To assess the hazed caused by the rail potential, the trend of the potential against earth must be considered transversely to the track. In Figure 12.16 the fundamental trend of voltage UTP between the track and point P and that of voltage UPE between P and the reference earth are shown in relation to the rail potential UTE. The voltage UTE cannot be shunted at a distance of 1 km. However, the part of the rail potential between the outer rail the track and a point 1 m apart must not exceed the permissible accessible or touch voltage according to EN 50 122-1, where only approximately 20% of it can be shunted. For the earthing of high voltage systems, HD 637 S1 assumes 50% of the potential and specify that the permissible accessible or touch voltage are considered to be compliant if the potential does not exceed twice the permissible accessible or touch voltage. This relation applied to the rail potential is taken into account in Table 12.12.