MCB vs RCBO vs RCD vs RCCB: Key Differences
What is the difference between an MCB, RCD, RCCB and RCBO? An MCB (miniature circuit breaker, to IEC 60898-1) trips on overload and short-circuit current only; an RCD — sold in some markets as RCCB (residual current circuit breaker) — trips on earth-leakage current only, per IEC 61008-1, typically at 30 mA or 300 mA; an RCBO combines both trip functions in one DIN-rail device. The practical consequence: an MCB will not open on a live-to-earth fault below its overload or short-circuit threshold, and an RCD will not open on a sustained overload or a phase-to-phase short-circuit. This article covers what each device senses, the 30 mA-vs-300 mA sensitivity split, a side-by-side comparison table, Vigi and DS201 add-on modules, and where each device is required.
What Each Device Actually Detects
Three different fault types exist on a final circuit: overload (sustained current above the cable's rating), short-circuit (a fault-level current spike), and earth-leakage (current escaping the intended L-N loop, usually through insulation breakdown or a person's body). An MCB senses the first two through mechanical means — a bimetal strip for overload, a solenoid for short-circuit. An RCD senses only the third, using a toroidal current transformer that has nothing to do with overload or short-circuit magnitude. Neither device substitutes for the other; an RCBO exists because most installations need both functions on the same circuit.
MCB: Overload and Short-Circuit Protection Only
An MCB's thermal element is a bimetal strip that bends under sustained overcurrent, opening the contacts after an inverse-time delay — the higher the overload, the faster it trips. Its magnetic element is a solenoid that trips instantaneously once current crosses a multiple of the rated current In, set by the curve: B at 3-5x In, C at 5-10x In, D at 10-20x In. Breaking capacity (Icn) is rated separately — 6 kA and 10 kA are the common commercial values. None of this reacts to leakage current. A 20-30 mA current diverted to earth through a person's body draws line current well below In on a 16 A or 20 A circuit; the bimetal strip does not move and the solenoid does not see a short-circuit-level spike. The MCB stays closed.
RCD (RCCB): Earth-Leakage Detection Only
An RCD has no thermal element and no magnetic overload trip. Its toroidal CT surrounds all live conductors (line and neutral); under normal operation the vector sum of currents through the window is zero, because every ampere going out on line returns on neutral. When current leaks to earth outside that loop, the sum stops being zero, an induced signal in the CT's secondary winding energizes a trip relay, and the mechanism opens. This means an RCD (also called RCCB, same device, different regional naming) will not open on a sustained overload or a bolted short-circuit between line and neutral — those currents balance through the CT window just like normal load current. That is why a stand-alone RCD is installed upstream of, or alongside, an MCB — never as its replacement.
Formula: Residual Current Detection — Source: IEC 61008-1, principle of operation via toroidal current transformer
IΔ = |IL1 + IL2 + IL3 + IN|
| Symbol | Description | Unit |
|---|---|---|
| IΔ | Residual (leakage) current sensed by the toroidal CT | A |
| IL1, IL2, IL3 | Instantaneous line currents passing through the CT window | A |
| IN | Instantaneous neutral current passing through the CT window | A |
RCBO: Both Functions in One Device
An RCBO houses the thermal-magnetic trip mechanism of an MCB and the toroidal-CT sensing of an RCD in a single body, on a single DIN-rail position (typically two modules wide for a 1P+N unit, the same footprint as an MCB plus a bolt-on leakage block). One reset lever, one part number, one set of terminals. The operational benefit is discrimination at the circuit level: if one RCBO nuisance-trips on an earth fault, only that final circuit drops — not the whole distribution board, which is what happens with a single shared RCD covering several MCB-protected circuits.
Residual-Current Sensitivity: 30 mA vs 300 mA
The residual-current rating (IΔn) determines what the device protects against, not how it protects. 30 mA is the personnel-protection threshold — the level widely adopted for additional protection against direct and indirect contact, with a required trip time under 300 ms at rated residual current for general-use (Type AC/A) devices. 300 mA is the fire-protection and insulation-monitoring threshold, used where the concern is a slow-developing leakage fault heating insulation over time rather than an immediate shock hazard, and commonly deployed with a time delay (Type S) at a main incomer for selectivity with downstream 30 mA devices. What we see in the field: some panel builders default to a time-delayed 300 mA RCBO or RCD at the incomer and 30 mA RCBOs on final circuits — this depends on the earthing system (TT installations need the incomer device for fault-loop reasons that TN systems often don't) and on whether the local wiring regulation treats the two thresholds as substitutable.
