The components of a SPD
From Electrical Installation Guide
The SPD chiefly consists of (see Fig. J45):
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. J45: 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.
|Component||Gas Discharge Tube (GDT)||Encapsulated spark gap|| Zinc oxide varistor ||GDT and varistor in series||Encapsulated spark gap and varistor in paralle|
|Operating mode||Voltage switching||Voltage switching||Voltage limiting||Voltage-switching and -limiting in series||Voltage-switching and -limiting in parallel|
(associated with varistor)
|LV network||LV network||LV network||LV network|
|SPD Type||Type 2||Type 1||Type 1 ou Type 2||Type 1+ Type 2||Type 1+ Type 2|
Fig. J46: Summary performance table
Note: Two technologies can be installed in the same SPD (see Fig. J47)
Fig. J47: The Schneider Electric brand PRD SPD incorporates a gas discharge tube between neutral and earth and varistors between phase and neutral