For high-voltage cable lines, the cross-bonding of screens is quite common. It is used not only for 110-500 kV lines, but also for 6-35 kV. The cross-bonding assumes that the six cable screens (1,2,3,4,5,6) must be removed from the three joints (A,B,C) and then connected to each other in a cross manner using a system of three jumpers installed inside a link-box. In practice, many questions arise, and among them, for example, the following two:
➡️which type of bonding cable should we use to connect joints and link-box: coaxial or single-core;
➡️what is the maximum allowable bonding cable length (the distance from the joint to surge arresters installed in the link-box and designed to protect the outer sheath of the main 6-500 kV cables from lightning/switching impulses induced there from the cores of the main cables).

The following answers are usually given to these questions (to reduce longitudinal voltage drop on bonding cables, and therefore to limit main cable sheath voltages more effectively):
❌ a coaxial cable is better because it has a lower inductance (L);
❌ the maximum allowed length is 10 m only (not more than 15 m).

In my opinion, both answers are not correct. And actually:
✅ from the point of view of impulse voltages, both types of cables behave the same way and have no advantages over each other (evaluation of the influence of the bonding cable type on ease of joint installation and joint reliability will not be discussed today);
✅ the maximum allowed length (10 m) is calculated based on misconceptions about bonding cable inductance (L) and the steepness of impulses (di/dt), and is therefore hardly correct.

Today we will only talk about the inductance (L). The inductance of a coaxial cable would be small (L=min) only if the currents in its core and screen were approximately equal to each other in magnitude and had opposite signs. However, the coaxial cable in the cross-bonding node has no such operating conditions. Most often, there is current in only one of two conductive elements of coaxial cable, and therefore the inductance (L) of a coaxial cable will not differ in any way from a system using two separate single-core cables.

If you are told that the inductance (L) of a coaxial cable is small, and therefore this cable is very good at cross-bonding nodes, I would advise you to treat this very carefully.

For instance, some documents suggest to use in calculations the inductance (L) of a coaxial cable at the level of 0.1-0.3 µH/m. Please note (!) that this is valid only for the unreal case when currents in its core and screen are equal and have opposite directions.

In fact, inductance (L) will be several times higher, up to 1-1.5 µH/m. All this leads us to the question – was the maximum “well-known” bonding cable length of 10 m (or 15 m) calculated correctly? Big doubts. The topic will be continued….

You can find more information about the impulse processes in Chapter 10.3 of the book