Comparison Table: MCB vs RCD (RCCB) vs RCBO
| Criteria | MCB | RCD / RCCB | RCBO |
|---|---|---|---|
| Detects | Overload + short-circuit | Earth-leakage only | Overload + short-circuit + earth-leakage |
| Sensing element | Bimetal (thermal) + solenoid (magnetic) | Toroidal CT (vector sum) | Bimetal + solenoid + toroidal CT |
| Governing standard | IEC 60898-1 (or 60947-2 industrial) | IEC 61008-1 | IEC 61009-1 |
| Typical trip setting | In x curve multiple (B 3-5x, C 5-10x, D 10-20x) | 30 mA or 300 mA IΔn | Curve multiple + 30 or 300 mA IΔn |
| Protects against electric shock | No | Yes | Yes |
| Protects cable from sustained overload | Yes | No — needs an upstream MCB | Yes |
| Board impact on nuisance trip | Only its own circuit | Every circuit under it, if shared | Only its own circuit |
Vigi and DS201: Turning an MCB into an RCBO — and Where Each Is Required
Two ways exist to get RCBO-level protection from an MCB platform you already stock. Schneider's Vigi module clips onto an existing Acti9 iC60 MCB without replacing it — the MCB keeps its original curve and breaking-capacity rating, and the Vigi block adds the toroidal CT and trip coil, available in 30 mA or 300 mA versions. ABB's DS201 takes a different route: it is a single part number built on the S200 platform with the RCD block factory-integrated, not a field-added accessory — functionally the same result, one fewer connection point. Both approaches avoid rewiring an entire final circuit to add leakage protection to a board that was designed with MCBs only.
Where an RCD or RCBO is required depends on the wiring regulation in force and the earthing system, but the recurring cases are: socket-outlet circuits accessible to unskilled users, bathroom and wet-area circuits, outdoor and construction-site circuits, and any circuit on a TT earthing system where the fault-loop impedance is too high for an MCB to guarantee disconnection within the required time on its own. Industrial motor and feeder circuits handled by skilled personnel under IEC 60947-2 are the cases most likely to run MCB-only, with earth-leakage protection handled separately at the switchboard level rather than per final circuit. Full selection logic — curve, breaking capacity, poles, and now leakage protection — is covered in the MCB selection checklist; how the underlying standards differ is in IEC 60898 vs IEC 60947 standards, and the broader trip-curve and breaking-capacity fundamentals are in the MCB tripping curves and MCB breaking capacity ratings guides.
Frequently Asked Questions
Can an RCD replace an MCB?
No. An RCD has no thermal or magnetic trip element, so a sustained overload or short-circuit passes through it undetected as long as the current balances between line and neutral. Cable protection still requires an MCB, either upstream or combined as an RCBO.
Do I need an RCBO on every final circuit?
Not universally — it depends on the local wiring regulation and the earthing system. Socket outlets, bathroom circuits, and outdoor circuits are the cases most commonly mandated; skilled-access industrial feeders under IEC 60947-2 are more often MCB-only with leakage protection handled elsewhere on the board.
Is RCCB the same thing as RCD?
Yes. RCD is the more general IEC term; RCCB (residual current circuit breaker) is the same device, more commonly used as a label in some regional markets. Both sense the same vector-sum leakage current and carry no overload rating of their own.
Is a Vigi module functionally identical to buying an RCBO?
For protection, yes — the combination of an Acti9 MCB plus a Vigi block behaves as an RCBO electrically. Mechanically it occupies the MCB's original module width plus the Vigi block's width, so confirm DIN-rail space before choosing it over a single-body RCBO or DS201-style integrated unit.
Why does my RCBO trip but the incomer's time-delayed RCD doesn't?
That is correct selectivity, not a fault. A time-delayed (Type S) device at the incomer is set to let the faster downstream RCBO clear the leakage on its own circuit first, isolating only the affected final circuit instead of the whole board.
Conclusion
The three devices answer three different questions. An MCB asks: is this current above the cable's overload or short-circuit rating? An RCD (RCCB) asks: does the current going out equal the current coming back? An RCBO asks both, in one housing. Specifying the wrong one for the job — an MCB where shock protection is required, or a stand-alone RCD with no upstream overcurrent device — leaves a real gap, not a redundant safeguard. For the full protection picture across curves, breaking capacity and pole configuration, see the MCB engineering guide and browse the current range of miniature circuit breakers available.
