Because of its form, the overhead electric-power lines are sort of objects that are pretty well exhibited to risk of lightning hits. Lightning that hit the overhead electric-power lines could make a higher voltage than insulators on overhead transmission lines may stand and then will come to skipping and short circuit. Current bow made on that way will not turn off for a long time because the voltage of phase conductor is enough for its maintaining.
When the lightning hits transmission line, travelling waves could appear, which will be taken to the plant or consumers and cause large damage. It could also cause a protection to turn on and shut down the line. For this reason, it is very important to obstruct those hits.

One of the fundamental ways for protection of overhead transmission line is putting the protecting rope, which is connected with transmission-line pylon and each pylon is good grounded. Protection rope should be installed over the conductor so they can be inside the protected area. As permeable distance depends on peak lightning current and overvoltages on conductors too. Question is what is the minimum value of current we should protect the conductor from? The protected area is always calculated for critical current while the lightning with smaller current than critical could still hit the conductor. Criterion is that the critical current should not make the overvoltages, which will be larger than voltages that an insulation of conductor can hold out. We protect the transmission lines from that critical current.
Calculations for determining the value of that current are complicated, but foundations are visible in next animation.

In praxis, with protecting rope are covered all transmission lines over 30 kV. Mostly, there are two protecting ropes. When lightning hits the protecting rope there could come to large drop of voltage while passing through pylon and grounded conductor. In that case phase conductors, which are on nominal voltage, would have a voltage much smaller than part of pylon they are hooked on. The insulation between pylon and conductor wouldn't stand that, and "recurrent skip" would appear. Such skipping could permanently damage the insulation and throw the line out of operation for a long time.
The resistance of grounded pylon should be smaller to prevent this. It should not be larger than 15 ohms.

There is one more device for decreasing that overvoltage named protecting sparklet. It is set parallel to insulator chain where are phase conductors and it's insulating the phase conductors from transmission-line pylon. The voltage of skipping on sparklet is smaller than insulation of transmission line could stand. The main function is to move the arch originated from recurrent ship on insulator chain and that way it protects the insulator from damage. The sparklets are usually installed on the transmission lines over 110 kV but sometimes on the nets of lower voltages, on especially critical places. Its function is also to make the more favourably distribution of electrical field around insulators chain and on that way it protects from different kinds of discharge (corona).