Circuit Breakers and Fuses - Protection Part 4 PDF

Circuit Breakers Introduction to Circuit Breakers A circuit breaker is an essential safety device in electrical systems. Its primary function is to protect electrical circuits from damage caused by overloads or short circuits by interrupting the flow of current when a fault is detected. They not onl...

Circuit Breakers Introduction to Circuit Breakers A circuit breaker is an essential safety device in electrical systems. Its primary function is to protect electrical circuits from damage caused by overloads or short circuits by interrupting the flow of current when a fault is detected. They not only protect equipment and lives but also ensure uninterrupted power supply by isolating faults efficiently. Understanding their operation, types, and maintenance is vital for engineers and electricians working in the field. Also, circuit breakers play a critical role in maintaining the safety and reliability of power distribution systems. Why Do We Need Circuit Breakers? 1. Protection Against Overloads: When too much current flows through a circuit, the wires can overheat, leading to insulation damage and fire hazards. Circuit breakers disconnect the power supply to prevent this. 2. Protection Against Short Circuits: Short circuits occur when live and neutral wires accidentally come into contact, causing a high current flow. Circuit breakers quickly cut the current to prevent system damage. 3. Ensuring System Reliability: By isolating only the faulty part of the system, circuit breakers ensure that the rest of the electrical system continues to function. Role of Circuit Breakers in Power Systems 1. Fault Isolation: Circuit breakers disconnect faulty sections of the power system, preventing cascading failures that could lead to widespread outages. (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.)  2. System Stability: By quickly removing faults, circuit breakers help maintain voltage stability and frequency balance in the power system. 3. Protection of Expensive Equipment: High-voltage transformers, generators, and switchgear are critical and costly components. Circuit breakers ensure these are protected from damage due to high current or fault conditions. 4. Integration of Renewable Energy: As grids increasingly incorporate renewable energy sources, circuit breakers enable smooth integration by handling varying loads and faults. Basic Components of a Circuit Breaker 1. Frame: The external casing that provides structural support and protection. 2. Contacts: Conductive elements that allow or interrupt the current flow. 3. Arc Extinguishing Mechanism: Suppresses the electric arc formed when the breaker interrupts the circuit. Operating Mechanism: Activates or deactivates the contacts manually or automatically. 4. Trip Unit: Senses fault conditions and triggers the breaker to operate. Types of Circuit Breakers Circuit breakers are categorized based on voltage, structure, and operating mechanisms. Here are the main types: 1. Based on Voltage Level Low Voltage Circuit Breakers (LV): Used in residential and commercial applications.  - Miniature Circuit Breakers (MCB): Compact and used for low-current circuits. - MCB rated current not more than 100A - MCB is used for 1-phase and 3-phase applications - Rated current :International Standard IEC (at ambient air temperature of 30 °C) are: (6,10,13,16,20,25,32,40,50,63,80 and 100) Amperes. Note: -The circuit breaker is labeled with the rated current in ampere, but without the unit symbol "A". Instead, the ampere figure is preceded by a letter "B", "C”, "D“, “K" or "Z" that indicates the instantaneous tripping current (the minimum value of current that causes the circuit-breaker to trip ) without intentional time delay (i.e., in less than 100 ms): Note : In=Nominal current Molded Case Circuit Breakers (MCCB): Handles higher currents and can be adjusted for specific requirements. Thermal or thermal-magnetic operation. Tripping current may be adjustable. Mainly is used for 3-phase circuits and for currents larger than 100A and up to 1600A. Mainly it is used in industrial applications to protect cables and equipment. Types of these C.Bs are shown below Dr 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. 2. Based on Operating Mechanism Magnetic circuit breakers: Based on a solenoid (electromagnet) whose pulling force increases with the current.  The contacts are held closed, in case of SC or overload the current in the solenoid increases beyond the rating of the CB, the solenoid's pull and releases the latch. This allows the contacts to open by spring action. Thermal-Magnetic Circuit Breakers: Combines thermal sensing for overloads and magnetic sensing for short circuits. Thermal breakers use a bimetallic strip, which heats and bends with increased current, and releases the latch. This type is commonly used with motor control circuits Electronic Circuit Breakers: Use microprocessors for precise fault detection.  