Lightning strokes have the ability to discharge thousands of amperes of current in very short time. This high current needs to be discharged quickly into the earth to prevent the potential of the tower from rising.
Back flashover occurs when the lightning which has struck the tower is unable to get discharged to the earth. This can occur due to high impulse resistance of the ground.
When a tower is struck by lightning, a travelling voltage is induced which moves many times between the top and the bottom of the tower , the potential of the tower is thus raised. The elevated voltage also appears on the cross arms of the towers. This can cause back flashovers if the insulators are unable to withstand the voltage surge.
Back flashovers can be avoided by improving the impulse resistance of the earth point of the transmission towers and improving critical flashover limits of the insulators.
Back flashovers are identified as line to earth faults.