2. Circuit Interruption Devices, EE 50031 notes in English,

Rajasthan Polytechnic (BTER) February 03, 2025

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2. Circuit Interruption Devices

Circuit interruption devices are essential components in electrical systems used to disconnect faulty circuits and protect equipment from damage. They operate by interrupting the flow of current when an abnormal condition (like a short circuit) occurs. Let's look at the different circuit interruption devices in detail.

2.1 Isolators

Isolators are mechanical devices used to disconnect a part of the electrical circuit when there is no current flowing. They are typically used for maintenance purposes and are placed in series with the circuit.

2.1.1 Vertical Break Isolator:

  • In a vertical break isolator, the contact point moves vertically to open or close the circuit.
  • It is commonly used in substations and overhead lines.
  • Vertical movement helps minimize arcing and ensures reliable operation in various weather conditions.

2.1.2 Horizontal Break Isolator:

  • In horizontal break isolators, the contacts are separated horizontally.
  • They are often used for medium- and low-voltage systems.
  • The design allows easy maintenance and inspection as the contacts are clearly visible when open.

2.1.3 Pantograph Type Isolator:

  • A pantograph isolator uses a pantograph mechanism (similar to train pantographs) to lift and separate the contacts.
  • It is mainly used in high-voltage transmission lines.
  • The pantograph type allows for efficient operation in high-voltage systems by reducing the wear and tear on the contacts.

2.2 HRC Fuses – Construction, Working, Characteristics, and Applications

HRC (High Rupturing Capacity) Fuses are used to protect electrical circuits from overcurrent conditions. They are designed to interrupt high fault currents without causing damage to the system.

Construction:

  • HRC fuses consist of a metal conductor (usually a wire or strip) enclosed in a ceramic or glass tube.
  • The tube is filled with sand or another arc-quenching material to extinguish the arc.
  • The fuse element inside the tube is designed to melt when the current exceeds a predetermined value.

Working:

  • Under normal conditions, the fuse remains intact and allows current to pass.
  • When an overcurrent (or fault condition) occurs, the fuse element heats up and melts, breaking the circuit and protecting the equipment downstream.
  • The arc formed when the fuse element melts is extinguished by the arc-quenching material inside the fuse.

Characteristics:

  • High Current Interrupting Capacity: HRC fuses can interrupt large fault currents without causing damage.
  • Fast Response Time: They provide quick disconnection in case of a fault, minimizing damage.
  • Reusability: Once blown, the fuse must be replaced.

Applications:

  • HRC fuses are used in power distribution systems, transformers, motors, and other electrical equipment to protect from short circuits and overloads.

2.3 Arc Formation Process

An arc is formed when a high current flows through the gap between two contacts that are about to be separated. This happens when the current tries to maintain the flow even after the contacts start to open.

  • As the contacts start to open, the resistance between them increases, causing the temperature to rise.
  • The high temperature ionizes the air (or other dielectric medium), allowing the current to continue to flow through the ionized path, forming an arc.
  • The arc must be extinguished quickly to avoid damage to the contacts and the surrounding environment.

2.4 Methods of Arc Extinction

The arc formed during the interruption of a circuit needs to be extinguished to avoid prolonged damage to the system. There are two main methods of arc extinction:

High Resistance Method:

  • In the high resistance method, the arc is quenched by introducing resistance into the arc path.
  • The high resistance increases the voltage drop across the arc, which reduces the current and eventually extinguishes the arc.
  • This method is used in low voltage circuit breakers.

Low Resistance Method:

  • In the low resistance method, the arc is extinguished by rapidly increasing the distance between the contacts.
  • This method is used in high-voltage circuit breakers, where the arc needs to be stretched to a point where the dielectric strength of the air (or another medium) can quench the arc.
  • This process occurs quickly to prevent the arc from sustaining itself.

