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RCBO- Residual Current Breaker with Over-Current

What is RCBO?

RCBO stands for “Residual Current Breaker with Over-current”. This electronic device combines the features of an MCB and a RCD (residual current device). An MCB protects us against over-current which includes:
i) Overload – this occurs when too many appliances draw power from the circuit, overwhelming it in the process.
ii) Short-circuit – this occurs when a contact is made between the live and neutral wires/conductors of a circuit. This causes huge amount of current to flow into the circuit, wreaking havoc.

An RCD provides protection to a circuit from residual current/ earth leakage. This occurs when bad wiring or human-caused actions creates a break in the circuit causing current to flow out and give electrical shock to human. RCBO provides protection from both of these faults.

Working of RCBO:-

In order to understand the working of RCBO, we must understand the working of both a MCB and a RCCB/RCD;

Miniature Circuit Breakers
An MCB provides protection from both overload and short circuit.
Overload protection is provided by the bimetallic strip around which a heater coil wounded to create heat depending on the flow of current. Bimetallic strips are chosen to provide particular time delays under certain overloads.

They are usually made of two types of metal, brass and steel. During slow continuous over-current, characteristic to overload, the bimetallic strip is heated by the electrical charge and thus bends, releasing the mechanical latch. The latch, which is connected to the operating mechanism causes the contacts to open and breaks the circuit.

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If a short circuit occurs, the huge surge in electrical current causes the displacement of the plunger from the solenoid, striking the trip lever, thus breaking the circuit.

MCB’s can be switched ON and OFF manually. This functionality is highly useful, especially during maintenance. The operation of MCB’s is affected by environmental temperature, particularly the deflection of the bimetallic strip. Hence, it is very important to choose appropriate MCB based on its ambient temperature.

MCB’s may be of single, two, three or four-pole version. In two, three and four-pole versions, the respective number of single poles MCBs are joined together and their knobs are combined so that fault cleared by anyone of the poles will open all the poles together.

Residual Current Circuit Breaker
A RCCB/RCD provides protection against earth leakage. To further explain, earth leakage can be human induced, i.e., if a person touches the live part of an appliance and it causes another path for the current to be grounded, causing electrocution.
The RCCB uses Kirchhoff’s Current Law as its working principle. The RCCB constantly monitors the amount of current flowing in the live and neutral conductors. For the safe functioning of the circuit, the amount of electric current flowing through the live conductor should be equal to the amount of current flowing inside the neutral conductor. Electrocution occurs when there is a difference in this amount. This can be due to an accidental outlet in the wiring or due to contact being made with the livewire of an appliance.

The RCCB senses this difference in the solenoid and within milliseconds the RCCB disconnects the supply of power to the circuit. This provides protection to the person. Most RCDs have a rating of earth leakage current of not more than 30 mA.

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The RCCB consists of a toroidal transformer consisting of three coils – a primary coil containing the live wire, a secondary coil containing the neutral wire and a sensing coil which is connected to the trip relay.

The primary and secondary coils generate equal and opposite fluxes under normal conditions. Whenever in case there is a fault and both the currents changes, it creates an out of balance flux, which in turn produces the differential current that flows through the sensing coil connected to relay.

An RCCB comes in two pole types – 2-pole and 4-pole.

Types of RCBOs:-

As mentioned earlier an RCBO combines an MCB and an RCCB.
MCB’s based on tripping curves are of the following kinds :
Type B- This kind of MCB is used in domestic or very light commercial establishments where the surge of power is limited.
Type C- These are normally used for high inductive loads where switching surges are high such as small motors and florescent lighting. They are able to trip five to ten times its rated current.
Type D- These trip instantly from ten to twenty-five times and are used in heavy industrial facilities such as factories using large winding motors, X-ray machines or compressors.

Based on no. of poles – single, two, three or four-pole versions.

RCCBs also have categories:

Generally, RCBOs are differentiated based on their RCCB configuration. Moreover, manufacturers produce RCBOs with a combination of Type-B/Type-C circuit breaker with a Type-AC/Type-A RCCB. If other combinations of RCCBs and MCBs are required then they must be manually and separately connected to the circuit.

Certain things to keep in mind:
o While selecting an RCBO, make sure to select the one that suits the configuration.
o Type AC RCCBs are standard in RCBOs, but the use of Type AC RCCBs when upgrading installations requires careful consideration for most circuits nowadays include appliances that can produce different types of residual fault current.
o RCBOs also need to be selected with overload, current rating and the short circuit capacity in mind.

Which one is better? – MCB vs. RCCB vs. RCBO

Major Manufacturers

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