AFDD vs MCB vs RCCB vs RCBO: Where Arc Fault Protection Fits in LV Panels

Every low-voltage panel quotation eventually runs into the same question from the specifier: we already have MCBs and RCDs, so why add an AFDD?

It is a fair question, and answering it with "for extra safety" loses the deal. The four device families — MCB, RCCB, RCBO and AFDD — do not overlap. Each one watches for a different electrical signature, and each one is governed by a different IEC product standard. A panel that is fully compliant on overcurrent and earth leakage can still be blind to the single most common ignition mechanism in electrical fires: the low-current series arc.

This guide breaks down what each device actually detects, where it belongs in the LV panel architecture, and how to specify arc fault protection without creating nuisance-trip problems on site.

Quick comparison: four devices, four fault types

The row that matters commercially is "blind to." Three of the four devices are blind to series arcing, and that is the gap AFDDs were created to close.

MCB: magnitude-based protection

An MCB reacts to how much current is flowing, not to the shape of the waveform. The thermal bimetal handles sustained overload; the magnetic coil handles short circuit. Curve selection (B, C or D) sets the magnetic threshold at roughly 3–5, 5–10 or 10–20 times rated current.

That model works perfectly for the faults it was designed for. It fails on arcs because an arc fault is a high-impedance event. A loose terminal, a screw-pierced cable or a cracked conductor inside a flex may sustain an arc that draws only 2–5 A on a 16 A circuit. The MCB sees a normal load. Meanwhile the arc itself is dissipating hundreds of watts into a few square millimetres of insulation, at temperatures well above the ignition point of PVC and timber.

For the overcurrent layer of the panel, our range covers both compact modular formats and higher breaking capacity variants:

• CDB6i Miniature Circuit Breaker — AC 50/60 Hz, up to 400/415 V, 1–63 A, with overload, short-circuit, control and isolation functions.
• DZ47s-80/80H Miniature Circuit Breaker — for boards where the prospective fault current demands higher Icn; the 80H variant is rated 10 kA (2P).
• DZ47sZ DC Miniature Circuit Breaker — 1P DC250V, 2P/3P DC500V, 6 kA, for PV strings and DC distribution.

If curve and Icn selection is the open question on your project, our earlier guide on MCB curve types, Icn and application examples works through the calculation.

RCCB: differential protection for people

An RCCB passes line and neutral through a toroidal core. Under healthy conditions the vector sum is zero. When current escapes to earth — through a person, damp insulation or a damaged sheath — the imbalance induces a signal that trips the device, typically at 30 mA for personal protection or 100/300 mA for fire and equipment protection.

Two limitations matter for panel design:

1. An RCCB provides no overcurrent protection. It must always sit downstream or upstream of an MCB or MCCB that protects it against short-circuit stress.
2. A series arc produces no residual current. The arcing current still returns through the neutral, so the vector sum stays at zero and the RCD stays closed. Only parallel arcs to earth are visible to an RCD; line-to-neutral parallel arcs are not.

Residual current type selection is the second common specification error. Type AC covers sinusoidal AC only; Type A adds pulsating DC, which is what you need behind most modern electronics; Type F and Type B cover frequency-mixed and smooth DC residual currents from VSDs, EV chargers and PV inverters.

RCBO: one module, two functions

An RCBO integrates residual current detection and overcurrent release in a single device, so each final circuit trips independently. Compared with a shared RCCB protecting a bank of MCBs, an RCBO limits the loss of supply to a single circuit — a decisive advantage in hotels, data rooms, clinics and any installation where a single leakage fault must not black out a whole board.

The trade-off is cost per way and module width. Panel builders typically use a hybrid architecture: group RCCB protection for lighting and general socket circuits, RCBOs for critical or high-leakage circuits.

For this layer, we supply:

• DZ47sLE Residual Current Operated Circuit Breaker — modular residual current protection for terminal distribution.
• DZ47sLES Small Leakage Protection Circuit Breaker — 230/400 V, up to 63 A, combining leakage, overload and short-circuit protection, with optional over- and under-voltage release for unstable grids.

Pole configuration, residual current type and sensitivity vary by model, so confirm the exact variant against the datasheet on the product page before you finalise the schedule. Background on the underlying principle is covered in What is a Residual Current Circuit Breaker?

AFDD: signature-based protection against fire

An AFDD does not measure magnitude or imbalance. It samples the current and voltage waveform continuously and runs pattern recognition on the high-frequency content. Fault arcs are chaotic: they show current-zero shoulders, broadband HF noise and characteristic randomness between half-cycles. Operational arcs — a light switch, a contactor, a brushed motor, a plug inserted under load — are short, repeatable and predictable. The device's algorithm has to separate the two, which is why IEC 62606 defines both mandatory trip tests and mandatory non-trip (unwanted tripping) tests.

Two arc types are relevant:

• Series arc — a break in one conductor, a loose terminal screw, a fatigued flex. Current is limited by the load, so it stays below the MCB threshold and produces no residual current. Invisible to MCB, RCCB and RCBO alike.
• Parallel arc — between line and neutral, or line and earth, through degraded insulation. It may eventually escalate into a short circuit, but the pre-escalation phase can smoulder for a long time.

