UFC 3 -520-01
June 10, 2002
operation. The curves represent a locus of a family of paired coordinates (current and
time) that indicate the period of time required for device operation at a selected current
value. Protective relay curves are usually represented by a single line only. C ircuit
breaker tripping curves, which include the circuit breaker operating time and the trip
device time, are represented as bands. The bands represent the limits of maximum and
minimum times at selected currents during which circuit interruption is expected. The
region above and to the right of the curve or band represents an area of operation.
Fuse characteristics are represented by a tolerance band bounded by minimum melting
time and total fault current interrupting time curves. A specific current a bove the fuse
current rating is expected to blow the fuse at some value between these times.
9-2.5.2 Figure 9-1 shows a time-current curve represented as a band. Reading
current along the y-axis of the time-current curve, the time or range of times in which
any device is expected to operate is shown on the x-axis. Notice that the x-axis is
shown in terms of the multiples of circuit breaker rating; this scale applies specifically to
the load breaker and its rating, and the feeder breaker current values ha ve been
converted to this scale to allow plotting both breakers on the same scale. It is often
easier to plot in terms of amperes rather than multiples of circuit breaker rating.