RCD Types AC, A and B vs MCB Trip Curves
An electrical schedule that says only “Type B breaker” is ambiguous. It may mean an RCD with Type B residual-current detection, or an MCB with a B overcurrent curve. Those are different functions, and confusing them can leave an electronic load with the wrong protection.
The direct answer is that AC, A and B usually describe residual-current waveform capability in an RCD specification. B, C and D describe magnetic overcurrent characteristics in an MCB specification. An RCBO combines functions, so its order can require both a residual-current type and an overcurrent curve. Specify each field separately.
Residual-current types describe the leakage waveform
A residual-current device compares current leaving and returning through its sensing arrangement. Type AC addresses sinusoidal alternating residual current. Type A additionally addresses pulsating DC residual current within its declared test scope. Type B covers additional waveforms including smooth DC within its declared scope; it is not simply a higher-current version of Type A.
The load matters. Rectifiers, variable-speed equipment and charging systems can introduce waveforms that a simple AC assumption misses. Obtain the equipment manufacturer’s protective-device instructions and follow installation rules for the destination market. A general claim that all electronic equipment requires Type B is just as unhelpful as assuming Type AC is always adequate.
| Marking | What it describes | What it does not establish |
|---|---|---|
| Type AC | Sinusoidal AC residual-current response | Overcurrent curve or DC suitability |
| Type A | AC and pulsating DC residual-current response | Full smooth-DC detection |
| Type B RCD | Additional waveforms including smooth DC within scope | A B-curve MCB characteristic |
Review the difference between RCBO and RCCB protection before allocating overload and residual-current functions.
MCB curves describe overcurrent operation
An MCB’s magnetic trip curve concerns high-current pickup, whereas its thermal element responds to sustained overload. The familiar B, C and D labels therefore answer a different question from Type AC, A or B on an RCD. Select the curve using load inrush, conductor protection and the available fault current needed to achieve disconnection.
Do not choose a curve with a higher magnetic threshold simply to conceal repeated trips. Investigate overload, starting conditions and faults. For low fault-current circuits, a higher threshold can affect the time required to disconnect a fault. Use the exact manufacturer’s time-current curves and the installation calculation.
The guide to residual-current protection describes the installation conditions that a device label alone cannot establish.
Specifying an RCBO without ambiguity
Write the current rating, poles, voltage, residual-current type, residual-current sensitivity, overcurrent curve and short-circuit rating as separate entries. For example, “Type A, curve B” is a coherent description because the two labels refer to different functions. It is not a Type B RCD specification.
| Project question | Required information |
|---|---|
| What current can the circuit carry? | Design load, conductor ampacity and device current rating |
| Which leakage waveform can occur? | Equipment data and residual-current type |
| Will inrush cause unwanted operation? | Overcurrent curve and starting envelope |
| Can the device interrupt the fault? | Short-circuit rating at circuit voltage |
| Is additional protection required? | Sensitivity and applicable installation rules |
Before replacing a device, check neutral routing and the complete circuit arrangement. A shared neutral can create residual-current operation that changing the curve cannot solve. Isolate and investigate through a competent person rather than treating a new label as a cure.
Standards and procurement evidence
IEC 61008-1 covers RCCBs without integral overcurrent protection; IEC 61009-1 covers RCBOs. IEC 60898-1 is a separate reference for AC household and similar overcurrent circuit-breakers. The IET’s RCD-type guidance explains why waveform selection follows the connected equipment. A catalogue standard reference does not itself certify a particular model.
Ask W9 Group for the exact device type, order code and supporting technical documents. Keep the waveform selection, curve and installation verification in the circuit schedule so a later replacement does not silently change the protection boundary.
Frequently asked questions
Is a Type B RCD the same as a B-curve MCB?
No. Type B RCD describes residual-current waveform detection, while a B curve describes magnetic overcurrent pickup. An installation may need both functions in separate devices or an appropriately specified RCBO.
Can Type A detect every DC leakage current?
No. Type A covers defined AC and pulsating DC residual currents, not unrestricted smooth DC. Check the load instructions and applicable rules for additional detection or a different RCD type.
Is Type AC suitable for every household circuit?
No. Connected equipment and local installation requirements can require another type. Select from the expected residual-current waveform, not the building label alone.
Will changing from a B curve to a C curve fix RCD trips?
It does not change residual-current waveform detection or sensitivity. Diagnose whether the trip is caused by overcurrent, leakage or wiring before altering the protective arrangement.
References
- IEC 61008-1:2024 — RCCBs without integral overcurrent protection.
- IET — Which RCD Type?.
- IEC 60364-6:2016 — installation verification.
For waveform and pole selection, contact W9 Group with the circuit topology, load equipment and destination-market rules.