Our AFDD Arc Fault Protection Circuit Breaker integrates arc detection with a thermal-magnetic overcurrent release in a 1P+N format:

Because overcurrent protection is built in, the device occupies the position of the final-circuit MCB rather than adding to it. Typical target applications are areas combining high occupancy with combustible content: hotels and dormitories, schools, libraries and museums, care facilities, timber-frame construction and heritage buildings.

Where the AFDD goes in the panel

Position matters more with AFDDs than with any other modular device, because detection quality degrades with distance and with the number of parallel loads sharing the monitored conductor.

A conventional protection hierarchy looks like this:

Incomer (ACB / MCCB)  →  main busbar
        ↓
Surge protection (SPD) at the origin
        ↓
Group RCCB (30 mA)  →  MCBs on final circuits
        ↓
RCBO on critical / high-leakage circuits
        ↓
AFDD at the origin of designated final circuits

Four rules keep the installation clean:

1. Install at the head of the final circuit it protects. An AFDD upstream of a sub-board monitors the aggregate of many circuits, and the arc signature of a 3 A series fault disappears into the noise of everything else on the busbar. It is a final-circuit device, not an incomer device.
2. Respect the circuit length limit in the manual. Detection sensitivity falls off along the cable run; every manufacturer states a maximum protected length.
3. Connect the neutral properly. The electronics are powered from the circuit. A missing or borrowed neutral means no detection, and the device must not be used with the load side of a TN-C system.
4. Coordinate with the upstream RCD. If a time-delayed or S-type RCD is used upstream for selectivity, verify discrimination so a downstream fault does not take out the whole board.

For the surge layer at the origin, pair the arrangement with a DZ47vY Surge Protector or DZ47sY Surge Protector. Surge events degrade insulation over time, and degraded insulation is exactly what produces arcs three years later.

Selection matrix by application

Regulatory status varies by market. Several national codes now require AFDDs on socket-outlet final circuits in higher-risk residential buildings, and IEC 60364-4-42 recommends them for premises with sleeping accommodation, fire-propagating structures and irreplaceable goods. Always verify the current edition of the wiring rules that apply to your project before finalising the specification.

Five specification mistakes that cost panel builders money

1. Treating the 30 mA RCD as fire protection. It addresses earth-fault ignition only. Series arcing is a separate mechanism.
2. Fitting the AFDD on the incomer. Cheaper per board, but the device cannot resolve a small series arc buried in aggregate load current.
3. Overlooking busbar compatibility. 1P+N devices need both poles fed. A standard single-pole comb busbar will not serve an AFDD or 1P+N RCBO without the matching accessory.
4. Ignoring derating and heat rise. Densely packed modular devices inside a sealed enclosure run hot. Verify against IEC 61439 temperature-rise requirements rather than assuming nameplate ratings at 30 °C.
5. Blaming the AFDD for a wiring defect. An AFDD that trips repeatedly on one circuit is usually reporting a real loose connection. Investigate terminations before replacing the device — 2.5 N·m torque on modular terminals is a routine root cause.

Procurement checklist for distributors and panel builders

Before placing an order, confirm:

• Product standard marks: IEC 62606 (AFDD), IEC 60898-1 (MCB), IEC 61008-1 (RCCB), IEC 61009-1 (RCBO)
• Icn versus prospective short-circuit current at the point of installation
• Curve type against inrush profile of the connected loads
• Residual current type: AC / A / F / B
• Module width and DIN rail budget across the whole schedule
• Terminal capacity, torque spec and comb busbar compatibility
• Operating temperature range and derating tables
• Available certificates, catalogues and user manuals for the submittal package
• MOQ, lead time and OEM branding options

Full catalogues, user manuals and certificate copies for the ranges above are available from our Download centre and Certificates pages.

FAQ

Does an AFDD replace an RCBO? Not by default. Our AFDD integrates arc detection with overcurrent protection but not residual current protection, so a 30 mA RCD or RCBO is still required upstream for shock protection. Some markets specify combined AFDD+RCBO units instead.

Can I install an AFDD and an RCBO on the same circuit? Yes, and it is a common arrangement. Keep the AFDD at the origin of the final circuit and verify discrimination with the upstream residual current device.

Why does an MCB not catch arc faults? Because arc current is limited by the load impedance. A series arc typically draws less than the MCB's rated current, so neither the thermal nor the magnetic release operates.

What is the difference between an RCCB and an RCBO? An RCCB detects residual current only. An RCBO adds overload and short-circuit protection in the same module, so it can protect a final circuit on its own.

How often should AFDDs be tested? Follow the test button interval in the user manual — monthly is the usual recommendation — and record the result as part of the installation's periodic inspection.

Specify the right protection layer for your project

Overcurrent, earth leakage and arc fault are three independent risks, and a compliant LV panel addresses all three deliberately rather than by assumption.

Delixi Electric supplies the complete terminal distribution range — MCBs, residual current devices, AFDDs, surge protectors, LV distribution boxes and LV switchgear — with IEC and CUL certification, factory catalogues for your submittals, and OEM branding for volume orders.

Send us your circuit schedule and we will return a device-by-device selection with datasheets. Request a quotation → | Become a distributor →