1) Selection of voltage protection level (UP)

The value of UP should not exceed the rated value of the impulse withstand voltage of the protected equipment, and UP requires that the insulation of the SPD and the protected equipment should be well matched.

In the low-voltage power supply and distribution system, the equipment should have a certain ability to withstand surges, that is, the ability to withstand impulse overvoltage. When the impulse overvoltage withstand voltage of various equipment of 220/380V three-phase system cannot be obtained, it can be selected according to the given index of IEC 60664-1 and GB 50057-1994 (2000 edition).

2) Selection of nominal discharge current In (impact current capacity)

The peak current of the 8/20 μs current wave that flows through the SPD. It is used for class II classification test for SPD, and also used for pretreatment of class I and class II classification test for SPD.

In fact, In is the maximum peak value of the inrush current that the SPD can pass the specified number of times (generally 20 times) and the specified waveform (8/20 μs) without substantial damage.

3) Selection of the maximum discharge current Imax (limit impact current capacity)

The peak current flowing through SPD, 8/20 μs current wave, used for class II classification test. There are many similarities between Imax and In. They both use the peak current of 8/20 μs current wave to do Class II classification test on SPD. The difference is also obvious. Imax only does one impact test on SPD, and SPD does not undergo substantial damage after the test; while In can do 20 such tests, SPD cannot have substantial damage after the test. Therefore, Imax is the current limit value of the impact, so the maximum discharge current is also called the limit impact current capacity. Obviously, Imax>In.