If the cable insulation is damaged, the current in the cable core (Ic) and the current in the cable screen (Is) can reach the value of the network short-circuit current. One of the important steps in selecting a cable core and screen is to check their resistance to short-circuit (fault) currents, taking into account the magnitude of the current (If) and its duration (tf). Where can we get the duration from? Unfortunately, IEC 60949 does not provide answers to this (and to many other questions). Today, let us try to talk about the fault time (tf) selection.

Simplified formulae for selecting the minimum cross-section of the core (Fc) and screen (Fs) are shown in the upper right corner. To use them, coefficients (Kc, Ks) are needed, depending on the properties of the materials (copper, aluminium), the initial temperature before the short circuit and the final temperature, to which it is not dangerous to heat the insulation. However, what time should we substitute into these formulae? It’s not exactly 1 second (which would be convenient), but then what?

If we do not complicate things, we can suggest four options for selection of the time (tf):
1️⃣ everything is OK, everything is fast and clear;
2️⃣ failure of the circuit breaker of the line (and breaker of transformer needs to be used);
3️⃣ tripping of the backup protection, and not a main protection;
4️⃣ everything has failed (both the line breaker and the main protection).

In fact, there are additional factors complicating the time calculation. For example:
✅ sometimes there is an automatic reclosure on the line (for example, if the cable line is a section of an overhead line);
✅ it is not always possible to divide (relay) protection into main and backup;
✅ the cable line is powered from two sides, and the short circuit point is supplied from two sides of the line, and each of the two sides has its own disconnection time.

When should we use each of these four options? In my opinion, options 2️⃣ or 3️⃣ should be used most often when checking cables for thermal stability. Option 1️⃣ can be used for short cables (less than 500 m, without cable joints) because these lines are more likely to be completely replaced after a short circuit rather than repaired.

Option 4️⃣ deserves special attention, because in some cases (when laying outdoors, in tunnels, or even in pipes/ducts) the cable may self-ignite if it has temperature above 400℃. This is the most difficult scenario, and it seems to me that we don’t pay enough attention to it.

A detailed look at the choice of cables in terms of thermal resistance and non-self-ignition is described in Part 5 of the book “High Voltage Cable Lines”, 2025.