RCD vs RCBO vs MCB: What's the Difference?

By ARCK Electrical · Trade counter, North Parramatta · Reviewed May 2026

Quick Q&A — click to expand

Does every circuit need an RCD now? +

Effectively yes for a domestic install. AS/NZS 3000:2018 with amendments requires 30mA RCD protection on essentially every final sub-circuit — lighting, GPOs, hot water, oven, AC, EV charger. There are narrow exemptions for specific life-safety and equipment-protection cases, but for a typical home you plan on RCD coverage across the board. Verify the current edition and any state-level requirements for the specific job.

Why use RCBO instead of RCD? +

RCBOs give you per-circuit selectivity. A single earth fault on the oven trips the oven RCBO only — it doesn't take out the kitchen GPOs and half the lights with it. That's the main reason boards have moved to RCBO-per-circuit. RCBOs cost more per module than separate MCBs plus a shared RCD, but the price gap has closed and nuisance trips drop dramatically.

What trip current should I use — 30mA? +

For personal protection on domestic final sub-circuits, 30mA is the rule. 100mA and 300mA RCDs exist for equipment protection and upstream selectivity in larger installations, but they aren't a substitute for 30mA on circuits where people will be using the equipment. Default to 30mA Type A unless you've got a specific reason to size differently.

Type A vs Type AC RCDs? +

Type AC trips on AC residual currents only. Type A trips on AC plus pulsating DC. Modern appliances with electronics — inverters, LED drivers, EV chargers, induction cooktops, variable-speed drives — can produce DC fault currents that Type AC misses entirely. AS/NZS 3000 amendments now treat Type A as the default for new domestic installs. EV charging usually needs Type A with DC residual monitoring, or Type B.

Three letters, three jobs. MCBs trip on overcurrent.

RCDs trip on earth leakage. RCBOs do both.

Get the wrong one in the wrong slot and you've either got a circuit that isn't safe to touch, or a board that's over-engineered. This is the version we run through at the counter when someone's planning a board or replacing an old one.

MCB — the overcurrent device

A miniature circuit breaker protects the cable and the load from overcurrent and short-circuit faults. If the current exceeds the breaker's rating for long enough, the thermal trip opens it.

If a short circuit pulls a high current instantly, the magnetic trip opens it instantly. An MCB doesn't care about earth leakage.

Touch a live conductor and a 16A MCB will sit there happily watching the current flow to ground through you, because it's nowhere near the trip threshold. What an MCB protects:

• The cable downstream of it, from running over its current rating
• The installation from short-circuit faults
• Connected equipment from sustained overcurrent

What an MCB doesn't protect: people, from electric shock.

RCD — the earth-leakage device

A residual current device measures the current going out on the active and the current coming back on the neutral. If those numbers don't match, it means current is going to earth somewhere — through a faulty appliance, damaged insulation, or a person.

When the difference exceeds the trip threshold (30mA for personal protection on a domestic install), the RCD opens within milliseconds. An RCD doesn't care about overcurrent.

Run a 32A load through a 25A circuit on a 30mA RCD and the cable will overheat with the RCD sitting there ignoring it, because the active and neutral currents still balance. What an RCD protects:

• People, from electric shock under earth-fault conditions
• The installation, from earth faults that don't draw enough current to trip an MCB

What an RCD doesn't protect: cables, from overcurrent.

RCBO — both in one module

An RCBO (Residual Current Breaker with Overcurrent) is a single device that does both jobs. It opens on overcurrent like an MCB and on earth leakage like an RCD.

From the cable's point of view it's a single point of protection. RCBOs come in narrow (1-module) and wider (1.5- or 2-module) formats depending on the brand and amp rating.

Clipsal Resi MAX, Hager Resi9, NHP and Connected Switchgear all make RCBOs that fit their respective boards.

