Selection RCD / RCBO
Online calculation and selection of type, rating, and differential current of RCD or RCBO based on room type, earthing system, and load according to IEC 60364.
⚙️ Parameters
RCD types — comparison
| Type | Protects against | Application | Cost |
|---|---|---|---|
| AC ← | Sinusoidal AC residual current | Lighting, simple household appliances | € |
| A | AC + pulsating DC | Inverter appliances, kitchen, bathroom | €€ |
| B | AC + DC + smooth DC | IT equipment, EV chargers, server rooms | €€€ |
| S | AC (with 40 ms delay) | Incoming fire-protection RCD (300 mA) | €€ |
How to calculate the rated current for an RCD
The rated current of the RCD is selected by the formula:
I = P / (U × cos φ)
where: P — load power in watts, U — mains voltage (220 V for single-phase), cos φ — power factor (0.95 for household appliances).
Example: bathroom — washing machine 2.2 kW + heated floor 1.5 kW = 3.7 kW.
I = 3700 / (220 × 0.95) = 17.7 A → select RCD 25 A (next standard rating above MCB C20). ΔI = 10 mA, type A (bathroom zone).
Key rule: the RCD rating must be not less than the rating of the protective MCB on the same line. If the MCB is C25 — the RCD is taken at 25 A or 32 A. A lower RCD rating will cause nuisance tripping at full load.
How to choose an RCD for an apartment or house?
RCD — residual current device — protects against electric shock and fire due to leakage current. RCBO (residual current circuit breaker with overcurrent protection) is an RCD + MCB in one housing.
Key parameters for selection: differential current (ΔI) — 30mA for human protection, 10mA for bathrooms, 300mA for fire protection; and type — AC for simple loads, A for inverter equipment, B for server equipment.
The RCD rating is chosen not less than the rating of the upstream MCB. For example, if the line has a C25 MCB, the RCD must be 25A or higher.
The number of groups under one RCD is limited by the total leakage current — it must not exceed 1/3 of ΔI. Typical leakage is 3-5 mA per group, so for a 30mA RCD, no more than 2-3 groups are recommended.