We know that protective grounding and protective zero connection are both for the protection of personal safety and equipment safety, and they are set for the same purpose. In terms of these two security protections, the protection of human life is the most important. Since they are all for the protection of people’s safety, why should they be divided into protective grounding and protective zeroing?
What is protective grounding
What we call protective grounding is to electrically connect the uncharged metal shell part of the electrical equipment to the ground . The purpose of this is to prevent electric shock accidents. For example, when we touch the shell of a charged device, the resistance of the protective grounding is very small, generally less than 4 ohms, and the resistance of our human body is generally around 1,000 ohms. Such a large human body resistance and a If a relatively small grounding resistance is connected in parallel, most of the current will pass through the grounding resistance, and the current flowing through the human body will be very small, generally less than the safe current, thus achieving the purpose of protecting personal safety. It can be seen that this grounding protection is based on the knowledge of parallel circuits . After a large resistor and a small resistor are connected in parallel , there will be a strong shunt effect. Safety. Protective grounding is mainly used in 220V/380V circuits where the neutral point of the power supply is not grounded. This grounding resistance is generally required by the government, generally below 4 ohms. If the capacity of the transformer we use is below 100KVA, then the grounding resistance can be below 10 ohms.
What is protection zero connection
As the name suggests, the protective zero connection is also a method to connect the metal shell of the electrical equipment that is usually uncharged to the neutral wire N of the power supply. It is generally used in 380V/220V three-phase four-wire circuits where the neutral point of the power supply is directly grounded. The working process of protective zero connection is different from that of protective grounding. After we use protective zero connection in a circuit where the neutral point is directly grounded, when the electrical equipment is operating normally, the shell of these electrical equipment has no voltage to the ground. Yes, there is no risk of electric shock if we touch it with our hands. When the shell of the electrical equipment leaks electricity, the live wire and the neutral wire of the metal shell of the electrical equipment are directly connected, forming a single-phase short circuit fault. This current is very large. As I just said, grounding The resistance of the circuit is very small. In this case, the short-circuit current in this circuit will cause the air switch , leakage protector and other protective devices in the circuit to trip, thus cutting off the circuit and avoiding the danger of electric shock. It can be seen that the protective zero connection and protective grounding are different in the process of protection mechanism.
Requirements for protective zero connection to the line
Since most of our daily life and industrial production use the three-phase four-wire power distribution method , there must be certain requirements in the protection and zero connection, so as to ensure the safety of electricity use. First of all, we need to know that in the same line, protective grounding and protective zero connection cannot be used at the same time. If these two protection methods are mixed, it may cause the air switch not to trip when a certain electrical equipment leaks, and the circuit cannot be broken. There is a risk of electric shock to persons who touch electrical equipment. Secondly, in the protection connection to the zero line, the grounding device must be reliable and firm. In addition to the grounding resistance meeting the requirements, then the zero line must have repeated grounding measures. Thirdly, the working neutral line in the line is electrically connected with the grounding line. It is not possible to install some easily disconnected nodes such as switches and fuses on the neutral line, and there must be certain requirements for the conductor area of the neutral line. In low-voltage lines, the thickness of the neutral wire should be the same as that of the live wire. Finally, we must pay attention to the protection of the zero-connected electrical equipment must have a reliable short-circuit protection device, such as air switch, leakage protector, etc., and each electrical equipment we use must have a separate protection line and zero trunk line To be reliably grounded.
