Switchgear and Protection MCQs 2026

Fuse protects against

1. Aboth overload and short circuit within its rating✓
2. Bonly voltage regulation of line✓
3. Conly Ferranti effect✓
4. Donly motor synchronizing only✓

Instantaneous overcurrent element in a relay operates

1. Aonly after 2 seconds always✓
2. Bonly on reverse power✓
3. Cwithout intentional time delay once current exceeds setting✓
4. Donly when voltage is maximum✓

Directional overcurrent relay requires

1. Aonly CT without PT✓
2. Bonly fuse melting curve✓
3. Cboth current and voltage inputs to determine power flow direction✓
4. Donly Buchholz gas pressure✓

Differential protection compares

1. Aonly voltage magnitude at one bus✓
2. Bonly frequency deviation only✓
3. Conly rotor speed of motor✓
4. Dcurrent entering and leaving the protected zone✓

Buchholz relay is installed on transformers to detect

1. Aoverflux due to high voltage only✓
2. Bdifferential current only on lines✓
3. Cdistance to fault on cable✓
4. Dincipient faults producing gas in the oil✓

Restricted earth fault (REF) protection is sensitive to

1. Aphase faults on long transmission lines only✓
2. Bearth faults within the transformer differential zone near the star point✓
3. Cmotor starting inrush only✓
4. DFerranti effect only✓

Distance relay measures apparent impedance to fault as

1. AV/I seen by the relay✓
2. Bonly current magnitude without voltage✓
3. Conly frequency✓
4. Donly zero-sequence voltage only✓

Plug setting multiplier (PSM) equals

1. Atime setting only✓
2. BCT ratio only✓
3. Cbreaking capacity only✓
4. Dfault current divided by relay pick-up current✓

Time setting multiplier (TSM) on an IDMT relay adjusts

1. ACT primary turns only✓
2. Bthe time dial to shift the inverse time curve✓
3. Ccircuit breaker arc voltage only✓
4. Dmotor slip only✓

Relay coordination on radial feeders ensures

1. Aall relays trip simultaneously always✓
2. Bonly backup relay ever operates✓
3. Cthe relay nearest the fault operates first✓
4. Dfuse never blows✓

Numerical protection relays offer

1. Aonly mechanical induction disc operation✓
2. Bprogrammable logic, self-test and event recording✓
3. Cno communication capability ever✓
4. Dmandatory oil immersion✓

Electromechanical induction disc relay operating time depends on

1. Aonly ambient temperature without current✓
2. Bonly line length in km✓
3. Conly SIL of line✓
4. Dcurrent magnitude and time dial setting✓

CT burden on relay must be kept within rating to avoid

1. Aincrease of SIL✓
2. BFerranti voltage rise✓
3. Csaturation and loss of accuracy in protection✓
4. Dmotor pull-out torque change✓

Overflux protection on transformers responds to

1. Aonly differential current magnitude✓
2. Bonly distance to fault✓
3. Conly negative-sequence current only on lines✓
4. DV/f ratio exceeding allowable limit✓

High-set instantaneous element is used to

1. Areplace all time-delayed elements always✓
2. Baccelerate clearance of close-in high-current faults✓
3. Cmeasure power factor only✓
4. Dcontrol excitation of alternator only✓

Stability of differential relay during external faults requires

1. Aopening of all circuit breakers✓
2. BCTs of adequate ratio and knee-point to avoid false differential current✓
3. Czero load current always✓
4. Dleading power factor at receiving end only✓

SF6 circuit breaker uses sulfur hexafluoride primarily because

1. Ait has high dielectric strength and excellent arc quenching capability✓
2. Bit is lighter than air only without arc benefit✓
3. Cit eliminates all fault current instantly without arc✓
4. Dit replaces the need for contacts✓

Vacuum circuit breaker extinguishes arc by

1. Arapid dielectric recovery in high vacuum between contacts✓
2. Boil spray on contacts only✓
3. Ccompressed air blast only✓
4. Dlong arc running in open air only✓

Air blast circuit breaker interrupts current by

1. ASF6 liquefaction only✓
2. Bvacuum pumping only✓
3. Chigh-pressure air jet axially along the arc✓
4. Doil decomposition only✓

Oil circuit breaker relies on

1. Avacuum alone without oil✓
2. Barc decomposition of oil generating gas flow to extinguish arc✓
3. Cpure SF6 without tank✓
4. Dno arc interruption medium✓

