• Home
  • Courses
  • Machines
    • Transformer
    • AC Motors
      • Induction Motor
      • Synchronous Motor
    • DC Motor
    • DC Generator
  • Power System
    • Circuit Breaker
    • Switchgear
    • Substation
    • Control System
    • Measurement
  • Electronics
  • Blog
  • Contact
    • Contact Us
    • Privacy Policy
Have any question?
[email protected]
RegisterLogin
StudyElectrical.Com
  • Home
  • Courses
  • Machines
    • Transformer
    • AC Motors
      • Induction Motor
      • Synchronous Motor
    • DC Motor
    • DC Generator
  • Power System
    • Circuit Breaker
    • Switchgear
    • Substation
    • Control System
    • Measurement
  • Electronics
  • Blog
  • Contact
    • Contact Us
    • Privacy Policy

Power System

Residual Current Protective Devices (RCCB, ELCB & RCBO)

  • Categories Power System, Circuit Breaker
RCD ELCB RCCB Working

Table of Contents

  • What is an RCD?
  • Operating Principle of a Residual Current Device
    • Test Circuit in RCD
    • Types of Residual Current Device
    • Working of a Residual Current Device
    • Related Articles

What is an RCD?

A Residual Current Device (RCCB, ELCB, RCBO, GFCI etc.) is a device that is designed to provide protection against electrocution or electrical fires by cutting off the flow of electric automatically when it senses a ‘leakage’ of electric current from a circuit. 

Residual Current Devices have the following Specifications 

  • Sensitivity – milliAmperes (mA) 
  • Current Rating – Amperes (A)
  • Short Circuit Rating – Kilo Amperes (kA)
  • Poles – 2 Pole or 4 Pole

Difference between RCD, RCCB & RCBO

Residual current protective devices are classified according to their various versions.
  • RCD is the generic term for all types of residual current protective device.
  • RCCBs are residual current operated circuit-breakers without integral overcurrent protection known in Germany as Fehlerstrom-Schutzschalter (FI-Schutzschalter).
  • RCBOs are devices which feature an integrated overcurrent protection unit for overload and short-circuit protection in addition to protection against residual currents.

Read GFCI (Ground Fault Circuit Interrupter) – Working, Types, Installing and Troubleshooting

Operating Principle of a Residual Current Device

The basic operating principle of a residual current device (RCCB, ELCB, RCBO etc.) is given in the figure below.

Residual%2BCurrent%2BCircuit%2BBreaker
Operating principle of Residual Current Circuit Breaker

When the load is connected to the supply through the  Residual Current Device (RCD), the line and neutral conductors are connected through primary windings on a toroidal transformer.

In this arrangement, the secondary winding is used as a sensing coil and is electrically connected to a sensitive relay or solid state switching device, the operation of which triggers the tripping mechanism. 

When the line and neutral currents are balanced, as in a healthy circuit, they produce equal and opposite magnetic fluxes in the transformer core with the result that there is no current generated in the sensing coil. (For this reason, the transformer is also known as a ‘core balance transformer‘). 

When the line and neutral currents are not balanced they create an out-of-balance flux. This will induce a current in the secondary winding which is used to operate the tripping mechanism. It is important to note that both the line and neutral conductors pass through the toroid. 

rcd%2Bworking
This is how RCD protects from electrocution.

A common cause of unwanted tripping is the failure to connect the neutral through the RCD, RCDs work equally well on single phase, three phase or three phase and neutral circuits, but when the neutral is distributed it is essential that it passes through the toroid. 

Test Circuit in RCD

A test circuit is always incorporated in the RCD. Typically the operation of the test

rcd

button connects a resistive load between the line conductor on the load-side of the RCD and the supply neutral. 

The test circuit is designed to pass current in excess of the tripping current of the RCD to simulate an out-of-balance condition. Operation of the test button verifies that the RCD is operational. It is important to note, therefore, that the test circuit does not check the circuit protective conductor or the condition of the earth electrode.


On all RCDs a label instructs the user to check the function of the RCD at regular intervals and to observe that the RCD trips instantly.

Types of Residual Current Device

There are different types of residual current devices available now. Some of them are mentioned below.

1. RCCB 

They are Residual Current Operated Circuit-Breaker without Integral Overcurrent Protection.
A mechanical switching device designed to make, carry and break currents under normal service conditions and to cause the opening of the contacts when the residual current attains a given value under specified conditions.

It is not designed to give protection against overloads and/or short-circuits and must always be used in conjunction with an overcurrent protective device such as a fuse or circuit-breaker.

2. RCBO 

RCBO is a Residual Current Operated Circuit-Breaker with Integral Overcurrent Protection.

A mechanical switching device designed to make, carry and break currents under normal service conditions and to cause the opening of the contacts when the residual current attains a given value under specified conditions.

In addition, it is designed to give protection against overloads and/or short-circuits and can be used independently of any other overcurrent protective device within its rated short-circuit capacity.

Working of a Residual Current Device

In an RCD, the line and neutral conductors of a circuit pass through a sensitive current transformer. If the line and neutral currents are equal and opposite, the core remains balanced (Figure A). 

