EMFs Around Shielded and Unshielded Cables

Underground cables are either direct buried in the soil or installed in ducts. Within these burial systems, they can be either unshielded or shielded with metal shielding units. The internal structure of an underground power cable already ensures that no electric field can escape, whilst additional external metal shielding acts to reduce the magnetic field. Here is the difference shielding makes in terms of magnetic fields:

• Unshielded cables: Unshielded cables buried at a depth of around 2 metres, produce very high magnetic fields when standing on the ground above the installation, at around 30 micro Teslas. This is because the short distance from the cable cannot be compensated for by the increased shielding effect of the soil. However, because of the earth, when standing at 15 metres from the line of the cable the magnetic field will have dropped close to zero.
• Shielded cables: Metal shielding units around a cable ensures that the magnetic field around it is almost zero, even right above the line of the cable.