Switchgear and Protection
(2020503C) - Solved Exam Paper
Group (B)
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Q.2 Discuss needs of current limiting reactors.
(धारा सीमित रिएक्टर की आवश्यकताओं का वर्णन करें ।)
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A current limiting reactor is a large coil (inductor) inserted in series with a power line. Its primary need is to limit the magnitude of short-circuit current.
The needs are:
- Protect Equipment: To prevent excessive fault currents from damaging expensive equipment like circuit breakers, transformers, and busbars.
- Use Lower-Rated Breakers: By reducing the maximum fault current, it allows for the use of circuit breakers with a lower (and less expensive) rupturing capacity.
- Improve Stability: It limits the severity of the fault, which helps maintain system voltage and synchronism, thus improving overall power system stability.
- Localize Faults: It can isolate the effect of a fault to a specific section, preventing it from propagating and causing a widespread blackout.
OR (अर्थवा)
(शिरोपरी लाईन में होनेवाले दोषों का वर्णन करें |)
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Faults on overhead lines are broadly classified into two groups:
1. Symmetrical Faults (Balanced):
- Three-Phase Fault (LLL): All three phases are short-circuited together.
- Three-Phase-to-Ground Fault (LLLG): All three phases are short-circuited to the ground.
2. Unsymmetrical Faults (Unbalanced):
- Single Line-to-Ground Fault (LG): One phase conductor falls to the ground or contacts the neutral. This is the most common type of fault (70-80% of all faults).
- Line-to-Line Fault (LL): Two phase conductors touch each other.
- Double Line-to-Ground Fault (LLG): Two phase conductors touch each other and also touch the ground.
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Q.3 Explain in brief the following terms:- Making current capacity and Rate of rise of re-striking voltage.
(निम्नलिखित शर्तों की संक्षेप में व्याख्या करें-मेकिंग धारा क्षमता एवं पुनर्प्रधर वोल्टता के बढ़ने की दर।)
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1. Making Current Capacity: This is the maximum peak value of current (including any DC offset) that a circuit breaker can safely close onto during a short circuit. When a breaker closes on a fault, the first current peak is the highest it will experience due to the transient DC component. The breaker must be mechanically strong enough to withstand the immense magnetic forces and close its contacts without welding shut. It is expressed as a peak value (kA-peak).
2. Rate of Rise of Re-striking Voltage (RRRV): After a circuit breaker interrupts the current at a current zero, the voltage across its open contacts does not return to normal instantly. It undergoes a high-frequency transient oscillation, known as the "re-striking voltage". The RRRV is the speed (in V/µs) at which this voltage rises immediately after the arc is extinguished. If the RRRV is too high, the dielectric strength of the gap between the contacts may not build up fast enough, causing the arc to re-ignite (re-strike).
OR (अथवा)
(शुन्यक परिपथ विच्छेदक के संचालन सिद्धांत का वर्णन करें, इसके कुछ लाभ लिखें।)
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Principle of Operation: A Vacuum Circuit Breaker (VCB) operates on the principle that a high vacuum (around 10⁻⁶ torr) has an extremely high dielectric strength. The entire contact assembly is sealed in a vacuum chamber called a "vacuum interrupter".
When the contacts are opened to interrupt a fault, an arc is drawn between them. This arc is formed from metal vapor boiled off the contact surfaces. As the AC current approaches a natural zero, the arc extinguishes.
The vacuum provides two key benefits:- There is no gas to ionize, so the arc is only sustained by the metal vapor.
- At current zero, the metal vapor disperses and condenses on the contacts and shielding in microseconds.
Advantages:
- High Reliability & Long Life: No arc by-products to degrade contacts.
- Minimal Maintenance: The interrupter is sealed for life.
- Fast Operation: Very quick dielectric recovery.
- Environmentally Friendly: No flammable oil or greenhouse gases (like SF6).
- Quiet Operation: No loud blast of air or gas.
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Q.4 Discuss the characteristics of protective relay.
(सुरक्षात्मक रिले की विशेषताओं का वर्णन करें ।)
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The main characteristics of a good protective relay are:
- 1. Selectivity (or Discrimination): The ability to distinguish between a fault in the protected zone and a fault outside of it. It must only operate for faults within its intended zone, leaving other healthy parts of the system operational.