Making capacity of a circuit breaker refers to

1. Aonly load current at unity pf✓
2. Bmaximum peak current it can close onto without excessive damage✓
3. Conly charging current of line only✓
4. Donly excitation current of alternator✓

Breaking capacity of a circuit breaker must

1. Aequal only full-load current✓
2. Bbe less than load current always✓
3. Cexceed prospective fault level at the installation point✓
4. Dignore short-circuit MVA always✓

Restriking voltage after current zero is the transient voltage appearing

1. Aonly at transformer no-load✓
2. Bonly during motor starting✓
3. Cacross breaker contacts during interruption✓
4. Donly on DC systems without arc✓

Rate of rise of restriking voltage (RRRV) affects

1. Asuccessful arc extinction in oil and air break CBs✓
2. Bonly cable charging current magnitude✓
3. Conly PID integral gain✓
4. Donly Buchholz gas volume only✓

Current chopping in vacuum CB on low inductive current can cause

1. Ahigh transient overvoltages due to rapid current interruption✓
2. Breduced insulation stress always✓
3. Czero restriking voltage always✓
4. Dautomatic reclosing failure only on fuses✓

Auto-reclosing on transmission lines attempts

1. Anever used on overhead lines✓
2. Bonly on underground cables always without blocking✓
3. Cautomatic closure after brief outage to restore permanent faults selectively✓
4. Dreplacement of all relays✓

Arc chutes in air break low-voltage breakers

1. Aincrease fault level✓
2. Blengthen and cool the arc for extinction✓
3. Creplace CT cores✓
4. Dmeasure distance to fault✓

Minimum oil circuit breaker (MOCB) differs from bulk oil CB by

1. Ausing smaller quantity of oil in interrupting chamber only✓
2. Busing no oil at all✓
3. Coperating only at EHV✓
4. Dhaving no moving contacts✓

SF6 breaker contact separation is accompanied by

1. Ahigh-pressure gas blast through the arc region✓
2. Bonly vacuum creation in main tank✓
3. Conly oil jet without gas✓
4. Donly fuse wire melting✓

Circuit breaker rated voltage must match

1. Aonly load current in amperes✓
2. Bonly motor slip✓
3. Conly cable length in metres only✓
4. Dhighest system voltage including insulation coordination margins✓

Operating mechanism of a CB may be

1. Aonly manual fuse wire replacement✓
2. Bspring charged, pneumatic or hydraulic depending on design✓
3. Conly permanent magnet only without motion✓
4. Donly Buchholz float movement✓

Successful interruption requires current to reach

1. Amaximum peak always without zero✓
2. Bnatural or forced zero with adequate dielectric recovery across contacts✓
3. CDC offset only without decay✓
4. Dinfinite frequency always✓

HRC fuse stands for

1. Ahigh reactive compensation✓
2. Bhigh rupturing capacity fuse✓
3. Chigh resistance conductor✓
4. Dhigh relay coordination only✓

Cartridge type HRC fuse contains

1. Aonly rewirable wire in open air✓
2. Bonly oil without element✓
3. Conly vacuum interrupter✓
4. Dsilver or copper element enclosed in silica sand filled tube✓

Cut-off current of a current-limiting fuse is

1. Amaximum instantaneous current reached before fuse clears✓
2. Bsteady-state load current only✓
3. Ctransformer magnetizing current only✓
4. Dmotor full-load current only✓

Inverse time characteristic of a fuse means

1. Atime fixed for all currents✓
2. Bfuse never blows on overload✓
3. Cblow time increases with fault current✓
4. Dhigher overcurrent clears in shorter time✓

Rewirable Kit-Kat fuse is considered obsolete because

1. Ait has highest rupturing capacity✓
2. Bit has lower breaking capacity and poor discrimination✓
3. Cit is used on 400 kV lines✓
4. Dit replaces numerical relays✓

Expulsion fuse used in outdoor distribution

1. Auses arc energy to expel gases that extinguish the arc✓
2. Brequires SF6 tank always✓
3. Coperates only on DC✓
4. Dcannot be used outdoors✓

Fuse coordination ensures

1. Aall fuses blow together always✓
2. Bbreaker never operates✓
3. Crelay never picks up✓
4. Dfault is cleared by the smallest protective device upstream of the fault✓

Current-limiting fuse reduces

1. Aline SIL to zero✓
2. Bexcitation reactance of alternator✓
3. CBode gain margin only✓
4. DI²t let-through energy seen by downstream equipment✓

Fuse rating in amperes indicates

1. Amaximum current the fuse can carry continuously without melting✓
2. Bminimum fault MVA only✓
3. Cdistance relay reach only✓
4. DPID proportional gain only✓

