RCCBDDRResidual Current ProtectionIEC 61008

How an RCCB works

An RCCB (Residual Current Circuit Breaker) — also known as an RCD or residual-current device — is the device that protects people from electric shock. It works on a completely different principle from an MCB: it detects the difference between the current flowing into and out of the circuit.

29 May 2026·11 min read·

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Photograph of an opened RCCB with its components labelled — Fig. 0 — Internal construction of an RCCB: zero-sequence transformer (toroidal core), tripping coil, test resistor and test button

The problem the RCCB solves

The current that can be lethal to a human is 30–100 mA — that is, 0.03–0.1 amperes. A 16A MCB has no way to detect such a tiny difference relative to the circuit’s rated current. That is why an MCB alone provides no protection against electric shock.

Fig. 1 — The MCB does not detect the fault current: 30mA is negligible compared to the 16A rating

According to medical data, current flowing through the human body for 1 second produces:

Current	Physiological effect
1 mA	Perception threshold — slight tingling
10 mA	Muscle contraction — let-go impossible
30 mA	Ventricular fibrillation possible — life-threatening
100 mA	Ventricular fibrillation certain — lethal

Source: IEC/TR 60479-1 — Effects of current on human beings and livestock

The working principle: the toroidal transformer

At the heart of an RCCB is a toroidal transformer — a ring-shaped ferromagnetic core through which both conductors pass: the line (L) and the neutral (N).

Under normal conditions, all the current entering on the line returns on the neutral: I_L = I_N. The two conductors generate equal and opposite magnetic fields that cancel out completely. The resulting flux in the core is zero.

RCCB circuit diagram with the toroidal core, line coil, neutral coil and tripping coil — Fig. 2 — RCCB circuit diagram: the LINE coil and the NEUTRAL coil pass through the same toroidal core; the imbalance induces a signal in the detection coil that energises the tripping coil

Fig. 3 — RCCB operation: on the left, normal operation (I_L = I_N, zero flux); on the right, a fault (I_L ≠ I_N, the relay trips)

When an insulation fault occurs (or a person touches the line conductor), part of the current escapes directly to earth without returning through the neutral. Now I_L ≠ I_N. The difference — the residual current — generates a magnetic flux in the core that induces a voltage in the secondary winding. This energises a sensitive relay that mechanically trips the RCCB.

Reaction speed: A standard RCCB (general type) trips in less than 40 ms at the rated residual current. An RCCB of type S (selective) has a deliberate delay of 40–500 ms, precisely to let the downstream device (on the circuit) trip first.

The standardised sensitivities

Fig. 4 — The standardised RCCB sensitivities and where each one is used

The I7-2011 standard requires 30 mA residual-current devices as a general rule for the protection of people on socket-outlet circuits and in bathrooms. The 300 mA sensitivity is accepted for fire protection on risers, where tripping at 30 mA would be too sensitive.

Types AC, A, F, B — why they matter

Not all fault currents are sinusoidal. Modern electronic equipment (computers, chargers, inverters, EV stations) generates currents with DC or high-frequency components. A type AC RCCB does not detect these current waveforms.

Fig. 5 — Type B is the most complete and is mandatory for EV stations and PV inverters

2023 updates to I7-2011: For electric-vehicle charging stations and grid-connected photovoltaic inverters, a type B RCCB is required, capable of detecting the smooth DC currents these systems generate.

The test button — what it checks and what it doesn’t

Every RCCB has a test button (T) that injects a small current directly into the tripping relay, bypassing the toroidal core. It checks whether the mechanical tripping mechanism works, but it does not check:

the accuracy of the toroidal sensor (it can degrade over time)
the actual tripping current (it may trip at 50mA instead of 30mA)
the continuity of the installation’s earth connection

I7-2011 recommends testing with the button at least once every 6 months. A full check (actual tripping current) requires a measuring instrument — an RCD tester.

What an RCCB does NOT do

An RCCB does not protect against overloads and short circuits. A 100A short circuit flows through the RCCB without tripping it — the L and N currents are equal (both large), so the difference stays zero. That is why, in practice, an RCCB is always paired with MCBs on each circuit or replaced by an RCBO (which combines both protections).

Normative reference

Reference standards: IEC 61008-1 (RCCBs without overcurrent protection), IEC 62423 (types AC/A/F/B), IEC/TR 60479-1 (effects of current on humans). The I7-2011 standard requires 30mA RCDs in Ch. 4.1.3.x for socket-outlet circuits, outdoor installations and bathrooms.

ElectroSchema

In the ElectroSchema distribution board, the RCCB is placed on the DIN rails with its sensitivity and type configured. The validation rules check that all socket-outlet circuits have a 30mA RCD (V02), that bathrooms have a 30mA RCD on the bathroom circuit (V34) and that RCCB–RCBO selectivity is respected (ratio ≥ 3:1 per Art. 4.1.5.2.8, rule V44).

Discussion

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