- 2. Speed: The ability to operate as fast as possible. Fast fault clearing minimizes damage to equipment, reduces stress on the system, and prevents loss of synchronism.
- 3. Sensitivity: The ability to detect even the smallest fault currents within its protected zone. The relay must be sensitive enough to operate reliably at its minimum fault condition.
- 4. Reliability: The ability to operate correctly and dependably when a fault occurs. It must not fail to operate (dependability) and must not operate incorrectly when there is no fault (security).
- 5. Simplicity: The relay should be as simple in its design and construction as possible. Simplicity makes it more reliable and easier to maintain.
- 6. Economy: The relay should provide maximum protection at a minimum cost, balancing the cost of the relay against the cost of the equipment it protects.
OR (अथवा)
(दिशात्मक रिले की आवश्यकता एवं संचालन को संक्षेप में व्याख्या करें ।)
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Need: A simple overcurrent relay operates whenever the current exceeds its setting, regardless of the direction of power flow. This is not suitable for complex networks like parallel feeders or ring mains.
In such systems, a fault at one point can be fed from two or more directions. A directional relay is needed to ensure correct selectivity. It must only operate for a fault in one specific direction (its "tripping direction") and block operation for faults in the opposite direction.Operation: A directional relay is essentially a power relay. It compares two electrical quantities:
- A Current (from a CT)
- A Reference Voltage (from a PT)
- Under normal conditions or for a fault in the "reverse" direction, the power flow (and thus the phase angle) is in one direction, and the relay does not operate.
- During a fault in the "forward" (tripping) direction, the power flow reverses, the phase angle relationship changes, and the relay operates (usually by enabling an associated overcurrent relay).
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Q.5 Explain in brief the various types of faults in conferred in transformer.
(परिणामित्र में होनेवाले विभिन्न प्रकार के दोषों को संक्षेप में व्याख्या करें ।)
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Faults in transformers can be broadly categorized as follows:
1. Internal Faults (Inside the transformer tank): These are the most dangerous as they can lead to fires and explosions.
- Winding Faults:
- Inter-turn Faults: Short circuit between turns of the same winding. Starts as a minor fault but generates intense local heat, decomposing oil and gas.
- Phase-to-Phase Faults: Short circuit between two different phase windings (e.g., R-Y).
- Phase-to-Ground Faults: Short circuit from a winding to the earthed core or tank.
- Core Faults: Breakdown of the insulation between core laminations, leading to eddy currents and severe overheating.
- Tank Faults: Oil leakage, which can lead to overheating and insulation failure.
2. External Faults (Outside the transformer): These are faults on the power system that the transformer is connected to.
- External Short Circuits: Faults (like LG, LL, LLL) on the transmission or distribution lines. These cause the transformer to carry heavy "through-fault" currents.
- Overloading: Sustained operation above the transformer's kVA rating, leading to overheating and premature aging of insulation.
OR (अथवा)
(प्रत्यावर्त्तक के भू-दोष संरक्षण का स्वच्छ आरेख के साथ वर्णन करें |)
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Earth fault protection for an alternator is crucial as most alternator faults are earth faults. A common method is the Restricted Earth Fault (REF) Protection scheme, which is a type of differential protection.
Principle: It compares the current flowing out of the alternator windings with the current flowing into the neutral.
- Normal/External Fault: The current entering the neutral (Iₙ) is exactly equal to the vector sum of the three phase currents (Iᵣ + Iᵧ + Iₛ). The differential current flowing into the relay is zero.
- Internal Earth Fault: If an earth fault occurs on a winding, the currents will be unbalanced. The current at the neutral (Iₙ) will no longer be equal to the sum of the phase currents. This difference (spill current) flows through the relay, causing it to operate.
Operation: As shown in the diagram, three CTs are placed on the phase terminals, and one CT is placed on the neutral-to-ground connection. The secondaries of all four CTs are connected in parallel to an overcurrent relay. This scheme is "restricted" to only see faults within the protected zone (from the phase terminals to the neutral).
