Leakage Circuit Breaker Function, Principle & 2026 Selection
Share
Why leakage protection still matters in 2026
In any PLC-controlled cabinet you wire on the shop floor, the leakage circuit breaker — also called a residual current device (RCD) or residual current circuit breaker (RCCB) — sits between the main breaker and the contactors that feed your motors, servo drives, and I/O racks. Get the sensitivity wrong and a single insulation fault can lock out an entire line for hours. Get the type wrong and nuisance trips stop production for no real reason at all. This Engineering Note refreshes the 2024 fundamentals of function, working principle, and key parameters against the 2026 IEC 60947-2 landscape, and adds practical selection rules we use every day in our multi-brand sourcing work.
> AI Summary
- A leakage circuit breaker (RCD/RCCB) monitors the algebraic sum of line and neutral currents and trips when residual current exceeds its rated sensitivity (IΔn), typically 30 mA for personnel protection and 300 mA for fire protection.
- Working principle is based on a toroidal zero-sequence current transformer (ZCT) feeding a residual operator — Type AC, A, B, or F classifications now cover everything from pure AC loads to modern VFD-driven motor circuits.
- 2026 selection hinges on IEC 60947-2 Annex M, which clarifies co-ordination with motor starters (AC-3 utilization) and DC-13 auxiliary circuits, plus immunity to high-frequency leakage from SiC/GaN inverters.
- Key parameters: rated current In, rated residual operating current IΔn, tripping time, short-circuit capacity Icn, and the new IΔn auxiliary powered (A type) self-monitoring feature on premium 2026 product lines.
For PLC panels with contactors, servo amplifiers, and soft-starters, choose Type A minimum, prefer Type B for VFDs, and coordinate IΔn with the upstream breaker for selectivity.
What changed since 2024
Three things shifted in the 2024–2026 window that affect how we specify leakage protection on industrial cabinets.
- IEC 60947-2 Annex M matured. The 2024 amendment clarified how RCDs coordinate with motor starters (AC-3) and DC-13 auxiliary contactor circuits. In 2026 most major vendors (Schneider, ABB, Eaton, Siemens) publish dedicated “RCCB + TeSys / 3RV” or “RCCB + Sirius” co-ordination tables in their online selectors — Schneider Modicon-compatible panels and Siemens Sirius-compatible panels each have their own Icn / Icu tables.
- Type F and Type B leakage breakers became the default for VFD cabinets. Modern Yaskawa Σ-7, Mitsubishi MR-J5, and Allen-Bradley PowerFlex 525 drives all produce smooth DC residual currents that a Type AC RCD simply cannot see — the result is a tripped breaker with no detectable ground fault.
- Self-test / auxiliary-powered monitoring is now standard on premium lines. A 2026 Type A-S or Type B-S RCD performs a periodic internal test of the toroid, electronics, and trip mechanism. For unmanned cabinets this is a real reliability gain — you find out the breaker is faulty before a person does.
Function: what a leakage circuit breaker actually protects
A standard miniature circuit breaker (MCB) only reacts to overcurrent — short circuits and overloads. It has no idea whether a person is touching a live conductor that has lost its insulation. The leakage circuit breaker exists to fill that gap. Its job is to detect small residual currents that flow to earth through a person, a wet surface, or a degraded cable, and to disconnect the supply fast enough that the let-through energy stays below the ventricular fibrillation threshold.
> Tip
For personnel protection on a 230 V or 400 V industrial socket, the rule of thumb from IEC 60479 is: trip at 30 mA within 30 ms. That combination gives a let-through energy below the fibrillation curve even for a hand-to-hand contact path.
Working principle: the toroid that watches the algebraic sum
Inside the breaker, all live conductors — line, neutral, and any auxiliary on a three-phase unit — pass through a toroidal (ring-shaped) current transformer. In a healthy circuit the vector sum of the currents passing in and returning is zero, so the toroid’s secondary winding sees no flux. The instant some current leaks to earth through a person, a wet floor, or a damaged cable sheath, that current returns through earth instead of the neutral. The vector sum is no longer zero. The toroid produces a small voltage, the residual operator compares it against the calibrated threshold IΔn, and within a few milliseconds the latching mechanism releases.
In 2026 product lines the analog electronics are replaced by a microcontroller with auto-calibration on power-up and a periodic self-test of the trip path. This is what the “S” suffix means on Type A-S and Type B-S breakers.
Type classification — AC, A, F, B at a glance
The most expensive mistake in 2026 leakage-breaker selection is buying a Type AC device for a cabinet that powers a VFD, an EV charger, or a photovoltaic inverter. The waveforms those loads produce contain DC residual components that saturate the toroid and either blind the breaker (no trip on real fault) or trip it on the leakage current of the line filter capacitors (nuisance trip).
| Type | Detects | Typical load | 2026 use case |
|---|---|---|---|
| AC | AC 50/60 Hz only | Resistive heating, lighting, socket outlets without electronics | Residential and simple commercial sub-distribution only |
| A | AC + pulsating DC components (phase-controlled) | Single-phase drives, washing machines, dimmer circuits | Minimum acceptable for any PLC or contactor cabinet |
| F | A + high-frequency leakage immunity, surge resistance | Single-phase VFD, HVAC with EC motors, heat pumps | Cabinets feeding 1-phase frequency converters |
| B | F + smooth DC residual currents | Three-phase VFD with active front end, PV inverter, EV DC charger, large servo drives | Required anywhere a 3-phase Yaskawa Σ-7 or Mitsubishi MR-J5 servo amplifier is downstream |
Technical parameters you actually look at
- Rated current In — the maximum continuous current the device can carry without nuisance tripping. Standard DIN-rail RCCBs go up to 125 A; for larger motor feeders you step up to a 4-pole molded-case device.
