Among the number of signal apparatus types used, the electric train detectors used for crossings are most frequently subjected to lightning strikes. Although various lightning-strike prevention measures have been proposed including the connection of earthing conductors for security devices to rails, it is not easy to judge whether these measures should be applied, as their effectiveness in reducing damage related to lightning strikes is unknown.
Although security devices are not grounded at present as a measure to prevent lightning-strike damage for electronic train detectors, the RTRI has proposed such a measure (i.e., grounding security devices), and has verified its effectiveness in suppressing lightning overvoltage through field tests. It was found that the class D grounding (100 ¶) used for telecommunication and signal apparatus suppresses lightning overvoltage. When the grounding resistance is reduced to 20 ¶, class D grounding cuts the lightning overvoltage to a half the amount found with the present measures for the same level of lightning, giving approximately double the lightning resistance (Fig. 1).
The RTRI also surveyed the frequency distribution of overvoltage occurrence under lightning conditions on crossings in the area known to have Japanfs highest lightning frequency, and established a formula to estimate the cumulative frequency per year of lightning occurrence accompanying overvoltage (Fig. 2). The formula gives a cumulative frequency of 1.05 times per year. When the 20 ¶ grounding is implemented, however, the overvoltage is reduced to about 20 kV or a cumulative frequency of 0.24 times per year, which is about a fifth of the present level. When the coefficient in the estimation formula is adjusted according to the frequency of lightning in the area in question, the effectiveness of lightning-strike damage prevention in the area can be evaluated. For this purpose, data on the frequency of lightning strikes in the area in question are available in the relevant maps released to the public.