain range, and reduce the danger of high voltage into low voltage.
2. Protective grounding
Protective grounding is the grounding carried out to prevent the metal shell of the electrical installation, the frame of the distribution device and the line tower from being electrified and endangering the safety of people and equipment. The so-called protective grounding is a protective wiring method that reliably connects the metal part of the electrical appliance (that is, the metal structure part insulated from the live part) that is not charged under normal circumstances and may be charged after the insulation material is damaged or under other circumstances.
2.1 Then the application range of protective grounding:
Protective grounded ones are suitable for ungrounded power grids. In such power grids, regardless of the environment, protective grounding measures should be taken for all metal parts that may present dangerous voltages due to insulation damage or other reasons, unless otherwise specified, including:
(1) Metal shells, bases and transmission devices connected to motors, transformers, switchgear, lighting appliances and other electrical equipment;
(2) Metal frames or reinforced concrete frames for indoor and outdoor power distribution installations, as well as metal barriers or fences near live parts;
(3) The metal frame or shell of the distribution panel, console, protective screen and distribution cabinet (box);
(4) The metal shell of the cable splice box, the metal sheath of the cable and the steel pipe of the wiring;
In addition, the metal towers and reinforced concrete towers of some overhead power lines, the secondary coils of transformers, etc., should also be grounded
3. Protection zero
Protective zero connection is a way to directly connect the metal part of the equipment that is not charged under normal conditions to the system with a wire. Adopt the protection zero connection method to ensure personal safety and prevent electric shock accidents.
3.1 The working principle of protection zero connection
It is an electrical safety measure that connects the metal shell of electrical equipment with the neutral line of the power grid to protect personal safety. In the zero grid with a voltage of less than 1000 volts, if the electrical equipment is energized due to insulation damage or accidents, a single-phase short circuit of the phase wire to the neutral wire is formed, and the protection device (automatic switch or fuse) on the line acts quickly to cut off the power supply, so that the metal part of the equipment will not have a dangerous voltage for a long time, which ensures personal safety. In the multiphase AC power system, the neutral point of the winding connected by the star is directly grounded so that it is equal to the earth, that is, the zero potential. The wire leading from the neutral point of the ground is called the neutral line. On electrical equipment powered by the same power supply, it is not allowed for some of the equipment to be connected to zero and the other to be grounded (see grounding). Because when the protective grounding equipment shell is live, if its grounding resistance r′D is larger, the fault current ID is not enough to make the protection device act, then due to the existence of the working resistance rD, the voltage U0=IDrD has been present on the neutral line, at this time, there is a dangerous voltage U0 on the shell of the protective grounding equipment for a long time, endangering personal safety.
4. The difference between working grounding and protective zeroing
Working grounding - is the residual electricity generated by electricity in the work, in order to prevent the residual electricity from hurting people, so that it can let the residual electricity into the earth body, the so-called working grounding; Any grounding that is necessary for the operation of the equipment is called working grounding. If not, the device will not work. For example: the neutral ground of a transformer.
Protection zero line - in fact, it is the ground wire, that is, when one of the wires touches the object, so that the leakage protection switch can trip in time without injuring people, the so-called protection zero line.
Both wiring methods play an important role in protecting personal safety.
5. The difference between protective grounding and protective zeroing
5.1 The principle is different
Protective grounding is to limit the voltage to the ground after the leakage of the equipment so that it does not exceed the safe range. In the high-voltage system, in addition to limiting the voltage to the ground, the protective grounding also has the effect of promoting the action of the power grid protection device in some cases; The protection of zero connection is to make the equipment leakage form a single-phase short circuit with the help of the zero connection line, promote the action of the protection device on the line, and cut off the power supply of the faulty equipment. In addition, in the case of a protected zero-connected grid, the protective neutral and repeated grounding also limit the voltage to ground when the equipment leaks.
5.2 The scope of application is different
Protective grounding is suitable for high and low voltage power grids that are generally not grounded, and also suitable for low-voltage power grids that have taken other safety measures (such as installing leakage protectors); The protection zero connection is only suitable for low-voltage power grids where the neutral point is directly grounded.
5.3 The line structure is different
If protective grounding measures are taken, there can be no working zero line in the power grid, and only a protective grounding line can be set; If protective zero measures are taken, a working zero line must be set up, and the working zero line must be used for zero connection protection. The protective neutral wire should not be connected to the switch and fuse, and when the fuse and other breaking appliances are installed on the working neutral line, the protective grounding wire or neutral wire must also be installed.