Breaker trip curves B C D K
Trip curves define the relationship between current intensity and response time of a circuit breaker. Types B, C, D and K are the most common standardized curves used in low-voltage thermal-magnetic circuit breakers; each is optimized for a specific range of starting currents and loads. The thermal part of the curve acts against overloads via a bimetal strip heated by the current, while the magnetic part responds instantaneously to high short-circuit currents.
Thermal tripping is based on the deformation of a bimetallic strip. When current flows, heat is generated by the Joule effect; the difference in expansion between the two metals causes a bending that, upon reaching a critical displacement, releases the trip mechanism. The relationship between the strip geometry and temperature is given by the following expression:
s = (α · L² · ΔT) / e
| Variable | Meaning | Typical unit |
|---|---|---|
| s | Displacement (bending) of free end | mm / in |
| α | Bimetal bending coefficient | 14 × 10⁻⁶ K⁻¹ (typical) |
| L | Active length of bimetallic strip | mm / in |
| ΔT | Temperature increase relative to rest condition | °C / °F |
| e | Total thickness of bimetal | mm / in |
The bimetal temperature depends on the heat generated by the current (Q ∝ I²·R·t). Consequently, in the overload region the trip curve follows an inverse-time law: the trip time decreases as current increases. The magnetic trip uses a coil whose electromagnetic field attracts a moving core when the current exceeds a preset threshold, achieving response times on the order of milliseconds (typically below 10 ms / 0.01 s).
Standardized trip curves
Section titled “Standardized trip curves”Standards IEC 60898 (circuit breakers for household and similar installations) and IEC 60947-2 (low voltage industrial circuit breakers) establish the magnetic trip thresholds and thermal test conditions. The most common curves are:
- Curve B: 3 to 5 times the rated current (In) for instantaneous trip.
- Curve C: 5 to 10 In.
- Curve D: 10 to 20 In.
- Curve K: 8 to 12 In (some manufacturers extend up to 14 In), defined in IEC 60947-2.
The thermal behavior is identical for all curves in domestic circuit breakers: it must not trip at 1.13 In within 1 hour (2 hours for rated currents above 63 A) and must trip within 1 hour (or 2 hours) at 1.45 In.
Curve B – fast response
Section titled “Curve B – fast response”The magnetic threshold is between 3 In and 5 In. This curve is the most sensitive of the common ones, designed for circuits with moderate short-circuit currents and where fast disconnection is required for small overcurrents.
- Magnetic trip time: less than 100 ms / 0.1 s for currents ≥ 5 In.
- For a 200% overload (2 In) the thermal trip time is around 60 s / 1 min (indicative values for In ≤ 63 A).
Curve C – general use
Section titled “Curve C – general use”The magnetic threshold ranges from 5 In to 10 In. It is the most widespread curve in conventional electrical installations.
- Magnetic trip time: less than 100 ms / 0.1 s from 10 In.
- With a 150% overload (1.5 In) the thermal trip occurs approximately between 100 s / 1.67 min and 200 s / 3.33 min for circuit breakers up to 63 A.
Curve D – loads with high starting current
Section titled “Curve D – loads with high starting current”The magnetic trip is set between 10 In and 20 In. It withstands the high starting peaks of motors, transformers, or equipment with magnetic cores without causing unwanted disconnection.
- Magnetic trip time: typically below 20 ms / 0.02 s once the threshold is exceeded.
- With a 150% overload the thermal time is similar to the other curves, since the thermal protection does not depend on the magnetic setting.
Curve K – motor and line protection with power electronics
Section titled “Curve K – motor and line protection with power electronics”Defined by standard IEC 60947-2, the magnetic trip range is from 8 In to 12 In (some commercial series reach up to 14 In). It has a slightly slower thermal characteristic to avoid tripping during prolonged starts, while maintaining fast response to solid short-circuits.
- At 10 In the magnetic trip occurs in less than 100 ms / 0.1 s.
- During a 120% overload (1.2 In) the thermal trip time may exceed 300 s / 5 min, allowing heavy starts.
