It is believed that “we cannot have a closed contour of magnetic material around an AC single-core cable”. However, in practice, there are issues with the application of such a rule. For example, would the following contours be dangerous?
➡️ it does not tightly cover the cable, but has large transverse dimensions;
➡️ it has a side wall which is not massive (thick), but made of a thin steel rod.

In fact, there are many places in the industry where a single conductor passes through some kind of obstacle having metal elements that create a closed contour (loop) of magnetic material. For example, a bushing of power transformer through its metal tank or cable through the wall.

However, the main focus of the industry is on the cables only, or rather on the cable terminations. This is not accidental. The fact is that terminations must have a solid foundation and therefore are installed on the massive metal elements. As shown in the photo.

In the photo, a metal frame with thick side wall was created around the cable, to place 4 insulators in its corners, on which the base plate and the termination itself were then planned to be mounted. By mistake, such a frame was in the form of a closed loop.

The termination installer discovered the error in time, disassembled the frame, and created a gap with a saw (then the frame will be reassembled). As a result, the frame began to look like a horseshoe, and, we can say, the installer acted as a blacksmith.

If the massive magnetic contour around the cable were closed, then a magnetic field (it “rotates” around the cable) would be closed in it, and therefore effects similar to what happens in the cores of power transformers would occur:
♨️ hysteresis losses;
♨️ eddy current losses.

In transformers, to reduce power losses in the massive core, it is made not monolithic, but consisting of thin plates isolated from each other. Similarly, with a cable, if it passes through a massive metal frame, then in such a frame there will be power losses and dangerous heating. To solve the problem, there are two options to choose from:
✅ make the contour consist of thin frames isolated from each other (as they do with power transformers);
✅ make a gap in this contour (as shown in the photo).

This means that it is the massive monolithic metal contours that pose the danger. If the cable passes through a wall of the building in which there is a closed contour of thin reinforcement (thin metal rods), it is unlikely that this will be enough to create dangerous heating. The problem will occur only in case of massive contour around a single conductor.

In general, to calculate the heating, we need to know:
👉 the size of the contour (the larger it is, the further away it is from the cable, and the smaller the magnetic field there will be);
👉 the thickness of the contour side wall (this is a thin rod or a thick frame).

But it’s better to always have a horseshoe. It is no coincidence that they say: A HORSESHOE – FOR LUCK.