Today's offices are full of electric or electronic equipment。 Each device needs a power cord and so near the desktops, under the tables, or near the cupboards, we find many electric cables。
The electromagnetic emissions of a simple power cord would not be so harmful if the cable was far enough from us. In fact if the office is quite big, and the cables are kept well far from the seats the EMF pollution is quite low.
But offices come in every size, some offices are small and crowded, so the cables are laying on the floor under the tables and the seats。
There is also another problem created by the interaction of the electric cables with the metal furnitures。
If we measure the low frequency electric and magnetic field at 5 meters from a cable laying on the floor, with nothing between the cable and the instrument, and of course with the instrument far from other EMF sources, we find that the fields are equal to the background values of the room。 Instead if the cable is near a 5 meters long metal cupboard and we measure the fields at the opposite side of the cupboard we find that the fields are much stronger。
The electric field near an unshielded cable is very high, and if the cable is at short distance from a metal cupboard this field induces electric vibrations into the metal, which propagate to the whole cupboard and reach also the opposite site.
In other words to provoke an electric vibration in a metal object there's no need of a tight contact with an electric conductor under voltage, it is enough that the voltage source is near to the metal object.
This effect causes unexpected EMF, we may think to be far enough from the cable, but a metal cupboard or other furniture between the cable and us brings some electric field on us.
The pending cables in front of the desk are harmful also for people who sits or stands in front of the desk. If the desk is placed in front of a wall one may think to hide the cables under the desk, but with the lots of instruments needed it is easy to create a concentration of cables with incredible high values of electric and magnetic fields.
Some of the power cords under the desk may be very near to the legs of the person who is sitting at the desk, and the electric field of the cables can cause serious harm that appear as muscle or bone inflammations.
Offices with a high density of computers and related machines often are build on special floors that have empty space below to contain the cables。
This solution is very good to provide electric sockets and also communications sockets to many points of the room, removing the cables from the pathways, but the cables are still there, just a few centimeters under our soles.
The metal structure that holds such floor is subjected to the electric fields of the cables and so it spreads the electric vibrations to the entire room. The underlaying structure also creates a vertical electric field, especially if the ceiling is also covered with similar metal structures. This vertical electric field is another source of pollution in such rooms.
A possible solutions to the electric fields created by power cords in offices is to use shielded cables.
The electric field is blocked almost completely by the shield of a shielded cable. A shielded cable do not induces electric vibrations in near metallic objects.
Shielded cables should be used also for the cables inside the raised floors, so the electric fields near the metallic structure of the floor would be eliminated almost totally.