The difference between type B, C and D circuit breakers
The difference between Type B, C and D circuit breakers
Short answer: B, C and D identify the instantaneous magnetic tripping characteristic of a miniature circuit breaker. For the same rated current, a B-curve device enters its magnetic tripping region at a lower current than a C-curve device, while a D-curve device requires a higher current still. B16, C16 and D16 breakers all have a rated current of 16 A. Choosing D16 does not increase the circuit’s continuous current capacity.
Selection depends on the normal load current, the current drawn during starting or energisation, and the installation’s fault conditions. The breaker must allow normal operation while providing suitable overcurrent protection. Simply choosing the device that trips least often does not establish that the circuit is adequately protected.
B, C and D tripping ranges: a B16, C16 and D16 comparison
The following table shows the general magnetic tripping bands for AC miniature circuit breakers covered by IEC/EN 60898-1. In is the breaker’s rated current, expressed in amperes.
| Trip curve | Magnetic tripping band | Example with a 16 A breaker |
|---|---|---|
| Type B | 3–5 × In | B16: 48–80 A |
| Type C | 5–10 × In | C16: 80–160 A |
| Type D | 10–20 × In | D16: 160–320 A |
These figures are not continuous operating current ratings. Nor do they mean that every breaker within a band trips at exactly the same current. Use the specific product’s time–current characteristic for detailed assessment. These ranges apply to alternating current; suitability, voltage limits and tripping behaviour for direct current must be checked separately.
Why thermal and magnetic tripping must be distinguished
Thermal protection responds to sustained overloads. Its operating time depends on the magnitude and duration of the current. Magnetic protection provides rapid operation at high currents and is particularly important for short-circuit protection.
At the same rated current, B, C and D characteristics share the standard thermal test limits; their main difference is in the magnetic region. Consequently, “Type D always trips more slowly” is an inaccurate description. Ambient temperature, heat from adjacent devices and previous loading can also affect thermal operation.
Where are B, C and D curves considered?
Type B: Considered for loads with little starting or energisation current above their normal operating current. Suitable resistive heating loads are one example. However, a circuit being located in a house does not automatically determine its trip curve. The connected equipment and installation conditions still need to be assessed.
Type C: Considered where a moderate starting or energisation current must be accommodated. Some motors and lighting circuits fall into this category. Both the magnitude and duration of a motor’s starting current must be compared with the breaker’s characteristic.
Type D: May be considered for certain transformers and motor applications with substantial inrush current. The installation must also satisfy the required fault disconnection conditions. A D-curve MCB does not automatically provide all the overload, phase-loss or stalled-rotor protection that a motor may require.
For LED lighting, total wattage is not enough. Electronic drivers can draw a brief, high inrush current despite having a low normal operating current. Converting watts to amperes does not, by itself, describe this transient. Check the driver manufacturer’s guidance on permitted luminaire quantities and protective devices.
What do C16, 16 A and 6000 mean?
In the marking C16, C identifies the trip curve and 16 identifies the rated current. Changing B16 to C16 changes the curve; changing B16 to B20 changes the current rating. The choice between 10 A, 16 A, 20 A and other breaker ratings is separate from the choice of curve.
Under IEC/EN 60898-1 marking conventions, 6000 inside a rectangle indicates a rated short-circuit capacity of 6 kA under the specified conditions. It does not mean that the device carries 6000 A continuously or waits until 6000 A before tripping. The rated operating voltage, pole arrangement, AC/DC suitability and applicable standard must also be checked.
Can a B-curve breaker be replaced with C or D?
Repeated tripping alone is not a sufficient reason. First determine whether operation results from an overload, an actual fault or normal starting current. Then assess the cable’s current-carrying capacity, installation conditions and protection arrangement. Increasing the breaker’s current rating likewise requires a separate technical assessment.
Both minimum and maximum prospective fault currents matter. At the lowest relevant fault current, the protection must disconnect within the required time. At the highest prospective short-circuit current, sufficient breaking capacity must be available. Moving from B to C or D raises the current required for instantaneous magnetic operation. Fault-loop impedance and the protective earthing arrangement therefore cannot be overlooked.
On a long cable run, circuit impedance can limit the fault current. The normal operating voltage drop also needs assessment. Checking voltage drop and verifying fault disconnection time are separate design checks.
Is a Type B MCB the same as a Type B RCD?
No. B on an MCB describes its magnetic tripping characteristic. B in a Type B RCD describes the residual-current waveforms that the device can detect. These are different classification systems.
An RCBO may carry C16 together with a 30 mA marking and a residual-current type designation. Each describes a different property. An ordinary B-, C- or D-curve MCB does not provide residual-current protection by itself.
What should be checked before changing the curve?
First establish whether the MCB or the RCD has tripped. A trip that occurs when a particular appliance starts can suggest an inrush issue, but the timing alone does not prove the cause. If a breaker trips again immediately after resetting, do not repeatedly reclose it.
Different curve letters do not guarantee selectivity. The aim is for the protective device nearest the fault to operate while other circuits remain supplied. If the main breaker trips before a downstream breaker, simply comparing B, C and D labels is insufficient. The manufacturer’s coordination and selectivity tables must be considered.