Switchgear Protections — Part 1 MCQs with Explanations

The induction cup relay operates based on the principle of electromagnetic induction, which produces torque in the relay mechanism. Current: Changes in current generate a magnetic field, influencing the operation of the relay. Voltage: Variations in voltage also affect the electromagnetic field and contribute to the relay's operation. Impedance: As impedance changes, it alters the relationship between current and voltage, impacting the electromagnetic torque and thus the relay's activation. Since the relay responds to changes in current, voltage, and impedance, all of the above factors contribute to its operation. Therefore, Option D is the correct answer.

The correct answer is Option B: arc extinguishers . Let's break down why: What is a circuit breaker? A circuit breaker is a safety device in electrical systems. Its main job is to interrupt the flow of current when a fault (like a short circuit or overload) is detected. Why it's not the other options: Option A: current carrying contacts called electrodes - While circuit breakers *do* have contacts that carry current, that's not their defining characteristic or primary function. Option C: circuits to break the system - This is too general. Circuit breakers are *part* of a circuit and *cause* the circuit to break under fault conditions. It's not that they are an entire circuit designed to break the system. Option D: transformers to isolate the two systems - Transformers isolate circuits using electromagnetic induction. Circuit breakers break circuits physically. These are completely different things. Option E: any of the above - Since A, C, and D are not the primary functions, this is incorrect. Why Option B is correct: When a circuit breaker opens, it creates an arc (a bright, hot electrical discharge) between the separating contacts. If this arc isn't controlled, it can damage the breaker and surrounding equipment, and potentially re-ignite the circuit. Therefore, a crucial function of a circuit breaker is to extinguish (put out) this arc . Circuit breakers use various methods (oil, air, vacuum, SF6 gas) to extinguish the arc quickly and safely. So, arc extinction is essential to the operation of the circuit breaker .

A short circuit in any winding of a transformer is primarily caused by insulation failure . The insulation in a transformer is responsible for preventing electrical contact between different windings and between windings and the core. Factors such as aging, overheating, moisture, and electrical stress can degrade the insulation, leading to its breakdown. Once the insulation fails, the winding conductors come into direct contact, resulting in a short circuit . Other factors like mechanical vibration, loose connections, and impulse voltage may contribute to insulation deterioration but are not the direct cause of a short circuit. Hence, the correct answer is Insulation Failure (Option B).

Distance relays are primarily used for the protection of transmission lines in power systems. They operate based on the ratio of voltage to current (i.e., impedance) seen at the relay location. When a fault occurs, the impedance between the relay and the fault point drops. The relay measures this drop and operates if the impedance falls within a preset value. Impedance relays are designed to detect this change in impedance and hence are best suited for distance protection. Split-phase relays are used for other specialized protection schemes and are not suitable for distance protection. Reactance relays are a type of distance relay, but they are more sensitive to resistive faults and are mainly used in specific conditions like high resistance ground faults. They are not as commonly used for general distance protection as impedance relays. Therefore, impedance relays are the standard and most widely used type of distance relays.

A fuse in a motor circuit primarily provides protection against short-circuit and overload conditions. Short-circuit Protection: In the event of a short-circuit, where a low-resistance path is created between two conductors, a large current surge flows through the circuit. This can damage the motor and other components. The fuse interrupts the circuit when the current exceeds its rated value, preventing damage by quickly melting and opening the circuit. Overload Protection: Overload conditions occur when the current drawn by the motor exceeds its rated value for an extended period. This can happen due to mechanical issues, such as increased friction, or electrical issues, such as voltage fluctuations. The fuse protects against overload by melting and opening the circuit when the current exceeds its rated value for a sustained period. Therefore, Option B correctly identifies that a fuse in a motor circuit provides protection against both short-circuit and overload conditions.

instantaneous is the correct answer to “Magnetic circuit breaker has ______ trip action.”. This question is standard in competitive examination syllabi for Switchgear Protections.

extinguish the arc is the correct answer to “In a circuit breaker the basic problem is to”. This question is standard in competitive examination syllabi for Switchgear Protections.

copper tungsten alloy is the correct answer to “The arcing contacts in a circuit breaker are made of”. This question is standard in competitive examination syllabi for Switchgear Protections.

repeated duty is the correct answer to “Air blast circuit breaker is used for”. This question is standard in competitive examination syllabi for Switchgear Protections.

