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The components of a SPD


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).

Fig.1 – 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.2 – Summary performance table

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