RCD vs RCBO vs Circuit Breaker: What Actually Protects You

Open any modern Australian switchboard and you will see a row of small switches with names that get used interchangeably and mean completely different things. Getting them straight matters, because one type protects your house wiring and another protects your life, and only one of them does both. Here is what each device does, in plain terms, and why the difference is the whole point.

Circuit breaker: protects the wiring

A circuit breaker, or MCB (miniature circuit breaker), watches how much current a circuit is drawing. If a circuit rated for 16A starts pulling far more, because too many appliances are on it or because of a dead short, the breaker trips and cuts the power before the cable overheats and starts a fire. That is its entire job: overcurrent protection for the cables in your walls. It is the modern replacement for the old ceramic rewireable fuse.

What a circuit breaker does not do is protect a person. If you touch a live conductor and current flows through you to earth, that leakage can be a tiny fraction of the breaker's rating, nowhere near enough to trip it, and still be more than enough to stop your heart. The breaker sees nothing wrong. This is the gap the RCD fills.

RCD: protects people

An RCD (residual current device), commonly called a safety switch, measures the current flowing out on the active and the current returning on the neutral. In a healthy circuit those are equal. If some current is leaking to earth, through a faulty appliance or through a person, the two no longer balance, and the RCD disconnects the supply in around 30 milliseconds. Domestic RCDs are set to trip at about 30mA, a level chosen to protect against fatal shock.

An RCD does not protect the cable from overload. That is why on its own it is not enough, and why for years a common setup was a bank of circuit breakers sitting behind one or two shared RCDs.

RCBO: both, per circuit

An RCBO (residual current breaker with overload) combines both functions in a single module. It gives one circuit its own overcurrent protection and its own earth-leakage protection at the same time.

The advantage over shared RCDs is important. When several circuits sit behind one RCD, a single earth fault, even a slightly damp outdoor light, trips the whole group, and you lose power to everything on it including your fridge and freezer, often while you are away. With per-circuit RCBOs, only the faulty circuit drops out and the rest of the house keeps running. It is also far easier to find the fault, because the device that tripped tells you exactly which circuit it is on.

RCBOs cost more per circuit, which is the main reason older boards used shared RCDs. When you are pricing a switchboard upgrade, asking for individual RCBOs rather than grouped RCDs is usually money well spent.

What the rules require

The Australian Wiring Rules, AS/NZS 3000, have progressively tightened the requirement for residual current protection. Under the 2018 edition, additional protection by an RCD is required for socket outlets and for final subcircuits including lighting in domestic installations. Practically, that means a compliant modern home has RCD protection on essentially all of its circuits, whether delivered by shared RCDs or by individual RCBOs. Older homes are often grandfathered until work is done, but any new circuit or board upgrade must meet the current rules. The AS/NZS 3000 wiring rules for buyers explains why this matters when you buy second-hand gear.

Reading what is written on the device

The markings on the front of a breaker tell you most of what you need to know, and they are worth being able to read before you buy anything second-hand.

• The current rating is the number with an A after it: 6, 10, 16, 20, 25, 32 and so on. It must match the cable the circuit is wired in, which is why you never simply fit a bigger breaker to stop nuisance tripping. A larger device on undersized cable removes the protection entirely.
• The curve letter in front of that number, usually B or C in domestic work, describes how tolerant the device is of the brief inrush current that motors and transformers draw at switch-on. Fitting the wrong curve produces a breaker that either trips whenever the air conditioner starts or is slower than it should be on a genuine fault.
• The kA rating is the fault current the device can safely interrupt. It has to suit the supply it is installed on, and it is not a number to guess at.
• The mA figure on an RCD or RCBO is the leakage current at which it trips, normally 30mA for domestic protection. A 100mA or 300mA device is intended for fire protection on a supply, not for protecting people, and it is not a substitute.

If a listing does not state these, ask for a clear photo of the device front before you commit.

Test them, and never trust a used one blindly

Every RCD has a test button. Pressing it should trip the device, and you should do this every three months or so. A safety switch that does not trip on the test button is not protecting anyone, and a surprising number of them fail silently over the years.

That is also why buying used protective devices is risky. An RCD or RCBO pulled from an old board might be worn, water-damaged or already failed, and you cannot tell by looking. If you do buy second-hand, it must be tested by a licensed electrician before it is energised, ideally with an instrument that verifies the actual trip current and trip time rather than just the test button.

Compliance

Installing, replacing or rearranging these devices in a switchboard is fixed electrical wiring work. In Australia it must be carried out by a licensed electrician, who will test the installation and issue a certificate of compliance. Do not fit your own breakers or RCBOs, even if a video makes it look simple. If you are choosing who to hire, our guide to finding a licensed electrician covers verifying the licence and getting the certificate.

For anyone planning a specific job, a properly protected board is the foundation everything else sits on, from LED downlights to an EV charger that draws serious sustained current. If you are sourcing your own breakers, RCBOs or a board, current listings are a good place to start, just remember the testing rule before anything goes live.