If the electrical equipment has grounding protection, when a single-phase grounding short circuit or leakage fault occurs, a large short circuit current or leakage current will be generated in the line, which will trip the superior protection device (circuit breaker or leakage circuit breaker) and automatically cut off the power supply of the faulty line.
The zero connection of protection is to make the equipment leak to form a single-phase short circuit with the help of zero line, so as to prompt the protection device on the line to act and cut off the power supply of the faulty equipment.
2, the scope of application is different.
Protective grounding is suitable for general low-voltage ungrounded power grids and low-voltage grounded power grids with other safety measures; Protective grounding can also be used in high voltage ungrounded power grid.
Protective zero connection is only applicable to low-voltage power grid with neutral point directly grounded.
3. The line structure is different.
In addition to the phase line, the protective grounding system only has protective ground wire.
In addition to the phase line, the zero protection system must have a zero line; When necessary, the protective zero line should be separated from the working zero line; Its important devices should also be grounded.
Advantages of protective zero connection: In the grounded power grid, it is much better to adopt protective zero connection than protective grounding in order to prevent the electrical equipment shell from being charged and hurting people.
Disadvantages of zero connection of protection: only the serious fault of connection between electrical enclosure and power supply live wire can be ruled out, but the leakage fault of electrical enclosure cannot be ruled out.
The advantages of protective grounding are: first, it reduces the grounding voltage of leakage equipment; Second, it reduces the risk of disconnection of the neutral trunk line; Third, when lines and equipment are short-circuited to the ground, the grounding resistance decreases, the short-circuit current increases, and the protection device moves faster; Fourthly, improving the lightning protection performance of overhead lines is conducive to limiting lightning overvoltage.
Disadvantages of protective grounding: it is difficult for the line to meet the required technical standards, and there are unsafe factors, but hidden dangers of accidents are buried.
Usage is divided as follows:
When the power system is directly grounded at one point, the exposed conductive part of the power receiving equipment is connected with the grounding point through the protection line (PE line);
When the construction site uses electricity and the external * * * uses low-voltage power grid, that is, the grounding electrode of the power system is not at the construction site, it is difficult to adopt protective grounding system, and only protective grounding system can be adopted.
When the construction site is powered by the low-voltage side of the power department and connected with the power line operators, the protective grounding system should be adopted according to the regulations of the local power supply department.
Extended data:
In daily life, in order to prevent the electrical shell from being charged, grounding measures are adopted for protection. When the grounding resistance is less than 4 ohms, the grounding circuit is protected if the voltage of the electrical enclosure is 220 volts at this time. According to the formula I=U/(RO+RG), the short-circuit current is 27.5A, where RO represents the grounding resistance of the neutral point of the transformer, which is generally called working grounding resistance.
In order to ensure the timely and effective protection of protective equipment, it is generally necessary to adjust the grounding short-circuit current, which is generally treated as 1.25 times of the automatic switch setting current or as 3 times of the blown fuse current. Through calculation, when the setting current is less than 27.5/ 1.25, the short-circuit current can be disconnected. ?
For the protection equipment, if the rated current value is greater than the above value, the protection equipment can not carry out the protection action in time and effectively. At this time, there will be a voltage to the ground on the shell of electrical equipment, and this voltage will last for a long time, and the electrical operators will be threatened by this voltage.
References:
Baidu Encyclopedia-Protect Zero Connection
Baidu encyclopedia-protective grounding