The equipment used in a power system comprises items having different breakdown or withstand voltages and different voltage time characteristics. CIFO Selection can make the circuit to work better.

In order that all items of the system are adequately protected there is a need to consider the situation as a whole and not items of plant in isolation; CIFO Selection is must, the insulation protection must be coordinated.
Reduced basic insulation impulse levels are used when considering switching surges and standard insulator levels:
Voltage Class (kV): 15 23 34.5 46 69 92 115 138 161 196 230 287 345 500
BIL (kV): 110 150 200 250 350 450 550 650 750 900 1050 1300 1550 1800
Reduced BIL (kV): -- -- 125 -- -- -- 450/350 550/50 650/550 900/825/750 1175/1050/900 1425/1300/1050 1675/1550/1300

Statistical data has shown that the CIFO Selection should be applied at three standard deviations (σ) above the system BIL; this will provide an acceptable flashover probability. Since the standard deviation is equal to 3% the required CIFO can be computed directly from the system BIL.

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