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Causes of cable failure and need of protection


    Cables are the mortar that holds the bricks of equipment in an electric system. They are not unlimited in their power capability and hence need to be protected to prevent being exposed to operation beyond their capability. Cable protection is required to protect the cable, personnel and the equipment. High temperature and thermal stresses are the main causes of cable failure.


    Influence of fault currents on power cables


    Cable conductor temperature rises rapidly due to the high I 2R losses in the cable under the short circuit conditions and could result in permanent damage of the cable insulation, shield and the possibility of a fire. A standard cable insulation structure is given in Fig. below.




    The temperature of the cable, while in service, depends on the load current, the ambient temperature and the installation conditions. The installation conditions include confined space, metallic or non-metallic ducts, bunched or flat configuration, spaced or touching, underground or overground, soil types, spacing between nearby cables etc. The designers shall take special care in assessing these conditions so that the cables are properly sized and rated against overheating while in normal service, overload and under fault conditions. The cable manufacturers specify these maximum permissible temperature limits based on the type of insulation used. Table. depicts these limits for typical insulating materials.




    Short circuit requirements of power cables


    It is important to identify the short circuit requirements of cables for maintaining their temperature under all operating and fault conditions within the maximum permissible limits. These requirements and the recommended necessary steps to be followed in the cable sizing and selection process are listed in Table.below.



    Factors affecting cable selection and ratings


    The above discussion briefly recognised the short circuit fault current influence on cable sizing and selection based on their ampere ratings. Proper selection and ratings ensures their suitability in the power system. Therefore, it may be concluded that the selection of power cables generally depends on the following factors.


    i) Operating voltage

    ii) Load current

    iii) Emergency loading requirements and duration

    iv) Fault current

    v) Fault clearing time (Protection relays & Fuses)

    vi) Allowable voltage drop

    vii) Ambient temperature for the particular installation configuration (Environmental conditions)

    viii) Cable route length

    ix) Method of cable installation (derating factors)  



    IEEE Std C37.04-1999. IEEE Standard Rating Structure for AC High Voltage Circuit Breakers

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