SPD-specific Disconnector (SSD): An external disconnector installed in series with the surge protective device (SPD) in low-voltage power systems, also known as a pre-SPD external disconnector. It is commonly referred to as the SPD-specific backup protection device. Its main functions are as follows:

1. Protection Against Short-Circuit: When an abnormal transient over-voltage causes the SPD to conduct and leads to a short circuit, this can result in a power outage or even a severe fire hazard. The SSD is designed to quickly disconnect the circuit if a short circuit occurs in the SPD. It is crucial that this external disconnector has the capability to interrupt the short-circuit current; otherwise, it may lead to arcing and severe fire risks.

2. Protection Against SPD Deterioration: If the SPD deteriorates due to lightning strikes or transient over-voltage impacts, and its breakdown voltage drops below the supply voltage, the power frequency leakage current may rapidly increase. When this current is only a few amperes, the SPD's thermal trip mechanism can disconnect the circuit. However, if the current exceeds 10A, the SPD may overheat and catch fire faster than heat can be transferred to the trip mechanism. Thus, the SSD must disconnect the circuit before the SPD catches fire to prevent fire hazards.

3. Avoiding False Tripping: The SSD ensures that the SPD's effectiveness in protecting electrical equipment from lightning is maintained. It prevents false tripping, ensuring that the lightning protection remains operational at all times.