Evaluate relay coordination for the minimum fault currents in Example 10.4. For the selected current tap settings and time dial settings,

(a) determine the operating time of relays at B2 and B3 for the 700-A fault current.

(b) Determine the operating time of relays at B1 and B2 for the \(1500-\mathrm{A}\) fault current. Are the fault-to-pickup current ratios \(\geqslant 2.0\) (a requirement for reliable relay operation) in all cases? Are the coordination time intervals \(\geqslant 0.3\) seconds in all cases?

Example 10.4

Data for the \(60-\mathrm{Hz}\) radial system of Figure 10.16 are given in Tables 10.3, 10.4, and 10.5. Select current tap settings (TSs) and time-dial settings (TDSs)

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Bus 1 2 3 S MVA Bus 1 2 3 TABLE 10.4 Breaker B1 B2 B3 TABLE 10.5 11.0 4.0 6.0 TABLE 10.3 Maximum loads-Example 10.4 Maximum Fault Current (Bolted Three-Phase) A 3000 2000 1000 Breaker Operating Time 5 cycles 5 cycles 5 cycles Lagging p.f. 0.95 0.95 0.95 Minimum Fault Current (L-G or L-L) A CT Ratio 400:5 200:5 200:5 2200 1500 700 Relay CO-8 CO-8 CO-8