Temperature classification and overcurrent protection requirements of electrical cables in explosive atmospheres
The electrical cables in the explosive atmosphere include a wide range of single core and multicore cables for high voltage or low voltage power distribution as well as for control and instrumentation. The electrical cables, except cables of intrinsically safe circuits, are normally not installed in zone 0 or zone 20. The instrumentation and lighting cables are mostly installed up to zone 1 or zone 21 areas. High voltage power cables are mostly passing through zone 2 or 22 or non-hazardous area.
Why are cables heating due to current flow?
The current flow in the conductor causes a temperature rise because of a heat dissipation. The heat dissipation is a result of an internal resistance of the conductor subject to current flow. The heat dissipation may also be a result of a current flow in electrical insulation as well because of energy losses due to the magnetic hysteresis of the materials. However, if suitable maintenance is exercised, the heat dissipation resulting from the current flow in insulation is negligible. The suitable maintenance includes, for example, high potential test followed by megger test, measurement of insulation dissipation factor and visual examination.
Electrical protection of cables
The electrical protection of cable includes overcurrent and overvoltage protection. The overcurrent protection is to ensure the electrical cable is not damaged by thermal or electro-dynamic effects of the overcurrent, as well as to limit the cable’s maximum surface temperature, which is required by the area classification. See IEC 60079-14/9.3.8, edition 5.0 and IEC 61892 – 7/7.8, edition 2.0 and IEC 61892-2 / 10.1.1. ed.2.0. The surface temperature limit of area classification depends on the ignition temperature of the explosive mixture. The increase of current or it’s duration, results in an increase of insulation temperature, because of heating by current flow in a conductor. Thus, to prevent ignition of the explosive mixture due to insulation failure or excessive temperature of the insulation, the duration, and magnitude of the overcurrent is limited by protective devices as demonstrated on the schematic below.Subsequently, the time-current curve of protection against overtemperature of cables’ surface is presented below.
Subsequently, the time-current curve of protection against overtemperature of cables’ surface is presented below.
How is the current magnitude and duration limitation selected to protect the cable?
The current magnitude and duration limitation is selected in accordance with: cross-sectional area of the conductor, the resistivity of the conductor, insulation material, type of installation, maximum ambient temperature and external sources of heating if such sources are present. The figure above presents the time current curve of a protective device. The trip time of the protective device is shorter than the time required to heat the insulation surface temperature to 135 degrees C at 70 degrees C ambient. Therefore the protection of the cable is suitable for temperature class T4.
Overcurrent and surface temperature of instrumentation cables
The duration of short circuits may range from a millisecond and less, up to minutes and hours. In the case of control and instrumentation cables, which have relatively small cross-sectional area and may have a significant length, the short circuit currents may not significantly exceed the nominal currents. Therefore, the duration of short circuits may be significant, due to the time delay of a protective device. The time delay of protective devices may not always be further limited due to coordination and selectivity requirements. In such circumstance, the insulation surface temperature may be critical.
Overcurrent and surface temperature of power cables
The power cables may have significant short-circuit current magnitude. Thus, the short circuit duration time is relatively low, usually limited to few or several cycles of voltage. Several voltage cycles equal around 50 ms in 60 Hz systems. Therefore, the time of heating the insulation by the conductor is negligible as well as the temperature rise, provided sufficient margins of cable capacity are included. However, the dynamic effects of high magnitude current flow are significant. Therefore, suitable protective measures, for example, sufficient spacing of cable cleats in the case of single core power cables are provided.
The criticality of the insulation temperature depends on the duration and magnitude of overcurrent which can heat up the insulation above the maximum surface temperature, as well as on frequency of occurrence of the explosive atmosphere. Therefore, the confirmation of surface temperature of the cables in hazardous area shall be considered case by case based on the specifications of electrical cables and in accordance with the area classification requirements.
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