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| Cables and electromagnetic fields |
Any current-carrying conductor, including a cable, radiates an electromagnetic field. Likewise, any conductor or cable will pick up energy from any existing electromagnetic field around it. These effects are often undesirable, in the first case amounting to unwanted transmission of energy which may adversely affect nearby equipment or other parts of the same piece of equipment; and in the second case, unwanted pickup of noise which may mask the desired signal being carried by the cable, or, if the cable is carrying power supply or control voltages, pollute them to such an extent as to cause equipment malfunction.
The first solution to these problems is to keep cable lengths in buildings short, since pick up and transmission are essentially proportional to the length of the cable. The second solution is to route cables away from trouble. Beyond this, there are particular cable designs that minimize electromagnetic pickup and transmission. Three of the principal design techniques are shielding, coaxial geometry, and twisted-pair geometry.
Shielding makes use of the electrical principle of the Faraday cage. The cable is encased for its entire length in foil or wire mesh. All wires running inside this shielding layer will be to a large extent decoupled from external electric fields, particularly if the shield is connected to a point of constant voltage, such as earth. Simple shielding of this type is not greatly effective against low-frequency magnetic fields, however - such as magnetic "hum" from a nearby power transformer. A grounded shield on cables operating at 2 kV or more gathers leakage current and capacitive current, protecting people from electric shock and equalizing stress on the cable insulation.
Coaxial design helps to further reduce low-frequency magnetic transmission and pickup. In this design the foil or mesh shield is perfectly tubular - i.e. with a circular cross section - and the inner conductor (there can only be one) is situated exactly at its center. This causes the voltages induced by a magnetic field between the shield and the core conductor to consist of two nearly equal magnitudes which cancel each other.
The twisted pair is a simple expedient where two wires of a cable, rather than running parallel to each other, are twisted around each other, forming a pair of intertwined helices. This can be achieved by putting one end of the pair in a hand drill and turning while maintaining moderate tension on the line. Field cancellation between successive twists of the pair considerably reduces electromagnetic pickup and transmission.
Power-supply cables feeding sensitive electronic devices are sometimes fitted with a series-wired inductor called a choke which blocks high frequencies that may have been picked up by the cable, preventing them from passing into the device.
