Building protection system

From Electrical Installation Guide

The role of the building protection system is to protect it against direct lightning strokes.

The system consists of:

  • the capture device: the lightning protection system;
  • down-conductors designed to convey the lightning current to earth;
  • "crow's foot" earth leads connected together;
  • links between all metallic frames (equipotential bonding) and the earth leads.

When the lightning current flows in a conductor, if potential differences appear between it and the frames connected to earth that are located in the vicinity, the latter can cause destructive flashovers.

The 3 types of lightning protection system

Three types of building protection are used:

The lightning rod (simple rod or with triggering system)

The lightning rod is a metallic capture tip placed at the top of the building. It is earthed by one or more conductors (often copper strips) (see Fig. J12).

Fig. J12 – Lightning rod (simple rod or with triggering system)

The lightning rod with taut wires

These wires are stretched above the structure to be protected. They are used to protect special structures: rocket launching areas, military applications and protection of high-voltage overhead lines (see Fig. J13).

Fig. J13 – Taut wires

The lightning conductor with meshed cage (Faraday cage)

This protection involves placing numerous down conductors/tapes symmetrically all around the building. (see Fig. J14).

This type of lightning protection system is used for highly exposed buildings housing very sensitive installations such as computer rooms.

Fig. J14 – Meshed cage (Faraday cage)

Consequences of building protection for the electrical installation's equipment

50% of the lightning current discharged by the building protection system rises back into the earthing networks of the electrical installation (see Fig. J15): the potential rise of the frames very frequently exceeds the insulation withstand capability of the conductors in the various networks (LV, telecommunications, video cable, etc.).

Moreover, the flow of current through the down-conductors generates induced overvoltages in the electrical installation.

As a consequence, the building protection system does not protect the electrical installation: it is therefore compulsory to provide for an electrical installation protection system.

Fig. J15 – Direct lightning back current