Three-core cables of medium voltage 6-35 kV, as a rule, are used only for small core cross-sections – up to 240 mm2 (although there are 300, 400 and more). Each of the three phases A, B, and C on top of XLPE insulation has its own individual screen, most often made of copper tape (foil) or copper wires. As a result, the three-core cable design resembles three single-core cables that were combined into a single structure during production.

The screens of a three-core cable usually have a minimum cross-section (this is done to lighten the cable and ensure its flexibility) and they are always grounded on both sides. There is a great variety of designs of three-core cables, but today let’s divide them into two simple groups:
✅ The screens of phases A, B, and C do not contact each other.
✅ The screens of phases A, B, and C contact each other.

It turns out that the absence/presence of contact between three screens affects many things:
1️⃣ The compactness of the cable, its cost, weight, flexibility.
2️⃣ The ability of screens to pass short-circuit currents.
3️⃣ Screen losses in normal mode.

Let us discuss 1-2-3 in more detail.

1️⃣ It is obvious.

2️⃣ If the screens do not contact each other, then in case of damage to the insulation of one of the phases of the cable, the fault current (If) from the core through the damaged insulation will enter the screen of this phase and then, along the screen, will go to the left (Is1) and right (Is2) to the nearest grounding. Since the cross-section of an individual cable screen (Fs) rarely exceeds 25-35 mm2, it is highly likely that the cable screen will be dangerously overheated by the current passing in it. This will require cable replacement along the route of the line.

If the screens contact each other, then in the event of damage to the insulation of one of the phases, the fault current will flow not in one screen (Fs), but in three contacted (equivalent cross-section 3Fs). Thus, contacting the screens significantly reduces the risk of overheating of the insulation. This is especially true in networks with an isolated neutral, but not only.

3️⃣ If the screens do not contact each other, then we have three single-core cables with screens grounded at the ends of the line. In such lines, the core current induces a current in the screen. The power loss ratio in the screen (Ps) and core (Pc) for a three-core cable can easily reach 0.05 (that is 5%). If the screens contact each other, then instead of three separate screens (and the associated “screen-screen” contours), in fact, we have one common screen in the form of a trefoil, in which there will no longer be circulating currents. Thus, we will have Ps/Pc=0, and it helps to increase a cable line ampacity by up to 5%.

So, pay attention to whether the cable screens contact each other. This is important for reasons mentioned in the post.

You can read more in the book.