For low-voltage power supply system, it is better to adopt hierarchical protection.

[the first line of Defense] shall be the high-capacity power supply anti surge protector connected between each phase of the incoming line at the entrance of the user's power supply system and the earth. Generally, this level of power protector is required to have a large impact capacity of more than 100kA / phase, and the required limiting voltage shall be less than 2800v. We call it class I power surge protector (SPD). These power surge protective devices are designed to withstand the large current and high-energy surge energy absorption of lightning and induced lightning, and can divert a large amount of surge current to the earth. They only provide the limiting voltage (when the impulse current flows through the SPD, the large voltage on the line becomes the limiting voltage) as the medium level protection, because the class I protector mainly absorbs the large surge current. They alone cannot completely protect the sensitive electrical equipment inside the power supply system.

[the second line of Defense] should be the power surge protector installed at the branch distribution equipment supplying power to important or sensitive electrical equipment. These SPDs can better absorb the residual surge energy passing through the surge arrester at the user's power supply inlet, and have a good inhibitory effect on transient overvoltage. The power supply surge protector used here requires a large impact capacity of more than 40ka / phase, and the required limiting voltage shall be less than 2000V. We call it class II power surge protector. The general user power supply system can meet the requirements of the operation of electrical equipment when it achieves the second level protection.

[the third line of Defense] a built-in power supply surge protector can be used in the internal power supply of electrical equipment to completely eliminate the transient overvoltage of small transient. The power supply surge protector used here requires a large impact capacity of 20KA / phase or less, and the required limiting voltage shall be less than 1800V. For some particularly important or sensitive electronic equipment, it is necessary to have the third level of protection. At the same time, it can also protect the electrical equipment from the transient overvoltage generated inside the system.