Aside from all the problems of determining if the lines are powered now, there is the fact that they may become powered in an instant with no notice.

Scenario: you need to cross the downed line. You check and are sure the line is dead. You take a couple of steps and are near the line when somewhere someone throws a switch or something else happens and the lines are now instantly powered and deadly.

Please, treat all downed lines as live at all times just like you treat all guns as loaded.