Other types of circuit breakers: Breakers for protections against earth faults to trip an overcurrent device such as: Residual Current Device (RCD) or Residual Current Circuit Breaker(RCCB) :- Disconnecting a circuit when detecting the electric current is not balanced between the phase conductor and the neutral conductor. Does NOT provide over-current protection. sometimes caused by current leakage through the body of a person who is grounded and accidentally touching the energized part of the circuit. Residual Current Breaker with Over-current protection (RCBO) combines the functions of an RCD and an MCB in one package. Earth leakage circuit breaker (ELCB) This detects earth current directly rather than detecting imbalance 3. Types of Circuit Breakers in Power Systems In power systems, circuit breakers are categorized based on their arc extinguishing medium and operating voltage: 1. Air Circuit Breakers (ACB): Commonly used in low- and medium-voltage applications. They use compressed air to extinguish arcs. Tripping current is adjustable This type of circuit breaker is used for very large current applications up to 6000A. Use case: Industrial facilities with medium-voltage power distribution.) 2. Vacuum Circuit Breakers (VCB): Extinguish arcs in a vacuum chamber, which has excellent insulating properties and long service life. These are ideal for medium-voltage systems.  These circuit breakers operate on a different principle from other breakers because:- -There is no gas to ionize when the contacts open. -They are silent and never become polluted -Several circuit breakers are connected in series. Their interrupting capacity is limited to about 30 kV for higher voltages and is often used in underground distribution Advantages: Free from arc and fire hazards. Low cost for maintenance & simpler mechanism. Low arcing time & high contact life. Silent and less vibration operation. Due to vacuum contacts remain free from corrosion. Disadvantages: High initial cost due to creation of vacuum. High cost and size required for high voltage breakers. (Use case: Medium-voltage substations or industrial power grids.) 3. SF6 Circuit Breakers: Use sulfur hexafluoride (SF6) gas, a highly effective arc-quenching medium, and are widely used in high-voltage applications. Several characteristics of SF6 circuit breakers Simply of the interrupting, does not need an auxiliary breaking chamber. Higher performance, up to 63 kA, with a reduced number of interrupting chambers. Possible compact solutions when used for GIS (gas insulation switchgear) or hybrid switchgear. Used in synchronized operations to reduce switching over-voltages Advantages Very short arcing period due to superior arc. Much larger currents as compared to other breakers. No risk of fire. Low maintenance, less size. No over voltage problem. No carbon deposits. Reliability and availability Low noise levels. Disadvantages Costly due to the high cost of SF6. SF6 gas must be reconditioned after every operation of the breaking. Additional equipment is required for this purpose (Use case: Transmission substations in power grids.) 4. Oil Circuit Breakers: Use insulation oil to extinguish the arc. While effective, they are being phased out in favor of SF6 and vacuum technologies due to environmental concerns. Bulk oil CB: Composed of a steel tank filled with insulating oil (for electric arc). For three phase there are thee movable contacts, actuated simultaneously by an insulated rod, open and close circuit.  When the circuit breaker is closed, the line current for each phase flows through the fixed contact. If high current is detected the tripping coil releases a powerful spring that pulls on the insulated rod causing the contacts to open. Minimum Oil Circuit breaker These circuit breakers contain minimum quantity of oil. The three phases are separated into three chambers. Advantages of Oil circuit breakers: Oil has good dielectric strength. Low cost. Oil is easily available. It has wide range of breaking capability. Disadvantages of Oil circuit breakers: Slower operation, takes about 20 cycles. It is highly risk of fire. High maintenance cost 5. Hybrid Circuit Breakers: Combine different technologies (e.g., gas and vacuum) for enhanced performance in high-voltage applications. Working Principle of Circuit Breakers 1. Normal Operation Under normal conditions, the circuit breaker allows electrical current to flow through it without interruption. This is achieved via its closed contacts, which provide a continuous conductive path. Key Components in Normal Operation: Contacts: These remain in a closed position, facilitating current flow. Trip Unit: Monitors the current to ensure it stays within safe operational limits. Housing and Insulation: Provide protection and ensure safety. Functionality: The trip unit remains inactive since no abnormal conditions are detected. 