Circuit Breakers Overview

Circuit Breakers

• Circuit breakers are safety devices in electrical systems, protecting circuits from damage caused by overloads or short circuits.
• They interrupt current flow when a fault is detected, preventing damage and fire hazards.
• Circuit breakers ensure uninterrupted power supply by efficiently isolating faults.
• They play a crucial role in maintaining safety and reliability of power distribution systems.

Protection Against Overloads

• When excessive current flows through a circuit, wires overheat leading to insulation damage and fire hazards.
• Circuit breakers help by disconnecting the power supply to prevent these issues.

Protection Against Short Circuits

• Short Circuits occur when live and neutral wires accidentally touch, causing a high current flow.
• Circuit breakers promptly interrupt the current to prevent system damage.

Ensuring System Reliability

• By isolating the faulty part of the system, circuit breakers ensure the rest of the electrical system continues operating.

Role of Circuit Breakers in Power Systems

• Fault Isolation: Circuit breakers disconnect faulty sections of the power system preventing widespread outages by isolating faults.
  • Example: In a power grid if a fault occurs in a transmission line, circuit breakers at both ends of the line trip to isolate the issue.
• System Stability: Circuit breakers aid in maintaining voltage stability and frequency balance in the power system.
• Protection of Expensive Equipment: Circuit breakers safeguard transformers, generators, and switchgear from damage caused by high current or fault conditions.
• Integration of Renewable Energy: Smooth integration of renewable energy sources into grids is aided by circuit breakers which can handle fluctuating loads and faults.

Basic Components of a Circuit Breaker

• Frame: Provides structural support and protection for the circuit breaker.
• Contacts: Conductive elements that allow or interrupt current flow.
• Arc Extinguishing Mechanism: Suppresses the electric arc formed when current is interrupted within the circuit.
• Trip Unit: Detects fault conditions and triggers the breaker to operate.

Types of Circuit Breakers

• Based on Voltage Level:
  • Low Voltage Circuit Breakers (LV): Used in residential and commercial applications.
  • Medium Voltage Circuit Breakers (MV): Used in industrial facilities and substations.
  • High Voltage Circuit Breakers (HV): Used in large power grids to control very high voltages.
• Based on Operating Mechanism:
  • Magnetic circuit breakers operate based on a solenoid (electromagnet) with pulling force increasing with current.

Miniature Circuit Breakers (MCB)

• Compact circuit breakers used for low current circuits.
• Rated current typically not more than 100A.
• Suitable for 1-phase and 3-phase applications.
• Uses International Standard IEC rated currents (6, 10, 13, 16, 20, 25, 32, 40, 50, 63, 80 and 100 Amperes).
• Different trip types (B, C, D, K, Z) correspond to instantaneous tripping currents.

Molded Case Circuit Breakers (MCCB)

• Handles higher currents, adjustable for specific requirements.
• Thermal or thermal-magnetic operation, with adjustable tripping currents.
• Typically used in 3-phase circuits for currents from 100A up to 1600A
• Commonly used in industrial applications to protect cables and equipment.

Medium Voltage Circuit Breakers (MV)

• Used in industrial facilities and substations.
• Typically used for controlling medium voltages.

High Voltage Circuit Breakers (HV)

• Used in large power grids for controlling very high voltages.
• Used commonly for large power grid systems.

Thermal-Magnetic Circuit Breakers

• Combines thermal sensing for overloads and magnetic sensing for short circuits
• Using a bimetallic strip which heats and bends with increasing current and releases the latch.

Electronic Circuit Breakers

• Employ microprocessors for precise fault detection.

Other Types

• Residual Current Device (RCD) or Residual Current Circuit Breaker (RCCB): Disconnects a circuit when detecting unbalanced currents (e.g., ground fault).
• Residual Current Breaker with Over-current protection (RCBO): Combines RCD and MCB functions.
• Earth leakage circuit breaker (ELCB): Detects earth current rather than imbalance.
• Air Circuit Breakers (ACB): Used in low and medium-voltage applications using compressed air to extinguish arcs.
• Vacuum Circuit Breakers (VCB): Used for medium voltage applications, using a vacuum chamber to extinguish arcs, excelling in insulation properties and long service life.
• SF6 Circuit Breakers: Use sulfur hexafluoride gas (SF6) as an arc-quenching medium in high-voltage applications; more compact and performant, higher current ratings.
• Oil Circuit Breakers: Use insulation oil to extinguish arcs but are declining in use due to environmental concerns.

Operational Requirements of Circuit Breakers

• High Breaking Capacity: Must handle large fault currents in high-voltage systems.
• Rapid Operation: Must clear faults within a short time (milliseconds) to prevent equipment damage.
• Remote Operation and Automation: Enable remote operation and integration with SCADA systems.
• Selective Tripping: Isolate only the faulty section of the grid, ensuring continued operation of the non-faulty parts.

Circuit Breaker Testing and Maintenance

• Routine Inspection: Visual checks for cracks, corrosion, overheating, loose connections, damaged wires, etc.
• Operational Testing: Simulates fault conditions to ensure proper tripping.
  • Primary Injection Testing
  • Secondary Injection Testing
• Maintenance Frequency: Varies depending on usage, ranging from monthly to annually or after critical events. -Lubrication of Moving Parts.
  • Insulation Resistance Testing.
  • Thermal Imaging.
  • Calibration of Trip Units.

Fuses

• Fuses are protective devices used to prevent circuit overload and short circuits.
• Fuse element material has a low melting point and high conductivity.
• Overload or short circuit causes current exceeding limiting value.
• Fuse element melts and breaks the circuit, protecting the connected circuits and equipment.
• Fuse element is rated by current capacity.
• Fuse has inverse time current characteristics, meaning it blows faster with higher currents allowing for control of short-lived faults.
• Types include Rewirable and Cartridge fuses, both rated by current.
• High Rupture Capacity (HRC) cartridge fuses are typically used for higher current applications due to their reliability, speed, and ability for higher rupturing capacity.
• Fusing factor is the ratio of minimum fusing current to the rating of the fuse.
  • This is important for operational decisions of which current ratings to use for fuses based on anticipated loads and risk tolerance.
• Different classes of fuses cater to specific applications and operating conditions.