Time Current Coordination Curves
Jim Phillips, P.E.Nov 7, 2006
Time Current Coordination Curves
Every protective device such as circuit breakers, relays and fuses have a time current curve (TCC) that defines the device’s unique tripping characteristic. The TCC of a 70 Amp circuit breaker is shown below. It has a horizontal axis that represents current in Amps and a vertical axis that represents time in Seconds. Both scales are logarithmic in nature which means the scale increases by multiples of 10. The traditional TCC begins with the horizontal scale at 0.5 and 1.0 Amps which is unrealistically low for many applications. By rescaling the axis with a multiplier of 10 or even 100, the scale can be increased to allow curves to be plotted in the 100’s or 1000’s of Amps. The horizontal scale on this TCC is scaled by a factor of 10. i.e. 1 on the scale now corresponds to 1 X 10 or 10 Amps, 10 = 10 x 10 Amps etc.
The upper portion of the curve represents the overload or the thermal region of the 70 Amp breaker. The flat horizontal region of the breaker curve at the lower right of the graph illustrates the breaker clearing time in the instantaneous portion of the breaker. The vertical band corresponds to the current where the instantaneous trip function begins. Time current characteristics are obtained from the device manufacturer and can usually be obtained from their web sites. Each device has a unique curve so the specific device’s curve must be used.
The instantaneous portion of the breaker requires higher current to operate and is often thought of as short circuit protection. If a current exceeds the instantaneous trip value, the breaker will trip with no intentional time delay, with some breakers tripping in ½ to 1 electrical cycle. Tripping any faster than ½ cycle would be considered current limiting and would require current limiting fuses or circuit breakers with a current limiting design.
Illustrated on the 70 Amp TCC is a time-current point of 400 Amps and 10 Seconds. If this 70 Amp circuit breaker saw 400 Amps flowing through it as illustrated in blue on the graph, the breaker would trip in approximately 10 seconds. The thickness of the band illustrates tolerance and the actual tripping time could be somewhat lower or higher based on the band. If the breaker saw less than 70 Amps, the current value would never intersect the trip curve which indicates the device should not trip. If the breaker saw 10,000 Amps of short circuit current ( 1K x 10 on the scale) the breaker would open instantaneously in approximately 0.017 seconds.
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