Water, electricity, gas meter reading systems, access control, intercom, alarm systems and other similar systems generally use low-frequency (pulse) signals or DC (AC) switch signals.

Such systems are divided into two categories: ungrounded systems and grounded systems.

The following are circuit protection solution for both systems:

1. R1 and R2 can use common metal oxide film resistors (4.3~5.1Ω) or PTC resettable fuses. 

2. The DC breakdown voltage of the gas discharge tube and TVS diode  is selected according to the signal voltage amplitude.

When there is no continuous DC voltage in the circuit, the TVS tube can be replaced by TSS. When the surge current is small, the ceramic gas discharge tube can be replaced by SPG.