MCB Tripping Curves B, C, D, K and Z Explained
What is an MCB tripping curve? A tripping curve is the IEC-defined band of instantaneous magnetic-trip current, expressed as a multiple of rated current (In), that determines how much inrush an MCB tolerates before it opens without delay. Get the curve wrong and the breaker either nuisance-trips on a healthy load's inrush or reacts too slowly to a genuine fault. Five curves cover almost every application: B (3-5x In) for resistive and lighting circuits, C (5-10x In) for general and mixed loads, D (10-20x In) for transformers and motors, K (8-12x In) for industrial inductive loads, and Z (2-3x In) for semiconductor and electronic-circuit protection.
What a Tripping Curve Actually Controls
Every miniature circuit breaker carries two separate trip mechanisms in one housing: a thermal element for overload and a magnetic element for short-circuit. The thermal element is slow by design. A bimetallic strip bends under sustained current and opens the contacts after a delay measured in seconds to minutes, and that behavior is roughly the same across curve letters for a given rated current. The curve letter changes only the magnetic side: the instantaneous multiple of In at which the solenoid fires and opens the contacts in under one cycle, typically 10 ms or less.
Two MCBs rated 16 A, one B-curve and one D-curve, protect against overload identically. They diverge entirely on how much inrush current each will absorb before tripping instantly.
How to Read a Time-Current Curve
Manufacturer curve sheets plot trip time on a log scale against current as a multiple of In, also on a log scale. Two regions matter. The sloped band on the left is the thermal zone: trip time falls as current rises, from hours near 1.13x In to seconds around 1.45x In, the test points defined in IEC 60898-1. The near-vertical line further right is the magnetic zone. Once current crosses the curve's lower bound, trip time collapses to milliseconds regardless of how much higher the fault current climbs.
Reading that vertical line's position is the entire exercise. A B-curve line sits at 3-5x In; move the same rating to a D-curve and the line shifts right to 10-20x In, with no change to the thermal band underneath. What we see in the field: installers often read only the amp rating on the label and skip the curve letter, then wonder why a "16 A breaker" nuisance-trips on one panel and behaves fine on another.
Formula: Magnetic Trip Threshold — Source: IEC 60898-1 / IEC 60947-2 (instantaneous trip requirements)
Im = k × In
| Symbol | Description | Unit |
|---|---|---|
| Im | Instantaneous magnetic trip current | A |
| k | Curve multiplier: 3-5 (B), 5-10 (C), 10-20 (D), 8-12 (K), 2-3 (Z) | dimensionless |
| In | Rated current of the MCB | A |
B, C, D, K and Z at a Glance
The table below is the reference point for every curve decision that follows. Trip multiple sets how much inrush the breaker survives before it opens instantly; the standard column sets which installations the curve letter is normally certified for.
| Criteria | B Curve | C Curve | D Curve | K Curve | Z Curve |
|---|---|---|---|---|---|
| Trip multiple (× In) | 3-5x | 5-10x | 10-20x | 8-12x | 2-3x |
| Governing standard | IEC 60898-1 | IEC 60898-1 | IEC 60898-1 | IEC 60947-2 | IEC 60947-2 |
| Typical loads | Lighting, long cable runs, resistive heating | General/mixed circuits, small motors | Transformers, motors, welding sets, capacitor banks | Industrial inductive loads, motor contactors | Semiconductor and electronic-circuit protection |
| Sensitivity to inrush | Highest (most sensitive) | Moderate | Low | Low-moderate, narrower band than D | Extreme (least tolerant) |
| Common setting | Residential/final circuits | Default for mixed panels | Heavy-inrush industrial feeders | Industrial motor circuits needing tight coordination | Control electronics, PLC I/O circuits |
B and C Curves: the Everyday Choices
B curve trips instantaneously between 3x and 5x In. That narrow band suits circuits where inrush stays close to running current: incandescent and resistive lighting, heating elements, and long cable runs where line impedance already caps fault current well below what a C curve would tolerate before tripping. On those circuits, B curve clears a fault faster and lets discrimination studies work with a tighter margin.
C curve, 5-10x In, is the default across most commercial and light-industrial panels because it survives the inrush of LED and fluorescent drivers, small motors, and mixed office/workshop loads without per-circuit selection. Some electricians default to C-curve everywhere, but that habit costs precision on the circuits where it was never the right fit.
