Single-core cables of medium voltage 6-35 kV often have simple grounding of the screens on both sides (“solid bonding”). As a result, the cable screens form closed contours, and the magnetic field of the core current induces currents in these contours, that is screen currents.

Induced currents (Is) in the screens pass through the screens 24 hours a day throughout the lifecycle of the line, and they cause power losses (Ps). Losses in the screens (Ps) increase the heating of the cable line, and they need to be paid (as well as power losses in the cable cores Pc).

The currents in the screens (Is) and the losses in the screens (Ps) are significantly dependent on two factors:
✅the screen cross-section (and the screen material);
✅the distance between the phases A, B and C (closed triangle ABC would be the best option).

Unfortunately, sometimes it is inconvenient for installers to lay the phases in closed triangle ABC (according to the design), and they lay the phases in a row or somehow else. This usually leads to dangerous consequences for the cable line.

The photo shows a case when a 35 kV line with three single-core cables per each phase was laid not ABC-ABC-ABC, but AAA-BBB-CCC. The rejection of the closed triangle ABC in favor of AAA led to an increase in induced currents (Is) and power losses (Ps) in the screens. As a result, the cable line was overheated and even caught fire (along with the polymer pipes in which the cables were located).

For example, let’s assume that for the closed triangle ABC, the losses in screens are small, and the ratio of losses in screens (Ps) and cores (Pc) is only Ps/Pc=0.2. It can be seen that losses in the screens are a secondary source of cable heating (since Ps<Pc).

When laying AAA, the “screen-screen” contours get increased area, and 50Hz currents in the screens (Is) can easily increase by, say, 3 times. Then the losses in screens (Ps) will increase by 3*3=9 times. Then the ratio of losses in screens and cores, in the conditions of the example, will be Ps/Pc=9*0.2=1.8. That is, losses in screens are 1.8 times (almost 2 times) greater than losses in cores. It means the screens are a main source of cable heating (since Ps>Pc).

Due to the violation of the design, the losses in the screens turned from a secondary source of cable heating into the main one. As a result, the total heat dissipation of the cable increases. This often leads to dangerous consequences – see the photo.

So, we have:
✅for ABC, the total losses are P=Pc+Ps=Pc+0.2Pc=1.2Pc
✅for AAA, the total losses are P=Pc+Ps=Pc+1.8Pc=2.8Pc

As we can see, when the installers replaced ABC with AAA, they increased the total losses in the cable line from 1.2Pc to 2.8Pc, that is, 2.8/1.2=2.5 times! What do you think happened to the cable?

If we want to have AAA-BBB-CCC, then it is forbidden to use two-side (“solid”) grounding of screens, and one-side grounding or other alternatives should be used.