Selection Principle of Surge Protector in Distribution Box

(1) The voltage protection level Up of SPD should always be less than the impulse withstand voltage Uc of the protected equipment, and greater than the maximum operating voltage Us of the power grid obtained according to the grounding type, that is, Us<Up<Uc. If the line is not shielded, it should be taken into account the line induced voltage, Uc should be considered as 80% of its value.

(2) The leads at both ends of the surge protector SPD and the protected equipment should be as short as possible and controlled within 0.5m.

(3) If the Up of the incoming SPD plus the induced voltage of the leads at both ends and the reflected wave effect are too high compared with the impulse withstand voltage of the protected equipment farther away from it, it needs to be added at this equipment. When the second-level SPD is installed, its nominal discharge current In should not be less than 8/20μs 3kA; when the incoming SPD is not more than 10m away from the protected equipment, if the SPD's Up plus the induced voltage of the leads at both ends is less than the equipment's 80% of Uc, in general, SPD may not be installed at the device.

(4) When there is a switchboard between the SPDs installed according to the requirements of point 3 above, if the Up of the first-level SPD plus the induced voltage of the leads at both ends cannot protect the equipment in the switchboard, it should be installed in the switchboard. The second-level SPD, its nominal discharge current In should not be less than 8/20μs 5kA.

(5) When multiple SPDs are installed on the line, the line length between the voltage switching SPD and the voltage limiting SPD should not be less than 10m, and the line length between the voltage limiting SPD should not be less than 5m. For example, the protected equipment is relatively close to the power distribution center, and more wires can be deliberately wound in the line laying.

(6) When the distance between the surge protector SPD at the incoming end and the protected equipment is greater than 30m, another SPD should be installed as close as possible to the protected equipment, and the current capacity can be 8kA.

(7) When selecting SPD, care should be taken to ensure that the SPD will not be burned due to power frequency overvoltage, because SPD is anti-transient overvoltage, power frequency overvoltage is transient overvoltage (ms level), and the energy of power frequency overvoltage is Hundreds of times the transient overvoltage energy, therefore, care should be taken to select an SPD with a higher power frequency operating voltage.

(8) Protection of SPD: each level of SPD should be protected, or fuses can be used for protection, and the breaking capacity of the surge protector is greater than the maximum short-circuit current at the place.

(9) In addition, when selecting SPD, attention should also be paid to: the response time is as fast as possible; the length of service life, the price factor, the maintainability, the size of the flow capacity, and the humidity resistance.