Answer Key
- The primary purpose of IEC 183 is to provide guidance on the selection of high-voltage cables for use in three-phase alternating current systems operating at voltages exceeding 1 kV. It helps users make appropriate choices regarding cable construction, insulation level, and conductor size based on specific service and installation conditions.
- Uo is the rated r.m.s. power-frequency voltage between each conductor and the screen or sheath for which the cable and its accessories are designed. U is the rated r.m.s. power-frequency voltage between any two conductors for which the cable and accessories are designed.
- Um represents the maximum r.m.s. power-frequency voltage between any two conductors that a system can sustain under normal operating conditions at any time and point. It is considered the highest voltage of the system and is a critical parameter for ensuring compatibility between the cable and the equipment it connects to, such as switchgear and transformers.
- Amendment 1 (1990) replaces the original Table I from the 1984 publication. This new table updates the specified relationship between the rated voltage of cables and accessories (Uo/U) and the corresponding highest voltage for equipment (Um).
- Category A systems are those where earth faults are cleared as rapidly as possible, within one minute. Category B systems are those where a single phase-to-earth fault condition can persist for a short period, typically not exceeding one hour. Category C comprises all systems that do not fall into categories A or B, often where faults may be intermittent or persist longer.
- A “Lightning overvoltage” is a phase-to-earth or phase-to-phase overvoltage at a given location caused by a lightning discharge or other event. Its wave-shape is standardized for insulation coordination purposes, and cables must be designed with a lightning impulse withstand voltage (Up) sufficient for the system’s protection levels.
- Operating conditions that must be specified include: (a) nominal voltage of the system, (b) highest voltage of the three-phase system, (c) system frequency, (d) type of system earthing, (e) maximum rated current (continuous, cyclic, or emergency), and (f) expected short-circuit currents.
- The three primary factors for selecting conductor size are: (a) the maximum temperature the cable will experience under various load conditions (continuous, cyclic, short-circuit); (b) the mechanical loads imposed on the cable during installation and in service; and (c) the electrical stress in the insulation, as a small conductor may result in unacceptably high stress.
- At high altitudes, the air density is lower than at sea level, which reduces its electric strength. This means that for terminations, the standard air clearances may be insufficient. For any altitude above 1000 m, the air clearances must be increased by a suitable amount to ensure proper insulation.
- According to the updated Table I, a cable with a rated voltage (Uo/U) of 36/66 kV corresponds to a “Highest voltage for equipment” (Um) of 72.5 kV.