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What is a Residual Current Device (RCD) Working & Application?

Residual current device, RCD
Residual current device, RCD
Resource: https://professional-electrician.com

A residual current device is a special safety device that protects you from electric shock. It does this by disconnecting the circuit when it detects an imbalance between live and neutral wire currents. This serves two functions: shock protection and the prevention of electric fires. Below, you will learn everything about the residual current device, how it works, and so on.

What is a Residual Current Device?

A residual current device, also commonly called RCD, is an electrical safety device that’s used to prevent electric shocks or electrocution by live conductors. It monitors the current flowing through the live and neutral wires.

If there’s a difference between the two, it means that current is leaking somewhere – perhaps because you’ve touched something live. The RCD device will then instantly switch off the electricity to prevent you from being electrocuted.

There are three main types of RCDs based on the place of installation:

  • The fixed/mains type, which is permanently installed in the distribution board
  • The outlet type, which is used to protect individual sockets; and
  • The portable type, which can be plugged into a socket outlet.

Mains RCD

These are intended to protect the whole installation by being connected with all the phase (live) conductors. The mains RCD is usually incorporated within the main distribution board but may be found as a free-standing unit fed from the distribution board.

Socket RCD

A socket RCD is built for use with individual outlets. These are commonly used almost anywhere, from domestic to industrial and construction sites. They provide convenience where it’s difficult to find a fixed RCD. Socket RCD protection is normally provided as a temporary measure and is not intended to be a permanent installation.

Portable RCD

This is a plug-in RCD that can be used with socket outlets when working with electrical equipment outdoors or where there is no fixed RCD protection. The portable RCD can be used by anyone, including professionals, and is an essential piece of safety equipment for anyone working with electricity.

What Does a Residual Current Device Do

The residual current device function is to provide a high level of protection against electric shock. This is because they can detect very small imbalances in the circuit and will trip (disconnect the power) long before you would feel effects from the current.

In fact, the residual current monitoring device can often detect imbalances that are too small for you to cause harm, but which could still be enough to cause discomfort. For this reason, RCDs are sometimes known as life-saving devices, and they really can save lives.

The residual current device construction
The residual current device construction
Resource: https://www.youtube.com/watch?v=HR5D8IDkyUU

How Does a Residual Current Device Work?

An RCD constantly monitors the current flowing through the live and neutral wires. If there is a difference between the two (known as leakage current), it means there is a problem somewhere in the circuit. The RCD will then switch off the power.

Here is the working of a residual current circuit breaker explained.

  • The RCD working principle is based on this one fact: the same current flowing in the live wire will always return through the neutral wire, as long as there is no leakage to earth.
  • If there is a leakage (for example, if you touch a live wire), the current will take an alternative route to earth through your body. This is what causes an electric shock.
  • The residual current device construction is composed of a current transformer. When there’s no leakage current, the magnetic fields generated by the current in the coils cancel out.
  • However, when there is a leakage current, the fields generated by the live wire are disturbed, creating an imbalance. It is this imbalance that activates the switch and trips the RCD, cutting off the power.
RCD Use in an electrical system
RCD Use in an electrical system
Resource: https://forums.overclockers.co.uk

When should you Use a Residual Current Device?

You should always use a residual current device when working with electrical equipment, whether you are a professional or a DIY enthusiast. Common situations that require RCD use include the electrical systems of homes, offices, factories, construction sites, and anywhere else where electrical equipment is being used.

RCD Use in the Home

In the home, RCDs are most commonly used in the distribution panel or in conjunction with socket outlets. This is because, in addition to preventing fatal shocks, the residual current device operation provides a convenient way to protect electrical equipment and circuits from being damaged by circuit faults.

You should also use a residual current device whenever you use electrical equipment outdoors, as this will provide a layer of protection in case of a leakage current.

When using an RCD, it’s important to remember that they are not a substitute for good electrical safety practices. RCDs should therefore be seen as an additional layer of protection, rather than a replacement for good safety habits.

RCD Use in Commercial Places

RCDs are also used in commercial places, such as retail stores. In these cases, they are usually integrated into the main switchboard. This provides protection for all the circuits in the building.

RCD use in these places can also include the socket outlet RCD, which protects individual circuits or pieces of equipment. This flexibility makes them ideal for use in places where there is a lot of electrical equipment, such as computers and photocopiers.

RCD Use in Industrial Settings

Residual current devices are also important protection gadgets in industrial settings, such as factories and warehouses. In these RCD applications, they are usually used to protect against electric shocks, fires, and damage to electrical equipment.

They are especially important in places where there is a risk of electrocution, such as in wet or damp areas. The industrial RCD is also often used in circuits that supply power to potentially dangerous equipment, such as welding machines.

Residual Current Device Cost

The residual current device price depends on several factors, such as the type of device, the brand, the features, and so on. However, you can expect to pay anywhere between $10 and $100 for a good quality RCD.

Portable RCDs are usually the most affordable option, while fixed RCDs tend to be more expensive. However, fixed RCDs offer a higher level of protection and are more convenient, so they are generally the better option.

Anyone who uses electrical equipment regularly should consider investing in an RCD, as they could potentially save your life. Also, given the varied cost of RCDs, there is always an option to suit every budget.


In conclusion, residual current devices are important gadgets when it comes to protection against electric shocks. They work by constantly monitoring the current flowing through the live and neutral wires, and will trip if there is a difference between the two (known as leakage current). Residual current devices are available in both portable and fixed options, which provides flexibility in terms of cost and convenience

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I am William, Electrical Engineering Author. Dedicated to writing technical articles on Timer Relay, Monitoring Relay, Surge Protection Device and other electrical devices. With 7 years of writing experience, I am committed to providing accurate and in-depth expertise to my readers.


    I want to know about construction, uses, maintenance and testing requirements of different types of RCDS.

    Can I use a portable RCD when using outdoor equipment and connect it to a socket of the home that has an inbuilt RCD?
    If there is already an inbuilt RCD is it ok to plug in a portable one for outdoor work?

    Come si misura il tempo di intervento dei RCD e con quale strumento? Grazie mille

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