Practice

On a 13.8 kV radial feeder, the downstream device (relay plus breaker) fully clears a 4800 A fault in 0.35 s. The upstream backup 51 relay has a pickup of 600 A and time dial 2, following the very-inverse curve $t = TD\left(\dfrac{19.61}{M^{2}-1}+0.491\right)$ s, where $M = I_{fault}/I_{pickup}$. The coordination margin between the two devices at this fault, and whether it meets a 0.3 s coordination time interval (CTI), are most nearly:

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