What Is a Residual Current Device (RCD) and How Does It Work? | Mr Switch
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What Is a Residual Current Device (RCD) and How Does It Work?

What Is a Residual Current Device (RCD) and How Does It Work?

May 13, 2021

Getting shocked by electricity is never any fun. 

In some cases, accidentally coming into contact with a surge of electricity can be fatal, and has often led to electrical hazards such as house fires and electrocution.

But there’s a way to prevent such terrible electrical accidents from happening to you, so you can stop an electrical hazard before it gets the chance to start.

The solution? A Residual Current Device (RCD).

What is a Residual Current Device (RCD) or Residual Current Circuit Breaker (RCCB)?

A residual electric current device (RCD), also known as a residual current circuit breaker (RCCB), is a device that cuts off power in the event of a leakage current and keeps you out of harm’s way. They’re designed to break the circuit immediately when it detects small leakage currents, helping to prevent you or your loved ones from getting electrocuted. 

RCDs are specifically set up to switch off the power supply within 25-40 milliseconds of any leakage current greater than 30mA. This is quick enough to avoid severely shocking you and sending your heart into an abnormal heartbeat rhythm, which is the most common cause of death by electric shock. 

Why Do You Need a Residual Current Device?

By law, all homes in Australia are required to have residual current devices installed as part of their electrical systems. 

Beyond being legally required, RCDs also ensure the safety and protection of your home from electrical disasters. Not only do they help to keep your house safe from electrical fires, but they also add another layer of security to all your belongings, including sensitive data that might be stored on some electronic devices.

More importantly, an RCD can save your life. In parts of your home where there’s a higher chance of being electrocuted (e.g. in wet areas with nearby power outlets), having a RCD installed could mean the difference between life or death in the event of an electric shock. 

How Does a Residual Current Device Work?

RCDs work by measuring the balance of electrical currents between two conductors using what’s called a “differential current transformer.” Basically, a residual current device uses a detector to track how much electricity is sent out against how much comes back through and checks to make sure the two amounts match. If it notices that the currents are unbalanced, that means there is a leakage of current, and it will quickly trip the circuit to prevent more electricity from flowing through to wherever the leakage is happening. 

What Are the Different Types of Residual Current Devices?

There are two types of RCDs: portable residual current devices and non-portable ones. While both offer protection against electrical hazards, they are not completely interchangeable, and should be used specifically for different situations. Here’s a breakdown of the advantages and disadvantages of each type of RCD:

Portable Residual Current Devices

Portable RCDs can be used in any socket outlet to protect individual appliances and pieces of equipment. They can also be used in combination with non-portable RCDs that are already installed on a given circuit. There are two kinds of portable residual current devices: plug connectors and stand-alone units.  

Plug Connector Type

This type of portable RCD is plugged into a wall socket and can be used to protect a single appliance or piece of equipment.

  • Adds a layer of protection to any equipment whether or not the circuit comes with an RCD installed
  • Allows the user to specify the power cut-off point to a particular appliance rather than the entire circuit
  • Can’t protect against faults in fixed-wiring
  • Needs to be tested frequently as it’s much more exposed to potential damage and abuse

Portable Stand-Alone Unit

The portable RCDs that are stand-alone units are typically installed in the circuit board, which allows them to protect multiple power outlets and any appliances and equipment that are connected through the outlets.

  • Stronger protection than plug types of portable RCDs
  • Protects multiple socket outlets and appliances as opposed to just one at a time
  • Can’t protect against faulty fixed-wiring
  • Not as cost-effective as non-portable RCDs if you’re looking to protect many pieces of equipment

Non-Portable Residual Current Devices

Non-portable RCDs are devices that are permanently installed on either a socket outlet or the switchboard. Below, we’ve listed out the unique advantages and disadvantages of the two different types of non-portable RCDs:

Non-Portable RCD Installed on Switchboards


Permanently protects against fixed-wiring faults 

Installable on new, modified or existing electrical system


Can result in an entire shut-off of your electrical system due to just one fault

Installation can be costly if you modifications need to be made to your existing switchboards

Non-portable RCD for Socket Outlets

  • Fixed protection against electric shock and similar hazards
  • Protects any device or appliance that’s plugged into the socket outlet
  • Protection is limited to whatever is plugged into the specific outlet
  • Must be replaced whenever the protected outlet becomes damaged

Are RCDs Easy to Maintain?

The ease of maintaining a residual current device depends on whether the RCD you’re using is portable or non-portable. 

You can check if a portable RCD is working every time you use it, as it’s meant to be removed with little fuss. For non-portable RCDs, you can test the functionality by pressing the safety switch marked as “T” or “Test.” 

Non-portable RCDs in particular should be checked quarterly, or every 3 months. Clicking the test button should shut down the power supply for the specific circuit in your home, indicating that the RCD is working properly. If the RCD is not working, you will need to consult with a licensed electrician to help you replace the RCD.


It’s easy to overlook the many safeguards that are put in place to help keep you and your home safe from electrical hazards. When it comes to residual current devices, it’s important to make sure that your electrical system is outfitted properly with RCDs and that each device is in good condition.

If you’re noticing issues with RCDs in your house, such as a residual current device that keeps tripping or is not working at all, make sure to contact your local electrical experts right away to help fix the issue. Our certified technicians are ready to replace all types of RCDs on any type of installation. Even if you’re not sure how to check if your RCDs are working properly, we can help inspect your electrical system to make sure you’re getting the protection you need.

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