Characteristics of Circuit Breaker Trip
Curves and Coordination
By
testguy
May 17, 2015
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Figure 1: Simplified time current curve. Photo: TestGuy
Time-current curves are used to show the amount of time required for a circuit
breaker to trip at a given overcurrent level.
Time-current curves are typically shown on a log-log plot. Figures along
the horizontal axis of the curve represent the continuous current rating
(In) for the circuit breaker. The figures along the vertical axis represent time in
seconds.
To determine how long a breaker will take to trip: find the current multiple of
(In) at the bottom of the graph. Next, draw a vertical line to the point where it
intersects the curve and then draw a horizontal line to the left side of the graph to
find the trip time. (See fig. 1)
The total clearing time of a circuit breaker is the sum of the breaker’s sensing
time, unlatching time, mechanical operating time and arcing time.
Curves are developed using predefined specifications such as operation at
an ambient temperature of 40°C, so keep in mind that the actual operating
conditions of the circuit breaker can cause variations in its performance.
Most curves have an information box that will define which circuit breaker the
curve applies to. This information box may also contain important notes from the
manufacturer such as the allowable deviation from trip times. (See fig. 2)
Figure 2. Circuit breaker time current curve with highlights. Photo: TestGuy
Overload Protection
The upper portion the time-current curve shows the circuit breaker’s thermal
response. The curve line indicates the nominal performance of the circuit
breaker.
In thermal magnetic breakers, a thermal overload occurs when a bi-metal
conductorinside the circuit breaker deflects after becoming heated by the load
current, de-latching the operating mechanism and opening the contacts.
The larger the overload, the faster the bi-metalic strip will heat up and deflect to
clear the overload. This is what is known as an “inverse time-curve.”
Figure 3: Combined LSIG Curve. Photo: TestGuy.
Long-Time Function
In electronic circuit breakers, the long-time function (L) simulates the effect of a
thermal bi-metal element. The nominal pickup point where an electronic trip
unitsenses an overload is roughly around 10% of the selected ampere rating.
Once picked up, the circuit breaker will trip after the time specified by the longtime delay adjustment has been achieved.
Short Circuit Protection
The lower portion of the time-current curve displays the short circuit response
of the circuit breaker. In thermal magnetic breakers, tripping place when
overcurrent’s of significant magnitude operate a magnetic armature inside of the
circuit breaker which de-latches the mechanism.
Instantaneous Function
In electronic circuit breakers, the Instantaneous (I) function simulates the
magnetic characteristic of a thermal-magnetic circuit breaker. This is achieved
through the microprocessor which takes samples from the AC current waveform
many times a second to calculate the true RMS value of the load current.
Instantaneous tripping occurs with no intentional time delay.
Short-Time Function
Some electronic circuit breakers may be equipped with a Short-time function
(S) which gives the circuit breaker a delay before tripping on a significant
overcurrent. This allows for selective coordination between protective devices
to ensure that only the device nearest to the fault open, leaving other circuits
unaffected. (See circuit breaker cooridnation below)
The I2t characteristic of the short time function determines the delay type. I2t IN
will result in an inverse-time delay that resembles the time/current characteristics
of fuses. This is similar to the long time function except with a much faster
delay. I2t OUT provides a constant delay, usually 0.5 seconds or less as noted
on the time-current curve. (See fig. 3)
Zone Interlock Function
Circuit breakers equipped with zone interlocking on short delay utilized with no
restraining signal from a downstream device will have the minimum time band
applied regardless of setting. This is sometimes referred to as the maximum
unrestrained delay.
When the instantaneous function is disabled, a short-time delay override is used
to instantaneously trip circuit breakers in the event of a significant short circuit.
This is called the short-time withstand rating and is represented on the trip
curve as an absolute ampere value.
Ground Fault Protection
Like the long-time function, the ground fault (G) element consists of
a pickup and delaysetting. When a phase-to-ground fault occurs, the sum of the
phase currents are no longer be equal because the ground fault current returns
through the ground bus. In a 4-wire system a fourth CT is installed on the neutral
bus to detect this imbalance.
When a current imbalance occurs, the circuit breaker will pick up if the
magnitude exceeds the ground fault pickup setting. If the breaker remains
picked up for the time specified by the ground fault delay, the circuit breaker will
trip. Ground fault protection is sometimes supplied with an I2t function which
operates under the same principle as short-time delay.
Ground fault protection requires the least energy to trip the circuit breaker.
When testing the overload or short circuit function of a circuit breaker the ground
fault protection will have to be disabled.
Use of the manufacturer's test kit or rewiring the neutral CT input is the preferred
method of primary-injection testing on a low voltage circuit breaker with ground
fault protection, otherwise two poles can be connected in series to provide
balanced secondary currents to the trip unit.
Recommended: Ground Fault Protection Systems: Performance Testing Basics
Circuit Breaker Coordination
Time-current curves are essential for the proper coordination of circuit
breakers. In the event of a fault, only the circuit breaker closest to the fault
should operate, leaving other circuits unaffected.
In the example below, three circuit breakers have been coordinated so that the
tripping time of each breaker is greater than the tripping time for
the downstream breaker(s)regardless of the fault magnitude.
Figure 4: Simplified circuit breaker coordination example. Photo: TestGuy.
Circuit breaker CB-3 is set to trip if an overload of 2000A or greater occurs
for 0.080 seconds. Circuit breaker CB-2 will trip if the overload remains
for 0.200seconds, and circuit breaker CB-1 if the fault remains for 20 seconds.
If the fault occurs downstream of breaker CB-3 it will trip first and clear the fault.
Circuit breakers CB-2 and CB-1 will continue to provide power to the circuit.
Each function of the trip unit should also be coordinated to prevent nuisance
trips. If a circuit breaker is feeding a piece of equipment with large inrush
currents for example, the instantaneous pickup value should be set higher than
the short time pickup value to prevent tripping when the equipment is energized.