Residual Current Device (RCD) - What It Is, How It Works, and How It Differs from a Circuit Breaker (by Capitan Farloc)

Ever heard someone say, “The power’s out, the breaker tripped”? But what exactly is a residual current device (RCD), commonly known as a “salvavita” in Italy? In this guide, we’ll explain what it is, how it works, and how it differs from a standard automatic circuit breaker. Everything you need to know to better understand your home’s electrical system—explained clearly and simply!

What is a Residual Current Device (RCD)

Electric current is a danger that should never be underestimated. It's vital to avoid attempting risky operations that are beyond our abilities or experience.
Today, we'll discuss one of the devices that plays a critical role in ensuring safety and protection against the risks posed by electric current. It's so significant that it is often called "Lifesaver," but its proper technical name is the "Residual Current Device (RCD)."

How the Residual Current Device (RCD) Works

The Residual Current Device operates on a simple principle: the algebraic sum of the currents flowing between the live and neutral wires in an electrical system must be equal to zero. Put more simply, the current (I2) flowing in the return wire (neutral) must match the current (I1) flowing in the active live wire.
If this condition isn't met (I2 - I1 is not equal to zero), it indicates that there is current leakage occurring outside the electrical system, such as towards the ground. The RCD measures this difference, and when it exceeds the threshold it is set to, the device trips and disconnects the circuit.
It's important to note that when we talk about "zero difference" or "no leakage," we are referring to a theoretical system. In practice, real electrical systems do have some leakage, which is why the RCD has a threshold value below which it doesn't trip.

Why the Residual Current Device (RCD) Trips

Essentially, this type of device trips because it detects current leakage towards the ground. This can happen due to direct contact with the electrical current, or it might occur because a component inside an appliance has lost its insulation and has started allowing some of the electrical current to pass to the appliance's metallic parts.
In light of this, it's clear how crucial it is (as well as legally required) to have a compliant electrical system with proper grounding connections.

Why Does the Residual Current Device (RCD) Sometimes Trip Without Reason?

There are certain situations where an RCD may trip even when there is no issue with the electrical system. A classic example is during a thunderstorm—lightning strikes can cause voltage surges that may trigger the RCD, especially in rural areas.
Other factors, such as specific appliances, can also lead to frequent tripping of the RCD. In these cases, the device may not have been properly sized, and it might be necessary to install an RCD with a higher threshold current. However, such adjustments should only be carried out by a qualified technician who can accurately identify the problem and recommend the most suitable solution.

How the Circuit Breaker (Thermal-Magnetic Circuit Breaker) Works

The automatic circuit breaker, technically known as the "Thermal-Magnetic Circuit Breaker," serves the dual purpose of protecting the electrical system from short circuits and current overloads.
To perform these two functions, it is equipped with two mechanisms:

1. An electromagnetic circuit that, through a small lever controlled by an electromagnet, immediately disconnects the electrical contacts in the event of a short circuit.
2. A thermal circuit that uses the deformation of bimetallic strips (also known as thermocouples) to trip the breaker when current consumption exceeds the maximum allowed. Unlike short-circuit protection, this system has a longer response time (the time it takes for the thermocouple to deform). This delay prevents the breaker from tripping due to the normal startup current required by devices like motors. As a result, the system allows the maximum current to be temporarily exceeded, provided it happens for a limited period.

WARNING: Even the Residual Current Device (RCD) Cannot Guarantee Safety

We've explained the working principle of the Residual Current Device (RCD) as a lifesaver, and how it intervenes when it detects leakage in the system, based on the assumption that this leakage indicates current discharging towards the ground.
We've also seen that the Thermal-Magnetic Circuit Breaker is essential to protect against overloads and short circuits. It plays a crucial role in preventing shorted wires from potentially causing fires, but on its own, it is completely insufficient and ineffective against electrocution hazards. This is why both types of devices must always be present in a household electrical system.
One final warning I want to emphasize is that the RCD cannot work miracles. If someone is isolated from the ground—for example, by wearing rubber-soled shoes or standing on a wooden chair or ladder—and were to touch both wires of the domestic network, there would be no leakage detected. This is because the electric current from the live wire would simply return via the neutral wire, with no leakage recorded. Therefore, the presence of an RCD must never lead us to assume that we can work on the electrical system without first switching off the main breaker.
Every time we need to perform any operation on the electrical system, even something as simple as changing a light bulb, we must first switch off the main breaker and ensure that the electricity is off.

NOTE: The illustrations in this article were kindly provided by Sergio Aureli, whom I thank for his patience and dedication.
A big thank you to all of you as well for following me here on my website!
Luciano (Capitan Farloc)