Complex sentence: Crossing pylons for crossing valleys require, if the topography of the valley allows an adequate right of way, not to be high; however the distance between conductors must be great enough to prevent knocking together the conductors at strong winds.
Split into simpler sentences: Crossing pylons for crossing valleys require, if the topography of the valley allows an adequate right of way, not to be very high. However the distance between conductors must be great enough to prevent knocking together the conductors at strong winds.