In the energy industry, many processes can not only be calculated on models, but also measured in an operating network. Measurements are important because they allow us to verify the models and, if necessary, adjust them. One of the areas where measurements in the network are practically impossible is impulse processes (lightning and switching). Therefore, talking about surge protection (using metal oxide surge arresters MOA and other means) is “a very slippery road”. But let us try.

Protection against lightning surges may be required for both the insulation of cables and their outer sheath, but today we will only talk about the insulation. Its protection may be required primarily where the cable line is a section of the overhead line (OHL). If we look at the transition points of mixed cable-overhead lines, we will see that there are:
✅ points without MOA (top photos);
✅ points with MOA (bottom photos).

Transition points without MOAs are not someone’s fault, but they are found all over the world. For example, the main regulatory document of the former USSR (Rules for electrical installations “PUE”) directly contains provisions that, for mixed lines, there is no need for MOA at the ends of cable section if its length exceeds 1.5 km. I have also personally encountered 115 and 230 kV transition points without MOA in countries such as Italy and Germany (I do not know the exact length of cable sections there, but it was clearly at least 500 m). Hence the question arises – where could the idea not to put MOA at the transition point come from?

As for PUE, it is well known. The fact is that it was assumed that the lightning waves affecting the cable section come from far away. In order for a lightning voltage to occur, the lightning wave must charge the cable’s capacitance. Charging process is a result of a series of refractions and reflections of the waves at the ends of cable section. The authors of PUE considered that, for cables with a length of more than 1.5 km (regardless of voltage class and core cross-section), the capacitance is so great that the energy of a wave is clearly not enough to charge the cable to a dangerous voltage.

The problem with such reasoning is that, in addition to a lightning strike somewhere at a distance from the transition point (to the tower (1), shield wire (2), phase wire (3)), a lightning strike may occur in the very top of the transition point (4). In this case, depending on the parameters of the lightning current, the inductance of the tower and its grounding resistance, the potential of the tower can reach 1000 kV or more, which is quite enough to break any of 6-220 kV cable terminations from a grounded flange onto a cable core. In this case, the breakdown of the termination will occur regardless of the length of the cable section.

So, surge arresters (MOA) at transition points (according to the lightning conditions) are needed regardless of the cable length.