Electrical circuit for protection against surge overvoltage of transients

A circuit containing a pair of spaced leads and first, second and third current paths across the leads. The first path defines a first stage containing a first zener diode in series with a first resistor. A second zener diode can be placed in parallel across the first resistor to prevent excessive buildup of voltage across the first resistor. The second path defines a second stage containing a varistor which acts to relieve the first stage with a higher power dissipation but at a slower reaction time. The third path defines a third stage containing a gas discharge tube in series with a second resistor. A varistor is in parallel with the second resistor of the third stage to limit the voltage across the second resistor due to impulse current. The circuit combine the zener diodes, the varistor and the gas discharge tube in a manner to ensure rapid response and high energy dissipation to incoming transient impulses. The circuit has a high impedance under non-transient conditions so that, under normal operation, no load current passes through the first, second and third stages.