Why need to install backup protector before SPD? Why not MCB or fuse? In fact, from a professional perspective, all of these three are acceptable, but MCB and fuses can cause high current misoperation and low current rejection in the actual application of the system. Both misoperation and rejection can have a significant or minor impact on the operation of our system. Therefore, in the electrical design standard for civil buildings GB51348-2019, 11.9.11 has clearly required the capabilities that backup protectors should have: 1 The expected short-circuit current of the SPD installation line; 2. Withstand the continuous opening of surge current through SPD; 3. Break the power frequency current that cannot be disconnected by the built-in thermal protection of SPD. Our company's self-developed and self-produced dedicated backup protection device has also been applied in various occasions, and the actual use effect is a qualitative leap compared to traditional backup protection.