Shielded cable is an electrical cable with one or more conductive shields made of metallic foil or woven wire to minimize electromagnetic interference. Power cords can be a major source of EMI, and the degree of shielding required depends on the ambient EMI in a given environment. The two most commonly used types of shielding are foil and stranded wire, with foil providing excellent coverage but being brittle, and braided wire being more expensive but easier to wire. Double shielding is used in areas with extremely high EMI.
Shielded cable is electrical cable in which the conductor and its surrounding insulating layer have been enclosed in one or more conductive shields. These are usually made from metallic foil or woven wire. A protective jacket typically forms an outer layer. This works to minimize the electromagnetic interference (EMI) emitted and received by the cable.
Power cords can be a major source of EMI in an environment. These can act as antennas in transmitting interference or as receivers in acquiring and conducting interference to other devices. The level of EMI is a result of the power requirements of any electrical equipment in operation. A factory floor would typically have a higher environmental EMI level than an office or residence. The degree of shielding required to mitigate interference will vary accordingly.
In signal cables, EMI was first recognized as Radio Frequency Interference (RFI). The effect is commonly referred to as noise. Coaxial cable is a familiar version of shielded cable, often used in radio frequency transmission, cable television distribution, and computer networks. Security systems in areas prone to high EMI use shielded cables to help prevent false alarms.
Shielding can mitigate electromagnetic interference in two ways. It can reflect interference or it can conduct interference to earth. EMI is either negated or so reduced in potency that it has no noticeable effect on the conductor.
The degree of shielding of the cable depends on several factors which mainly include the ambient EMI in a given environment. The cost and diameter needed, as well as the weight and flexibility of the cable, are all additional factors. An unshielded cable may be acceptable for use in an industrial environment if the surrounding environment can be strictly controlled.
The two most commonly used types of shielding are foil and stranded wire. Foil shielding uses a thin layer of aluminum or copper backed by a polyester coating for added strength. This technique provides excellent coverage for the conductors, although the film is quite brittle. Connecting a connector and grounding can be difficult. A drain wire, which is uninsulated wire, is sometimes used to terminate and ground a foil shield.
A braided screen is a mesh woven from copper wire. This does not provide effective coverage like foil. The conductivity of the copper and the greater mass of the wire mesh work together to make braid an effective choice. It’s also much easier to wire a connector and terminate to ground. This type of cable is more expensive than foil and increases the likelihood of size, weight, and flex issues.
In areas with extremely high EMI, a double shielding approach has been used. Shielded cable has an inner foil shield and an outer layer of braided copper. This approach takes advantage of the greater foil coverage and superior conductivity of the copper braid.
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