LIGHTNING AND TRANSIENT OVERVOLTAGES The users or electronic equipment, telephone and data-processing systems must face the problem of keeping this equipment in operation spite of transient overvoltages induced by lightning. **There are several reasons **: Intégration of electronic components makes the equipment more vulnerable. Interruptions of service are unacceptable Data transmission networks cover large areas and are exposed to more disturbances Lightning, investigated since Benjamin Franklin's first research in 1749, has paradoxically become a growing threat to our highly electronic society. The lightning is a natural phenomenon, defined as electrical shock between two zones of opposite polarity, between the cloud and the ground. The result is a current lasting some tenths of microseconds which will generate side effects, transient overvoltages, which are much more destructive than the discharge itself. The development and the increasing brittleness of electronic or computing equipment entail a bigger sensibility with surges. The lightning constitutes a real threat for companies’ equipment but also for independent profession or even private individuals. Against lightning itself , The technique of protection consists in capturing the discharge to divert it from its initial target. The lightning will be got, for example, according to the technique of the 'lightning conductor' or the “meshed cage' and will save the site, however your electronic equipment will not be protected against the side effects. Direct effects, At the moment of the discharge, there is an impulse current flow that ranges from 1,000 to 200,000 Ampere peak, with a rise time of about few microseconds. This direct effect may be considered as a small factor in damaging electric and electronic systems, because it is highly localized. The best protection is still the classic lightning rod or Lightning Protection System (LPS), designed to capture the discharge current and conduct it to a particular point.