If there is an earth fault the neutral current will be lower than the line current. This imbalance produces an output from the current transformer which is used to trip the RCD and so break the circuit (Figure B).

RCD ELCB RCCB Working
Working Principle of Residual Current Device (RCD)

In practice, the main Miniature Circuit Breaker (MCB) for the premises will probably trip, or the service fuse so the situation is unlikely to lead to catastrophe, but it may be inconvenient. RCDs normally don’t offer protection against current overloads: RCDs detect an imbalance between the live and neutral currents. A current overload, however large, cannot be detected. If a line neutral fault occurs (a short circuit, or an overload), the RCD won’t trip, and may get damaged. 

It is now possible to get an MCB and RCD in a single unit called an RCBO. Replacing an MCB with an RCBO of the same rating is generally safe. RCBO is Residual Current Circuit Breaker with Overcurrent Protection.

Standard ratings available are 25A, 40A, 63A and 100A 

Standard Sensitivities available are 30mA. 100mA, 300mA and optionally 10mA is also available. Electromechanical RCCBs are designed to operate on normal supply waveforms and cannot be guaranteed to operate where none standard waveforms are generated by loads. The most common is the half-wave rectified waveform sometimes called pulsating dc generated by Computers, Speed control devices (VFD) and even dimmers. 

Related Articles

  • Construction of High Voltage Current Transformer
    Basic Types of Current Transformers

    Current Transformers (CT’s) are instrument transformers that are used to supply a reduced value of…

  • protective zones in power system
    Protective Zones in Power Systems

    In this post, we will look into protective zones in the power system and why…

  • Current Transformer construction
    Current Transformers - Windings and Rated Insulation Levels

    A current transformer is an instrument transformer in which the current ratio is within the…

  • Constant 2BCurrent 2BDriver 2BDesign 2Bbased 2Bon 2BLM317
    Constant Current Driver Design based on LM317

    Using a resistor to limit the current can be used for small LEDs. For such…

  • Types-of-Electric-current
    Types of Electric Current - Steady (DC), Varying & Alternating (AC) Current

    The directed flow of free electrons (or charge) is called the electric current. The electric…

  • Current
    What Are Current, Resistance and Voltage?

    Voltage, current, and resistance are three properties that are fundamental to almost everything you will…

  • Share:
author avatar
Electrical Engineer

Previous post

Insulators, Conductors and Semiconductors (with Band Diagram)
May 30, 2015

Next post

Systems of Wiring - Introduction to Electrical Wiring
May 30, 2015

You may also like

percentage_reactance transformer
Percentage Reactance in Power System (Transformer, Generator or Reactor)
16 January, 2023
Load Curve and Load Duration Curve
Load Duration Curve in Power Station
29 September, 2022
Double end break disconnect switch
Disconnect Switches (Isolator) in Substation: Types, Design and Working
1 September, 2022

    5 Comments

  1. read review
    February 9, 2019
    Reply

    I am constantly searching online for ideas that can facilitate me. Thanks!

  2. administratiekantoor
    May 18, 2019
    Reply

    Thank you for explaining the difference between RCCB and RCBO

  3. Un Whish
    May 18, 2019
    Reply

    Love this site– very informative and lots to consider!

  4. P.Imran khan
    September 23, 2019
    Reply

    can u tell me failures of rccb

  5. recharge able
    October 3, 2019
    Reply

    Great, thanks for sharing this blog.Really looking forward to read more. Cool.

Leave A Reply Cancel reply

Your email address will not be published. Required fields are marked *

Search Here

From Blog

capacitor charging and discharging
Charging and Discharging a Capacitor
29Feb2020
Sulphur Hexafluoride SF6 Circuit Breakers Construction Working and Advantages
SF6 Circuit Breakers – Construction, Types and Working
17Jul2014
assignment
Are New Standards Required Amid Electrical Safety Threats?
05Jan2022
maxresdefault
An Introduction to the Arduino for Beginners
11Apr2016

Categories

  • Alternator
  • Arduino
  • Basic Electrical
  • Battery
  • Books
  • Cables
  • Capacitor
  • Career
  • Circuit Breaker
  • Control System
  • DC Generator
  • DC Motor
  • Drives
  • Electric Vehicles
  • Electrical Circuits
  • Electrical Machines
  • Electrical Relays
  • Electrical Safety
  • Electronics
  • Embedded System
  • Exams
  • Generation
  • Guest Post
  • HVDC
  • Instrumentation
  • Interview Questions
  • Locomotives
  • Measurement
  • Microcontroller
  • Objective Questions
  • PCB
  • PLC
  • Power System
  • Problems and Solution
  • Projects
  • Resistor
  • Signals and Systems
  • Single Phase Motors
  • Substation
  • Switchgear
  • Synchronous Motor
  • Three Phase Induction Motor
  • Transformer
  • Transmission Line
  • Uncategorized

Copyright © 2021 Study Electrical, Inc.

© StudyElectrical.Com 2021

Login with your site account

Lost your password?

Not a member yet? Register now

Register a new account

Are you a member? Login now