The cable conductor is one of the most important components of a power cable, including the conductor core, metal shielding layer, and metal sheath. In practice, we are most concerned with the conductor core and the metal shielding layer. All grades of power cables must have a conductor core, usually made of high-purity copper or aluminum. The conductor cross-section can be circular, elliptical, sector-shaped, or hollow circular. Small cross-section conductors are generally composed of a single wire, while larger cross-section conductors are composed of multiple strands of wire twisted together. Cables of medium and high voltage grades have a metal shielding layer, and some low-voltage cables with special requirements also have a metal shielding layer. For rubber and plastic power cables, the metal shielding layer is usually made of copper wire or copper tape.
Basic Measurement Principle
As shown in the figure, a DC power supply E is applied to both ends of the resistor under test. By measuring the circuit current I and the voltage U across the resistor under test, the resistance value RX can be measured using Ohm's law R=U/I.
In practice, since the resistor under test RX is a cable conductor, its resistance is small, usually below a few ohms. To eliminate the influence of the voltage and current meter leads and their contact resistance with the resistor RX on the test results, two measurement methods are generally used: the bridge method and the four-terminal constant current method.
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