Safety switches (RCDs) explained: types, ratings, and what they actually protect

The "safety switch" is the single piece of equipment in your house most likely to save someone's life — and most people couldn't tell you what it actually does, where it sits, or whether they even have one.

This is a plain-English explanation of what a safety switch is, how it works, the different types and ratings, and what AS/NZS 3000:2018 (the Wiring Rules) requires for a modern installation.

What a safety switch actually does

A safety switch — known technically as a Residual Current Device (RCD) — sits in your switchboard and watches the current flowing through a circuit. Specifically, it compares the current going out on the active conductor with the current coming back on the neutral conductor. In a healthy circuit those two numbers are equal: every electron that goes out comes back.

When something goes wrong — a damaged cord, a faulty appliance, a person touching a live wire — some of the current finds an alternative path back to earth. That path bypasses the neutral, so the RCD sees an imbalance between active and neutral. As soon as that imbalance exceeds the device's rated trip current (typically 30 milliamps), the RCD disconnects the circuit.

It does this in about 30 milliseconds. That's faster than your heart can fibrillate from an electric shock — which is why a safety switch on a power circuit is the difference between "I got a hell of a fright" and a fatality.

A safety switch does not detect overload or short circuit. That's the job of a circuit breaker (MCB) — a different device, often sitting next to the RCD on the same switchboard. We'll come back to that distinction later, because it's where most of the confusion lives.

"Safety switch" vs RCD vs RCBO — the same family, different names

Australian electricians and consumer-facing language use different names for what is essentially the same family of devices:

• Safety switch — the consumer-facing name. The one your power company, your electrician, and your insurer will all use.
• RCD (Residual Current Device) — the technical name. The one printed on the device itself and used in AS/NZS 3000.
• RCBO (Residual Current Breaker with Overload protection) — an RCD and an MCB combined into a single device. Common in modern switchboards because it gives every circuit its own dedicated overload and earth-leakage protection in one unit.

If your switchboard has a row of small modular devices each with a TEST button, those are most likely RCBOs. If you have one or two larger devices upstream of several circuit breakers, those are standalone RCDs protecting a group of circuits. Both are "safety switches" in everyday speech.

The relevant device standards are AS/NZS 3190 for RCDs and AS/NZS 61009 for RCBOs.

Sensitivity ratings — 10 mA, 30 mA, 100 mA, 300 mA

Not all safety switches are created equal. The number that matters most is the rated residual operating current — the size of the earth-leakage current at which the device will trip. The standard ratings are:

• 30 mA — the residential and commercial default. Sized to protect people from electric shock. Required by AS/NZS 3000 on most final sub-circuits in homes and similar installations.
• 10 mA — extra-sensitive, used in special locations where the consequence of a shock is more severe. Healthcare, swimming pools, certain medical and educational settings.
• 100 mA / 300 mA — too coarse to protect a person from shock. These are used for fire protection and equipment protection, where the goal is to catch sustained earth-leakage currents that could ignite insulation or damage gear, rather than to protect a person.

The 30 mA figure isn't arbitrary. Below about 30 milliamps through the body, the heart is unlikely to fibrillate provided the exposure is brief. Above that, fibrillation risk climbs sharply. A 30 mA RCD that disconnects within 30 milliseconds keeps the human body well inside the survivable envelope.

If your switchboard has a device labelled "I△n = 30 mA" — that's a 30 mA RCD. If it says 100 mA or 300 mA on a residential power circuit, the protection is not at personal-shock level and is worth a conversation with your electrician.

RCD types — AC, A, F, B

The other characteristic that matters is the type of fault current the RCD can detect. Modern electronics — variable-speed motors, solar inverters, EV chargers — produce fault currents that look quite different from the simple sine wave of a 1980s washing machine, and not every RCD can see them all.

Type	Detects	Where you find it
AC	Sinusoidal AC fault currents only. Cannot reliably detect DC components.	Largely obsolete for new work. May still be present in older switchboards.
A	AC plus pulsating DC fault currents.	The modern minimum. Default specification on AS/NZS 3000 installations from 2018 onwards.
F	Type A plus multi-frequency residual currents from single-phase variable-speed drives.	Specialist applications — variable-frequency motor drives, some heat-pump hot-water systems.
B	All of the above plus smooth DC residual currents.	Required for some EV chargers, certain solar inverters, and DC-fault-prone equipment. Per AS/NZS 3000 §7.9 and the EV charger manufacturer's instructions.

For a regular Melbourne home with no EV charger, the safety switches in a 2024-era switchboard will be Type A across the board. If you're installing a wall-mounted EV charger, your electrician will check the manufacturer's data sheet — some require a Type A device upstream and use an internal DC-fault detector, while others require a Type B RCD in the switchboard. Getting this wrong is one of the most common compliance issues we see on retrofit EV jobs.

What AS/NZS 3000 requires

AS/NZS 3000:2018 — the Australian and New Zealand Wiring Rules — sets out which circuits must have RCD protection. The short version: in a modern domestic or similar installation, almost every 230 V final sub-circuit up to 32 A needs a 30 mA RCD.

That covers:

• All general-purpose power circuits (where your power points live)
• All lighting circuits
• Hardwired circuits supplying ovens, cooktops, hot water systems, washing machines, dishwashers, and air conditioners
• Outdoor power circuits and bathroom circuits

The 32 A cap is significant because some larger appliances (some ducted air-conditioners, some commercial kitchens) sit on circuits above 32 A. Those have their own rules. But for everyday residential wiring, the answer is "all of it".

