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The components of a SPD

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General rules of electrical installation design
Connection to the MV utility distribution network
Connection to the LV utility distribution network
MV and LV architecture selection guide for buildings
LV Distribution
Protection against electric shocks and electric fires
Sizing and protection of conductors
LV switchgear: functions and selection
Overvoltage protection
Energy Efficiency in electrical distribution
Power Factor Correction
Power harmonics management
Characteristics of particular sources and loads
PhotoVoltaic (PV) installation
Residential and other special locations
ElectroMagnetic Compatibility (EMC)


The SPD chiefly consists of (see Fig. J48):

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. J48Diagram 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 parallel
DB422511.svg DB422512.svg DB422513.svg DB422514.svg DB422515.svg
Operating mode Voltage switching Voltage switching Voltage limiting Voltage-switching and -limiting in series Voltage-switching and -limiting in parallel
Operating curves DB422516.svg DB422517.svg
  • 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

Fig. J49Summary performance table

Note: Two technologies can be installed in the same SPD (see Fig. J50)

Fig. J50The Schneider Electric brand iPRD SPD incorporates a gas discharge tube between neutral and earth and varistors between phase and neutral