Most domestic fires in the UK are due to electrical fires, the majority of which are caused by faulty electrical appliances connection, faults in installations and degradation or aging of the electrical switchboard. Additionally, 368 yearly deaths in the UK are caused by accidental electrocution.

The statistics seem dire and depressing, but they can be avoided by regularly checking your electric switchboard and connected appliances and installing a Residual Current Device (RCD) or Residual Current Circuit Breakers with overcurrent (RCBO). These devices offer protection against ground faults and earth leakage, which are common causes of electrical fires and shocks.

What exactly is an RCD and RCBO? How do they work? And is there any difference between them? Let’s answer these questions.

(RCD stands for Residual Current Device and is commonly referred to as such in the industry.)

Introduction to Residual Current Devices and Circuit Breakers

Residual Current Devices (RCDs) and circuit breakers are fundamental to maintaining electrical safety in homes, businesses, and industrial environments. RCDs, sometimes referred to as residual current circuit breakers, are designed to detect residual currents that may signal earth faults or other potentially dangerous situations within electrical systems. By quickly responding to these residual currents, RCDs help prevent electric shocks and reduce the risk of electrical fires.

Circuit breakers, on the other hand, are engineered to protect electrical circuits from overcurrent conditions, such as short circuits or overloads, which can cause significant damage to wiring and connected devices. Understanding how residual current devices and circuit breakers function—and the differences between them—is essential for ensuring the right level of protection in any electrical installation.

What Is a Residual Current Device (RCD)?

An RCD is a type of circuit breaker that can automatically open the breaker in case of an earth fault. In a healthy circuit, the current flow is balanced and there is no earth leakage, which is the normal operating condition for an RCD. This breaker is designed to protect against the risks of accidental electrocution and fire caused by earth faults. Electricians also call it RDB (Residual Current Breaker) and RCCB (Residual Current Circuit Breaker).

RCDs detect current leakage, which can occur due to faults or damaged insulation, and disconnect the circuit to prevent harm.

This type of breaker always has a push-button for the breaker test. You can choose from 2 or 4 poles, current rating (Amp rating) from 25 A up to 100 A, tripping curve B, Type A or Type AC (note: Type AC RCDs are commonly used but may have limitations for certain installations), and mA rating from 30 up to 100 mA.

Why Should You Use an RCD?

Ideally, it would be best to use this type of breaker to prevent accidental fires and electrocution. Any current going through a person more significant than 30 mA can drive the heart into ventricular fibrillation (or throwing the heart’s rhythm off)—the most common cause of death through electric shock. An RCD stops the current within 25 to 40 milliseconds before an electric shock could occur. RCDs are especially important in situations where there is a risk of direct contact with live electrical parts, as they provide critical protection in these scenarios.

By contrast, conventional circuit breakers such as MCB/MCCB (Miniature Circuit Breaker) or fuses break only when the current in the circuit is excessive (which can be thousands of times the leakage current an RCD responds to). An RCD can quickly disconnect the circuit to prevent harm. A small leakage current coursing through a human body can be enough to kill you. Still, it would probably not increase the total current enough for a fuse or overload the circuit breaker and not fast enough to save your life. RCDs protect against electrical faults, including those that may not cause a large overcurrent, by detecting leakage currents and disconnecting power rapidly in the event of a fault, enhancing safety.

What Is an RCBO?

An RCBO is a breaker with overcurrent protection and residual current detection, combining both functions in a single unit. This device integrates the features of an MCB and RCD, acting as a circuit breaker with overcurrent protection—similar to miniature circuit breakers—but with added earth leakage detection. An RCB, as discussed above, can sense the overcurrent caused by the short circuit. RCBOs use a bimetallic contact to provide overload protection. RCBOs can come in 1 pole, 1 + neutral or two poles as well as with an amp rating from 6 up to 100 A, tripping curve B or C, Breaking capacity 6K A or 10K A, RCD type A, A & AC.

Why Use an RCBO?

You need to use an RCBO for the same reasons we recommend an RCB – to save you from accidental electrocution and prevent electrical fires. An RCBO offers protection against both overcurrent and earth leakage faults, providing an added benefit over standard RCDs. It provides overload protection as well as earth leakage detection, ensuring your circuits are safeguarded from multiple types of electrical faults. The RCBO performs two functions: it acts as both an RCD and a circuit breaker, so it offers comprehensive safety by combining both an RCD's residual current detection and a circuit breaker's overcurrent protection in one device.

Overcurrent Protection: The Key Difference

A crucial distinction between RCDs and circuit breakers lies in their approach to overcurrent protection. Circuit breakers are specifically designed to interrupt the flow of electricity when they detect overcurrent situations, such as those caused by short circuits or overloaded circuits. This helps prevent damage to the electrical system and reduces the risk of fire. In contrast, RCDs focus on identifying residual currents that indicate earth faults, offering protection against electric shocks but not against overcurrent.

For those seeking both types of protection in a single device, residual current circuit breakers with overcurrent protection—commonly known as RCBOs—are the ideal solution. RCBOs combine the safety benefits of both RCDs and circuit breakers, providing dual protection against residual currents and overcurrent conditions. This comprehensive protection is especially valuable in environments where both earth faults and overcurrent hazards are a concern.

How Do RCDs and RCBOs Work?

RCDs and RCBOs both operate on the principle that a current entering through a live cable is the same coming out through the neutral wire by measuring the current balance between two conductors using a differential current transformer. 

