An RCD (residual current device, or safety switch) cuts the power when current leaks to earth, usually through a person, but does nothing about overloads. An RCBO does the same leakage job and also trips on overload and short circuit, all in one device protecting a single circuit. The RCBO is the modern choice because one fault no longer blacks out several circuits.
Both devices sit in your switchboard, both have a test button, and both get called a "safety switch" by most people. They are not interchangeable, though, and the difference explains why some houses lose half their power when one appliance faults while others lose a single room.
What does an RCD actually do?
An RCD watches for current going somewhere it should not. In a healthy circuit, every bit of current that flows out along the active wire comes back along the neutral. The RCD compares the two continuously. If the return is short, the missing current is leaking to earth, possibly through someone touching a faulty appliance, and the RCD disconnects the supply within a fraction of a second.
The residential devices in Australian homes trip at a leakage of 30 milliamps, which is small enough to stop a shock before it does serious harm. A standalone RCD has no idea how much current the circuit is carrying in total. Plug in five heaters and overload the cable, and the RCD will happily let it cook, because nothing is leaking. Overload protection comes from a separate circuit breaker, often called an MCB, sitting beside it.
On most boards built before the last decade or so, one RCD protects a group of circuits. A typical arrangement has two RCDs, each guarding several breakers underneath it.
What is an RCBO?
An RCBO is an RCD and a circuit breaker combined into one device that protects one circuit. The name stands for residual current breaker with overcurrent protection. It trips on earth leakage like an RCD, and it trips on overload or a short circuit like an ordinary breaker.
Because each circuit gets its own RCBO, a fault stays where it starts. If the washing machine develops leakage, the laundry circuit goes off and the rest of the house carries on. Under the shared RCD arrangement, the same fault drops every circuit on that RCD, which can mean the fridge, the kitchen power points and half the lights.
RCBO vs RCD: how do they compare?
The core difference is scope: an RCD guards a group of circuits against leakage only, while an RCBO guards one circuit against leakage, overload and short circuit. Here is how the three devices on a typical Sydney switchboard line up.
| Device | Earth leakage (shock) | Overload and short circuit | What goes off when it trips |
|---|---|---|---|
| RCD (safety switch) | Yes | No, needs a breaker as well | Every circuit connected under it |
| Circuit breaker (MCB) | No | Yes | One circuit |
| RCBO | Yes | Yes | One circuit |
Neither device offers better shock protection than the other. Both trip at the same 30 milliamp threshold. The RCBO improves containment, and it also cuts nuisance tripping. Small, normal leakage from appliances adds up across every circuit sharing an RCD, and enough of it will trip the RCD with no single fault anywhere. Give each circuit its own RCBO and those small leakages stop adding together. We cover that pattern in detail in our guide to an RCD that keeps tripping.
How can you tell which one your switchboard has?
Look for the test buttons and count the circuits beneath each one. Every RCD and every RCBO has a test button, usually marked "T" or "Test". Plain circuit breakers do not.
- Shared RCDs. Two or three devices with test buttons, each followed by a row of plain breakers. The RCD is often wider than the breakers and labelled with a rating like "40A 30mA" and the words "safety switch".
- RCBOs. A test button on every circuit, or nearly every one. The label usually shows both a breaker rating and a leakage rating on the one device, for example "C16" or "16A" alongside "30mA".
- No test buttons at all. The board has no shock protection, which is common behind ceramic rewireable fuses and on older breaker boards. That is worth fixing regardless of anything else in this article.
Read the labels with the cover open, but leave the devices alone. Switching things off to trace circuits is best left to an electrician, particularly on an older board.
Not sure what your switchboard is protecting?
DM Electrical can tell you what your board has, which circuits are protected and what is missing. Based in Brookvale, Level 1 and Level 2 ASP accredited, available 24/7 across the Northern Beaches and wider Sydney.
Is a shared RCD still allowed?
Yes, an existing board with shared RCDs is not illegal just because newer boards use RCBOs. The AS/NZS 3000 Wiring Rules require 30 milliamp RCD protection on the final circuits in a home, and they limit how many circuits can share a single RCD, which in a house means no more than three. Many older boards were installed under earlier editions and carry more circuits per RCD than that. They are allowed to stay as they are until the switchboard is altered or replaced, at which point the new work has to meet the current rules.
That is why a switchboard replacement today is almost always built around RCBOs. It is the simplest way to put every circuit under shock protection without the grouping limits, and it avoids the nuisance tripping that comes with sharing.
When does the choice actually matter?
It matters most on circuits where losing power has a cost, and on circuits that need a particular type of protection. These situations come up most often.
- Fridges and freezers. On a shared RCD, a fault on the laundry circuit can switch off the fridge while you are away. On an RCBO, it cannot.
- Home offices, medical equipment and security systems. Anything that should keep running while another part of the house faults is a good reason to separate circuits.
- EV chargers. A charger circuit needs an RCD type that also detects DC leakage, which a standard household RCD does not. It is fitted on its own dedicated circuit with protection chosen for the charger.
- Adding a new circuit. Most new circuits now go in on an RCBO. If the board is already full of shared RCDs, that can be the point where replacing the board makes more sense than adding to it.
Can you swap an RCD for RCBOs?
Often, yes, if the board has the physical space and is in good condition. Each circuit needs its own RCBO, so the board has to hold one device per circuit rather than one RCD per group. Some older enclosures simply do not have the room, and some are old enough that the whole board is better replaced. Either way, it is switchboard work for a licensed electrician, who tests the finished installation and issues a Certificate of Compliance (CCEW). Our switchboard upgrade page covers what a full replacement involves, and a residential electrical check will tell you whether your existing board is worth converting.
Frequently asked questions
Is an RCBO better than an RCD?
For most homes, yes. Both trip at the same 30 milliamp leakage level, so neither gives better shock protection. The RCBO wins on containment: it protects one circuit, so a fault switches off that circuit alone, and small appliance leakages on different circuits no longer add up to nuisance trips. It also includes overload protection, which a standalone RCD does not.
Is an RCBO a safety switch?
Yes. "Safety switch" is the everyday name for any device that trips on earth leakage, and an RCBO does that. It is a safety switch and a circuit breaker in one device, whereas a standalone RCD is only the safety switch part and needs a separate breaker for overload protection.
How often should I test my RCD or RCBO?
Press the test button every three months. The device should trip straight away, and you then switch it back on. If it does not trip, or will not reset, have it checked by an electrician, because a safety switch that fails its test is not protecting that circuit.
Do I have to replace my RCDs with RCBOs?
Not if the existing board is working and has not been altered. Older boards with shared RCDs can stay as they are. The current rules apply when the switchboard is altered or replaced, and in practice that is usually when a board is converted to RCBOs.
The bottom line
An RCD and an RCBO both stop an electric shock the same way. The difference is how much of the house goes dark when something faults, and whether overload protection sits in the same device. If your board has a couple of RCDs guarding everything, it is not unsafe, but it is the reason one faulty appliance can take out the fridge. If it has no test buttons at all, that is the part to fix first. Get in touch and we will look at your board and tell you where it stands.