Let us discuss the length of the bonding cable and also such a thing as the steepness of current/voltage impulses that creates voltage drop.

Industry documents suppose that the voltage in the joint can be determined as a sum of residual voltage of SVL (sheath voltage limiter) and the voltage drop across the bonding cable, which depends on:
➡️ linear inductance (we talked last time);
➡️ the length of the bonding cable (supposedly up to 10 m);
➡️ the steepness di/dt of the impulse.

When discussing steepness, somehow only lightning processes are mentioned, and not a word is said about switching. It means, the following facts are ignored:
✅ most cable lines are so far removed from overhead lines (OHL) or air-insulated switchgear (AIS), that they do not undergo any lightning processes;
✅even if cable lines are exposed to lightning processes, this does not give us the right to forget about switching, especially in 220-500 kV networks.

For instance, I know of several 220 kV cable lines where the cross-bonding joints were damaged when energized:
➡️ The joints were made by leading European manufacturers (TOP 5), and installed by highly qualified specialists.
➡️ The length of the bonding cables was less than 10 m.
➡️ Only those cross-bonding joints were damaged that were closest to the circuit breaker that supplied voltage to the line.

The investigation did not reveal any problems with the joint or its installation. The only problem was the impulse processes while switching the line using modern best-quality SF6 switchgear (GIS).

The steepness of a lightning impulse depends very much on the parameters of lightning, the place of strike to the OHL (phase or ground wire), and the distance from the switchgear. However, the steepness of the switching impulse is always almost “infinite”: there is no voltage on the line, and then it suddenly appears.

The phase-to-ground voltage wave moves along the cable, and the impulse steep front loses its steepness worse compared to a wave along OHL. So is it any wonder that the first joint is damaged – the one where the network voltage wave came without having time to reduce its steepness.

Let’s renew the documents that talk about lightning only. The switching steepness can be many times higher, and processes in the joint depend on:
➡️ the number and rated power of transformers on the busbars;
➡️ the number of other cable lines on the busbars;
➡️ the distance to the first cross-bonding joint;
➡️ the type of switching (the presence of initial charge);
➡️ the circuit breaker adjustment (the time shift between the phases A,B,C).

My advice to those who believe that bonding cable up to 10 m long guarantee the safety of the joint is not to believe this, but to approach the issue much more deeply. Especially when there is:
✅ switching on the 220-500 kV cable lines;
✅ a small distance between circuit-breaker and the first cross-bonding cable joint.

Please read Chapter 10.3 of the book