Without lightning surge voltage, that is, under normal power frequency voltage, the voltage distribution of MOV (metal oxide varistor) and GDT (ceramic gas discharge tube) is mainly affected by their electrical characteristics.

1. GDT (Ceramic Gas Discharge Tube):

  GDT has extremely high parallel impedance (about 1TΩ) and extremely small parallel capacitance (below 1pF) under normal working conditions.

  When the voltage applied across the GDT is lower than the gas ionization voltage (i.e., the "glow" voltage), the leakage current of the GDT is very small (typically less than 1pA) and the parallel capacitance hardly changes.

  Without transient overvoltage, GDT is almost non-conductive, the voltage at both ends is close to the system's power frequency voltage.

2. MOV(Metal oxide varistor):

  Under normal working conditions, MOV has a small leakage current (10mA) and a large parallel capacitance (about 45pF).

  When the voltage is lower than the threshold voltage of MOV, its leakage current is small, but as the voltage increases, the leakage current increases.

  Without transient overvoltage, due to the high impedance characteristics of GDT, the voltage across the MOV will be very low and almost non-conductive, which effectively alleviates the performance degradation of the MOV.

3. Serial application:

  When the GDT and MOV are used in series, the GDT acts as a switch when there is no transient overvoltage, isolating the MOV from the system, making the MOV have almost no leakage current.

  In this way, the system's power frequency voltage mainly falls on the GDT, while the voltage across the MOV is very low, almost close to 0V.

  This connection method can effectively prevent the MOV from aging due to leakage current in the absence of transient overvoltage, extending its service life.

  In summary, under normal power frequency voltage, in the series-connected GDT and MOV, the voltage is mainly distributed on the GDT, while the voltage across the MOV is very low andcan be alomost ignored. This configuration can effectively protect the MOV, extend its service life, and improve the reliability and practicality of the entire protection circuit.