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 4.7.6 Coordination Curves. On a coordination curve, time 0 is considered as the time at
which the fault occurs, and all times shown on the curve are the elapsed time from that point.
For a radial system, all the devices between the fault and a fault current source experience the
same current until one of the protective devices interrupts the circuit. After interruption, relay
overtravel, and circuit breaker and relay reset times, are examined. This determines whether any
device other than the one nearest the fault will continue to operate under reduced current and trip
a backup protective device. The series devices are considered coordinated if the source side
protective device nearest the fault is the only device to operate.
4.7.6.1 Representation. A coordination curve is arranged so the region below and to the
left of the curve represents an area of no operation. The curves represent a locus of a family of
paired coordinates (current and time) which 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. Circuit 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 fault current is expected to blow the fuse at some value
between these times.
4.7.6.2 Time-Current Curve. Figure 4-7 shows a time-current curve represented as a band.
Time t2 is the maximum time from the initiation of the current flow I within which operation
of the device and circuit breaker is assured. Time t1 is the time from initiation of the current
flow I within which the current must be returned to normal to prevent the device under
consideration from operating due to the thermal or mechanical momentum of the protective
device. Reading current along the abscissa of the time-current curve, the time or range of times
in which any device is expected to operate corresponds to the ordinate or ordinates of the curve
plotted. Usually circuit breaker curves begin at a point of low current close to the trip device
rating or setting and an operating time of 1000 seconds. Relay curves begin at a point close to
1-1/2 times pickup and the corresponding time for this point. Curves usually end at the
maximum short-circuit current to which the device under consideration can be subjected. A
single curve can be drawn for any device under any specified condition, although most devices
(except relays) plot an envelope within which operation takes place. This envelope takes into
consideration most of the variables which affect operation. Some of these variables are ambient
temperature, manufacturing tolerances, and resettable time delay.
4.7.7 Coordination Study. There are four steps involved in a coordination study.
4.7.7.1 One-Line Diagram. The first requisite for a coordination study is a one-line
diagram'of the system or portion of the system involved. This one-line diagram should show the
following data:
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