
System Protection 255
Secondary side rated line current = 100 × 1000 ÷ √3 × 208 = 277.6 A
General purpose CB or fuse rating on secondary side = 1.3 × 277.6 = 361 A
Primary side rated line current = 100 × 1000 ÷ √3 × 480 = 120.3 A
General purpose CB or fuse rating on primary side = 2* × 1.3 × 120.3 = 313 A
Or, time-delay CB or fuse rating on primary side = 1.3 × 120.3 = 156.4 A
* Factor 2 in the general purpose circuit breaker or fuse rating on the primary
side is to avoid nuisance tripping of the circuit under the magnetizing inrush cur-
rent when the transformer is first connected to the supply lines. Time-delay circuit
breaker or fuse on the secondary ...