Circuit protection
MCB Inrush & Trip Curve Checker (B / C / D)
Check whether a chosen MCB rating and curve will ride through the load inrush without nuisance tripping, and still clear the prospective fault current in the instantaneous magnetic band. Enter the rating, curve, inrush peak, and fault level to compare against the B, C, and D magnetic-trip ranges of IEC 60898-1.
What the curve controls
The B, C, and D letters set the instantaneous magnetic trip band — the multiple of rated current at which the MCB trips within milliseconds. Below that band the breaker only responds on its slower thermal element; above it, it trips instantly.
A usable curve has to satisfy two opposite conditions at once:
- Ride through inrush: the peak inrush should stay below the lower edge of the band, or the breaker nuisance-trips every time the load energises.
- Clear the fault fast: the prospective fault current should reach or exceed the upper edge of the band, so a real fault is cleared in the instantaneous region rather than on the thermal curve.
Choosing between B, C and D
| Curve | Magnetic band | Typical load |
|---|---|---|
| B | 3–5 × In | Resistive, lighting, long cable runs — low inrush, low fault level |
| C | 5–10 × In | General distribution, sockets, small motors — the common default |
| D | 10–20 × In | Transformers, large motors, capacitor banks — high inrush |
On a weak supply or a long final circuit, a high curve can fail to reach its magnetic threshold on an earth fault, so the disconnection time is set by the thermal element and may exceed the limit. That is why B and C curves are preferred where the fault level is low, and the earth-fault-loop impedance must always be verified against the required disconnection time.
Frequently asked questions
My motor breaker keeps tripping on start — what should I change?
The start inrush is likely reaching the magnetic band. Move to a higher curve (C→D) or a higher rating if the cable allows, so the lower band edge sits above the inrush peak. Confirm the fault level still reaches the new, higher band.
Does a higher curve make the breaker “stronger”?
No. The curve only shifts the instantaneous threshold. Breaking capacity (Icn) is a separate rating and must still exceed the prospective fault current.
Why can a D curve be unsafe on a long circuit?
If the fault current at the far end never reaches 10 × In, the breaker will not trip magnetically and disconnection relies on the thermal element, which can be too slow. Use a lower curve or verify the loop impedance.
Select the MCB
With the rating and curve screened, match the breaking capacity and pole count to a device for your market.
Screening estimate for guidance only. The final device, curve, and breaking capacity must be confirmed by a qualified person against the manufacturer’s time–current curve, the measured fault level, and the earth-fault-loop impedance.