The Mersen lightning arrester enclosures are designed to protect railway networks against disturbances incurred by lightning, in the feeding lines in the substations along the railtrack (pole enclosure) as well as in the power lines mounted on the roof (pantograph enclosure). These protection systems are to be intalled between the feeding line and ground. The usual solution consists of placing an air breaker and a non-linear resistance in silicium carbure between the line and the ground. The system presents several drawbacks : It does not age well, and it becomes passing after several operations. This malfunction triggers the opening of the line circuit breaker, which interrupts the service. The new solution is made up of the combination in series of a non linear zinc oxyde resistance and a current limiting fuse. The zinc oxyde resists better to ageing thanks to a better non linearity coefficient. In spite of this, should a lightning surge be violent enough to damage the lightning arrester, then the fuse will isolate the faulty equipment, leaving the feeding of the railway line in service. One of the essential elements of the system is that it is the first with a distant warning signal of the lightning arrester being isolated. The railway network operator has the advantage of detecting very rapidly the protections. Mersen proposes to use a zinc oxide varistor combined with a fuse and an electric signaling system. Thanks to this system, it is now possible.