In the low voltage distribution system diagram of the substation, improper placement of Surge Protective Devices (SPDs) in the transformer low-voltage receiving cabinet can result in non-compliant installation of SPDs inside the distribution cabinet, failing to meet the specification requirement that the lead length of SPDs at both ends exceeds 0.5m. Generally, in low voltage distribution cabinets, phase busbars are located at the top of the cabinet, and the PE busbar is at the bottom, with a cabinet height of 2.2m. If wiring is done from the top of the cabinet, it is difficult to ensure that the total length of SPD wiring does not exceed 0.5m. The potential consequence could be lightning-induced overvoltage on SPDs, leading to internal fires in the distribution cabinet. The diagram below illustrates this situation.

▲ Improper Installation Position of SPDs in Distribution Cabinets

Regarding the length of the leads at both ends of the SPD, Article 5.4.3(8) of the "Technical Specification for Lightning Protection of Electronic Information Systems in Buildings" GB50343-2012 clearly stipulates: when Surge Protective Devices are installed at various positions in power lines, the connecting wires of SPDs should be short and straight, with a total length not exceeding 0.5m. The Up of the power SPD should be less than the rated impulse withstand voltage Uw of the protected equipment.

Generally, a safety margin of 20% should be added, meaning the effective voltage protection level Up should be less than 0.8Uw. In other words, the voltage protection level Up/f of the SPD, including the induced voltage at both ends of the connecting wires, should be less than the insulation impulse withstand voltage of the system and equipment and should not exceed 80% of the withstand voltage level of the protected equipment. In actual SPD installations, there are often many limiting conditions, inevitably leading to problems such as excessive length of SPD leads, resulting in the installed equipment failing to meet technical specifications.

Lightning current belongs to high-frequency current. The substantial fluctuation of transient lightning current can generate a strong voltage drop across the small inductance of the grounding wire. When lightning current acts, the actual clamping voltage Up/f of the protective branch is usually divided into two parts: Up and (UL1 + UL2), as shown in the diagram below:

▲ Voltage between phase lines and equipotential bonding straps That is:

Where UL1 + UL2 is the induced voltage of the SPD leads L1 and L2, and Up is the protection voltage of the SPD itself. The voltage drop on the conductor depends on the voltage drop on the resistance R and inductance L. Reducing the resistance and inductance of the conductor is an effective means of reducing voltage drop. As shown in the diagram, when SPDs are correctly installed in distribution cabinets, the voltage drop on the grounding wire mainly depends on the inductance of the grounding wire, with little influence from the resistance. And the inductance mainly depends on the length of the conductor. Therefore, shortening the length of the grounding wire is a more effective way to reduce the voltage drop on the grounding wire than increasing the cross-sectional area.

▼ Correct Installation Position of SPDs in Distribution Cabinets

The impulse voltage withstand level Uw of general electrical equipment is 2.5kV or 4.0kV. When the length of the SPD connecting wires causes the effective voltage protection level to exceed the upper limit withstand voltage, the SPD not only fails to protect the equipment but also poses a hazard. Therefore, the standard stipulates: the connecting wires of surge protectors should be straight, and their length should not exceed 0.5m.

To ensure that SPDs can promptly and reliably limit voltage under transient overvoltage, the design should emphasize: "the connecting wires of surge protectors should be straight, and their length should not exceed 0.5m"; meanwhile, in construction, SPDs should be installed towards the lower part of the distribution cabinet, allowing the PE end of the SPD to be connected to the nearest grounding busbar of the distribution cabinet, and short leads should be used from the upper end of the SPD to the phase busbar, enabling the total length of the power phase line and grounding wire to be less than 0.5m.