- Winding Faults:
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Q.6 Mention the various methods of arc extinction in circuit breaker. Describe any one of them.
(परिपथ विच्छेदक में आर्क बुझाने की विभिन्न विधियों का उल्लेख करें एवं किसी एक विधि का वर्णन करें ।)
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The arc is extinguished by increasing the dielectric strength of the medium between the contacts faster than the RRRV.
Methods of Arc Extinction:
- High Resistance Method: The arc resistance is increased (by lengthening, cooling, splitting the arc) until the current is so low that it cannot maintain the arc. This is used in low-capacity AC breakers and DC breakers.
- Low Resistance (or Current Zero) Method: This is used in all modern high-power AC breakers. The arc is left to continue until the natural current zero of the AC wave. At this instant, the medium is rapidly "de-ionized" to prevent the arc from re-striking in the next half-cycle.
Description (Low Resistance Method in SF6 Breaker):
In an SF6 circuit breaker, the arc extinction is achieved by de-ionizing the arc path at current zero.
As the contacts separate, an arc is drawn in the SF6 gas. A high-pressure blast of SF6 gas is forced to flow along the arc (either axially or cross-blast).
SF6 is an electronegative gas, meaning its molecules have a high affinity for free electrons.
As the current approaches zero, the SF6 gas:- Cools the arc (excellent heat transfer).
- Captures free electrons (electronegativity), forming heavy, immobile negative ions.
OR (अथवा)
(संचरण लाईन की सुरक्षा के डिफरेंशियल पायलट तार विधि का वर्णन करें।)
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The Differential Pilot Wire Method (e.g., Merz-Price Voltage Balance) is a form of unit protection for transmission lines. It protects a specific section of the line and is not affected by faults outside this section.
Principle: It compares the current entering one end of the line with the current leaving the other end.
- Normal/External Fault: The current entering (I₁) is equal to the current leaving (I₂).
- Internal Fault: The current entering (I₁) is not equal to the current leaving (I₂), as some current is diverted to the fault.
Operation:
- Identical CTs are installed at both ends (A and B) of the transmission line.
- The secondaries of the CTs are connected to each other through a pair of cables called "pilot wires".
- A relay is connected at each end, in series with the pilot wires.
- During an external fault, I₁ and I₂ are equal. The CTs produce equal secondary voltages, which balance each other. No current flows through the relays.
- During an internal fault, I₁ and I₂ are different. The CT secondary voltages are unequal, causing a circulating current to flow through the pilot wires and operate the relays at both ends, tripping the line.
Group (C)
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Q.7 Describe the construction, working and application of HRC fuses.
(एच.आर.सी. फ्यूज की संरचना, कार्यविधि एवं उपयोगों का वर्णन करें |)
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Construction: An HRC (High Rupturing Capacity) fuse consists of:
- A heat-resistant body, usually made of ceramic.
- Two metal end-caps for connecting to the circuit.
- A fuse element (e.g., silver or copper) that connects the end-caps. The element often has several sections with "notches" to create specific melting points.
- A filling powder (e.g., quartz sand, silica) that surrounds the element.
- An indicator pin (in some types) that pops out when the fuse blows.
Working:
- Normal Operation: Current flows through the fuse element, which has low resistance and does not overheat.
- Overload/Short Circuit: The high fault current melts the fuse element at its weakest points (the notches).
- Arc Extinction: As the element melts, an arc is formed. The intense heat of the arc melts the quartz sand, turning it into a high-resistance glassy substance. This substance effectively quenches the arc by cooling it and absorbing its energy, thus interrupting the circuit. This entire process is extremely fast (within milliseconds).
Application: HRC fuses are used for backup protection against high short-circuit currents in:
- Low and high voltage distribution systems.
- Motor protection (as backup for a thermal relay).
- Capacitor banks and transformer protection.
OR (अथवा)
(गैस इन्सुलेटेड स्विच गियर का वर्णन करें ।)
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Gas Insulated Switchgear (GIS) is a type of compact, metal-enclosed switchgear. In GIS, all major components – including circuit breakers, busbars, isolators, and current/voltage transformers – are housed inside a sealed, earthed metal enclosure. This enclosure is filled with Sulphur Hexafluoride (SF6) gas under pressure.
