The following table shows the values of electric field measured near a non shielded and a shielded power cord both in the same conditions, with the cable inserted in the wall socket at 220 V AC and the other end disconnected, quite far from other objects。 Of course the wall socket has the correct ground connection。 The instrument used is sensitive to the AC electric field in the band 16 Hz - 2 KHz。
|Distance (cm)||Non Shielded (V/m)||Shielded (V/m)|
The electric field around the cable is almost the same whatever device is attached to the cable, it depends mainly on the voltage of the wall socket.
The magnetic field instead depends greatly on the current flowing in the cable so if the cable is not connected, or it is connected to a device but the device is off, the magnetic field is almost zero.
The following table shows the values of magnetic field measured near a non shielded and a shielded power cord that are connected to a typical office computer, which is turned on.
|Position||Non Shielded (nT)||Shielded (nT)|
As we can see the shield reduces slightly also the magnetic field, however it can't block it because the magnetic field can only be absorbed by ferromagnetic materials。
The major advantage of the shielded power cord is the nearly complete elimination of the alternate electric field, which reaches several hundreds V/m near the unshielded cord, and which can be harmful if the cord is near your body, or if the cord passes near a metallic object that can spread the field to a larger distance.