MCB Curve B, C or D - which one do you need?
The tripping curve of a miniature circuit breaker defines the fault current at which the fast magnetic trip reacts. This guide explains curves B, C and D to IEC/EN 60898-1, matches them to typical load types and shows how inrush current drives the choice.
View circuit breakersWhat does the tripping curve of an MCB mean?
A miniature circuit breaker (MCB) has two trip elements: a thermal bimetal trip against sustained overload and a magnetic trip against short circuit. The tripping curve describes only the behaviour of the fast magnetic trip, meaning at what multiple of the rated current In the breaker cuts off almost instantly.
The thermal trip works the same for B, C and D and protects the cable from overheating. The difference lies only in the magnetic range: type B trips early, type D trips late. This lets you match the breaker to the expected inrush current of the load so it does not nuisance-trip at every switch-on.
Which curve suits which load?
The deciding factor is the inrush current of the load. Resistive loads such as heaters or incandescent lamps draw barely more than their rated current, so type B is enough. Inductive and capacitive loads, however, produce short current peaks that a type B breaker would misread as a short circuit.
- Type B: socket circuits, lighting, residential and office areas, general loads.
- Type C: motors, pumps, transformers, LED and fluorescent gear with ballast.
- Type D: large transformers, welding sets, X-ray units, UPS with very high inrush.
- Motor starting current is often 5 to 8 times the rated current - type C absorbs it.
- Capacitors and switch-mode supplies create very short, high charging peaks.
Why does the disconnection condition matter?
For the MCB to disconnect fast enough under fault, the short-circuit current at the end of the cable must reach at least the magnetic trip value. IEC 60364‑4‑41 requires a 0.4 s disconnection time for final circuits. On long cables or with high line impedance, a type D breaker can miss this condition.
That is why you check the expected short-circuit current (loop impedance) during selection. A B16 already trips magnetically from about 80 A, a D16 only from roughly 160 to 320 A. If the fault current is too low, the cable must be shorter, the cross-section larger or the curve lower.
Frequently asked questions
What do the letters B, C and D stand for?
They denote the tripping characteristic, the current band in which the fast magnetic trip reacts: B at 3‑5x In, C at 5‑10x In, D at 10‑20x In. The rated current is a separate value.
Which MCB do I use for a motor?
Usually type C, because a motor starting current can reach 5 to 8 times the rated current. A type B breaker would nuisance-trip at switch-on. For very high starting current, type D may be needed.
Is type D always the safe choice?
No. Type D needs a high fault current to trip quickly and can worsen personal protection on long cables. You pick the lowest curve the load will tolerate.
Can I replace B with C if the breaker trips often?
Only after checking. Frequent tripping may indicate real overload. Switching to C is permissible if cable cross-section, fault current and the disconnection condition to IEC 60364‑4‑41 are still met.
Looking for the right circuit breaker?
We supply MCBs with curves B, C and D in all common rated currents and breaking capacities - tested to IEC/EN 60898-1.
Standard-tested
All MCBs comply with IEC/EN 60898-1.
Full range
Curves B, C and D available from stock.
Correctly sized
Advice on inrush current and disconnection.
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Electrical specialists help with the selection.