The components of a SPD

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. J48: 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.

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Component	Gas Discharge Tube (GDT)	Encapsulated spark gap	Zinc oxide varistor	GDT and varistor in series	Encapsulated spark gap and varistor in parallel
Characteristics
Operating mode	Voltage switching	Voltage switching	Voltage limiting	Voltage-switching and -limiting in series	Voltage-switching and -limiting in parallel
Operating curves
Application	Telecom network LV network (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. J49: Summary performance table

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

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Fig. J50: The Schneider Electric brand PRD SPD incorporates a gas discharge tube between neutral and earth and varistors between phase and neutral

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