Unlike a circuit breaker, a fuse

1. Amust be replaced after one fault operation✓
2. Bcan reclose automatically always without inspection✓
3. Cmeasures impedance to fault✓
4. Dprovides differential protection alone✓

Drop-out fuse cutout on distribution poles allows

1. Avisible open isolation when fuse carrier drops after operation✓
2. Bmeasurement of positive-sequence impedance only✓
3. Cautomatic PID tuning✓
4. Ddistance protection zone extension✓

Striker fuse operates an indicator or tripping mechanism by

1. Ameasuring V/f on transformer only✓
2. Bcomputing symmetrical components only✓
3. Cgenerating PWM for inverter only✓
4. Dmechanical movement when fuse element blows✓

Selectivity between series fuses requires

1. Aidentical ratings always✓
2. Bdownstream fuse always slower✓
3. Cno coordination study✓
4. Dupstream fuse has higher rating or slower characteristic than downstream✓

Phase-a to ground voltage during SLG fault is

1. Aalways equal for all phases✓
2. Balways maximum on faulted phase only without ground✓
3. Capproximately zero at fault location while other phases remain finite✓
4. Dundefined without sequence networks✓

IDMT overcurrent relay characteristic means

1. Aoperating time decreases as fault current increases above pick-up✓
2. Btime is fixed regardless of current✓
3. Crelay never operates on overload✓
4. Dpick-up is independent of plug setting✓

Sand filling in HRC fuse cartridge helps

1. Aincrease fault current magnitude✓
2. Bquench the arc and absorb energy during interruption✓
3. Creduce breaking capacity✓
4. Deliminate the need for element✓

Fuse element material is often silver because

1. Ait never melts✓
2. Bit increases fault level✓
3. Cit has stable melting characteristics and good conductivity✓
4. Dit replaces CT saturation curve✓

Fortescue theorem states that

1. Aany unbalanced three-phase phasors can be resolved into three balanced sets✓
2. Bonly positive sequence exists always✓
3. Czero sequence requires no ground path✓
4. Dnegative sequence rotates same as positive✓

Positive sequence network is used for

1. Aonly double line-to-ground faults without line✓
2. Bonly DC circuits✓
3. Conly single-phase motors without neutral✓
4. Dbalanced three-phase faults and normal balanced operation✓

Negative sequence components produce

1. Areverse rotating field harmful to rotating machines✓
2. Bno effect on induction motors✓
3. Conly DC offset in transformers✓
4. Donly Ferranti voltage rise✓

Zero sequence current flows only when

1. Asystem is perfectly balanced always✓
2. Bdelta winding carries zero sequence freely always✓
3. Cno ground exists anywhere✓
4. Da return path exists for the zero sequence network (usually ground/neutral)✓

Delta-connected transformer winding blocks

1. Apositive sequence only✓
2. Bzero sequence current from passing through the delta✓
3. Call sequence networks equally✓
4. Dnegative sequence only always on all designs✓

Single line-to-ground (SLG) fault involves

1. Aonly positive sequence alone✓
2. Ball three sequence networks connected in series at fault point✓
3. Conly negative sequence alone✓
4. Dno zero sequence ever✓

Line-to-line (LL) fault involves

1. Azero sequence only✓
2. Bpositive sequence only✓
3. Cpositive and negative sequence networks in series✓
4. Dno sequence analysis needed✓

Sequence impedances Z1, Z2, Z0 of a static line typically satisfy

1. AZ0 = 0 always✓
2. BZ1 = Z2 and Z0 > Z1 for overhead lines with ground return✓
3. CZ1 = Z0 always without ground wires✓
4. DZ2 = infinity always✓

Unbalanced fault analysis using symmetrical components requires

1. Aonly per unit change of base✓
2. Bonly Bode plot of open-loop system✓
3. Conly SCR latching current test only✓
4. Dinterconnection of sequence networks according to fault type✓

Negative sequence heating in synchronous machines is severe because

1. Ano current flows in rotor✓
2. Bonly stator heats without rotor effect✓
3. Crotor sees double-frequency induced currents relative to forward field✓
4. Dmachine runs as induction generator only✓

Conversion from abc phase quantities to symmetrical components uses

1. Aonly Laplace transform of DC signal✓
2. Bonly Fourier series of square wave without phase✓
3. Conly PID derivative term✓
4. Dlinear transformation with operator a = e^(j120°)✓