D and K Curves: High-Inrush and Industrial Inductive Loads
D curve (10-20x In) exists for loads whose starting current genuinely reaches that range: transformer energizing, where magnetizing inrush can hit 10-15x rated current for a few cycles, direct-on-line motor starts, welding transformers, and capacitor-bank switching. Undersizing with a B or C curve here means nuisance tripping on every start, not occasionally — every single time the load switches on.
K curve, 8-12x In under IEC 60947-2, sits inside the D band but narrower. It exists for industrial motor and inductive circuits where the tighter tolerance improves coordination with upstream and downstream devices: discrimination is easier to hold when the trip band spans 4x instead of 10x. Industrial ranges such as ABB's S200P offer K alongside standard B/C/D within the same physical footprint of miniature circuit breakers, so switching curve letter does not mean switching product family.
Z Curve: Protecting Electronics and Semiconductor Circuits
Z curve trips at 2-3x In, the most sensitive of the five. It protects loads that cannot absorb even moderate transient current without damage: semiconductor devices, control electronics, PLC input/output modules, and measurement circuits. A 2x In threshold is close to the load's own operating current, which is the point.
This is not a curve chosen for the cable's sake; it is chosen for the load's sake. That depends on the electronics behind the breaker being the weak link in the circuit, not the wiring ahead of it.
Consequences of Choosing the Wrong Curve
Undersizing the curve — B where C or D belongs — produces nuisance tripping: a healthy motor start, transformer energization, or capacitor inrush reads to the breaker as a fault, and the magnetic element opens instantly on a load that was never faulty.
Oversizing the curve — D where B belongs — produces the opposite failure. The magnetic element waits for 10-20x In before it reacts, so a genuine low-level fault on a lighting circuit can sit unresolved long enough to heat cable insulation before the breaker responds. Neither error shows up on a datasheet review of amp rating alone; both only surface once the circuit is loaded and running.
See how to choose the right MCB tripping curve for a load-by-load selection walkthrough, and the IEC 60898-1 vs IEC 60947-2 standards comparison for which curve letters each standard actually certifies.
Frequently Asked Questions
What's the practical difference between B and C curve MCBs?
B trips at 3-5x In, C at 5-10x In. Both handle overload the same way through an identical thermal element; the difference is purely how much inrush the magnetic element tolerates before it fires instantly. C survives higher-inrush starts that would nuisance-trip a B curve on the same circuit.
Can a D curve MCB be used on a normal lighting circuit?
It can be installed, but it works against fast fault clearance on a low-inrush circuit. A fault current that would trip a B curve instantly may sit within the 10-20x band of a D curve and take longer to clear, extending the time cable insulation is exposed to fault heating.
Is K curve the same as D curve?
No. Both handle industrial inrush, but K's 8-12x In band is narrower than D's 10-20x In band, per IEC 60947-2. The narrower band gives tighter, more predictable coordination with other protective devices on the same circuit.
When should a Z curve MCB be used instead of B?
Choose Z when the load itself, not the cable, is the sensitive component: semiconductor devices, control electronics, and PLC circuits that cannot tolerate transient current above 2-3x In without damage.
Does the curve letter affect breaking capacity in kA?
No. Curve letter (B/C/D/K/Z) and breaking capacity (Icn or Icu, in kA) are independent ratings. A single MCB range can offer the same breaking-capacity tier across several curve letters; check the MCB breaking capacity guide for how that rating is chosen separately.
Which standard governs which curve?
B, C, and D are defined in IEC 60898-1 for household and similar installations. K and Z are more commonly found under IEC 60947-2, which covers industrial circuit breakers for skilled personnel. Many industrial-grade MCBs carry dual marking to both standards.
Conclusion
Curve letter is not a secondary spec to skim past on a datasheet. It sets the exact multiple of rated current at which the magnetic element clears a fault in under one cycle, and picking the wrong one produces either nuisance tripping on healthy inrush or a fault that lingers too long on the wire. Match the curve to the load's actual starting behavior, not to habit. See the MCB engineering guide for how curve selection fits into full circuit design, and MCB vs RCBO vs RCD vs RCCB differences for where curve-only protection stops and residual-current protection begins.
