How to select the appropriate protection for the metering port of an electrical power measurement switch (PMS)?

The metering ports (voltage/current input terminals) of a PMS are directly exposed to the field and may be subject to surges, EFTs, ESDs, and other threats. The selection of protection devices must be determined based on the port type, signal level, and test level. For voltage sampling ports (high impedance), the first stage can use a gas discharge tube (GDT) such as 2R600L-8×6 (DC breakdown voltage 600V) to discharge high current. The second stage uses a TVS diode such as SMBJ6.5CA (bidirectional) for clamping. The third stage uses a series PTC such as SMD2920-300-16V for current limiting. For current sampling ports (low impedance), due to their low signal level, large resistors should not be connected in series. The main protection relies on a TVS diode (such as SMBJ6.5CA) and the high CMRR of the subsequent differential amplifier. All protection devices should be installed as close to the terminals as possible. Simultaneously, the impact of parasitic capacitance of the protection devices on high-frequency signals must be considered; for example, a low-capacitance TVS diode (e.g., <50pF) should be selected. After selection, the device must pass the corresponding level of testing and verification according to IEC 61000-4-5 (surge), IEC 61000-4-4 (EFT) and IEC 61000-4-2 (ESD).