2. Fault Detection When an abnormal condition such as an overload, short circuit, or ground fault occurs, the circuit breaker’s trip unit detects this anomaly. How Faults Are Detected: Thermal Overloads: Excessive current causes a bimetallic strip to bend due to heat, triggering the breaker to trip. Magnetic Overloads: In a short circuit, the high surge of current generates a magnetic field strong enough to activate the trip mechanism. Electronic Trip Units: Modern breakers use sensors and microprocessors to detect current abnormalities and issue a trip signal. Reaction Time: Detection is typically instantaneous in the case of short circuits, while overloads might take longer to allow for temporary surges. 3. Interruption Once a fault is detected, the breaker trips, which separates its internal contacts to interrupt the current flow. Arc Formation and Extinction: As the contacts separate, an electrical arc forms due to the high voltage. The arc is dangerous and needs to be extinguished quickly to avoid damage. Arc Extinguishing Mechanisms: When the breaker trips, separating the contacts causes an electric arc. This arc must be extinguished efficiently to ensure safety. Techniques include: Air Blast: Forces air through the arc. Vacuum Interruption: Contains and extinguishes the arc in a vacuum chamber. Oil Breakers: Submerges contacts in oil to dissipate the arc. SF6 Gas: Uses sulfur hexafluoride gas to cool and quench the arc. Breaking Speed: The mechanism operates within milliseconds to ensure safety. 4. Manual or Automatic Reset After the fault is addressed, the circuit breaker must be reset to restore the electrical supply. This can be done: Manually: By physically turning the breaker back to its closed position. Automatically: In systems with an automatic resetting mechanism, the breaker resets after confirming that conditions have returned to normal. Testing After Reset: It's critical to ensure the fault has been resolved to avoid re-tripping. Operational Requirements of Circuit Breakers in Power Systems Circuit breakers in power systems must meet stringent operational demands: 1. High Breaking Capacity: Must handle large fault currents that can occur in high-voltage systems. 2. Rapid Operation: Faults must be cleared within milliseconds to prevent equipment damage and ensure system stability. 3. Remote Operation and Automation: Modern circuit breakers can be operated remotely, often integrated with SCADA (Supervisory Control and Data Acquisition) systems for real- time monitoring and control. 4. Selective Tripping: Circuit breakers are designed to isolate only the faulty section of the grid, leaving the rest operational. Advantages of Circuit Breakers Faster operation compared to fuses. It can be reset and reused.  Provide both manual and automatic control. Offer adjustable settings for specific requirements. Testing and Maintenance 1. Routine Inspections: Routine inspections help ensure the physical integrity and readiness of the circuit breaker. Tasks Involved: Visual Checks: Look for cracks, discoloration, or signs of overheating on the breaker’s housing. Contact Points: Inspect connections for tightness and any signs of corrosion or looseness that may cause arcing or resistance. Wiring and Insulation: Check for damaged or degraded wires or insulation. Labeling and Markings: Verify that all labels are intact and legible to avoid misidentification. Frequency: Typically performed annually for residential breakers and more frequently in industrial settings, depending on environmental factors like dust, moisture, and vibration. 2. Operational Testing: Operational tests simulate fault conditions to ensure the breaker trips properly and protects the system. Methods Used: Primary Injection Testing: Real current is injected through the breaker to simulate actual fault conditions. Verifies the overall functionality of the breaker and its trip unit.  Secondary Injection Testing: Simulates faults electronically by injecting signals into the trip unit without engaging the main contacts. Faster and safer for verifying electronic trip mechanisms. Manual Tripping: Manually trigger the breaker using the test button or mechanism to ensure the tripping system responds correctly. Key Checks During Testing: Trip time and accuracy for different fault conditions (overload, short circuit, etc.). Reliability of resetting mechanisms after tripping. 