Rules for Surge Protective Device Installation in Different Earthing Systems

Rules for Surge Protective Device Installation in Different Earthing Systems

A variety of documents deal with the subject of surge protective device installation: for example in France, standard NF C 15-531 focuses on the installation rules of LV surge protective devices and standard NFC 15-100 covers all the LV electrical installations.

To evaluate the risk, a formula based on scientific criteria is proposed to engineering and design departments.This formula takes account of the characteristics of the site and the environment:

1. Lightning density,

2.Type of distribution network

3. Site topography,

4. Presence of lightning rods, if any.

Whatever system is used, if a lightning risk is present, all electrical installations must befitted with surge protective devices , whose composition may vary according to the type of earthing system.

Figures bellowed- Choice of surge protection mode (common or differential) according to the electrical installation earthing system as per NF C 15-443.

These differences are due to:
Whether or not differential mode surges are treated,The maximum steady state voltage Uc:

Between live conductors and the earth:

• Ue> 1.5 Un in the TT and TN earthing systems,
• U> √3 Un in the IT earthing system;