2.5 Arc Voltage, Recovery Voltage, Re-striking Voltage, and RRRV

  • Arc Voltage: This is the voltage across the contacts during the arcing process. It is higher than the normal operating voltage because of the ionized air path through which the current flows.

  • Recovery Voltage: After the contacts open and the arc is extinguished, the voltage across the contacts recovers to the normal line voltage. If the recovery voltage is high, it may cause the arc to reignite.

  • Re-striking Voltage: This is the voltage that appears across the contacts after the arc has been extinguished. If it is too high, it can cause the arc to strike again.

  • RRRV (Rate of Rise of Re-striking Voltage): This refers to how quickly the recovery voltage rises after the arc is extinguished. A high RRRV increases the likelihood of re-ignition of the arc.

2.6 Working and Applications of High Tension (HT) Circuit Breakers

High Tension (HT) circuit breakers are used for high-voltage applications (typically above 1000V) and play an important role in protecting electrical systems from faults.

2.6.1 Sulphur Hexafluoride (SF6) Circuit Breaker:

  • Working: SF6 circuit breakers use sulphur hexafluoride gas as an insulating medium. When the contacts of the breaker open, the SF6 gas is compressed and forced through the contacts, which helps to extinguish the arc formed by the current.
  • Applications: SF6 breakers are widely used in power substations and high-voltage transmission systems because of their excellent dielectric properties and ability to handle high fault currents.

2.6.2 Vacuum Circuit Breaker:

  • Working: In a vacuum circuit breaker, the contacts open in a vacuum, which provides excellent insulation. The absence of air prevents the formation of an arc, and the arc is quickly extinguished by the vacuum environment.
  • Applications: Vacuum circuit breakers are typically used in medium-voltage applications, such as in industrial plants and distribution systems.

2.7 Working and Applications of Low Tension (LT) Circuit Breakers

Low Tension (LT) circuit breakers are used for lower voltage systems (below 1000V) and protect circuits in homes, offices, and industrial applications.

2.7.1 Air Circuit Breaker (ACB):

  • Working: Air circuit breakers interrupt the current by using air to extinguish the arc when the contacts open. They are typically used in high current low-voltage applications.
  • Applications: Used in distribution boards and motor protection.

2.7.2 Miniature Circuit Breaker (MCB):

  • Working: MCBs are used to protect circuits from overcurrent and short circuits. They automatically trip when the current exceeds a safe limit.
  • Applications: Common in residential buildings, commercial offices, and small industrial setups.

2.7.3 Moulded Case Circuit Breaker (MCCB):

  • Working: MCCBs provide protection against overloads, short circuits, and earth faults. They can handle higher currents than MCBs and are adjustable.
  • Applications: Used in industrial and commercial electrical systems.

2.7.4 Earth Leakage Circuit Breaker (ELCB):

  • Working: ELCBs detect leakage currents to the ground and trip the circuit if the leakage exceeds a preset threshold, preventing electric shock.
  • Applications: Used in buildings to protect against electrical faults that may cause harm to personnel.

2.8 Selection of LT and HT Circuit Breakers (Ratings)

When selecting circuit breakers for both LT and HT systems, the following factors must be considered:

  • Current Rating: The circuit breaker should be rated for the maximum current that the circuit will carry.
  • Breaking Capacity: The maximum fault current that the circuit breaker can safely interrupt.
  • Voltage Rating: The maximum operating voltage that the breaker can handle.
  • Type of Protection: Whether the breaker needs to protect against overcurrent, short circuits, or earth leakage.

2.9 Selection of MCCB for Motors

When selecting MCCBs for motors, the following factors should be considered:

  • Motor Full Load Current: The MCCB should be rated slightly higher than the motor's full load current to avoid tripping during normal startup.
  • Inrush Current: Motors draw high inrush currents during startup, so the MCCB should have a delayed trip function to handle this surge.
  • Overload Protection: The MCCB should provide protection against motor overload conditions to prevent damage.
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