Typical applications
Section titled “Typical applications”| Curve | Typical load | Type of installation |
|---|---|---|
| B | Lighting circuits, general purpose outlets with low starting current | Homes, offices |
| C | Power outlets, small motors, luminaires with electronic ballast | Tertiary installations, commercial |
| D | Motors with high starting torque, transformers, welders | Industry, workshops |
| K | Induction motors, lines with variable frequency drives or rectifiers | Industrial plants, pumps, fans |
Summary table of trip thresholds
Section titled “Summary table of trip thresholds”| Curve | Magnetic trip (multiple of In) | Thermal trip (no trip / mandatory trip) | Test current range |
|---|---|---|---|
| B | 3 to 5 In | 1.13 In / 1.45 In | 3 – 5 |
| C | 5 to 10 In | 1.13 In / 1.45 In | 5 – 10 |
| D | 10 to 20 In | 1.13 In / 1.45 In | 10 – 20 |
| K | 8 to 12 In (manufacturer: up to 14 In) | 1.05 In / 1.2 In (indicative values) | 8 – 12 |
Thermal protection formula
Section titled “Thermal protection formula”Overload protection follows an inverse-time behavior governed by the Joule integral. For the thermal zone of the curve, neglecting heat dissipation, the following approximate relationship holds:
I² · t = K
| Symbol | Meaning | Unit |
|---|---|---|
| I | Effective current flowing through the circuit breaker | A (amperes) |
| t | Time required to reach the trip temperature | s (seconds) |
| K | Constant that depends on the bimetal construction, its mass, and the mechanical trip point | A²·s |
The constant K is characteristic of each rating and model. During the standardized test, the circuit breaker must not trip before 1 h (or 2 h) at 1.13 In, which implies that the corresponding I²·t product is below the threshold of energy accumulation sufficient to deform the bimetal to the trip point; with 1.45 In, on the other hand, that threshold is exceeded and tripping occurs within the specified time.
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”How long does a curve B circuit breaker take to trip under a solid short-circuit of 5 In?
Section titled “How long does a curve B circuit breaker take to trip under a solid short-circuit of 5 In?”The magnetic trip occurs in less than 100 ms / 0.1 s. With currents above 5 In the response is even faster, reaching values of 20 ms / 0.02 s.
What current does a curve C circuit breaker need to trigger instantaneous trip?
Section titled “What current does a curve C circuit breaker need to trigger instantaneous trip?”The magnetic zone acts from 5 In, but guaranteed trip occurs at 10 In, with a typical time of 10 ms / 0.01 s.
How long does a curve D circuit breaker take to open with a 150% overload?
Section titled “How long does a curve D circuit breaker take to open with a 150% overload?”The time is similar to the other curves (the bimetal is common) and is around 120 s / 2 min for ratings ≤ 63 A, as long as the magnetic protection does not operate.
How quickly does a curve K circuit breaker respond to a starting current of 8 In?
Section titled “How quickly does a curve K circuit breaker respond to a starting current of 8 In?”If the current remains below 12 In and the starting time is short, the magnetic trip does not occur; if it exceeds 12 In, the opening time is less than 100 ms / 0.1 s.
What is the no-trip tolerance for curve B at 1.13 In?
Section titled “What is the no-trip tolerance for curve B at 1.13 In?”At 1.13 In the circuit breaker must not trip in less than 1 hour (2 hours for In > 63 A), which means maintaining the admissible load during that interval without interruption.
What is the difference in trip time between 3 In and 5 In on a curve B?
Section titled “What is the difference in trip time between 3 In and 5 In on a curve B?”At 3 In the circuit breaker may take several seconds to trip from start, while at 5 In the trip is practically instantaneous (less than 100 ms / 0.1 s), marking the transition between thermal and magnetic zones.
References
Section titled “References”- engineeringtoolbox.com: https://www.engineeringtoolbox.com/bimetallic-strips-d_1755.html
- electrical4u.com: https://www.electrical4u.com/circuit-breaker-operation/