Time-Current Curves

Time-current curves:
similar to the one shown on the following page, are used to show how fast a breaker will trip at any magnitude of current. The following illustration shows how to read a time-current curve. The figures along the bottom (horizontal axis) represent multiples of the continuous current rating (In) for the breaker. The figures along the left side (vertical axis) represent time in seconds.
Time in Seconds Multiple of In





To determine how long a breaker will take to trip at a given multiple of In, find the multiple on the bottom of the graph and 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 six times In for .6 seconds. Note that the higher the current, the shorter the time the circuit breaker will remain closed. Time-current curves are usually drawn on log-log paper. Many time-current curves also show the bandwidth, tolerance limits, of the curve.
From the information box in the upper right hand corner, note that the time-current curve illustrated on the next page defines the operation of a Siemens MG frame circuit breaker. For this example, operation with an 800 ampere trip unit is shown, but, depending upon the specific breaker chosen, this circuit breaker may be purchased with a 600, 700, or 800 amp continuous current rating.

Overload Protection:
The top part of the time-current curve shows the continuous
Current performance of the circuit breaker. The black line shows the nominal performance of the circuit breaker and the gray band represents possible variation from this nominal performance that can occur even under specified conditions.

Using the example of an MG breaker with an 800 amp continuous current rating (In), note that the circuit breaker can be operated at 800 amps (1.0 times In) indefinitely without tripping. However, the top of the trip curve shows that an overload trip will occur in 10,000 seconds at 1000 amps (1.25 times In). Additionally, the gray area on either side of the trip curve shows the range of possible variation from this response.

Keep in mind that this trip curve was developed based upon predefined specifications, such as operation at a 40°C ambient temperature. Variations in actual operating conditions will result in variations in circuit breaker performance.

Instantaneous Trip:
The middle and bottom parts of this time-current curve show
Instantaneous trip (short circuit) performance of the circuit breaker. Note that the maximum clearing time (time it takes for the breaker to completely open) decreases as current increases. This is because of high-speed contact designs which utilize the magnetic field built up around the contacts. As current increases, the magnetic field strength increases, which speeds the opening of the contacts?

This circuit breaker has an adjustable instantaneous trip point from 3250 to 6500 amps, which is approximately four to eight times the 800 amp continuous current unit rating. This adjustment affects the middle portion of the trip curve, but not the top and bottom parts of the curve. The breaker shown in this example has a thermal-magnetic trip unit. Circuit breakers with solid-state trip units typically have additional adjustments


True RMS Sensing
Some solid state circuit breakers sense the peak values of the current sine wave. This method accurately measures the heating effect of the current when the current sine waves are perfectly sinusoidal. Frequently, however, the sine waves are harmonically distorted by non-linear loads. When this happens, peak current measurement does not adequately evaluate the true heating effect of the current.







Siemens solid state trip unit circuit breakers incorporate true root-mean-square (RMS) sensing to accurately sense the effective value of circuit current. True RMS sensing is accomplished by taking multiple, instantaneous “samples” of the actual current wave shape for a more accurate picture of its true heating value.





The microcomputer in Siemens solid state trip unit breakers samples the AC current waveform many times a second, converting each value into a digital representation. The microcomputer then uses the samples to calculate the true RMS value of the load current. This capability allows these circuit breakers to perform faster, more efficiently and with repeatable accuracy.

Being able to monitor true RMS current precisely is becoming more important in today’s electrical distribution systems because of the increasing number of power electronic devices being used that can distort the current waveform.


Adjustable Trip
Curves One of the key features of solid state trip unit circuit breakers is the ability to make selective adjustments to the circuit breaker’s time-current curve. The time-current curve shown here is for a circuit breaker in the SJD6-SLD6 family.





Solid State Circuit Breaker
Adjustments The type of trip unit included in a circuit breaker determines the specific time-current curve adjustments available. . The following illustration and associated table describes the adjustments available.