When AS/NZS 3000 requires RCD protection

The short version of the current rule (AS/NZS 3000:2018 with amendments): essentially every final sub-circuit in a domestic installation needs 30mA RCD protection. Lighting, GPOs, hot water, oven, AC, EV charger — all of it.

That covers:

• Socket-outlet final sub-circuits
• Lighting final sub-circuits
• Directly-connected equipment final sub-circuits in domestic installations
• Equipment in damp areas, kitchens, bathrooms

There are some specific exemptions and special cases (life-safety circuits, refrigeration in certain commercial contexts, etc.), but for a typical domestic board, plan on 30mA RCD coverage on every outgoing circuit.

This is the rule that's pushed RCBO take-up over the last decade. The old model — one or two RCDs covering a whole row of MCBs — meant a single earth fault could drop multiple circuits, and a single nuisance trip would knock out half the house.

RCBOs per-circuit fix both problems.

RCBO vs MCB-plus-RCD — how to plan a board

Two ways to meet the rule:

Option 1: RCBO per circuit. Every outgoing way has its own MCB+RCD in one module. Cleaner, simpler, more selectivity (a trip on the oven doesn't take out the lights), more expensive per pole.

Option 2: RCD groups covering multiple MCBs. A pair of 4-pole RCDs (one per "row") each cover a group of MCBs. Cheaper hardware, fewer modules on the board, but a single earth fault drops every circuit in the group.

Most domestic boards we sell into right now go RCBO-per-circuit. The price gap has closed, customers expect minimal nuisance trips, and the boards are simpler to fault-find.

A typical Sydney domestic board now looks something like:

• Main switch, 80A or 100A 2-pole
• 1× 6A RCBO — lighting (front)
• 1× 6A RCBO — lighting (back)
• 1× 20A RCBO — GPO circuit 1
• 1× 20A RCBO — GPO circuit 2 (kitchen)
• 1× 20A RCBO — laundry
• 1× 20A RCBO — bathroom heater
• 1× 20A RCBO — hot water
• 1× 32A RCBO — oven
• 1× 20A RCBO — split-system AC
• 1× 32A RCBO — EV charger (where fitted)
• Surge protection device on the main

Pole count: the number of modules each device takes. MCBs are typically 1 pole (1 module wide).

RCDs are 2-pole (1P+N) or 4-pole (3P+N). RCBOs are typically 1 or 1.5 modules depending on the brand.

Trip currents — 30mA, 100mA, 300mA

For personal protection (people touching live parts), the rule is 30mA. That's the trip threshold AS/NZS 3000 demands for final sub-circuits in a domestic install.

You'll also see 100mA and 300mA RCDs — those are for equipment protection, fire prevention, and selectivity in larger installations (e.g. an upstream 300mA RCD on a sub-board with downstream 30mA RCBOs on each circuit).

They're not a substitute for 30mA personal protection where AS/NZS 3000 calls for it.

Type A vs Type AC

The other distinction worth knowing:

• Type AC RCDs trip on AC residual currents. They're the older standard.
• Type A RCDs trip on AC and pulsating DC residual currents — important for circuits feeding equipment with electronics (inverters, EV chargers, LED drivers, washing machines, induction cooktops, variable-speed drives).

Many modern domestic appliances have rectifiers and can produce DC fault currents that a Type AC RCD will miss. AS/NZS 3000 amendments have pushed Type A as the default for new domestic installs.

For EV charging the standard is even stricter — Type A combined with DC residual current monitoring, or a Type B RCD. If you're stocking the board, default to Type A.

Order from ARCK

We stock Clipsal Resi MAX, Hager Resi9, Connected Switchgear and NHP RCBOs, MCBs and RCDs — Type A and Type AC, 6A through 63A, 1P, 2P, 3P and 4P. Same-day pick-up from North Parramatta, full board kits and individual modules.

Browse the circuit protection range, or call the counter on (02) 9890 9693 if you're sizing a board and want a hand. Mon–Fri 6:30am–5pm, Sat 7:30am–1pm.