How should surge protection be designed in stages for electrical power measurement switches (PMS)?

Surge protection for PMS needs to be designed in stages according to the IEC 61000-4-5 standard to cope with different levels of lightning strike induction and switching transients. A typical three-stage protection (stages 1-2-3) works in coordination:

Stage 1 (coarse protection) is located at the power input, using a gas discharge tube (GDT) such as 2R600L-8×6 or a varistor (MOV) such as 14D511K to discharge most of the energy.

Stage 2 (intermediate protection) is located at the board-level power input, using a varistor (such as 7D390K) or a TVS diode (such as SMCJ33CA) to further clamp the voltage.

Stage 3 (fine protection) consists of a TVS diode (such as SMBJ5.0CA) and decoupling capacitors near the chip's power pins.

Each stage needs to be decoupled through inductors (such as common-mode inductors CMZ1211-501T) or resistors to achieve energy coordination. Communication ports also require staged protection, such as using a GDT+TVS combination. The design requires calculating the current carrying capacity and response time of each stage of the device to ensure that the preceding stage operates first. Through hierarchical design, the PMS can withstand surge impacts of 4kV common mode and 2kV differential mode, protecting the internal circuitry.