for the span which crosses the aerial tramway. At crossings at which the aerial tramway runs above the power line, the line is frequently installed on special masts in the crossing range, which scaffold the line in the area of the aerial tramway crossing. Such a measure is not necessary according to power line regulations, but it is often done because, in case of aerial tramway failure, it is possible to rescue people from the tram without switching off the overhead line. Such constructions may be seen at 110 kV power line crossings of the Penkenbahn at Mayrhofen, the Patscherkofelbahn at Innsbruck and south of Zermatt. Overhead line crossings of broad rivers and straits Overhead line crossings of broad rivers and of straits, if the terrain on both sides is relatively even, frequently consist of four pylons: two particularly substantial anchor pylons for bracing the conductors of the crossing section, and two tall carrying masts to keep the line high over the water. These pylons have broader cross-bars and greater distances between the cross-bars than the other pylons of the line, in order to prevent the conductor cables from striking against each other during strong winds. In contrast to normal pylons, the two carrying masts at both ends of the crossing are frequently equipped with flight safety lamps, and have stairways for easy access to the top. Overhead line crossings of rivers and straits with spans of over 2 km are frequently prohibitively expensive to build and operate; because of the danger of wind-induced oscillatory movements of the conductor cables, it is necessary either to install very large leader distances or to mount insulators between the conductors in the area of the span. Bundle conductors, which are used for almost all extra-high voltage lines, are more susceptible to oscillations from wind forces than single conductors. Therefore, single conductors must be used for the crossing section, which means the crossing section of the power line determines the maximum transmittable power. Further, one cannot build pylons arbitrarily high at either end of the crossing section, and there is a usually a considerable minimum height because of ships crossing under the line, so there is often a high mechanical tension in the conductors at long spans. This tension requires conductors made largely of steel, which have a worse electrical conductivity than the common overhead line conductors consisting of copper, Aldrey or aluminum-encased steel,