When designing cable lines, we use software to check ampacity, taking into account the key influencing factors:
✅ cross-section of cable elements;
✅ the mutual arrangement of phases and circuits;
✅ screens’ bonding type;
✅ laying conditions;
✅ environmental parameters.

It turns out that almost every software known in the cable industry calculates ampacity perfectly, and also helps with calculating induced currents and voltages in screens. However, software does not calculate one important parameter – the amount of power losses in screens and their cost. At the same time, the availability of such information on “cost of losses” is extremely important for choosing the optimal screens’ bonding type and, therefore, for determining the final value of the ampacity of the cable line. Let us explain this using an example.

If the line is made with single-core cables, the user can usually choose two options for calculating ampacity:
1️⃣ with losses in the screens (solid bonding of the screens);
2️⃣ without losses in the screens (single-point grounding of the screens or cross-binding of the screens).

In case 1️⃣, the ampacity is less than in case 2️⃣. If two values of ampacity differ more than by 10%, the user usually decides to abandon the solid bonding in favor of an option without losses in the screens. Next, the software helps him calculate the induced voltage on the screen:
✅ in case of single-point grounding of the screens;
✅ in case of cross-binding of the screens.

Based on the results of comparing these voltages with acceptable ones, the user will understand which of the screen grounding is the most optimal.

It can be seen that the design of the cable line focuses on two things:
➡️ ampacity;
➡️ induced voltage (if solid bonding is rejected).

It turns out that these two calculations are not enough. In fact, software should show the user one more value – the annual cost of power losses in the screens. With solid bonding of screens, this cost can be significant and will help the user make a more reasonable decision about a particular screen grounding/bonding type.

The slide shows an example of a calculation using the methodology from the book. For this data, the user of software will receive that when the screens are solidly bonded, the ampacity is 94% of the ideal case when there are no losses in the screens. That is, ampacity decreased by only 6%, and the user will of course decide to make a solid bonding. Which would be incorrect.

If we calculate the cost of losses in screens (most software will not show this), then we see that in a year it can reach 3 thousand euros (or up to 100 thousand euros over the lifecycle of the cable line). This is quite enough to understand that solid bonding is a bad idea, and it is necessary to apply a lossless grounding for the screens. Since the line is only 300 meters, single-point grounding will be quite acceptable.

Interestingly, for 6-35 kV lines, the use of link boxes and sheath voltage limiters (SVL) is redundant in many cases. It is enough to protect the staff from touching the screens, for example, by using tape or heat shrink.

Well, let me please make a conclusion – would be great if developers of software would add a calculation of the cost of power losses in screens. It’s not so difficult, but it will help users make a more informed decision about the best design of the cable line.

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