The components of a SPD
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 |
| Characteristics | |||||
 |
 |
 |
 |
 | |
| Operating mode | Voltage switching | Voltage switching | Voltage limiting | Voltage-switching and -limiting in series | Voltage-switching and -limiting in parallel |
| Operating curves |  |
 |
|||
| Application |
(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