If your switchboard pre-dates 2018 — or 2007, or 1991, depending on which round of regulation it last saw — the coverage might be partial or absent. A common pattern in Melbourne homes from the 1990s and 2000s is one or two RCDs covering only the power circuits, with the lighting circuits sitting unprotected on plain MCBs. That's not a 2018-compliant installation, even if it's legal to leave alone until you renovate.

For more on what triggers a switchboard upgrade and what one involves, see the signs your switchboard needs upgrading.

The TEST button — monthly, takes 10 seconds

Every RCD and RCBO in your switchboard has a TEST button. Pressing it simulates an earth-leakage fault and forces the device to trip. If the device trips, the test passed. If nothing happens, the device has failed and needs to be replaced — and the circuit it was protecting has had no earth-leakage protection for some unknown length of time.

The recommended interval is monthly. It takes about ten seconds per device. The catch is that pressing the button cuts the power to whatever's downstream, so:

• Pick a time you don't mind your fridge briefly losing power (it's fine for a few seconds).
• Save and close anything important on the computer.
• Be ready to walk through the house and reset clocks, the microwave, and a few other appliances.

Press TEST. The device should snap to the OFF position. Push it back ON. Done.

If a device doesn't trip when you press TEST, don't ignore it. The mechanism has failed and the device is no longer protecting you, even though it'll happily pass current. Call your electrician.

Common reasons for nuisance tripping

A safety switch that trips for no obvious reason is doing exactly what it's designed to do — it's seen earth leakage somewhere. The job is to find out where. Common culprits, in rough order of frequency:

• A faulty appliance — most often the kettle, toaster, fridge, washing machine, or anything with a heating element. The fix is to unplug appliances one at a time, reset, and see which one triggers it.
• Bathroom heater or heated towel rail — older bathroom heat lamps and towel rails are notorious for accumulated moisture leakage, especially in the first hour after a hot shower.
• Outdoor power points — water ingress, garden lights, pool pumps, or pond pumps that have been sitting outdoors for a few summers.
• Long lighting circuits with many fittings — every fitting has a tiny amount of natural leakage to earth, and on a long circuit with twenty downlights, the cumulative leakage can sit close to the 30 mA threshold even when nothing is faulty. Splitting the circuit fixes it.
• A failed appliance neutral-to-earth contact — when a neutral wire inside an appliance touches the earthed metal case, it creates a permanent earth path that trips the RCD as soon as the circuit is energised.
• Storms and power surges — a nearby lightning strike or grid event can push a transient through the installation that an RCD will read as a fault. Reset and watch.
• Genuine wiring fault — degraded cable insulation, a damaged conductor, or a rodent-chewed cable in a roof space. Less common than appliance faults but the most serious.

If your safety switch trips repeatedly with no obvious cause, the diagnostic process — known as fault finding — involves systematically isolating circuits and appliances until the source is found. It's the kind of thing a fault-finding visit is designed for.

What you should not do is keep resetting an RCD that's tripping on a real fault, or — worse — bypass it. The RCD is the last line of defence against electric shock; resetting it five times until it stays on means you've just spent five chances at survival.

"Safety switch" is not a "circuit breaker"

This is the single most common point of confusion in customer conversations, so it's worth being explicit.

A circuit breaker (MCB) trips on overload (too much current for the cable) or short circuit (active touches neutral or earth directly). Its job is to protect the cable from melting and starting a fire.

A safety switch (RCD) trips on earth leakage (current taking an unintended path to earth, often through a person). Its job is to protect the human body from electric shock.

These are different jobs and they require different devices. A modern compliant circuit needs both. There are three ways to deliver them:

1. Group RCD upstream of MCBs — one larger RCD protecting several MCB-equipped circuits. Older but legal pattern. Downside: a fault on any one circuit knocks out everything downstream of the RCD.
2. One RCD per circuit — RCD plus a separate MCB on each circuit. Used to be common; rarer now.
3. One RCBO per circuit — combined RCD and MCB in a single device on every circuit. The modern preference. A fault on one circuit doesn't take any others down with it. Diagnosis is easier. Switchboard real estate is more efficient.

For a deeper comparison of these three devices and when to use which, see RCBO vs MCB vs RCD.

What to do next

If you've read this far, the practical questions are:

1. Do you have safety switches at all? Open your switchboard cover and look for devices with TEST buttons. If there are none, you have no RCD protection — and that's not a 2018-compliant installation.
2. Are they on every circuit, or just the power points? A common Melbourne pattern is partial coverage. If your lighting circuits are on plain MCBs with no test button, the lighting is unprotected.
3. What type are they? Type AC devices on a modern installation should be replaced with Type A. If you're planning an EV charger, the type question becomes more pointed.
4. Do they actually work? Press TEST on each one. If anything fails to trip, it needs replacement.

If any of those questions surfaces an answer you don't like, a safety inspection will walk through the whole installation, document where the gaps are, and quote what it would take to bring everything to current standards. Sometimes the answer is replacing one device. Sometimes it's a switchboard upgrade — and the spare capacity that comes with it is usually overdue for other reasons too.

We're a Victorian Registered Electrical Contractor (REC-22849), based in Nunawading, working across Melbourne's eastern suburbs. The safety switch isn't an upsell — it's the floor.