Introduction: Why MCB Protection Curves Matter
Miniature circuit breakers (MCBs) look simple on the outside, but choosing the correct type is far from straightforward. Two MCBs may have the same rated current, yet one is labeled B, another C, or D. These letters define the breaker’s protection curve, which directly impacts how fast it trips under overload or short circuit conditions.
For engineers, installers, and maintenance professionals, understanding the protection curve is essential. The wrong choice can cause nuisance tripping, insufficient protection, or even equipment damage. ONCCY, a global manufacturer of electronic protection devices, provides a complete range of MCBs designed for residential, commercial, and industrial applications.
This guide explains what protection curves are, how thermal and magnetic releases work, how temperature impacts performance, and—most importantly—how to select the right curve for your system.
What Is a Protection Curve?
A protection curve is the time-current characteristic of an MCB. It indicates how quickly the breaker trips when the current exceeds its rated value (In).
At 1.13 × In: the breaker must not trip within 1 hour (ensuring normal full load operation).
At 1.45 × In: the breaker must trip within 1 hour (protecting conductors from overheating).
At 2.55 × In: the breaker must trip within minutes (ensuring safety under heavy overload).
In short, the curve determines the breaker’s sensitivity to overloads and short circuits.
Thermal vs. Magnetic Release: Two Layers of Protection
Thermal Release
Uses a bimetal strip that bends when heated.
Responds slowly (seconds to minutes).
Protects against long-term overloads.
Strongly affected by ambient temperature.
Magnetic Release
Uses an electromagnetic coil that instantly separates contacts.
Responds in milliseconds.
Protects against short circuits or inrush currents.
Curve designation (B, C, D, etc.) defines the multiple of rated current needed to trip.
How Ambient Temperature Affects MCBs
The performance of thermal release depends on temperature. At higher temperatures, the bimetal strip bends faster, causing earlier tripping.
According to GB/T 10963.1:
Maximum ambient temperature: 40 °C instantaneous
Average temperature (24 hours): 35 °C
If the distribution box runs hotter than these limits:
Increase the breaker rating by one step.
Use an MCB with adjustable thermal settings.
Improve enclosure ventilation.
Ignoring temperature effects often leads to nuisance tripping in summer months.
Understanding MCB Curves: B, C, D, K, Z, and DC
B-Curve (3–5 × In)
Trips at 3–5 times rated current.
Suitable for resistive or light inductive loads such as lighting and general outlets.
C-Curve (5–10 × In)
Trips at 5–10 times rated current.
Common in small motors, pumps, and fans with moderate startup currents.
D-Curve (10–20 × In)
Trips at 10–20 times rated current.
Designed for large motors, transformers, and UPS systems with heavy inrush.
K-Curve (8–12 × In)
Optimized for industrial loads such as motors with frequent starts or rectifiers.
Provides reliable protection without nuisance tripping.
Z-Curve (2–3 × In)
Extremely sensitive.
Best for electronic devices, PLCs, control circuits, and railway electronics.
DC Curve (Specialized)
Designed for direct current applications such as solar PV, battery storage, and EV charging.
Includes arc-quenching chambers to handle DC arcs, which are harder to extinguish than AC arcs.
⚠️ Important: Do not use AC MCBs in DC systems. DC arcs require specialized construction.
Why Measuring Inrush Current Is Essential
Different loads behave differently at startup:
Incandescent lamps: startup current ≈ rated current.
Motors & compressors: startup current can be 5–10 times rated.
LED lights: high inrush due to capacitor charging.
If you select too sensitive a curve (e.g., B curve for a motor), the breaker will trip immediately during startup. Always measure or estimate inrush current before choosing a curve.
Verifying Minimum Short-Circuit Current
Selection doesn’t end at the curve. You must also verify that the minimum short-circuit current (Isc) in the line is greater than the breaker’s instantaneous trip threshold.
As a rule of thumb:
The available short-circuit current should be at least 1.25 × the lower limit of the chosen curve.
Example: For a C-curve (5–10 × In), Isc should exceed 6.25 × In.
If not, the breaker may fail to trip during an actual short circuit, compromising safety.
Common Mistakes in Selecting MCBs
LED lights tripping on startup: Caused by high inrush, usually resolved by switching from B to C curve or adding surge protection.
Motors tripping instantly: Curve too low (B or C). Use D or K curve, or install a soft starter.
Frequent summer tripping: Caused by high enclosure temperature. Either upgrade the breaker rating or improve cooling.
Using AC breakers in DC systems: Dangerous, as arcs may not extinguish. Always choose DC-rated MCBs.
Step-by-Step MCB Selection Guide
Determine rated current (In): Based on conductor size and ambient conditions.
Check overload limits: Ensure 1.13 × In doesn’t trip, and 1.45 × In does.
Measure inrush current: Match load characteristics with B, C, D, K, or Z curve.
Verify short-circuit current: Confirm Isc ≥ 1.25 × curve’s lower threshold.
Adjust for temperature: If enclosure exceeds 35–40 °C, increase rating or improve cooling.
Test in operation: Observe startup and tripping behavior, and fine-tune if necessary.
FAQs
1. What is the difference between thermal and magnetic trip in MCBs?
Thermal trip protects against overloads over time, while magnetic trip provides instantaneous short-circuit protection.
2. Why are there different curves like B, C, and D?
Each curve defines the inrush multiple at which the MCB trips. Choosing the right one depends on the load type.
3. Can I use an AC MCB in a DC application?
No. DC requires a specialized breaker due to the difficulty of extinguishing DC arcs.
4. How does temperature affect an MCB?
Higher ambient temperatures make the breaker more sensitive, leading to earlier tripping.
5. What happens if I choose the wrong curve?
The breaker may trip unnecessarily during startup, or fail to trip during a fault, risking damage.
Conclusion: Choosing the Right Protection Curve with ONCCY
Selecting the right MCB protection curve is not just a technical detail—it’s the difference between safe, reliable operation and constant headaches from nuisance trips or hidden risks. By understanding protection curves, load inrush behavior, and environmental effects, you can make informed decisions.
ONCCY provides a full portfolio of MCBs covering B, C, D, K, Z, and DC curves, designed to meet international standards and adapt to diverse applications—from household lighting to industrial motors and renewable energy systems.
Looking for the right miniature circuit breaker for your project?
Contact ONCCY today for expert guidance, custom solutions, and reliable MCBs tailored to your needs.