In 6-35 kV network, suppose we have busbars ABC, with two lines, and a ground fault has occurred in phase A of a line (1).

In case of isolated neutral, the ground fault current (Igf) is determined by the EMF (Ea) and the capacitances of all network to the ground, and therefore it is a capacitive current (Icap). Usually, this current is small and is not enough for tripping a current protection, and we have to use zero-sequence protection (3I0). It can be seen from the left diagram that:
⚡ for faulty line, current 3I0 goes “to the line”;
⚡ for normal line, current 3I0 goes “to the busbars”.

The difference in the sign of 3I0 helps to find the faulty line. The difference in 3I0 values (regardless of sign) can also be used, but not always. For example, in the case of a network consisting of only two lines of equal length, the values of 3I0 are equal for (1) and (2), and it is not possible to use them to find a faulty line. Problems will also arise in the case when a fault is unstable (of arc type), since 3I0 will have a complex form. It can be seen that finding and disconnecting the faulty line is a difficult task. However, there are additional features that depend on the type of network.

➡️ OVERHEAD network
In overhead networks, no one is in a hurry to turn off the fault, because there is a hope that, due to the small value of Igf=Icap (less than tens of Amperes), arc will self-extinguish, and the network will return itself to normal operation. This is also a rational scenario because overheads often power remote consumers who do not have backup power sources, and have a single circuit which cannot be lost.

➡️ CABLE network
In cable networks, Igf=Icap can reach hundreds of Amperes, and, for such a current, there are no problems with finding a faulty line. Also, with such high currents, an unstable fault (arc) is unlikely. At the same time, any delays with disconnection are useless and even harmful because:
🔹 the fault is not capable of self-extinguishing (this was possible for overheads);
🔹 cable lines have two circuits, and there is no problem for consumer to disconnect one;
🔹 prolonged passage of fault current increases the size of the damage and may affect neighbouring cables of phases, and also may cause electrical injuries and fires.

However, instead of fast finding and disconnecting of a faulty cable line, sometimes there is an idea to decrease the ground fault current (Igf) using a reactor. But it can be seen from right diagram, that reactor makes 3I0 currents in (1) and (2) of the same sign which create a problem for finding a faulty line.

We can conclude that reactor reduces the ground fault current (good), but makes it more difficult to find a faulty line (bad). This is all acceptable for overhead networks where there is no rush in fault finding, but not acceptable for cable network where we need to disconnect quickly.

For cable networks, only two types of neutral are preferable:
✅ isolated;
✅ resistively grounded.