INTRODUCTION

The impact of a lightning stroke on any conductor like electrical or telecommunication cables produces transient overvoltages which are characterized by a short duration, rapid amplitude growth and high peak values (up to more than hundred kV).

The discharge of a lightning flash is propagated within a radius of several kilometers and its extension raises the potential which leads to the induction of significant surge voltages in the cables located in the ground and in the earth terminations.

Lightning is not the only reason for surge voltages. They are also caused by:

Switching of electrical loads by the power utilities.
Switching of high power machines.
Electrostatic discharges.

In an installation all the conductors which enter from outside are potentially facilitating the way for surge voltages, which can lead to wrong functioning in the power supply of the connected systems.

The surge voltages caused by lightning can reach the installation in the following manners:

PROPAGATION MODES

Common or asymmetric mode:
Perturbations between an active conductor and earth (phase-earth or neutral-earth), with risk of dielectric damage.

Differential or symmetric mode:
Perturbations between active conductors, (phase-phase or phase-neutral), especially dangerous for computer equipment.

A protector performs like a switch controlled by voltage. If the voltage is higher than the rated voltage of the electrical line to be protected, then the protector changes its state to low impedance and derives current to earth. The usual state of the protector is being in high impedance, so that the protector is transparent for the installation.

THE PERFECT PROTECTOR

The perfect protector should derive the maximum current (Imax) generated by the surge voltage and the residual voltage appearing between its ends (Ures) should be less than the voltage which can be withstood by the equipment to be protected.

In practice there is no protector which fullfils both requirements perfecty.

Achieving a high discharge capacity and low residual voltage in the same protector is not possible. Thus, using a single protector often cannot assure the protection of the whole installation if there are sensitive equipments. The solution is to use 2 or more coordinated protectors.

The first protector (P1) is selected in order to achieve the maximum possible discharge capacity (Imax).

The second and following protectors depending on the required Up are installed in the more sensitve lines and as close as possible to the equipment which has to be protected.