Why electrical equipment must have grounding protection or zero protection measures

I, the concept of grounding protection grounding protection is often referred to as protective grounding, is the metal casing of electrical equipment and grounding body connection to prevent damage to the insulation of the electrical equipment to make the shell charged, the operator contact equipment shell and electrocution. In the neutral point is not grounded in the low-voltage system, under normal circumstances, a variety of power devices are not electrically exposed metal parts, in addition to the provisions should be grounded. Second, the concept of zero protection in order to prevent electrical equipment due to insulation damage and personal risk of electric shock, the metal casing of electrical equipment connected to the neutral point of the power transformer is called zero protection. In the neutral point is not directly grounded in the low-voltage power network, the power device should be used low-voltage zero protection. In the neutral point is not directly grounded in the low-voltage power network, the power device should be used low-voltage grounding protection. By the same generator, the same transformer or the same section of the bus power supply in the low-voltage power network, it is not appropriate to use both grounding protection and zero protection. Third, grounding and zeroing compared to the protection of grounding and protection of zero is to maintain personal safety of the two technical measures, the difference is: First, the principle of protection is different. The basic principle of low-voltage system protection is to limit the leakage equipment to ground voltage, so that it does not exceed a certain safety range; high-voltage system protection, in addition to restricting the ground voltage, in some cases, there are also contributing to the role of the system in the action of the protective device. Protection of the main role of the zero line is to zero the line so that the equipment subcardiac formation of single-phase short circuit, prompting the line of protective devices to act quickly. Second, the scope of application is different. Protective grounding applies to the general low-voltage ungrounded grid and other security measures taken by the low-voltage grounding grid; protective grounding can also be used for high-voltage ungrounded grid. Ungrounded power grid does not have to use the protection of zero. Third, the line structure is different. Protective grounding system in addition to the phase line, only the protective ground. Protective zero system in addition to phase line, there must be zero line; if necessary, the protective zero line should be separated from the working zero line; its important devices should also have a ground. It is generally best not to consider at the same time, is the need for two grounding has to meet the requirements of the grounding resistance, otherwise the consequences are endless.