Two miniature circuit breakers (MCBs) with the same current rating can behave very differently when a motor starts or a transformer is switched on. The difference is the trip curve, shown by the letter printed in front of the rating: B16, C16 or D16, for example. This article explains what the B, C and D curves mean, how to read them, and where each one fits.
Two ways an MCB trips
An MCB combines two protection systems:
- Thermal trip for overloads: a bimetal heats up with the current and trips the breaker after a delay that gets shorter as the current rises. It protects the cable against currents moderately above the rating that last for minutes or hours.
- Magnetic (instantaneous) trip for short circuits: a coil pulls the mechanism open almost immediately once the current passes a threshold.
The trip curve letter describes where that magnetic threshold sits, as a multiple of the rated current In. The thermal part is largely the same for B, C and D breakers of the same standard.
B, C and D curves under IEC 60898-1
IEC 60898-1, the standard for circuit breakers for household and similar installations, defines three instantaneous tripping ranges. Below the lower limit the breaker must not trip instantaneously; at or above the upper limit it must trip instantaneously. Between the two, it may trip either way, depending on the individual device.
| Curve | Instantaneous trip range | Typical uses |
|---|---|---|
| B | above 3 × In up to 5 × In | Resistive loads, lighting and outlets, long cable runs |
| C | above 5 × In up to 10 × In | General commercial and industrial circuits, control circuits, small motors, fluorescent lighting |
| D | above 10 × In up to 20 × In | High inrush: transformers, motors, solenoids |
Values for guidance. Typical uses are general examples; the circuit design decides.
Worked example: instantaneous trip currents
The table below multiplies each curve's range by common ratings. For a C16 breaker, for example, the instantaneous trip happens somewhere between 80 A and 160 A.
| Rated current In | Curve B | Curve C | Curve D |
|---|---|---|---|
| 6 A | 18-30 A | 30-60 A | 60-120 A |
| 10 A | 30-50 A | 50-100 A | 100-200 A |
| 16 A | 48-80 A | 80-160 A | 160-320 A |
| 20 A | 60-100 A | 100-200 A | 200-400 A |
| 32 A | 96-160 A | 160-320 A | 320-640 A |
Values for guidance, computed as the rated current multiplied by the lower and upper limits of each curve.
B, C and D trip curves, schematic, for guidance.
The thermal region
For overloads, IEC 60898-1 uses two conventional currents: the breaker must not trip at 1.13 × In within the conventional time, and must trip at 1.45 × In within the conventional time (one hour for ratings up to 63 A). The exact curve between these points and the instantaneous range is published by each manufacturer as a time-current chart; check the current edition of the standard and the data for the exact model.
How to choose a curve
- Start with the load's inrush current. If the inrush current can reach the lower limit of the instantaneous range, the breaker may trip at switch-on. Motors, transformers, power supplies with large input capacitors and LED drivers in groups all draw short inrush peaks.
- Then check the fault current. A lower curve trips instantaneously at a lower fault current. On long cable runs, where the available fault current at the far end is limited, a B or C curve is more likely to clear a fault quickly than a D curve.
- Match the curve to the existing design. When replacing a breaker, use the same curve unless a qualified person has reviewed the change. A D curve that hides an inrush problem may no longer protect the cable on a fault.
- Check selectivity. In a panel with several levels of breakers, curve and rating together decide which breaker opens first.
K and Z curves
Some industrial miniature breakers built to IEC 60947-2 are offered with other characteristics, commonly called K and Z. A Z curve has a low magnetic threshold for protecting sensitive electronics and semiconductors; a K curve is aimed at motor and transformer circuits with a magnetic threshold above that of a C curve and a tighter thermal response. Their ranges are not the same as the IEC 60898-1 curves and vary by manufacturer, so check the manufacturer's data for the exact values.
A note on North American ratings
The B, C and D letters come from IEC practice. In the United States and Canada, a miniature breaker may be listed as a branch-circuit breaker or only as a supplementary protector, and the two cannot be used interchangeably. Our article on UL 489 branch breakers and UL 1077 supplementary protectors explains the difference, and the one on short-circuit current ratings (SCCR) covers the breaking-capacity side. For the general difference between MCBs and molded-case breakers, see Circuit breaker - MCB - MCCB.
Key takeaways
- The curve letter sets the instantaneous (magnetic) trip range as a multiple of the rated current.
- Under IEC 60898-1: B is 3-5 × In, C is 5-10 × In, D is 10-20 × In.
- Choose the lowest curve that does not trip on the load's normal inrush, and confirm that the fault current at the end of the cable reaches the instantaneous range.
- When replacing a breaker, keep the same curve, rating, number of poles and listing unless the design is reviewed.
Work on electrical equipment must be done by qualified personnel following the applicable electrical code and the manufacturer's instructions.
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