3. Arc Chamber Cleaning: The arc chamber is a critical part of the breaker that extinguishes the electrical arc during tripping. Maintaining it ensures reliable operation. Steps for Cleaning: Disassemble with Care: Open the breaker causing to access the arc chamber without damaging the internal components. Remove Residue: Use a soft brush or compressed air to remove carbon deposits, dust, or debris. Avoid abrasive tools that may damage the chamber lining. Inspect Components: Check for signs of wear or pitting on the arc contacts and chamber materials. Replace it if necessary. Reassemble Securely: Ensure all components are returned to their correct positions and tightened appropriately.  Frequency: It should be done periodically based on the number of operations or environmental conditions. Industrial breakers may require more frequent cleaning due to higher usage. Additional Maintenance Practices Lubrication of Moving Parts: Apply appropriate lubricant to hinges, latches, and other moving components to reduce wear and ensure smooth operation. Insulation Resistance Testing: Test the insulation of the breaker using a megohmmeter to verify that there are no leakage paths or deteriorations. Thermal Imaging: Use infrared cameras to identify hotspots that could indicate loose connections or overloaded components. Calibration of Trip Units: Periodically, it calibrates electronic trip units to maintain accuracy in fault detection. Maintenance Frequency Routine Inspections: Monthly to annually, depending on the operating environment. Detailed Testing and Maintenance: Every 1-3 years or after a major fault or tripping event.  Fuses Introduction Fuse Is advice used to protect the circuits and equipment’s against overload and short circuits. Fuse element materials have low melting point, high conductivity. Fuse operation based on the heating effect of the current when flows through element. In normal operation the heat dissipated rapidly into the surrounding air and element remains below its melting point. When a fault occurs in the electric circuit, the current exceeds the limiting value and cannot be dissipated heat is fast enough, and elements melts and breaks the circuit. Its rating can start from a few mA to several kA. Many forms and shapes depending on its application. Fuse has inverse time current characteristics. Low voltage fuses 1- Rewirable Fuse (made from porcelain or ceramic insulator). Advantages: Low cost, wire may be easily available. Disadvantages: wrong size of wire to be fitted in fuse cause wrong operation at high current circuit which may be dangerous for the circuit protected and not adequate for electrical arc extinguish. 2. Cartridge Fuse: Advantages: The wire is enclosed in a cartridge-type container. The wrong size of fuse cannot be fitted since it with different sizes for different current. The fuse wire does not deteriorate and is more reliable in operation 3- High Rupturing Capacity (H.R.C) It is a cartridge-type with silver element connected between two end-contacts of a ceramic tube filled with a special quartz powder. This type of fuse is very reliable in performance and does not deteriorate and has a high speed. When the fuse blows are a silver (which rapidly melt) produced with the filling powder high- resistance material are formed in of operation and the path of the arc, causing it to be extinguished. ability to safely interrupt short circuit currents of much higher values (higher rupturing capacity). Fusing Factor: Fusing factor is the smallest current that will cause the fuse element to melt. Fusing factor is the ratio of a fuse's minimum fusing current. This may varying between 1.25 and 2.5 times the current rating, The current rating is the nominal rated current in Amps marked on the fuse body that the fuse will carry continuously without deteriorating. 𝑅𝑎𝑡𝑒𝑑 𝑚𝑖𝑛𝑖𝑚𝑢𝑚 𝑓𝑢𝑠𝑖𝑛𝑔 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 Fusing factor = 𝐶𝑢𝑟𝑟𝑒𝑛𝑡 𝑟𝑎𝑡𝑖𝑛𝑔 Classification of LV fuses: There are four classes, depending on fusing factors 1- Class P: Having a fusing factor of 1.25 or less This class is to protect circuits with small overloads. 2-Class Q fuses: Used for circuits with small over-currents and higher values of overload. And it’s divided into two classes Class Q1—fusing factor between 1.25 and 1.5 Class Q2—fusing factor between 1.5 and 1.75  3- Class R fuses: Used to protected a circuit against large overcurrent’s only. (Mainly is back-up protection) 4- Class R- Fusing factor between 1.75 and 2.5 Example : What is the minimum rated current of the 20A Q2 fuse that will be operated? Rated minimum fusing current= Fusing factor * Current rating = 20AX1.5=30A or = 20AX1.75=35A minimum fusing current rated to operate is between 30 and 35Amps Fuse symbols: High Voltage Fuses Same characteristics and operation as LV fuse but differing in size and shape