An RCBO (residual current breaker with overcurrent protection) combines residual current detection with overload and short circuit protection in one 18 mm module, compliant with IEC 61009-1. The most important marking is the RCD type. Type AC is no longer permitted for new installations in Australia and is restricted in the UK, making Type A RCBOs the preferred choice for many modern applications.
What an RCBO Protects Against
One RCBO replaces two devices in a single 18 mm slot. It monitors the imbalance between line and neutral, and also trips on overload and short circuit. IET guidance describes an RCD as using coils wound around a ferrite toroid core, with all live conductors of the protected circuit forming past the residual-current sensing arrangement.
That construction is why the device responds to leakage current rather than load current. When the outgoing current no longer matches the returning current, the difference induces a voltage in the trip coil and the contacts open.
The overcurrent side operates on a different principle. A bimetal strip handles slow overloads, and a magnetic element handles short circuits, which is where the B or C curve marking comes in.
Width is the other half of the argument. A 1P+N unit occupies one 18 mm module, the same slot an MCB would take, so a whole board can move to per-circuit leakage protection without a larger enclosure.
Type A or Type AC: The Marking That Decides Compliance
Regulation 531.3.3 of BS 7671 restricts the use of Type AC devices to fixed equipment where the load current is known to contain no DC component. The current version of the standard, BS 7671:2018+A4:2026, has been in force since 15 April 2026 and was published jointly by the IET and BSI.
Why Type AC RCBO Sensing Fails on Modern Loads
Read that restriction carefully. A socket circuit can supply anything a user chooses to plug in, so no one can reliably claim to know the DC content of its load current. In practice, the rule drives almost every final circuit onto Type A.
The technical reason is desensitisation. A Type AC sensor responds only to sinusoidal AC residual current. Add a DC component from a alimentatore switching or an LED driver, and the core can partially saturate, so the device may be slow to trip or fail to trip altogether.
A Restriction on Use, Not a Ban on Sale
Note the wording, though. This is a restriction on use, not a product ban. Type AC units remain legal to sell in the UK, which is exactly why buyers still receive them in mixed consignments.
Australia went further than simply restricting them. Under clause 2.6.2.2.2 of AS/NZS 3000:2018 (the Wiring Rules), Type AC RCDs must not be installed anywhere in Australia from 1 May 2023, and only Type A, Type B and Type F devices may be fitted.
Australia’s Type AC Ban and Its Two Dates
Western Australia’s Building and Energy division gives the reason plainly: Electronics, solar generation and similar products now introduce a significant DC component into circuits that Type AC sensors were never designed to detect.
The transition timeline is worth understanding, because it explains the two dates often cited. Amendment 2 to AS/NZS 3000 was published on 30 April 2021 with a 24-month window during which either type could be installed. That window closed on 30 April 2023.
Which Installations the Type A Rule Covers
Temporary supplies are covered by the rule as well. Builders’ power poles count as installations, since electronic equipment used on site is plugged directly into them.
New Zealand falls under the same wiring rules, since AS/NZS 3000 is a joint standard. A Type A unit complies both with the Australian ban and the narrower UK restriction, which is why exporters increasingly stock one type rather than two.
Enforcement is not theoretical. In 2023 guidance, Queensland’s Electrical Safety Office confirmed it would apply the AS/NZS 3000 Amendment 2 rules with no exemptions for projects already under way, and warned that fitting the wrong safety switch type means a contractor has failed to meet their duties under the Electrical Safety Act 2002.
For an exporter the risk sits upstream of the site. A distributor who takes Type AC stock into a market that has moved to Type A is left holding inventory that cannot legally be installed, and the cost lands on the supply chain rather than on the electrician.
Replacement work carries a carve-out worth knowing. Swapping a socket outlet, luminaire or heater in a fixed location does not by itself require replacement of the existing RCD, provided correct operation is confirmed.
Five specification items decide whether a unit suits the destination market:
Tipo RCD
Valore di corrente residua
Curva di intervento
Capacità di rottura
Configurazione dei poli
Australian regulators added a sixth in 2023. Only RCDs carrying the Regulatory Compliance Mark or an approval number may be used in Australian installations.
Six RCBO Markings That Decide Market Fit
Work down this list in order, because each line narrows the field. Type comes first, since it is the only one of the six that can make an otherwise correct unit illegal in the destination market. The remaining five are engineering suitability rather than legal compliance.
Marcatura
Cosa controllare
Specifica comune
Tipo RCD
Type A, B or F for new work
Tipo A
Corrente residua
Livello di protezione aggiuntivo
30 mA
Curva di intervento
Load inrush behaviour
B o C
Capacità di rottura
Icn against prospective fault current
6 kA o 10 kA
Bastoncini
Module width and neutral switching
1P+N at 18 mm
Marchio di approvazione
Destination market acceptance
RCM or approval number
B Curve or C Curve: Matching the Trip Band to the Load
Curve choice derails more orders than type selection does. Under IEC 60898-1 a B curve opens in the magnetic band at 3 to 5 times rated current and suits resistive and lighting loads, while a C curve trips between 5 and 10 times rated current for transformer and motor inrush.
Specify the pole configuration explicitly as well. Module width alone does not tell you whether the neutral is switched or only sensed, and the two configurations behave differently on a TT or PME supply.
Get the curve wrong and the circuit either nuisance trips on switch-on or fails to clear a fault quickly enough. The full selection logic is covered in our guide to how B, C and D curves behave under inrush.
FAQ
Cosa significa RCBO?
RCBO stands for residual current breaker with overcurrent protection. The device is manufactured to IEC 61009-1 and combines what an RCCB and an MCB do separately. It detects earth leakage, overload and short circuit, and it fits a single 18 mm DIN rail module in the common 1P+N format.
Posso sostituire un MCB con un RCBO?
Mechanically it often fits, but it is not a like-for-like swap. The replacement must suit the consumer unit it goes into, and mixing brands can take the assembly outside the manufacturer’s type-tested design. The circuit needs testing before and after the change, so in the UK this is notifiable work in most cases.
Do I need an RCBO on every circuit?
Not always, though the trend runs that way. An RCBO limits a fault to one circuit, whereas a shared RCCB drops every circuit on its busbar. Where a board carries more than roughly a dozen ways, a shared RCCB feeding MCBs is usually cheaper on parts, so the choice is cost against continuity.
What is the difference between an RCBO and an RCD?
RCD is the family name for any device that detects residual current, and it includes both RCCBs and RCBOs. An RCBO is the member of that family with integral overcurrent protection. An RCCB has no overload or short-circuit trip of its own, so it must work alongside MCBs to protect the cable.
Are single-pole RCBOs safe?
They are certified, but they behave differently from double-pole units. A single-pole device with neutral sensing detects a neutral-to-earth fault yet interrupts the line conductor only. On a TT or PME system many designers prefer double-pole so both conductors break. Check the pole marking, not just the module width.
What breaking capacity should an RCBO have?
Match Icn to the prospective fault current at the board. In household distribution 6 kA is common, while 10 kA gives headroom close to a large transformer or where the supply is stiff. The rating is verified in accordance with IEC 61009-1 at a stated reference ambient, so a hot panel shifts actual behaviour.
Conclusione
The RCBO decision now has two layers. The electrical layer is familiar: rated current, curve, breaking capacity and pole count. The regulatory layer is newer and less forgiving, because a unit that was a standard stock item five years ago cannot be installed in Australia at all, and cannot be used on a socket circuit in the UK. Check the type marking before you check the price. TOMZN publishes the RCD type, curve and Icn in accordance with IEC 61009-1 for every unit in the range.
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