It’s August, the month of vacations, and people don’t have time for cables. As a result, the cables have to remind of themselves in all possible ways, even by making different sounds. Today is one such example. Please watch (listen to) the VIDEO. Do you hear?
 
The video shows a 0.4 kV cable line consisting of 4 single-core cables – three operating phases (A,B,C) and neutral (N). The cable line is laid in the basement, partly in the air, partly in metal trays. It doesn’t seem to be anything special, but where does the sound come from, similar to the sound that usually comes from power transformers?

Measurements of currents in phases (A,B,C) and neutral (N) showed that the system is balanced as it should be, and there is no current in neutral (N). What happens then?

It turns out that we are not dealing with a simple group of four single-core cables (A,B,C,N), but with a group of armored cables. Moreover, the armour of these single-core cables is made of steel tapes, which are magnetic. Everyone knows that there should be no closed circuits made of magnetic materials around single-core cables, but for some reason someone ignored this basic rule of the cable industry and made single-core cables with magnetic armour. What is the result?
 
As a result, the magnetic field of each single-core cable, caused by the current in its conductor, is concentrated in the armour, as is the case with the magnetic flux of power transformers, which is concentrated in their core. Is it any wonder that the cable vibrates in the same way as the transformer core does?
 
It reminds us of an old joke about a professor asking a student, “How does a transformer work?” and the student replies– “Whooooo….” Now this joke can be told in the cable industry. Listen to the cables, and it can tell you a lot about the project. Professional cable works in silence, without witnesses!
 
By the way, are there any advantages from the fact that the cable vibrates? Definitely 🙂:
1️⃣ It’s easier to find a cable in the ground, because there’s enough vibration source.
2️⃣ It’s easier to find a cable in the dark, because it’s enough to follow the sound.
3️⃣ It’s easier to make sure that there is current in the cable – just listen.
4️⃣ More heat from the cable on cold winter.
5️⃣ We can use this cable’s surface as a shaker for a summer milkshake!
 
Have a nice summer August! Starry nights are especially good in August!

COMMENT, JOKING APART:
So, when can we use magnetic tape armour?
❌ Single-core cable
✅ Two-core cable (1 phase + neutral)
✅ Three-core cable (3 phases)
✅ Four-core cable (3 phases + neutral)

You can discuss this post on LinkedIn, in the “High Voltage Cable Lines” group, by following the link to the post.