ELF SHIELDING

Distribution Lines and Shielding Cable Trays

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

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

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

Fig. 4 - Coloured map of the magnetic induction of a three-phase line with 400 mm2 cables, with an ampacity of 605 A, placed within a shielding channel.

Fig. 4a - Comparison between the protection distance at 3 μT (m) with and without shielding channel.