Key Features & Principles:
- Insulation: SF6 gas is used as the primary insulating medium between live parts and the earthed metal enclosure. SF6 has a dielectric strength about 2-3 times that of air.
- Arc Quenching: SF6 gas is also used as the arc extinction medium inside the circuit breaker component, as it is highly electronegative and a superb arc quencher.
- Compactness: Because SF6 is such a good insulator, the physical distances between components can be drastically reduced. This makes GIS up to 10 times smaller than conventional Air Insulated Switchgear (AIS).
- Modularity: GIS is built in standardized, factory-assembled modules (e.g., a circuit breaker module, a busbar module), which are then connected on-site.
Advantages:
- Huge space saving: Ideal for cities, underground substations, or offshore platforms.
- High Reliability & Safety: All live parts are protected from the environment (pollution, moisture, animals), reducing faults and increasing operator safety.
- Low Maintenance: The sealed-for-life design requires very little maintenance.
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Q.8 Explain with the help of neat diagram the construction and working of Buchholz relay.
(बुकहोल्ज रिले कि संरचना एवं कार्यप्रणाली का स्वच्छ आरेख की सहायता से व्याख्या करें ।)
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A Buchholz relay is a gas and oil-actuated safety device mounted in the pipe connecting the main tank of a transformer to its conservator tank. It protects against all types of internal faults.
Construction: It consists of an oil-filled chamber. Inside, there are two hinged floats (or buckets):
- An Upper Float (Alarm): This float operates a mercury switch connected to an alarm circuit.
- A Lower Float (Trip): This float (along with a baffle flap) operates a mercury switch connected to the transformer's trip circuit.
Working: The relay operates in two ways:
- Incipient (Slow) Faults: (e.g., inter-turn faults, core overheating). These faults cause slow decomposition of the transformer oil, generating gas. The gas bubbles rise, travel towards the conservator, and get trapped in the relay's upper chamber. As gas accumulates, the oil level in the chamber drops, causing the Upper Float to tilt and close the ALARM circuit.
- Severe (Sudden) Faults: (e.g., heavy short circuits, bushing flashover). These faults cause a rapid generation of gas and a violent surge of oil from the main tank to the conservator. This oil surge strikes the Lower Float and baffle, causing it to tilt and close the TRIP circuit, which instantly disconnects the transformer.
OR (अथवा)
(एक स्वच्छ आरेख की सहायता से प्रत्यावर्त्तक के लिए मर्ज-प्राइज संरक्षण का वर्णन करें ।)
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Merz-Price Protection (or Percentage Differential Protection) is the primary protection scheme for alternator windings against phase-to-phase and phase-to-ground faults.
Principle: It is a differential protection scheme. It compares the current entering one end of a winding with the current leaving the other end.
- Normal/External Fault: The current entering (I₁) is equal to the current leaving (I₂). The relay does not operate.
- Internal Fault: The current entering (I₁) is not equal to the current leaving (I₂). This difference (I₁ - I₂) flows through the relay, causing it to operate.
Construction & Working: As shown in the diagram (for one phase):
- Two identical CTs (CT1 and CT2) are placed at opposite ends of the alternator winding.
- The secondaries of the CTs are connected in series opposition.
- An overcurrent relay (the "Operating Coil") is connected to the midpoint of the CT secondary circuit.
- During normal operation or an external fault, the secondary currents (i₁ and i₂) are equal and circulate between the two CTs. No current flows through the operating coil (i₁ - i₂ = 0).
- During an internal fault, I₁ and I₂ become unequal (or flow in opposite directions). This unbalances the secondary currents (i₁ and i₂). The spill current (i₁ - i₂) flows through the operating coil, causing the relay to trip the alternator.
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Q.9 Describe the construction and working of SF6 circuit breaker.
(SF6 परिपथ विच्छेदक के संरचना एवं कार्यप्रणाली का वर्णन करें ।)
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Construction: An SF6 circuit breaker consists of a sealed interrupter chamber filled with SF6 gas. The main parts are:
- Interrupter Chamber: A sealed vessel containing the contacts, arc quenching nozzle, and SF6 gas.
