RCD: Application & Function

Table of contents

1. What is an RCD?
2. How Does an RCD Work?
3. RCDs: Protection for People and Equipment
4. Differences Between an RCD and a Circuit Breaker
5. Where Are RCDs Used?
6. RCDs – For Maximum Safety

What is an RCD?

What exactly is an RCD? Well, the “R” stands for “Residual,” the “C” for “Current,” and the “D” for “Device. RCDs are the most commonly used components in residual current protection systems.

This component is a central safety feature in electrical installations. An RCD detects what is known as residual current—electricity that escapes through an unintended and unplanned path. This can happen, for example, when a person accidentally touches a live circuit. Additionally, cable damage, insulation faults, sudden moisture exposure, or lightning strikes can also trigger the breaker.

As soon as an unwanted current flow is detected, the RCD interrupts the power supply within milliseconds, significantly reducing the risk to human safety.

How Does an RCD Work?

How exactly does an RCD function? In simple terms, the protective mechanism of an RCD is based on measuring the differential current between the live conductor and the neutral conductor. Under normal conditions, this current flow is balanced—meaning the amount of electricity flowing in matches the amount flowing out. However, if residual current occurs and the outgoing current is lower than the incoming current, the RCD detects this discrepancy and immediately disconnects the circuit.

This is made possible by measuring the magnetic fields generated by the current flow. A specially designed coil system triggers a switch mechanism to cut off the electricity when faults are detected. Generally, an RCD trips at a rated residual current of approximately 30 mA, disconnecting all poles.

RCDs: Protection for People and Equipment

The primary purpose of an RCD is to protect human life. Even small residual currents can be life-threatening—an RCD cuts off the power supply before serious injuries occur.

Additionally, RCDs significantly reduce the risk of fire. Residual currents caused by damaged wiring or faulty devices can lead to smoldering fires. By disconnecting the circuit at an early stage, this risk is minimized. In combination with fire alarm systems, RCDs also contribute to fire prevention.

Differences Between an RCD and a Circuit Breaker

A common misconception is that RCDs and circuit breakers serve the same function. While a circuit breaker protects against overload and short circuits by providing thermal and magnetic protection for devices, an RCD specifically prevents dangerous residual currents caused by damaged cables or accidental contact with live parts. Both types of breakers are essential components of a safe electrical installation.

Where Are RCDs Used?

RCDs are mandatory in private households as well as in commercial and industrial installations. Their use is particularly crucial in wet areas such as bathrooms and kitchens. Since 1984, their installation has been required by law in bathrooms, damp rooms, and outdoor areas according to DIN VDE 0100-701.

Whenever a new power circuit is installed, such as when adding a new socket, an RCD is also required.

Modern installations should assign RCDs to circuits in a way that prevents a single fault from shutting down the entire system. In other words, if an issue occurs, not all circuits in the building should be disconnected. This principle is also mandated in Germany, as outlined in:

• DIN 18015-1 – Planning Electrical Installations in Residential Buildings
• DIN 18015-2 – Minimum Equipment Requirements in Residential Buildings
• TAB 2007 – Technical Connection Conditions for Low-Voltage Networks

Regular testing as part of routine maintenance and proper installation ensure that the protection mechanism functions reliably at all times.

RCDs – For Maximum Safety

RCDs are an essential part of any safe electrical installation. They detect residual currents within milliseconds and cut off the power supply before dangerous electric shocks or fires can occur. When combined with circuit breakers, they provide comprehensive protection for both people and equipment.

Their installation is legally required, especially in wet areas and new constructions. However, retrofitting existing installations is also highly recommended to enhance safety. Modern electrical systems also implement strategic circuit distribution to prevent unnecessary power outages in unaffected areas.

Whether for residential use or commercial applications—choosing high-quality RCDs means investing in reliable protection while ensuring compliance with safety regulations.