The factors to be considered for evaluating the suitability of a cable for a particular application are load, system voltage, cable insulation, short circuit rating, environmental conditions, sheath and protective coverings, heat dissipation losses, economic considerations etc. The following points are important
- Maximum continuous current rating
- System Earthing
- Voltage drop
- Conditions of installation
- Expected Short Circuit Level of System
- Temperature rise
- Economic evaluation
1. Maximum continuous current rating
While choosing the conductor size for continuous rating, proper care should be taken of all the rating factors depending on actual conditions of installation. Like any other equipment selection, current carrying rating of the cable is governed by a permissible rise in temperature of the cable which the insulation provided to the cable should have to withstand. This depends on the amount of heat produced and the surrounding temperature of the soil. The current rating of the cable apart from the above factors also depends on the large number of factors such as the method of cable laying employed, the spacing between the cables, number of cores of the cable and thermal conductivity of the soil.
The type of system: Earthed or un-earthed?Is it solid grounding resistance/reactance grounding? The unearthed system will require full insulation from the core to the ground and the cable will be costly compared to the earthed system. For un-earthed cables, more insulation is provided in order to withstand the higher voltage stresses during the short circuit faults.
3. Voltage drop
The cable should be selected such a way that at full load, the voltage drop should be within the permissible limits.
4. Conditions of installation
Methods of installation, estimated thermal resistivity of soil, type of covering, type of armouring, the need if any for additional corrosion protection.
5. Expected Short Circuit Level of System
On the basis of the expected short circuit current and time of clearance, an appropriate conductor size for the cable may be selected. The cables should be selected such that it has to withstand stresses and temperature rise in the event of short circuit faults
6. Temperature rise
During short circuit temperature of cable rises, cables should be able to withstand the rise in temperature for a prescribed duration without getting damage. It has to allow continuous current during normal operation without rising in the temperature and should be within desired limits.
7. Economic evaluation
One of the important factors to be considered while choosing the cable is cost evaluation.