RCD and RCBO selection rules
Residual-current waveform type, sensitivity, pole arrangement, current rating, trip curve, and breaking capacity are separate decisions and must all match the circuit.
IB ≤ In ≤ IZIcn ≥ prospective short-circuit currentKinrush = Iinrush / Inwhere:
- IB
- Circuit design current[A]
- In
- RCBO rated current[A]
- IZ
- Corrected conductor current-carrying capacity[A]
- Icn
- Rated short-circuit breaking capacity[kA]
- IΔn
- Rated residual operating current[mA]
Type A, F, or B refers to residual-current waveform detection; B, C, or D curve refers to overcurrent magnetic behavior. The labels must not be confused.
How to select an RCD or RCBO
Define the load, waveform risk, circuit role, conductors, design current, cable ampacity, inrush, and prospective fault current. Select residual-current type and sensitivity separately from overcurrent curve and breaking capacity.
- Choose RCD/RCCB or RCBO function.
- Select Type A, F, B, or an approved EV solution.
- Choose sensitivity and poles.
- Check Ib ≤ In ≤ Iz.
- Verify curve, fault level, leakage, and neutral routing.
Application starting points
Type A is a common starting point for modern general circuits, Type F may suit selected single-phase inverter appliances, and Type B addresses smooth-DC risk where equipment and rules require it. EV charging may use Type A with verified 6 mA DC detection or another approved solution.
| Application | Starting direction |
|---|---|
| General final circuit | Type A, 30 mA |
| Inverter appliance | Type F where specified |
| EV/PV/VFD | Manufacturer and code-specific Type A/F/B solution |
| Upstream | Selective 100 or 300 mA context |
Sensitivity and selectivity
Thirty milliamperes is widely used for additional personal protection on final circuits. One hundred or 300 mA devices are commonly upstream for discrimination or fire-risk protection and do not normally replace required 30 mA final-circuit protection.
Current, curve, poles, and breaking capacity
The rounded current rating must protect the conductor. Pole arrangement must pass every relevant live conductor through residual sensing. RCBO curve must tolerate normal inrush while still meeting fault-disconnection rules, and breaking capacity must exceed prospective fault current.
Assumptions
- IEC-style final and distribution circuits
- Type and sensitivity are starting recommendations
- Equipment manufacturer instructions and local wiring rules take precedence
Important Warnings
- EV, PV, VFD, medical, TT, and special installations require product and jurisdiction-specific verification.
- Do not confuse Type B residual-current detection with a B-curve overcurrent trip characteristic.