There are three points for the performance requirements of the SPD backup protector. First, break the expected short-circuit current of the SPD installation line; second, withstand the surge current passing through the SPD without breaking; third, break the power frequency current that cannot be broken by the built-in thermal protection of the SPD, quoted from GB 51348 "Civil Buildings" Electrical Design Standards.

In terms of 8.4.3.2 and 8.4.4.4 of the test standard GB18802.11-2020 for surge protectors, there are 8/20 waveform impulse current tests. In these tests, the surge protector and its backup protector are required to withstand the impulse current of 8/20 waveform of the magnitude of Imax. During the test, the backup protector is required not to trip. In order to find the cooperative relationship between the SPD and the backup protector, it is necessary to find the maximum inrush current withstand level of the backup protector.

Using I²t to calculate the waveform and compare it with the I²t (1ms) provided by the backup protector manufacturer is a possible method to estimate the single surge withstand capability of the fuse. I²t can be estimated from the peak value of the shock, which can be seen from the following formula:
---For 10/350 wave: I²t=256.3×I²crest
---For 8/20 wave: I²t=14.01×I²crest

Here, the unit of Icrest is kA, and the unit of I²t is A²·s.

In the test method described in GB18802.11-2020, the backup protector must not only withstand a single shock, but also withstand a complete sequence (preconditioning test and action load test). These shocks can reduce the performance of backup protectors, thereby reducing their ability to withstand a single shock. In order to pass the above test, the test display should be multiplied by a reduction factor of 0.5-0.9 on the basis of the single impact resistance value.

Although the GB18802.11-2020 standard provides a clear basis for the selection of backup protectors, in actual working conditions, SPD failures will still occur, backup protectors will not operate, and even SPDs will catch fire. The specific reason is due to the blind spot in the cooperation between the SPD and the backup protector. Since the fusing coefficient of a fuse is usually 1.5-2.0, it means that the larger the current, the faster the fusing speed, but at a small current, the fuse cannot operate. After our test, in the case of failure of the internal device of the SPD, the current of 5A can cause the SPD to catch fire.