There are two main forms of circuit protection: surge protection and overcurrent protection. Selecting the right circuit protection device is the key to achieve efficient and reliable circuit protection design. When selecting circuit protection devices, we need to understand the role of each circuit protection device. When selecting the circuit protection device, we should know that the circuit protection should not interfere with the normal behavior of the protected circuit. In addition, it must prevent any transient voltage from causing repeated or unique unstable behavior of the whole system.

There are three common devices for circuit protection: GDT, MOV and TVs.

Under normal working conditions, the parallel impedance GDT is about 1T Ω, and the parallel capacitance is less than 1pf. When the potential applied across the GDT is lower than the ionization voltage of the gas (i.e., "luminescence"), the small leakage current (usually less than 1 PA) and the small GDT capacity almost change. When the GDT reaches the incandescent voltage, its parallel impedance will drop sharply, so the current flows through the gas. The increase of current will cause a large amount of gas to form plasma, which will further reduce the voltage on the equipment to about 15V. When the transient power supply no longer provides plasma current, the plasma will disappear automatically. The net GDT effect is the arc damping effect, which can reduce the voltage during the transient event to below about 15V within 1ms. One of the main advantages of GDT is that it forces most of the energy to be consumed in the impedance of the transient power supply, rather than in the protection device or protection circuit. The voltage at which the GDT is triggered depends on the increase in signal voltage (DV / DT), GDT electrode spacing. The device can withstand currents up to 20 Ka.

GDT has two forms: monopole and tripole. The three pole GDT is a seemingly simple device that can maintain the balance of the differential line pair at the critical moment of disaster: small asymmetry can make the transition pulse priority clutch and one side of the balanced feeder, thus generating a huge differential signal. Even if the transition event occurs symmetrically on the balanced feeder, the small difference in the response characteristics of the two safety devices will lead to destructive pulse amplitude at the system input. The three pole GDT provides a differential device and two parallel devices in the pipeline of the common gas volume. Any condition that makes the electrode pair conductive will make all three electrodes conductive, because the gas state (insulation state, ionization state, or plasma state) determines the behavior of the discharge tube.