RCBO Wiring Diagram

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An RCBO combines earth-leakage (residual current) protection and overcurrent (short-circuit and overload) protection in a single module, replacing a separate RCD and MCB on one circuit.

A Residual Current Breaker with Overcurrent protection — universally abbreviated RCBO — integrates two independent protective functions into one DIN-rail device. Understanding how it differs from a plain RCD is essential before wiring one.

The residual current element works identically to a standalone RCD: it continuously monitors the vector sum of current flowing out through the line (live) conductor and returning through the neutral conductor. Under fault-free conditions the two currents are equal and opposite, and the sum is zero. If a person touches a live conductor and current leaks to earth through their body, the returning neutral current is smaller than the outgoing line current; the imbalance — as little as 30 mA for a Type A or AC RCBO — is detected by the toroidal sensing transformer, which triggers the tripping solenoid in under 40 ms.

The overcurrent element is a conventional miniature circuit breaker mechanism. It incorporates a bimetallic strip that bends with sustained overload current (inverse time-delay trip) and an electromagnetic solenoid that trips the mechanism instantaneously on short-circuit current exceeding the rated breaking capacity (typically 6 kA or 10 kA to IEC 60898-2).

Because both functions share one device, an RCBO is individually assigned to a single circuit. This is its key advantage over a single-pole RCD protecting multiple downstream MCBs: a 30 mA earth-leakage trip on one RCBO does not black out the entire consumer unit. Each circuit remains independent.

RCBOs are manufactured in three sensitivity ratings relevant to most installations: 30 mA (personal protection, mandatory for socket circuits and locations under IEC 60364 and BS 7671), 100 mA (fire protection on equipment circuits), and 300 mA (equipment protection). Type AC devices detect sinusoidal AC leakage only; Type A also detects pulsating DC leakage (required for circuits feeding electronic equipment, EV chargers, and modern appliances). Type F adds protection against high-frequency leakage.

Wiring is straightforward: line-in to the terminal marked Line or L (top), neutral-in to the N terminal (also top on double-pole or combined units), line-out to the circuit, neutral-out to the circuit. Many single-module RCBOs use a combined neutral bar rather than a separate neutral terminal — consult the device datasheet.

How to wire rcbo wiring diagram

Isolate, lock off, and verify dead Switch off the main isolator of the consumer unit. Apply a lock-off device if working in a shared premises. Use a calibrated voltage indicator to prove all busbars and terminals are dead before touching any conductors. Do not rely on the main switch alone — verify.
Identify the circuit conductors Locate the line (live), neutral, and earth conductors for the circuit being protected. Confirm conductor cross-section is appropriate for the RCBO rating (e.g., 2.5 mm² for a 20 A circuit). Label conductors if not already identified.
Connect the incoming supply to the RCBO top terminals Insert the line conductor into the Line-in (L) terminal and the neutral conductor into the N-in terminal. On a single-module RCBO, the neutral feed typically connects to the common neutral bar and then passes through the integral neutral link into the sensing toroid.
Connect the outgoing circuit conductors to the RCBO load terminals Insert the circuit line conductor into the load-side L terminal and the circuit neutral into the load-side N terminal. Ensure both conductors pass through the device's residual current sensing toroid as indicated by the device wiring diagram.
Connect the circuit protective conductor (earth) Connect the circuit earth conductor to the consumer unit earth bar. The RCBO does not carry the earth conductor internally — earth must be continuous from the source to the load independently.
Test the RCBO before energising With the circuit energised and the RCBO in the ON position, press the integral TEST button. The device must trip immediately. Reset and confirm the test result is recorded in the installation certificate or test schedule. A device that does not trip on test must be replaced.

Specifications

Rated current range	6 A, 10 A, 16 A, 20 A, 25 A, 32 A, 40 A (common sizes)
Residual current sensitivity	30 mA (personnel protection), 100 mA (fire protection), 300 mA (equipment protection)
Trip time at rated residual current (30 mA)	< 300 ms; < 40 ms at 5× rated residual current (IEC 61008/61009)
Breaking capacity	6 kA or 10 kA (prospective short-circuit current) per IEC 60898-2
Device type	Type AC (AC leakage only), Type A (AC + pulsating DC), Type F (AC + pulsating DC + high-frequency)
Operating voltage	230 V AC single phase (IEC standard)
Module width	Single module (18 mm) or 1.5 module (27 mm) — device-specific

Safety warnings

An RCBO provides earth-leakage and overcurrent protection but does not eliminate shock risk from live-to-neutral contact (line-to-line in three-phase systems). Personal protection requires both the RCBO and adherence to safe working practices.
Always isolate the main switch and verify dead using a calibrated voltage indicator before installing or modifying an RCBO. Consumer unit busbars remain live even when individual circuit breakers are off.
Do not pass the circuit earth conductor through the RCBO — doing so routes return earth-fault current through the sensing toroid and will cause persistent nuisance tripping or failure to trip under genuine fault conditions.
All electrical installation work must comply with IEC 60364, BS 7671, AS/NZS 3000, NEC/NFPA 70, or the applicable national wiring standard. In most jurisdictions, consumer unit work must be carried out or inspected by a licensed electrician.
This page provides illustrative reference information only. Verify device selection, installation, and testing against the applicable wiring standard and the device manufacturer's installation instructions.