You can imagine this as a toroidal transformer with three coils. The first two coils are wired in opposite ways. One wire is connected to the live wire, and the other to the neutral wire. You then have a third coil (trip coil) connected to a solenoid that will electromechanically switch the breaker off.

If the current entering the circuit is equal to the outgoing current, there is no current flowing in the third coil, so the breaker stays ON (in normal condition). If you have an earth leak, the current flow goes to the ground. Earth leaks can be caused by:

• A live wire touching the equipment frame.
• Wet equipment.
• Someone touching the live cables.
• Degradations in isolation material.

If there is a leak, an imbalance happens – the incoming current is higher than the outgoing current. The sudden current flow into the third coil activates the solenoid switching the breaker OFF. 

RCD and RCBO Applications

RCDs and RCBOs are widely used across a range of applications to enhance electrical safety and prevent electrical accidents. In domestic settings, RCDs are commonly installed to protect against earth faults and reduce the risk of electric shocks, making them a vital component in safeguarding families and homes. In commercial settings, such as offices and construction sites, where the risk of electrical accidents and fires is higher, both RCDs and RCBOs are used to provide reliable protection.

RCBOs, with their dual protection capabilities, are particularly favored in modern installations and areas with a higher risk of electrical fires, as they offer comprehensive protection for individual circuits. In industrial environments, where electrical loads are greater and the consequences of faults can be severe, RCBOs ensure both equipment and personnel are protected from earth faults and overcurrent conditions. By choosing the right combination of RCDs and RCBOs, it is possible to achieve a higher level of safety and minimize the risk of electrical hazards in any setting.

What Is the Difference Between an RCD and an RCBO?

The main difference between both these circuit breakers is that the RCBO is equipped with an overcurrent detector. At this point, you might be thinking about why they market these separately if there seems to be only one main difference between them? Why not sell only kind in the market?

Traditionally, two devices—an RCD for earth fault protection and an MCB for overcurrent—are used together as protective devices in electrical installations. However, RCBOs combine both functions in a single device, offering more comprehensive protection.

Whether you choose to use an RCBO or an RCD depends on the installation type and budget. For example, when there is an earth leak in a switchboard using all RCBO breakers, only the breaker with the faulty switch will go off, while other circuits remain unaffected. With a single RCD protecting multiple circuits, a fault in one power circuit can cause the RCD to trip, disconnecting other circuits as well and resulting in unnecessary power loss across the entire installation.

If budget is an issue, you can configure three or four MCBs under one residual current device. Installing an RCD at the main switchboard can protect the entire installation, but this setup does not provide individual circuit protection, so a fault in one circuit can impact other circuits. You can also use it for special applications like a jacuzzi or hot tub installation, where power circuits may require individual protection to prevent outages. These installations require faster and less activation current, generally 10mA.

Ultimately, whichever breaker you want to use depends on your switchboard design and budget. Proper selection of protective devices is essential for electrical installations to ensure earth fault protection, compliance, and optimal safety for both equipment and human life. However, if you’re going to design or upgrade your switchboard to stay in regulation and ensure the best electrical protection for both the equipment asset and human life, make sure to get in touch with a reliable electrical specialist.

General Q&A

What causes most domestic electrical fires in the UK, and how can they be prevented?

Most domestic electrical fires in the UK are caused by faulty electrical appliances, poor connections, installation faults, and aging or deteriorating switchboards. These issues can lead to dangerous situations like electrical fires or accidental electrocution, which is responsible for around 368 deaths annually in the UK. To prevent these risks, it's important to regularly inspect your electrical switchboard and appliances. Installing devices like RCDs (Residual Current Devices) or RCBOs (Residual Current Circuit Breakers with Overcurrent) adds a crucial layer of protection by quickly shutting off power in case of faults.

How does an RCD protect against electrocution and fire?

An RCD monitors the balance of electrical current between the live and neutral wires. If there's a difference—such as electricity leaking to the ground due to a fault or someone touching a live wire - it indicates a potential earth fault. The RCD responds by cutting the power within 25 to 40 milliseconds, fast enough to prevent serious electric shock or fire. Unlike traditional circuit breakers that trip only when current levels are too high, an RCD detects much smaller leaks that can still be deadly, offering significantly better protection.

What's the difference between an RCD and an RCBO, and when should you use each?

The key difference is that an RCBO combines the functions of an RCD and an MCB (Miniature Circuit Breaker). While an RCD protects against earth faults, an RCBO adds protection against overcurrent and short circuits. RCBOs offer individual circuit protection, meaning only the affected circuit trips in the event of a fault. This makes them ideal for modern installations, especially where continuity is important, like in kitchens or offices. RCBOs used to be more expensive but have reduced in recent years, costing less than £10. Its much better to have each circuit individually protected with an RCBO.

Conclusion on Residual Current Protection

In summary, residual current devices and circuit breakers—including those with overcurrent protection, such as RCBOs—play a crucial role in maintaining electrical safety across all types of installations. Understanding the key differences between these protective devices, particularly in terms of overcurrent protection and earth fault detection, is essential for selecting the most suitable solution for each application. Whether for domestic, commercial, or industrial use, the choice between RCDs and RCBOs should be guided by the specific risks present, such as the likelihood of earth faults, overcurrent conditions, and the need for comprehensive protection. By prioritizing electrical safety and using the appropriate protective devices, we can significantly reduce the risk of electrical accidents, fires, and injuries, ensuring a safer environment for everyone.