Example 7.5
Consider the 2000 hp motor installation shown in Figure 7.14. The CT ratio is selected to provide
some margin above the trip setting so meters will not read off-scale. Normally, overcurrent relays
are set at 125% of full load and the CT ratio should allow less than 5.0 A for this condition. If the
motor is vital to the operation of the plant, advantage is taken of the motor service factor which
is 115 %. This results in a maximum load of 245 A × 1.15 = 282 A and a relay pickup setting of
1.25 × 282 = 352.5 A. Select a CT ratio of 400:5 (80:1).
51 51 51G
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