Time-current curves are graphical representations of the trip characteristics of a circuit breaker. These curves show the relationship between the magnitude of the current flowing through the circuit breaker and the time it takes for the circuit breaker to trip.
This information is critical for ensuring that the circuit breaker provides adequate protection for the equipment it is connected to. Time-current curves are typically provided by the manufacturer of the circuit breaker and are based on industry standards such as the Institute of Electrical and Electronics Engineers (IEEE) and the National Electrical Manufacturers Association (NEMA).
How to read a Time-current Curve
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, 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.
Figure 1: Simplified time current curve. Photo: TestGuy
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.
Real world circuit breaker time current curve example with highlights. Photo: TestGuy
The upper portion of the time-current curve shows the circuit breaker’s thermal response, the curved line indicates the nominal performance of the circuit breaker.
In thermal magnetic breakers, a thermal overload occurs when a bi-metal conductor inside 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-metallic strip will heat up and deflect to clear the overload. This is what is known as an “inverse time-curve.”
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 unit senses 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.
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.
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.
Figure 3: Combined LSIG Curve. Photo: TestGuy.
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 coordination 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.
Zone Interlock Function
Circuit breakers equipped with zone interlocking on short delay 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.
Related: Zone Selective Interlocking (ZSI) Basic Principles
Ground Fault Protection
Like the long-time function, the ground fault (G) element consists of a pickup and delay setting. 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.
Example of a 4-Wire Residual Ground Fault Protection System. Photo: TestGuy.
Ground fault protection requires the least energy to trip the circuit breaker, often times with trip values set well below the long time pickup setting. When testing the overload or short circuit function of a circuit breaker, the ground fault protection will need to be disabled or “moved out of the way” for other functions to operate.
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.
Related: 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.
Simplified example of circuit breaker trip coordination. 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 0.200 seconds, 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.
Related: Electrical Power System Coordination Studies Explained