The components of a SPD

The SPD chiefly consists of (see Fig.1):

1) one or more nonlinear components: the live part (varistor, gas discharge tube, etc.);

2) a thermal protective device (internal disconnector) which protects it from thermal runaway at end of life (SPD with varistor);

3) an indicator which indicates end of life of the SPD;

Some SPDs allow remote reporting of this indication;

4) an external SCPD which provides protection against short circuits (this device can be integrated into the SPD).

Technology of the live part

Several technologies are available to implement the live part. They each have advantages and disadvantages:

•Zener diodes;

•The gas discharge tube (controlled or not controlled);

•The varistor (zinc oxide varistor).

The table below shows the characteristics and the arrangements of 3 commonly used technologies.

Fig.2 – Summary performance table