Tools needed

Calibrated voltage indicator (approved to GS38 or equivalent)
Lock-off device and safety tags
Insulated screwdrivers (flat blade, DIN terminal size)
Wire strippers with adjustable stop
Bootlace ferrule crimping tool
Digital multimeter
Torque screwdriver (RCBO terminals require correct torque per datasheet)

Common mistakes

Running the neutral conductor directly to the neutral bar and bypassing the RCBO neutral terminal — this defeats the residual current sensing and provides no earth-leakage protection.
Selecting a Type AC RCBO for circuits feeding variable-speed drives, EV chargers, or modern electronic equipment — these produce pulsating DC leakage that Type AC devices cannot detect. Use Type A or Type F.
Sharing a neutral between two circuits where one passes through an RCBO — the imbalance caused by the other circuit's current flowing in the shared neutral causes persistent nuisance tripping.
Failing to press the TEST button after installation and assuming the device works — RCBOs can be damaged during transit or wiring. The test must be performed and recorded.
Torquing RCBO terminals by feel rather than using a torque screwdriver — under-torqued connections increase resistance and cause overheating; over-torqued connections crack the terminal block.

Troubleshooting

RCBO trips immediately on reset: Cause: Persistent earth-leakage fault or short circuit on the connected circuit — could be damaged cable insulation, water ingress at a fitting, or a faulty appliance Fix: Disconnect all loads and cables from the RCBO load terminals. Reset. If it holds, reconnect cables without loads; if it trips, the cable is faulty. If it holds with cable only, reconnect loads one at a time to isolate the faulty appliance.
RCBO does not trip when TEST button is pressed: Cause: The device is faulty — the test circuit simulates a 30 mA imbalance and the tripping mechanism should operate. A failure to trip indicates a defective device. Fix: Replace the RCBO immediately. A device that fails the TEST button check must not be left in service.
RCBO trips intermittently under normal load: Cause: Accumulated capacitive leakage from multiple appliances, damp wiring insulation, Type AC device on circuit with pulsating DC leakage, or marginal insulation resistance degrading when the conductor heats under load Fix: Perform an insulation resistance test on the circuit (500 V DC, minimum 1 MΩ to IEC/BS standard). Check whether leakage correlates with a specific appliance. Upgrade to Type A if circuit feeds electronic equipment.

Frequently asked questions

What is the difference between an RCD and an RCBO?

An RCD (Residual Current Device) detects earth-leakage current only and protects multiple circuits downstream. An RCBO (Residual Current Breaker with Overcurrent) combines RCD earth-leakage sensing with a miniature circuit breaker's overcurrent and short-circuit protection in one device assigned to a single circuit. A plain RCD provides no protection against overload or short circuit.

What current sensitivity rating should I choose for a socket circuit?

BS 7671 (UK), IEC 60364, and most national regulations require 30 mA RCD or RCBO protection on socket-outlet circuits accessible to the public or used in domestic premises. The 30 mA threshold is chosen because currents above approximately 30 mA can cause ventricular fibrillation, while the trip time at rated current is under 40 ms.

Can I replace an existing MCB with an RCBO of the same current rating?

Yes, provided the RCBO fits the consumer unit's DIN-rail format, its rated current and breaking capacity match the circuit requirements, and you verify wiring polarity when reconnecting. Some consumer units require manufacturer-specific RCBO types. Always isolate the main switch and verify dead before removal.

Why does my RCBO trip randomly with no obvious fault?

Nuisance tripping is often caused by accumulated earth-leakage current from multiple appliances on one circuit (capacitive leakage in filters, switched-mode power supplies, motor drives), a Type AC device on a circuit with pulsating DC leakage (upgrade to Type A), damp wiring insulation, or a failing appliance with degraded insulation. Isolate loads one by one to identify the source.

Does the RCBO need a separate neutral for each circuit?

Yes. An RCBO must monitor the complete current path of its circuit — both line and neutral must pass through the device's sensing toroid. The neutral must not be shared with other circuits downstream of the RCBO, as shared neutrals cause false current imbalances and nuisance tripping.

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