- Rated residual operating current IΔn — the threshold at which the device must trip. 30 mA for personnel, 300 mA (or 500 mA) for fire protection upstream, 100 mA is a common compromise on mixed industrial socket circuits.
- Breaking capacity Icn / IΔm — the prospective fault current the device can interrupt safely. In industrial cabinets this should at least match the Icu of the upstream MCB.
- Tripping time — instantaneous for IΔn ≥ 2× threshold (≤ 30 ms), short time-delay (S or G type) for discrimination with downstream devices.
- Number of poles — 2P for single-phase, 4P for three-phase with neutral. Industrial practice in 2026 is to use 4P even on three-phase-without-neutral feeders so the neutral terminal is available for control transformers and 24 V SMPS returns.
- Auxiliary powered (A type) vs. voltage-independent — voltage-independent (the older electro-mechanical design) works even if the neutral is lost; auxiliary-powered A-type can be more sensitive but needs the control supply to function.
! Warning
Type AC RCDs must not be used to protect any circuit that contains a three-phase rectifier, an active front-end drive, or a PV inverter — even a small one. The smooth DC residual current saturates the toroid and the device becomes blind to ground faults. If your cabinet powers a PowerFlex 525 or a Schneider Altivar ATV320, specify Type B (or Type F for single-phase variants) and verify with the drive manufacturer’s EMC leakage data.
Selection walk-through — a 2026 PLC cabinet
Consider a typical European PLC cabinet feeding one 3-phase 7.5 kW motor through a Schneider TeSys contactor, one Yaskawa Σ-7 servo, and a 24 VDC power supply for the PLC and HMI.
- Select a 4-pole Type A-S RCCB on the main feed, rated 25 A or 40 A depending on the transformer upstream, IΔn = 30 mA, Icn ≥ 10 kA to match the MCB.
- Fit a Type B 4-pole RCCB (or a Type F 4-pole if the drive is single-phase) on the circuit feeding the servo amplifier.
- Use a 100 mA selective S-type RCCB upstream of the main breaker for fire protection and to provide discrimination with downstream 30 mA devices.
- Verify co-ordination tables with the contactor and soft-starter vendor — the 2026 IEC 60947-2 Annex M tables cover exactly this use case.
Cross-reference: what to specify on your BOM
| Cabinet role | RCCB type | Sensitivity | Typical cross-reference (Schneider / ABB / Eaton / Siemens) |
|---|---|---|---|
| Main personnel protection | Type A-S | 30 mA | Acti9 iC60H — comparable lines across all four vendors |
| VFD / servo feed | Type B-S | 30 mA | Type B variants of the same DIN-rail series; verify against drive EMC data |
| Upstream fire protection | Selective S-type | 300 mA / 500 mA | Varies by installation; consult your local authority having jurisdiction |
Testing and maintenance in 2026
Press the test button every six months on a personnel-protection device, every quarter on a fire-protection device. On Type A-S / B-S units the internal self-test will catch most faults between manual tests, but a manual trip test is still required by most jurisdictional regulations. Log the test with date, operator, result. If the device fails to trip, replace it — do not bypass.
Sourcing the right device
Most of the time you can specify a current-production RCCB from any of the four major vendors and have it on site within a week. The challenge comes when you are maintaining a 1990s cabinet and need to match an existing installation: the frame size, the mounting depth, the neutral terminal orientation, and the auxiliary contact footprint are all slightly different between families and across generations. KOEED keeps both current and legacy leakage-breaker lines in stock across our partner brands and can cross-reference an obsolete reference against its modern equivalent within 24 hours.
Frequently asked questions
What is the difference between an RCD and an RCCB?
Both terms describe a residual-current circuit breaker that trips on earth-leakage current. An RCCB only detects residual current and relies on a separate MCB; an RCBO combines both.
Why does my VFD cabinet nuisance-trip a Type AC RCCB?
Line filter capacitors in any modern three-phase drive produce high-frequency leakage above 30 mA. A Type AC device sees this as a fault and trips. Use Type F or Type B instead.
Can I install a 30 mA RCCB upstream of a 300 mA RCCB without coordination problems?
Yes, if you select a selective (S-type) upstream device. The S-type adds a short intentional time delay so the downstream 30 mA breaker trips first on a fault inside its zone, while the upstream 300 mA S-type only trips on faults in the cable between the two breakers.
Do I still need a residual-current device if I already have an insulation monitor on an IT earthing system?
On an IT system the first fault is handled by the insulation monitor, but a second fault on a different phase becomes a short through a person. Standards still require an RCD at the outlet.
What changed between the 2024 and 2026 versions of IEC 60947-2 for leakage breakers?
The 2024 amendment added Annex M for co-ordination with motor starters. The 2026 edition tightened immunity tests for SiC/GaN inverter leakage and clarified self-monitoring labelling.
Specifying leakage protection on a new panel?
Send your line diagram or single-line diagram to Moritta@KOEED.COM. Current production, EOL stock, and cross-references for nine brands — one quote within 24 hours.
Send My BOM →Related on KOEED Blog
- The structure and working principle of relays and their application in motor control
- Use of fuses and thermal relays in electrical circuits
- Contactor model significance and selection precautions
- Illustration of the principle and structure of electromagnetic contactor
- KLUBER industrial lubricants — greases, oils, chain oils
KOEED Engineering Team
Industrial automation editors at KOEED. We write about PLC sourcing, motor-circuit co-ordination, leakage protection, and legacy system support. Reach the team at Moritta@KOEED.COM.