Surge protective device(SPD) Principle

Data:2018-12-06 15:05Author:adminClick:

    Surge protection device(SPD) is designed to limit transient overvoltages of atmospheric origin and divert current waves to earth, so as to limit the amplitude of this overvoltage to a value that is not hazardous for the electrical installation and electric switchgear and controlgear.

SPD eliminates overvoltages
    in common mode, between phase and neutral or earth.

    in differential mode, between phase and neutral.

In the event of an overvoltage exceeding the operating threshold, the SPD

    conducts the energy to earth, in common mode;

    distributes the energy to the other live conductors, in differential mode.


The three types of SPD
Type 1 SPD

    The Type 1 SPD is recommended in the specific case of service-sector and industrial buildings, protected by a lightning protection system or a meshed cage.

    It protects electrical installations against direct lightning strokes. It can discharge the back-current from lightning spreading from the earth conductor to the network conductors.

    Type 1 SPD is characterized by a 10/350 µs current wave.


Type 2 SPD

    The Type 2 SPD is the main protection system for all low voltage electrical installations. Installed in each electrical switchboard, it prevents the spread of overvoltages in the electrical installations and protects the loads.

    Type 2 SPD is characterized by an 8/20 µs current wave.


Type 3 SPD

    These SPDs have a low discharge capacity. They must therefore mandatorily be installed as a supplement to Type 2 SPD and in the vicinity of sensitive loads.

    Type 3 SPD is characterized by a combination of voltage waves (1.2/50 μs) and current waves (8/20 μs).


SPD normative definition




Note 1: There exist  +  SPD (or Type 1 + 2 SPD) combining protection of loads against direct and indirect lightning strokes.
Note 2: some  SPD can also be declared as .