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What is RCBO?

What is RCBO?


Who would bother to install two devices when only one gadget can serve the purpose?

What is RCBO?

Residual Current Breaker with Over-Current is shortly known as RCBO and is the combination of RCB and MCB. As its name suggests, it offers a protective layer against the frequently occurring faults in any electrical circuit, be it a commercial or industrial circuitry system.

Do you remember the faults that caused damages to an electrical system?

Let’s take a glimpse at the system of electrical faults.

Residual current or earth leakage: As the name itself delivering the meaning, it occurs when there is an accidental break in a circuit through poor electrical wiring. It is a fact that electricity chooses the easiest route to travel; when a person or conductor gets in touch with the live war, the current alters its path and passes through the person to cause electric shocks, which often leads to death.

Over-Currenteveryone is familiar with it. Over-currents takes two forms.

  1. Overload: What would happen when the load applied to an electrical system is greater than its rated value? This occurs when too many devices are connected to the circuit exceeding the rated value of the cable.
  2. Short-circuit: More dangerous than any other fault is a short circuit when the live and neutral wire gets connected, resulting in a complete disaster. The current makes a loop in the circuit, multiplying the current value to the thousands of amperes; you can guess the magnitude of loss.

Faults are highly grim, right?

If not considered and tackled appropriately, these forms of faults can lead to an entire disaster.

RCBO is the breaker that counters these faults and provides us with an extraordinary layer of protection.

It is worth it to be understood and applied to our circuit, nodding along with me, right?

Let’s start knowing more about RCBO.

What is an RCBO?

One of the most crucial parts of an electrical circuitry system is protecting against any current leakage, especially earth leakage. To minimize the number of electric shocks and the resultant injuries, the Written Regulation Authority state that all electrical circuit must have protection against any leakage. For that reason, a breaker is imperative that will sense any leakage and trip the circuit immediately. RCBO is the combination of an RCB and MCB. RCBO protects against overload and short-circuiting, which cause current leakage to the earth. RCBO sense that imbalance and trigger to trip the circuit minimizing the perils of short circuit to a greater extent.

It is worth opening an RCBO and knowing what’s inside of it, right?

 Let’s get started without any further ado.

Construction of RCBO

The construction of RCBO is comparatively easy to understand as there are two compartments of an RCBO, an MCB and RCCB.

 MCB Compartments: MCB compartments contains multiple requirements that are necessary to trip the circuit.

Let’s dig deep and find what the MCB compartment of an RCBO contains.

  • Bimetallic contact: over-loading of an electrical system is a common phenomenon. Bimetallic contact is the device that an RCBO uses to trip the circuit in case of current leakage due to over-loading. It is designed so that it doesn’t change the physical property for the nominal current flow.
  • Arc Chute: the safety of the breaker itself while tripping the circuit is extremely important. Arch chute is an arch quenching equipment that is used to protect the breaker while it is operating.
  • Magnetic coil: it is used to trip the breaker when the circuit is getting over-loaded. 
  • Terminal screws: as the name suggest, terminal screws are used to connect the breaker with external circuits.
  • Handle knob: it’s a nob which helps in indicating the current status of the breaker. Different positions of the handle tell the breaker position, such as ON or OFF.

RCCB compartment: the second part of an RCBO is this RCBB compartment. It contains the following ingredients.

  • CBCT: Core Balanced Current Transformer detects any imbalance in the current flowing in the circuit. So it is clear that the live and neutral wire passed through the CBCT.
  • Control Circuit: whenever a fault occurs, the control circuit is used to give a tripping command to a coil that operates and trips the breaker.
  • Test Button: as the name suggests, it is used to check the working condition of the breaker.

How does it operate?

Before we dig deep and find out how the different components of an RCBO operate, let’s learn what principle RCBO follows to act.

Do you understand the series and parallel connection of a circuit?

 When the circuit is connected in series, the same amount of current passes through all of the attached equipment, but the voltage is not always the same. However, when the circuit is connected in parallel, the voltage across all of the parallel attached equipment is the same, but the current is divided, and so it’s not the same. When it comes to the internal circuitry of an RCBO, the following equipment or organs are connected in series.

  • Short circuit tripping coil
  • Bimetallic contact
  • Input terminal
  • Output terminal

The body of an RCBO is designed so that the output terminals are connected at the top side while the input terminals are connected at the bottom.

Should move toward knowing how the different part of an RCBO operates?

Let’s begin.

Earth Fault Operation

Well, this function is senses and performed by the RCCB compartment only. As the name suggests, CBCT always detects the vector sum of the live and neutral currents of an RCBO. When the circuit is running at normal conditions, it means that there are no imbalances of currents. According to Kirchoff’s rule, the current entering a point in a circuit must be equal to the current leaving the point. Hence, since the line current is equal to zero, the current output of CBCT will be equal to zero. Hence, the circuit remains stands.

However, when the circuit is running at normal condition, it means that now any of the neutral or live wire is in touch with the earth resulting in leakage current, leaving an imbalance between the current values. That imbalance is detected by the CBCT, which send a signal to the control circuit, pushing it to operate and trip the circuit.

Overload and Short Circuit

Bimetallic contact is designed to trip when there is a substantial amount of heat is produced. Under normal conditions, the heat generated in the bimetallic contact is not sufficient to trip the circuit as it is designed so. But when there is a fault, in that case, the current flowing through the circuit is high, resulting in generating heat; when the heat value is high, the bimetallic contact will bend and allow to ignite the tripping process of the circuit.

How does it operate under instantaneous overcurrent?

This is where MCB comes in and perform its duty. When there are instantaneous currents, the solenoid coil of MCB operates to trip the circuit.

Under normal conditions, the magnetic field generated is too negligible to be sensed by the coil. However, when there are faults conditions, the currents raises to 5 to 10 higher than their rated value leading towards a high value of the magnetic field, which pulls the plunger, which ultimately trips the circuit.

Before we embark on writing the advantages and demerits of an RCBO, let’s clear some ambiguities and have a crystal clear picture of the difference between an RCBO and other breakers.


With almost similar functionality as RCBO, RCCB does not support the fault such as overloading and short-circuiting. However, RCBO provides full protection against all kinds of faults, be it short-circuiting, earth leakage or overloading.

How to choose the best RCBO?

Countless companies are producing RCBOs in plenty. Choosing the best market and the best product is crucial, no doubt at all. However, Hager and Siemens are the two top-level companies offering the best RCBOs for a lowered price.

How can one decide and pick the best-suited RCBO for a circuit? Three factors can help one choose the best product.

  1. Check the rated current
  2. Closely analyze the tripping characteristics
  3. Check for the number of poles requirements

For household applications, the leakage current of 30 mA and type B or C is perfectly suitable.

Advantages of RCBO

  • It protects against almost all kinds of faults, including over current, short circuit and earth leakage
  • No need to keep spare fuses, wires and insulation material
  • Compact size
  • Re-usable
  • Easy installation
  • It helps to maintain a high degree of safety

Demerits of RCBO

  • It is costlier than MCB and RCCB
  • It doesn’t support DC circuits
  • The current range cannot be adjusted.

Wrapping up

 One of the most demanding factors of an electrical circuit is its protection from all sorts of faults that often causes severe damages to both human and worldly things alike. Countless breakers serve the protection purpose, but RCBO stands exceptional for several reasons. It’s a compact breaker comprised of two important breakers, RCB and MCB. Instead of installing two circuits, one can install RCBO and get the circuit fully safe and secure from faults.

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