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An electrical fault should be cleared as fast as possible while containing the outage to the smallest possible area. This requirement, known as 'selective coordination,' is met when overcurrent protective devices installed closest to the fault open before protective devices installed upstream.
One method of ensuring selective coordination is to examine each overcurrent device's time-current curve (TCC) and verify for any value of current, that the protective device closest to the fault clears faster than any upstream device.
To interpret the curves, locate a value of current on the bottom axis and follow that line vertically until it intersects the first device curve (Fig. 1). That first curve will be the first device to trip. Continue vertically to the next curve. That is the next device to trip. The time difference between the rightmost edge of the first curve and the leftmost edge of the second curve is called the Coordination Time Interval (CTI). If a point is discovered where an upstream device opens before the downstream device opens, that system is not selectively coordinated (Fig. 2).
Fuses
Can you use current limiting (CL) fuses to achieve selective coordination? Certainly! But as with all engineering, there are compromises to consider. One concern is increased arc flash hazard. Most people assume that CL fuses reduce the arc flash hazard, at least compared to breakers. But a CL fuse's arc flash performance degrades as the fault current is...