- Contacts: A pair of fixed and moving contacts. The moving contact is a hollow cylinder, while the fixed contact is a solid rod. Both have arc-resistant tips (copper-tungsten).
- Arc Quenching Nozzle: A special nozzle (often made of PTFE) that directs the flow of SF6 gas onto the arc.
- Gas System: In "puffer" type breakers, a moving cylinder and a fixed piston are used to compress the gas during the opening operation, creating the high-pressure blast.
- Operating Mechanism: A high-speed mechanism (e.g., spring-operated) to separate the contacts rapidly.
Working:
- Closed Position: The contacts are closed, and current flows normally. The chamber is filled with SF6 gas at a moderate pressure.
- Opening (Arcing): When a fault is detected, the operating mechanism pulls the moving contact. An arc is drawn between the fixed and moving contacts. As the moving contact (a hollow cylinder) moves, it slides over a fixed piston, compressing the SF6 gas in a "puffer" cylinder.
- Arc Extinction: This highly compressed SF6 gas is forced at high velocity through the nozzle and flows axially along the arc. The gas cools the arc and, being highly electronegative, it captures the free electrons. At the natural current zero, the arc path is rapidly de-ionized and its dielectric strength builds up, extinguishing the arc.
- Open Position: The contacts are fully separated, insulated by the high-pressure SF6 gas.
OR (अथवा)
(न्यूनतम तेल परिपथ विच्छेदक की संरचना एवं सिद्धांत का वर्णन करें )
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A Miniature Circuit Breaker (MCB) is an automatic switch that protects a low-voltage circuit from damage caused by overloads and short circuits. It is a modern replacement for a fuse.
Construction: An MCB consists of:
- A molded case (plastic housing).
- An operating lever (switch) to manually turn it ON or OFF.
- A latching mechanism that holds the contacts closed.
- A pair of fixed and moving contacts.
- An Arc Chute (or arc splitter) to extinguish the arc.
- Two trip mechanisms:
- A Bimetallic Strip for overload protection.
- A Solenoid (Electromagnet) for short-circuit protection.
Working Principle: The MCB has two modes of tripping:
- Overload Protection (Thermal): During a sustained overload (e.g., 15A in a 10A circuit), the current flows through the bimetallic strip. The strip heats up, bends, and after a time delay, it pushes the latching mechanism, tripping the breaker. This delay prevents tripping from harmless inrush currents.
- Short-Circuit Protection (Magnetic): During a short circuit, a very high current (e.g., 100A) flows. This current instantly energizes the solenoid, which creates a strong magnetic field. The solenoid's plunger strikes the latching mechanism, tripping the breaker instantaneously (within milliseconds) to prevent damage.
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Q.10 Describe different faults on bus bar and transmission lines.
(संचारणा लाईन एवं बस-बार के विभिन्न दोषों का वर्णन करें ।)
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Faults in both locations are classified similarly, but their causes differ.
1. Faults on Transmission Lines: These are very common due to the long, exposed nature of the lines.
- Unsymmetrical Faults:
- Single Line-to-Ground (LG): Most common (70-80%). Caused by insulator failure, lightning, or a conductor falling to the ground.
- Line-to-Line (LL): Caused by high winds causing conductors to swing and touch.
- Double Line-to-Ground (LLG): A conductor breaks and falls, contacting another conductor and the ground.
- Symmetrical Faults (LLL or LLLG): Rare, but severe. Can be caused by a tower collapsing or all three phases coming into contact.
2. Faults on Busbars: These are very rare but extremely severe because a busbar is the central connection point for many circuits. A busbar fault can cause a total substation blackout.
- Single Line-to-Ground (LG): Most common bus fault. Caused by failure of a support insulator due to moisture or pollution, or an animal bridging the gap.
- Phase-to-Phase (LL): Caused by insulator failure or a foreign object (e.g., a tool) being dropped across two busbars.
- Three-Phase (LLL): The most severe, can be caused by mechanical failure or an isolator being operated under load, causing a flashover.
