How to suppress lightning overvoltage in a power measurement switch (PMS)?

PMS (Power Supply Management) devices installed outdoors or in industrial power grids are susceptible to overvoltages from direct or induced lightning strikes. These surges can be extremely powerful, requiring multi-stage coordinated surge protection for suppression.

The first stage (building-level) is typically handled by an external surge protector (SPD), while PMS focuses on subsequent stages. At the PMS power input, a varistor (MOV) such as 14D511K or a gas discharge tube (GDT) such as 2R600L-8×6 is installed, with a current capacity greater than 20kA (8/20μs).

The second stage uses a varistor (e.g., 7D390K) or a TVS diode (e.g., SMCJ33CA) at the board-level power input.

The third stage uses a TVS diode for the chip power supply (e.g., SMBJ5.0CA). Each stage is isolated from the others by a common-mode inductor CMZ1211-501T or a decoupling inductor to ensure energy is discharged sequentially. Signal ports also require a GDT+TVS protection combination, such as RS485 ports using a 2R090L-8×6 GDT and an SMBJ6.5CA TVS.

All protective devices must have short, thick grounding wires connected directly to the grounding busbar. The PMS should be able to withstand the combined wave test of 10/700μs (communication lines) and 1.2/50-8/20μs (power lines) as specified in IEC 61000-4-5 to ensure survival under lightning strikes.