A shielded cable is a type of electrical cable with one or more cables inside, enclosed by a conducting layer that shields the cables from external electrical noise and interference. When reed switches are tested, shielded cables should be used to connect the device up to the test equipment to ensure accurate measurements.
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A shielded cable or screened cable is an electrical cable that has a common conductive layer around its conductors for electromagnetic shielding. This shield is usually covered by an outermost layer of the cable. Common types of cable shielding can most broadly be categorized as foil type (hereunder: metallised film), contraspiralling wire strands (braided or unbraided) or both. A longitudinal wire may be necessary with dielectric spiral foils to short out each turn.
The shield acts as a Faraday cage – a surface that reflects electromagnetic radiation. This reduces both the interference from outside noise onto the signals and the signals from radiating out and potentially disturbing other devices (see electromagnetic compatibility). To be effective against electric fields (see also capacitive coupling), the shield must be grounded. The shield should be electrically continuous to maximize effectiveness, including any cable splices. For high frequency signals (above a few megahertz), this extends to connectors and enclosures, also circumferentially: The cable shielding needs to be circumferentially connected to the enclosure, if any, through the connector or cable gland.
Some types of shielded cable use the shield as the return path for the signal. As contrasting examples, coaxial cable does, whereas twinax cable does not.
High voltage power cables with solid insulation are shielded to protect the cable insulation, people and equipment.