Connecting lines in parallel is the correct answer to “The steady state stability of the power system can be increased by”. This question is standard in competitive examination syllabi for Switchgear Protections.

both (A) and (B) is the correct answer to “The inductive interference between power and communication line can be minimized by”. This question is standard in competitive examination syllabi for Switchgear Protections.

overload is the correct answer to “A thermal protection switch can protect against”. This question is standard in competitive examination syllabi for Switchgear Protections.

delayed trip action is the correct answer to “Thermal circuit breaker has”. This question is standard in competitive examination syllabi for Switchgear Protections.

0.003 sec is the correct answer to “The time of closing the cycle, in modern circuit breakers is”. This question is standard in competitive examination syllabi for Switchgear Protections.

Open circuit is the correct answer to “Transmission line has reflection coefficient as one.”. This question is standard in competitive examination syllabi for Switchgear Protections.

any of these is the correct answer to “Short-circuit currents are due to”. This question is standard in competitive examination syllabi for Switchgear Protections.

3.3kV and 11kV is the correct answer to “Resistance grounding is used for voltage between”. This question is standard in competitive examination syllabi for Switchgear Protections.

slow is the correct answer to “Discrimination between main and back up protection is provided by the use of relays which are”. This question is standard in competitive examination syllabi for Switchgear Protections.

heavy loads is the correct answer to “Thermal overload relays are used to protect the motor against over current due to”. This question is standard in competitive examination syllabi for Switchgear Protections.

both (A) and (B) is the correct answer to “When a transmission line is energized, the wave that propagates on it is”. This question is standard in competitive examination syllabi for Switchgear Protections.

all above is the correct answer to “Overload relays are of type.”. This question is standard in competitive examination syllabi for Switchgear Protections.

all of the above is the correct answer to “H.R.C. fuse, as compared to a rewirable fuse, has”. This question is standard in competitive examination syllabi for Switchgear Protections.

internal short-circuits is the correct answer to “The inverse definite mean time relays are used for over current and earth fault protection of transformer against”. This question is standard in competitive examination syllabi for Switchgear Protections.

Lightning arresters is the correct answer to “Which of the following devices will receive voltage surge first travelling on the transmission line”. This question is standard in competitive examination syllabi for Switchgear Protections.

5 to 10 sec is the correct answer to “The relay with inverse time characteristic will operate within”. This question is standard in competitive examination syllabi for Switchgear Protections.

a resistance decreasing with voltage rise across the arc is the correct answer to “Arc in a circuit behaves as”. This question is standard in competitive examination syllabi for Switchgear Protections.

in series with circuit is the correct answer to “A fuse is connected”. This question is standard in competitive examination syllabi for Switchgear Protections.

unity is the correct answer to “The reflection co-efficient at the open circuited end of a transmission line.”. This question is standard in competitive examination syllabi for Switchgear Protections.

100 A is the correct answer to “Protection by fuses is generally not used beyond”. This question is standard in competitive examination syllabi for Switchgear Protections.

10 is the correct answer to “The ground wire should not be smaller than No ______ copper.”. This question is standard in competitive examination syllabi for Switchgear Protections.

power is the correct answer to “Directional relays are based on flow of”. This question is standard in competitive examination syllabi for Switchgear Protections.

directional is the correct answer to “Admittance relay is _______ relay.”. This question is standard in competitive examination syllabi for Switchgear Protections.

recovery is the correct answer to “The voltage appearing across the contacts after opening of the circuit breaker is called______voltage.”. This question is standard in competitive examination syllabi for Switchgear Protections.

low melting point and -low specific resistance is the correct answer to “The material used for fuse must have”. This question is standard in competitive examination syllabi for Switchgear Protections.

33 kV is the correct answer to “For cost and safety, the outdoor substations are installed for voltages above”. This question is standard in competitive examination syllabi for Switchgear Protections.

inversely proportional to the square of the current is the correct answer to “The torque produced in induction type relay (shaded pole structure) is”. This question is standard in competitive examination syllabi for Switchgear Protections.

Zero is the correct answer to “What will be the reflection co-efficient of the wave of load connected to transmission line if surge impedance of the line is equal to load”. This question is standard in competitive examination syllabi for Switchgear Protections.

absorbed by the circuit of relay is the correct answer to “Burden of a protective relay is the power”. This question is standard in competitive examination syllabi for Switchgear Protections.

not directly related with is the correct answer to “The line insulation is the insulation level of the station equipment.”. This question is standard in competitive examination syllabi for Switchgear Protections.

positive sequence currents only is the correct answer to “A balanced 3-phase system consists of”. This question is standard in competitive examination syllabi for Switchgear Protections.