Time-current curves are used to show how fast a breaker will trip at any magnitude of current. The following illustration shows how a time-current curve works. The figures along the bottom (horizontal axis) represent current in amperes. The figures along the left side (vertical axis) represent time in seconds.
To determine how long a breaker will take to trip at a given current, find the level of current on the bottom of the graph. Draw a vertical line to the point where it intersects the curve. Then draw a horizontal line to the left side of the graph and find the time to trip. For example, in this illustration a circuit breaker will trip when current remains at 6 amps for 0.6 seconds.
It
can be seen that the higher the current, the shorter the time the
circuit breaker will remain closed. It can be seen from the
time-current curve on the following page that actual time-current
curves are drawn on log-log paper, and the horizontal line is in
multiples of the breakers continuous current rating.
From the information box in the upper right hand corner, note that the
time-current curve illustrated on the following page defines the
operation of a CFD6 circuit breaker.
For this example a 200 ampere trip unit is selected.
The top part of the time-current curve shows the performance of the overload trip component of the circuit breaker. Time-current curves are shown as bands, and the actual performance of any one breaker can fall anywhere within the band. Using the example CFD6 breaker and 200 ampere trip unit, the time the breaker will trip for any given overload can easily be determined using the same procedure as previously discussed.
For example, the breaker will trip between 25 seconds and 175 seconds at 600 amps with a 40XC ambient temperature, which is 3 times the the trip unit rating.
This is illustrated by the time-current curve below.
The bottom part of the time-current curve shows the performance of the instantaneous trip component (short circuit) of the circuit breaker. The maximum clearing time (time it takes for breakers to completely open) decreases as current increases. This is because of the blow-apart contact design which utilizes the magnetic field built-up around the contacts.
As current increases the magnetic field strength increases, which aids in opening the contacts. This circuit breaker has an adjustable instantaneous trip point from 4.5(900 A) to 10(000 A), which is 4.5 to 10 times the 200 A trip unit rating. If the trip point adjustment is set to minimum 4.5 (900 A), and a fault current of 900 amps or greater occurs, the breaker will trip within 1 cycle (16.8 ms). If the trip point setting is set to maximum 10 (2000 A), and a same fault current of 900 amps cause the breaker trip approximately from 12 to max 55 seconds.
A
greater fault current will cause the breaker to trip faster.
In some 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 long-time delay adjustment has been achieved.
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.
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.
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)
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.
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.
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.
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.