Conventional low voltage distribution network





    The devices used in the incomer should be capable of:

    1. Switching and carrying normal currents (generally above 1200 A);
    2. Withstanding abnormal currents for a short duration in order to allow downstream devices to operate;
    3. Interrupting the maximum value of the fault current generated in the system;
    4. Ensuring the safety to the operating personnel;
    1. Inter-locking with downstream equipment; and
    2. Facilitating easy


    In the past, oil circuit breakers (OCBs), re-wireable fused isolators and air circuit breakers (ACBs) were the commonly used devices. However, ACB has been acknowledged as an ideal device for incomer in terms of the safety, reliability and maintenance needs of the system. This is mainly due to its various characteristics like quick-make, quick-break stored energy type reliable mechanism, safety interlocks/indications, ease of maintenance and its ability to withstand fault currents for a specified duration (1-3 secs) thereby allowing the feeder device to isolate the faulty branch of the network and ensuring reliable supply to healthy areas.

    The following features make ACBs popular amomg users:

    • Simplicity;
    • Efficient performance;
    • High normal current rating up to 6000 A; and
    • High fault withstanding capacity upto 63


    The following are the limitations of ACBs:

    • Long tripping time;
    • Bulky in size;
    • High initial cost; and
    • Requirement of additional floor space due to their bulky





    The devices installed as parts of a sub-incomer should have the following characteristics:

    • Ability to achieve economy without sacrificing protection and safety;
    • Capability to withstand abnormal currents; and
    • Need for relatively less number of inter-locking indicating accessories since it covers a limited area of

    ACBs and switch fuse units (SFUs) are, to a large extent, being used as sub-incomers along  with modern devices like moulded case circuit breakers (MCCBs).





    Feeder protection covers all load centres like motor control centres, lighting switchboards and industrial load centres. The choice of feeder protection device based on the different conventional feeder load centres are discussed below.




    • Switchgear Engineering
    • Medium voltage Switchgear Handbook

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