Double line-to-ground (LLG) fault uses

1. Apositive sequence only✓
2. Bno zero sequence if ungrounded system only in some cases✓
3. CDC component only✓
4. Dpositive, negative and zero sequence networks interconnected✓

Balanced three-phase load carries

1. Aequal negative and zero sequence always✓
2. Bonly positive sequence current under normal conditions✓
3. Conly zero sequence✓
4. Dunbalanced sequence always✓

Grounding system design affects primarily

1. Azero sequence impedance and fault current magnitude for earth faults✓
2. Bonly positive sequence only on delta systems without ground✓
3. Conly motor slip under load✓
4. Donly inverter switching frequency only✓

Symmetrical components simplify fault studies because

1. Athey eliminate the need for per unit✓
2. Bthey remove all resistances✓
3. Csequence networks are often uncoupled except at fault point✓
4. Dthey set all voltages equal✓

Impedance relay measures

1. Aonly real power flow direction without voltage✓
2. Bonly third harmonic only✓
3. Conly DC resistance of tower footing only✓
4. Dapparent impedance V/I to the fault point✓

Mho relay characteristic on R-X diagram is

1. Aa circle passing through origin oriented to forward faults✓
2. Ba vertical line only at all settings✓
3. Ca horizontal line at negative reactance only always✓
4. Dunrelated to impedance plane✓

Zone 1 distance protection typically covers

1. Aentire adjacent line without delay always✓
2. Bonly transformer winding only✓
3. Cabout 80% of protected line length instantaneously✓
4. Donly busbar differential zone✓

Zone 2 of distance relay provides

1. Abackup coverage beyond line end with intentional time delay✓
2. Binstantaneous tripping for all faults on adjacent lines without coordination✓
3. Conly reverse power protection✓
4. Donly REF on transformer✓

Zone 3 distance element acts as

1. Ainstantaneous primary only without settings✓
2. Bfar backup with longest time delay✓
3. Cmotor stall protection only✓
4. DBuchholz gas detection only✓

Power swing blocking in distance relays prevents

1. Aall internal transformer faults✓
2. Ball SLG faults always✓
3. Cfuse blowing on overload✓
4. Dtripping during stable power swings without fault✓

Carrier aided distance schemes use

1. Aonly DC injection in rotor✓
2. Bonly cable Murray loop only✓
3. Ccommunication between line ends for permissive or blocking tripping✓
4. Donly PID anti-windup only✓

Reach setting of distance relay is specified in

1. Aonly amperes of full-load current✓
2. Bonly motor RPM only✓
3. Cohms or percent of line positive-sequence impedance✓
4. Donly per unit sag only✓

Fault resistance can cause distance relay to

1. Aalways overreach without exception✓
2. Bignore all faults✓
3. Cmeasure only zero sequence✓
4. Dunderreach compared to metallic fault assumption✓

Blinders on distance relays exclude

1. Ainternal differential current only✓
2. Btransformer magnetizing inrush only without harmonic restraint✓
3. Cload encroachment into tripping characteristic✓
4. DFerranti effect on open end only✓

Quadrilateral distance characteristic allows

1. Aonly circular mho without resistance coverage adjustment✓
2. Bonly instantaneous overcurrent without voltage✓
3. Cindependent control of resistive and reactive reach✓
4. Donly fuse time-current curve only✓

Permissive overreach transfer trip (POTT) uses

1. Aonly local zone 3 without communication✓
2. Bonly Buchholz relay contacts✓
3. Creceived permissive signal from remote end to trip on internal fault✓
4. Donly open-loop PID step response✓

Step distance scheme applies

1. Amultiple impedance zones with increasing time delays✓
2. Bsingle instantaneous zone only always✓
3. Conly DC machine commutation only✓
4. Donly cable capacitance grading only✓

Distance relay is primarily applied on

1. Alow-voltage lighting circuits only✓
2. Bonly battery rooms without CT✓
3. Ctransmission lines and EHV feeders✓
4. Donly open-loop water level control only✓

Voltage memory in distance relays helps

1. Aincreasing Ferranti rise✓
2. Bmaintaining directional and impedance measurement during close-in faults with voltage collapse✓
3. Cmeasuring motor slip only✓
4. Dreplacing symmetrical components entirely✓

Switch-on-to-fault (SOTF) logic in distance protection allows

1. Ablocking of all zone 1 always✓
2. Bfast tripping when breaker closes onto a fault✓
3. Cdisabling carrier channel permanently✓
4. Dmeasuring only third harmonic in transformer only without restraint✓