Modern MV-HV cable has conductive screen. Such screen, after grounding, make it possible to equalize the electric field in the cable insulation and eliminate this field outside the cable. In practice, the question arises – how to choose the screen cross-section?

Cable factories have their own opinion. Let’s assume that a screen is made out of wires of specific cross-section. The figure shows that if the factory had made the screen cross-section equal to 25 mm2, the number of wires (red) would not be enough to evenly cover the insulation and solve the problems of the electric field. Therefore in this example, it is necessary to increase the number of wires, and hence to increase the screen cross-section to 50 or 95 mm2.

However, does the designer need to raise the screen cross-section beyond the factory’s value? Yes! The fact is that the cross-section of the screen (and the core) must be able to pass short-circuit currents of the network and at the same time not cause dangerous overheating of the insulation. We are not talking about an external short circuit (when the current is divided equally between “n” parallel cables), but rather about internal damage.

In case of internal damage, the short-circuit current passes through the core to the breakdown point, then enters the screen and then returns along the screen towards the network. The current heats the core and the screen not only at the point of damage, but along the entire route from the supply network to the damage. If the cross-section of the core and the screen are insufficient, then it will be necessary not only to repair the original damage, but also to completely replace a significant section of the route. Therefore, it is important that the cross-section of the screen and the core are capable of passing short-circuit currents, taking into account their clearance time.

The core and the screen are adjacent to the same XLPE insulation, which we are trying to avoid overheating. However, the allowable temperatures are different:
➡️ 250°C, core;
➡️ 350°C, screen.
The temperature for the screen is allowed to be higher, since the screen cools down quickly enough, removing heat through the cable sheath/jacket to the outside.

In order for the core and screen not to overheat beyond the permissible temperatures, their cross-sections must meet the conditions shown in the table. To use the formulae, we need to know the magnitude of the short-circuit current and its clearance time.

The main questions arise only about time. The fact is that we want to set the time to 1s, since it is convenient to extract the root from it. However, the real times are either much less than 1s (main protection) or more (backup protection). So what time should we choose? Unfortunately, there is very little information about it. I tried to present my thoughts in Part 5 of the book, giving specific recommendations on when to take the main protection and when the backup.

The issue of thermal stability is very serious. In fact, we need to talk not only about time, but also:
1️⃣ the type of a short circuit (three-phase, single-phase etc);
2️⃣ accounting for the aperiodic component of the current;
3️⃣ the type of the network neutral (grounded, isolated etc);
4️⃣ cable design (single-core, three-core etc);
5️⃣ non-adiabatic nature of processes (heat removal during a fault).

The questions 1-4 are discussed in detail in the book. As for the fifth one, I am a sceptic of this topic, and I intend to make a separate post about it. Today, my main task is to show that it is unacceptable to use the time of 1 second. I regret to admit that I have seen a lot of projects where the designer uses exactly this time, detached from reality.

By the way, the more time we take, the larger the screen cross-section, the greater the currents and losses in the screens will be in normal operation mode of the cable line. Therefore, increasing the time not only makes the cable itself more expensive, but also creates problems for us in normal operation related to screen losses. Hence the problems with choosing the screen grounding (both-sides, single-side, cross-bonding).

Therefore, time is not just money, it’s a lot of money!