Unipolar power cables are commonly used for high current distribution lines in industrial and civilian environments. A classic example is the power supply of air conditioning system engines where it is common to find more cables used in parallel, adding to thousands of amperes. The exposure level caused by induction is clearly to be kept within the quality target (3 μT), but sometimes more stringent limits are required (0.1 μT near electronic microscopes for example).

Figure 3 shows the coloured map of the magnetic induction of a three-phase line with 400 mm2 cables with an ampacity of 605 A.

It can be seen that in order to stay below 3 μT the distance from the centre of the line is about 1.4 m. Table 4 shows the buffer zone values associated with the induction of 3 μT for lines up to 2000 A consisting of single core cables in parallel.

TABLE 4

Thermal capacity of the line (A) Nominal diameter of the conductors (mm2) Phase layout Protection distance 3 μT (m)
88 16 RST 0.24
117 25 RST 0.3
144 35 RST 0.37
175 50 RST 0.45
222 70 RST 0.55
269 95 RST 0.65
312 120 RST 0.74
355 150 RST 0.83
417 185 RST 0.95
490 240 RST 1.1
530 300 RST 1.21
605 400 RST 1.39
834 2x185 RRSSTT 1.9
980 2x240 RRSSTT 2.2
1251 3x185 RRRSSSTTT 2.85
1470 3X240 RRRSSSTTT 3.3
1668 4X185 RRRRSSSSTTTT 3.8
1960 4X240 RRRRSSSSTTTT 4.4

Shielding channels have a high shielding performance with an average screen factor of approx 30. Figure 4 shows a coloured map of the magnetic induction of a three-phase line with 400 mm2 cables with an ampacity of 605 A.

The comparison with induction levels in the absence of shielding is obvious. The reduction in levels of induction involves a significant reduction in buffer zones. Table 5 shows the bands with respect to the different lines inserted into the shielding channel while Figure 4 shows the comparison between the buffer strips with and without channel shielding.

TABLE 5

Thermal capacity of the line (A) Nominal diameter of the conductors (mm2) Phase layout Protection distance 3 μT (m)
88 16 RST -
117 25 RST -
144 35 RST -
175 50 RST -
222 70 RST -
269 95 RST -
312 120 RST -
355 150 RST 0.15
417 185 RST 0.17
490 240 RST 0.2
530 300 RST 0.21
605 400 RST 0.25
834 2x185 RRSSTT 0.35
980 2x240 RRSSTT 0.4
1251 3x185 RRRSSSTTT 0.52
1470 3X240 RRRSSSTTT 0.6
1668 4X185 RRRRSSSSTTTT 0.69
1960 4X240 RRRRSSSSTTTT 0.8