When lightning strikes the transmission lines, a travelling wave appear. It means that lightning current divides into two separate currents and each of these currents goes to one part (side) of transmission line. Those currents are shaped as a wave, they quickly achieve their peak value but their decreasing lasts much longer.

Travelling waves spread along the transmission lines with speed of light (300000 km/s). Because of the resistance of the lines, their peak value of current decreases as they move. Also, steep of their current decreases with time. That fact is explained with enhancement of electrical resistance for very high voltages and currents. A part of the wave with small value of current and voltage moves with speed of light (all electromagnetic waves travel with speed of light). Another part of wave with high values of current and voltage is slows down due to appearance of corona. That way corona lowers the steep of the current.

When travelling wave comes to any crossing, it divides itself into several smaller waves and each of them proceeds to other branch. Value of current in each branch depends on total resistance in that branch. If the resistance is smaller, the current is higher and vice versa. That fact is good for us when travelling wave spreads through the protecting rope. The rope is grounded on every post and a part of travelling wave is conducted to ground.

The main danger for transmission lines is appearance of high voltage between conductor and the grounded post or its construction. That voltage can cause a spark and discharge from the conductor to grounded construction of the post. That danger increases with the speed of current change (di/dt), or with higher value of steep of current of the travelling wave that is the same thing. The most inconvenient circumstance is that that way started current will not stop flowing after travelling wave is gone. Nominal voltage of the transmission lines is high enough to keep it flowing. If that situation last long, the insulators that separate conductors from the construction could be damaged or destroyed and whole transmission line shut down for long time.