OR (अथवा)
(निम्नलिखित टर्म की व्याख्या करें- अ) रिले टाइम ब) रिसेट धारा स) प्लग सेटिंग मल्टीप्लाइर)
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a) Relay Time: This is the time delay between the instant the actuating quantity (e.g., fault current) exceeds the relay's pickup value and the instant the relay's contacts close to send a trip signal to the circuit breaker. For an instantaneous relay, this time is very short (milliseconds). For a time-delay relay, this is the intentionally set time (e.g., 0.5 seconds).
b) Reset Current: This is the value of the actuating quantity (e.g., current) at which the relay contacts return to their normal (open) position after operating. It is always lower than the pickup (operating) current. The "Reset Ratio" (Reset Current / Pickup Current) is always less than 1, preventing the relay from "chattering" or oscillating near the pickup value.
c) Plug Setting Multiplier (PSM): This is a term used for electromagnetic overcurrent relays. It defines the severity of the fault relative to the relay's setting.
PSM = (Actual Fault Current) / (Relay Pickup Current)
The "Pickup Current" is determined by the Plug Setting (e.g., 50%, 100%, etc.) and the CT secondary rating (e.g., 5A).
Example: If CT is 100/5A and Plug Setting is 50%, the Pickup Current = 50% of 5A = 2.5A. If the fault current is 25A, then PSM = 25A / 2.5A = 10.
The PSM is used to find the relay's operating time from its time-current curve. - Unsymmetrical Faults:
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Q.11 Describe construction and working operation of electromagnetic relay.
(विद्युत चुम्बकीय रिले की संरचना एवं कार्य की विधि का वर्णन करें ।)
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An electromagnetic relay operates on the principle of magnetic attraction or induction. A common type is the "Attracted Armature" relay.
Construction:
- An Electromagnet: A soft iron core wrapped with a coil. This coil is the "actuating" coil, connected to the secondary of a CT.
- An Armature: A movable piece of iron, hinged and held open by a restraining spring.
- Contacts: A set of fixed and moving contacts. The moving contacts are attached to the armature. These are connected to the circuit breaker's trip circuit.
Working Operation:
- Normal Condition: The normal operating current flowing through the coil is insufficient to create a strong magnetic field. The restraining force of the spring is greater than the magnetic force, so the armature stays open, and the trip contacts are also open.
- Fault Condition: During a fault, a large current flows through the coil. This produces a very strong magnetic field. The magnetic attractive force on the armature becomes greater than the spring's restraining force.
- Tripping: The electromagnet pulls the armature towards it. This movement causes the moving contacts to close, completing the trip circuit. This sends a signal to the circuit breaker, which then opens and clears the fault.
OR (अथवा)
(निम्नलिखित में से किन्हीं दो पर संक्षिप्त टिप्पणियाँ लिखें- अ) एयर परिपथ विच्छेदक ब) थर्मल रिले स) सममित दोष )
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a) Air Circuit Breaker (ACB): An Air Circuit Breaker is a circuit breaker that operates in air at atmospheric pressure. Its primary use is in low-voltage applications (below 1000V), such as in large industrial plants and commercial buildings.
Its arc extinction method is high resistance. When the contacts separate, the arc is drawn into a set of "arc chutes." These arc chutes are insulated metal plates that split, lengthen, and cool the arc. This rapidly increases the arc's resistance, causing it to be extinguished.
A similar type, the Air-Blast Circuit Breaker (used at high voltages), uses a high-pressure blast of compressed air to extinguish the arc (a "current zero" method).b) Thermal Relay: A thermal relay is a protective device that protects equipment (mainly motors) from overload conditions. It operates based on the heating effect of the current.
It contains a bimetallic strip. During an overload, the high current heats this strip. Because the strip is made of two metals with different thermal expansion rates, it bends. After a certain time delay (proportional to the current), this bending action mechanically triggers a latch, opening a set of contacts and tripping the motor starter. This time delay is its key feature, allowing it to "ride through" the temporary high current of a motor starting, but tripping on a sustained overload.c) Symmetrical Fault: A symmetrical fault is a fault in a three-phase power system where all three phases are affected equally. The system remains balanced (though short-circuited).
The two types are:- Three-Phase Line-to-Line (LLL): All three conductors are shorted together.
- Three-Phase to Ground (LLLG): All three conductors are shorted together and to ground.
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