The screen (and core) must withstand the short-circuit (fault) current (If), taking into account its time (tf). For XLPE- and EPR-insulated cables, the temperature during a short-circuit should not exceed 250°C. In the lack of information, this temperature is applicable for both the core and the screen. However, according to IEC 61443, if the screen does not have contact with polymers (and this is typical), then the temperature of the screen can be much higher (350°C). This is understandable, since the screen cools faster than the core located in the centre of the cable.

Improving the accuracy of thermal calculation and the screen cross-section (Fs) selection can be achieved by clarifying:
✅ “formula” Fs=function(If, tf);
✅ the current (If);
✅ the time (tf).

We talked about time (tf), and now let’s talk about the rest.

Clarification of Fs=f(If,tf) is possible if we take into account the non-adiabatic nature of the process of heating the screen with short-circuit current, i.e., take into account that some heat transfers from the screen into the surroundings. This calculation method is proposed in IEC 60949 and usually helps to show the possibility of increasing the permissible short-circuit current by 10-20%, which would seem good. However, it’s not that simple.

This IEC has several features.
1️⃣The method is based on a system of empirical coefficients, which were obtained in very limited (!) number of experiments.

2️⃣The research that formed the basis of the IEC 60949-2008 was conducted before 2008, that is 20 years ago (!), but the industry has changed over the years.

3️⃣The calculation results significantly depend on the initial data, which, as a rule, are still unknown at the design stage of the cable line:
➡️the diameter of each wire;
➡️number of wires;
➡️the gap between the wires;
➡️presence/absence of gap filling;
➡️materials in contact with the wires;
➡️the presence of tapes superimposed spirally on top.

Even if the designer somehow managed to accurately guess the numerous design features of the cable that would eventually be purchased during the construction of the cable line, and even if he correctly interpreted the confusingly stated IEC provisions, it is still at best an opportunity to clarify the short-circuit currents by 10-20%.

Is it worth spending time and effort on non-adiabatic processes, which cannot be trusted and changes the result, at best, by 10-20%, or is it better to start clarifying the selection of the screen (core) cross-section by understanding the values of current (If) and time (tk), which can change the result ten times more (by up to 100-200%)?

For instance, the effect of the aperiodic component reaches 20-40%. Please ask yourself, when was the last time you took aperiodic component into account? And this is only the first of many (!) ELEPHANTs. Then why should I care about FLY that gives me only 10-20%? You can read more in Part 5 of the book.

CONCLUSIONS

IEC 61443 and other documents (as well as basic concepts of physics) state that the permissible temperatures for the core and the screen are not necessarily equal to each other. For the screen, for most modern cables, the temperature can be higher than for the core. It can be up to 350°C, and not only 250°C.

IEC 60949-2008 is based on old and limited research, and in IEC itself it is written that “The major problem has been a lack of experimental results.” But most important is that the result of calculating the cable for thermal stability depends to a much greater extent not on the non-adiabatic nature of the processes (it gives only 10-20%), but on another:
✅ the aperiodic component (changes the result up to 20-40%);
✅ the clearance time (changes the result significantly, up to 100-200%).

Let’s start considering the aperiodic component. Let’s work out right rules for selecting the time (it is not correct to take time just equal to the main or backup protection, since everything is more complicated – see the book; and it is even more absurd to take the time equal to 1 s, which is done in many projects).

Of course, you can perform calculations according to IEC 60949-2008, but please, do not forget about many ELEPHANTs.