With a gradual introduction of electronic protection systems sensitive to the overvoltage into the railway traffic, it is necessary to address also their protection, especially against dangerous effects of lightning strokes that may cause extensive damages to this equipment. A railway electronic signalling and interlocking system is a complex system that must be highly reliable and responsive in accordance with the needs of safe railway traffic. Individual functional units are located both inside and outside buildings, at the railway stations and on the lines, i.e. in the open environment where the induction from the lightning (impulse) currents has a great effect, both from the short and long-distance discharges. Seeking the optimum overvoltage protection thus requires a very special approach that is influenced mainly by the two basic aspects:
• The overvoltage protection installation must not influence a functional reliability and operational safety of the electrical signalling
• Acquisition and operating costs should not be excessive.
On electrified tracks the overhead lines act as a protective lightning network and the lightning strokes are mostly caught by exposed parts of the overhead lines. By this the outdoor parts of the interlocking boxes are protected to a great extent. It must be, however, taken into account that the effects of the lightning impulses may be transferred via a decentralized electronic interlocking power supply system from the overhead line not only to the UNZ power supply source but also to the interlocking electric circuits.