mediastrip g7 · 2007. 10. 26. · currentcarryingcapacities in a,indicatedinthe appro-priate...

MEDIAstrip G7Selecting cross-sections

Cross-section design de-pends on many factors.Normally, operating cur-rent carrying capacitiesand short circuit cur-rents are the 2 parametersaffecting cross-section de- - Laying depths:sign.In the 1st case, conduc-tor's temperature in nor-mal and permanentconditions shall notexceed insulation's maxi-mum design temperature(90°C). Therfore, normalcurrent carrying capacityof system shall be com-pared to current capaci-ties indicated in cable'sspecifications, suitablyfactored with the cor-recting coefficientes.In the 2nd case, conduc-tor's temperature atthe end of a short circuitshall not exceed insula-tion's short circuit design

temperature (250°C).

The cross-section to be u-sed is the largest between ted and grounded at both ends.the 2 values above.

CURRENT CARRYINGCAPACITIES

Current carrying capacitiesin A, indicated in the appro-priate tables were calcula-ted according to the CEI20-21 (IEC287) standard.They were calculatedassuming the followingvalues:

- Maximum conductortemperature (90°C)

- Ambient temperature forinstallation in open air (30°C)

- Ambient temperature forunderground burial (30°C)

- Metallic screen interconnec-

DATA FOR CALCULATING U = 3÷10 kV → 0,8 mU = 15÷30 kV → 1,0 mU = 45 kV . 1,2 m

- Thermal resistivities (Rt)

Ground → 1°Cm/WHEPR insulation → 4,5°Cm/WPVC sheath → 5°Cm/W

- Single core cableslaid on a horizontalplane have a center-to-center distance of 2diameters