Understanding RCDs and RCBOs: Essential Components for Electrical Safety


In the realm of electrical safety, two terms frequently pop up: RCDs and RCBOs. 

While they may sound complicated, they play a crucial role in safeguarding both people and property against electrical hazards. 

In this comprehensive guide, we’ll delve into the components of a switchboard and unpack the definitions of RCDs and RCBOs. We’ll explore why they are indispensable and discuss how to test them. We will also shed light on the requirements for rental properties in Victoria (VIC). 

Let’s embark on a journey toward better understanding and implementing electrical safety measures.

Components of a Switchboard:

Before we delve into the specifics of RCDs and RCBOs, it’s essential to grasp the basics of a switchboard (which is where these live).

Essentially, a switchboard serves as the ‘nerve centre’ of electrical distribution in a property.

A drawing of a house with a diagramDescription automatically generated

As per the above image, electricity travels into the property to the switchboard. This then goes to a ‘master switch’ called the main switch.

Then the switchboard divides the electricity from the main switch into different circuits. These circuits are powered by switches called circuit breakers. 

If you turn the circuit breaker off, then that circuit will not longer have electricity running to it. 

There are a few reasons we split the single incoming cable with electricity into multiple circuits. One key reason is that if one circuit faults, only that circuit is affected. Not every electrical item in the property.

This is where RCDs and RCBOs come into play.

What is an RCD?

RCD stands for Residual Current Device. 

It is a device that swiftly disconnects the electrical supply during an electrical fault to prevent serious harm. 

These faults can occur when a person accidentally touches a live wire. It can also happen when an appliance malfunctions, causing leakage of current to earth. 

The Residual Current Device RCD constantly monitors the flow of electricity through a circuit. If it detects any imbalance indicating a leakage, it instantly cuts off the power supply. It then averts potential electrocution or fire hazards. This is as opposed to a circuit breaker which does not have the same ‘sensitivity’ to faults.

RCDs are installed on a switchboard and protect multiple (generally up to 3) circuits from leaky current. 

In the above example, the oven and 2x lighting circuits are protected by the RCD. As such, if there is an earth fault in one of those circuits, the RCD will ‘trip’ and shut power to all 3 circuits for safety purposes.

As you can imagine, this may be of nuisance. If an oven is faulty, then electricity 2x additional lighting circuits will be cut off. While this is for safety reasons, there is a better way! 

This is where RCBOs are introduced.

What is an RCBO?

RCBO stands for Residual Current Circuit Breaker with Overcurrent Protection. 

It combines the functions of an RCD and a circuit breaker in a single unit. Like an RCD, it detects any leakage of current to earth and disconnects the circuit. This is as opposed to a circuit breaker which does not have the same ‘sensitivity’ to faults.

Additionally, they protect against earth leakage and overcurrents, such as overloads or short circuits. This dual functionality makes RCBOs versatile and highly effective in enhancing electrical safety.

A drawing of a switchboardDescription automatically generated

In the above example, each circuit breaker has an ‘in-built RCD’ protecting that circuit. As such, if the oven is faulty, then only the oven circuit will trip. (Leaving the other 2 circuits available for use unlike with the RCD example above).

Pro tip: You can tell if a circuit breaker is an RCBO by the small ‘T’ test button on the circuit breaker. This can be on the top or the bottom of the breaker (usually the bottom). This applies to miniature circuit breakers as well. (MCB)

Why Are They Important?

The importance of RCDs and RCBOs cannot be overstated when it comes to electrical safety. Here’s why they are indispensable:

  1. Human Safety: RCDs and RCBOs significantly reduce the risk of electric shock and electrocution by swiftly disconnecting the power supply in the event of a fault.
  2. Fire Prevention: By promptly cutting off power during electrical faults, these devices help prevent electrical fires, safeguarding lives and property.
  3. Compliance: Adhering to regulations and standards regarding electrical safety is mandatory for property owners and landlords. For Victorian rental properties, all power and lighting circuits are required to be RCD/RCBO protected. Hardwired appliance (like ovens, cooktops, electric hot water circuits) are recommended only. 

How to Test Them:

Regular testing of RCDs and RCBOs is crucial to ensure they are functioning correctly. Here’s a simple procedure to test them:

  1. Press the “Test” button on the RCD or RCBO.
  2. This should simulate a fault and cause the device to trip, cutting off the power supply.
  3. If the device trips as expected, it indicates that it’s working correctly. If not, it may need to be repaired or replaced by a qualified electrician.

While conducting the above trip test is important, it does not always confirm that the RCD or RCBO are working correctly. To do this, we measure trip time to verify that the circuit breaker works under fault conditions.

There are also two further tests we measure.

Fault Loop Impedance Test:

Purpose: This test checks if your electrical system can handle a fault (like a short circuit) without causing harm. It ensures that if something goes wrong, the power shuts off quickly to prevent damage or danger.

How it Works: We mimic a fault and measure how much resistance the electrical system offers. If it’s too high, it means there’s a problem that needs fixing to keep things safe.

What it Tells Us: It helps ensure that protective devices like circuit breakers can do their job effectively during a fault.

Measurement of Earth-Electrode Resistance for Exposed-Conductive-Parts:

Purpose: This test makes sure that metal parts in your electrical system (like equipment casings) are properly connected to the ground to prevent shocks.

How it Works: We drive metal rods into the ground and measure how well they connect to the earth. If the connection is weak, it could mean a risk of shock during a fault.

What it Tells Us: It ensures that metal parts stay safe by providing a low-resistance path to the ground in case of a fault.

What’s Required for Rental Properties in VIC:

In Victoria, rental properties must comply with strict regulations to ensure the safety of tenants. Regarding RCD/RCBO protection, the following requirements apply:

  1. All rental properties must have RCD/RCBO protection installed on all power and lighting circuits.
  2. Landlords are responsible for ensuring that RCDs/RCBOs are installed and regularly maintained in their rental properties and checked as a part of an electrical safety audit every 2 years.
  3. Compliance inspections may be conducted, and failure to comply with regulations can result in penalties or legal consequences.


Q: Can I install RCDs and RCBOs myself?

A: While DIY installation might seem feasible, it’s illegal to be conducted by an unqualified electrician. We highly recommended to hire a licensed electrician for installation and testing to ensure compliance with regulations and safety standards.

Q: How often should RCDs and RCBOs be tested?

A: RCDs and RCBOs should be tested at regular intervals as per manufacturer recommendations or local regulations. For rental properties, we check these every 2 years.

Q: Do RCDs and RCBOs require maintenance?

A: Yes, regular maintenance and testing are essential to ensure the proper functioning of RCDs and RCBOs. This may include visual inspections and testing with specialized equipment.

In conclusion, RCDs and RCBOs are indispensable components of electrical safety systems, offering crucial short circuit protection against electric shocks and fires. Understanding their functionality, testing procedures, and regulatory requirements is essential for property owners, landlords, and tenants alike. By prioritizing electrical safety measures and ensuring compliance with regulations, we can create safer living and working environments for everyone.

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