10. Higher Voltages. 1230

For circuits operating at or above 2,000 volts between conductors, a conductive shield may surround each insulated conductor. This equalizes electrical stress on the cable insulation.

The individual conductor shields of a cable are grounded at the ends of the cable, and at locations along the length if voltage rise during faults would be dangerous.

Cables for power distribution of 10 kV or higher may be insulated with oil and paper, and are run in a rigid steel pipe, semi-rigid aluminum or lead sheath.

A high voltage cable is designed for 400 kV. Large center conductor carries the current, smaller conductors act as a shield to equalize the voltage stress in the thick polyethylene insulation layer.

Many multiconductor cables have a bare or insulated grounding or bonding wire for connection to the ground. Electrical power cables are often installed in raceways, including electrical conduit and cable trays, which may contain one or more conductors.

All electrical cables are somewhat flexible, allowing them to be shipped to installation sites wound on reels or drums. Where applications require a cable to be moved repeatedly, such as for portable equipment, more flexible cables called "cords" or "flex" are used.

11. Shielded Cable. 1386

A shielded or screened cable is an electrical cable of one or more insulated conductors enclosed by a common conductive layer. The shield may be composed of braided strands of copper (or other metal), a non-braided spiral winding of copper tape, or a layer of conducting polymer.

Usually, this shield is covered with a jacket. The shield acts as a Faraday cage to reduce electrical noise from affecting the signals, and to reduce electromagnetic radiation that may interfere with other devices.

By twisting two conductors of a balanced-line signal circuit into a twisted pair, some cancellation of inductively coupled noise is obtained. However, a metallic shield layer over the twisted pair provides better suppression of noise.

Coaxial cable is used at higher frequencies to provide controlled circuit impedance, but the outer tubular conductor is also effective at reducing coupling of noise into a circuit.

The common method to wire shielded cables is to ground only the source end of the shield to avoid ground loops. However, in airplanes special cable is used with both an outer shield to protect for lightning and an inner shield grounded at one end to eliminate hum from the 400 Hz power system.

The use of shielded cables in security systems provides some protection from power frequency and radio frequency interference, reducing the number of false alarms being generated.