The short answer
An RCD (residual current device, or safety switch) cuts the power when it detects current leaking to earth, such as through a person. In a home it usually protects a group of circuits, up to three. An RCBO does the same job but also includes a circuit breaker, so it protects one circuit against electric shock, overload and short circuit in a single device. The practical difference: with shared RCDs, a fault on one circuit can switch off several; with an RCBO on each circuit, only the faulty circuit goes off. Both meet the 30 mA requirement of the Wiring Rules when correctly installed.
What an RCD does
A safety switch constantly compares the current going out on the active conductor with the current coming back on the neutral. If some of it is leaking away to earth, the two don't match, and the RCD switches off. SafeWork NSW describes RCDs as protecting against electric shock and giving some protection against electrical fires. Household RCDs required by AS/NZS 3000 are rated at no more than 30 mA.
An RCD doesn't protect a cable from overload on its own. On a typical board with separate RCDs, each RCD sits in front of a group of ordinary circuit breakers (MCBs), and the breakers handle overload and short circuits.
What an RCBO does
RCBO stands for residual current breaker with overcurrent protection. It combines an RCD and a circuit breaker in one module and protects a single circuit. So each circuit has its own earth leakage protection and its own overload protection.
Side by side
| Shared RCD (with MCBs) | RCBO on each circuit | |
|---|---|---|
| Protects against electric shock (30 mA) | Yes | Yes |
| Protects against overload and short circuit | No, the MCBs do that | Yes, built in |
| Circuits per device | Up to 3 in a home | 1 |
| A fault on one circuit switches off | Every circuit on that RCD | Only that circuit |
| Finding the faulty circuit | Harder, several circuits drop together | Easier, the tripped RCBO shows you |
| Space on the board | Fewer devices | One device per circuit |
Why one fault can black out half the house
Under the rules for homes, there must be at least two RCDs, with no more than three final subcircuits on any one RCD, and lighting circuits split across RCDs. So with shared RCDs, a single faulty appliance on the outdoor power might also switch off the kitchen power and the fridge, or one bank of lights.
That kind of nuisance trip is a real pain on a hot Wagga day when the fridge and air conditioner go off, or on a farm where a freezer or pump is on the same RCD as a shed circuit. With an RCBO on each circuit, the fault only takes out the circuit it is on.
Type AC, Type A and Type B
Not all RCDs are the same type, and the types matter in NSW:
- Type AC detects AC leakage only. Type AC RCDs can no longer be installed in new electrical work in NSW (since 1 May 2023) and can no longer be sold here. Building Commission NSW can fine electricians up to $1,000 for installing one, or for not replacing one when required.
- Type A detects AC faults and pulsating DC faults. This is the standard type for new household work. NSW Advisory Note 1/2025 says a new or upgraded circuit for a heat pump hot water system must use a Type A RCD.
- Type B is for circuits with a higher risk of DC leakage. NSW names EV chargers and solar inverters as examples. For an EV charger, the safe wording is Type B, or Type A where the charger has built-in 6 mA DC fault detection.
The ban is on installing Type AC in new work. If a circuit is being altered or the board replaced, the new protection must be the right type, and Nelson will tell you if an existing device needs replacing as part of the job. More detail is on the Building Commission NSW RCD compliance page.
Which should you choose?
For a new switchboard, we fit an RCBO on every circuit. It costs more than a couple of shared RCDs, because each circuit needs its own device, but you get:
- no more half-the-house blackouts from one bad appliance;
- faster fault finding, because the tripped RCBO points to the circuit;
- each circuit fully protected in its own right.
Because there's one RCBO per circuit, the number of circuits is the main thing that drives the price of a new board. See switchboard upgrade cost for how that fits into the total, or run the instant estimate.
If you only need one new circuit on an existing board, say for an air conditioner, that circuit needs its own RCD protection at the switchboard. An RCBO is often the neatest way to add it, if there is room.
What the rules say
In short (and see safety switch rules in NSW for more):
- AS/NZS 3000:2018 requires 30 mA RCD protection on all final subcircuits in new domestic installations, located at the switchboard.
- If you alter or add a final subcircuit, only that circuit needs RCD protection, not the whole house.
- If a switchboard is replaced for any reason, every applicable final subcircuit from it needs RCD protection.
Whether that protection comes from shared RCDs or RCBOs is a design choice. Both comply when installed correctly.
Common questions
Is an RCBO better than an RCD?
For most homes, yes, in practice. Both give the same 30 mA shock protection, but an RCBO protects one circuit and includes overload protection, so a fault only switches off that circuit.
Can I replace my RCD with RCBOs?
Yes, if there is room on the board and the board is in good condition. It is electrical wiring work, so it must be done by a licensed electrician and certified with a CCEW. On an old or crowded board, a switchboard upgrade is often the better option.
Is a safety switch the same as a circuit breaker?
No. A circuit breaker protects the cable from overload and short circuit. A safety switch (RCD) protects people from electric shock by detecting current leaking to earth. An RCBO does both.
How do I know if my safety switch is Type AC or Type A?
The type is usually marked on the front of the device. If you can't tell, Nelson can check it when he is there. Don't open the switchboard cover yourself.
Why does my RCD trip several circuits at once?
Because a shared RCD protects up to three circuits, and when it detects a leak on any one of them, it switches off all of them. See safety switch keeps tripping for what to do.