The components of a SPD

From Electrical Installation Guide

The SPD chiefly consists of (see Fig. J50):

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).

Fig. J50 – Diagram of a 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. J51 – Summary performance table
Component Gas Discharge Tube (GDT) Encapsulated spark gap Zinc oxide varistor GDT and varistor in series Encapsulated spark gap and varistor in parallel
DB422511.png DB422512.png DB422513.png DB422514.png DB422515.png
Operating mode Voltage switching Voltage switching Voltage limiting Voltage-switching and -limiting in series Voltage-switching and -limiting in parallel
Operating curves DB422516.png DB422517.png
  • Telecom network
  • LV network

(associated with varistor)

LV network LV network LV network LV network
SPD Type Type 2 Type 1 Type 1 or Type 2 Type 1+ Type 2 Type 1+ Type 2

Note: Two technologies can be installed in the same SPD (see Fig. J52)