Switchgear and Protection MCQs 2026

Fuse protects against

1. A both overload and short circuit within its rating ✓
2. B only voltage regulation of line ✓
3. C only Ferranti effect ✓
4. D only motor synchronizing only ✓

IDMT overcurrent relay characteristic means

1. A operating time decreases as fault current increases above pick-up ✓
2. B time is fixed regardless of current ✓
3. C relay never operates on overload ✓
4. D pick-up is independent of plug setting ✓

Instantaneous overcurrent element in a relay operates

1. A only after 2 seconds always ✓
2. B only on reverse power ✓
3. C without intentional time delay once current exceeds setting ✓
4. D only when voltage is maximum ✓

Directional overcurrent relay requires

1. A only CT without PT ✓
2. B only fuse melting curve ✓
3. C both current and voltage inputs to determine power flow direction ✓
4. D only Buchholz gas pressure ✓

Differential protection compares

1. A only voltage magnitude at one bus ✓
2. B only frequency deviation only ✓
3. C only rotor speed of motor ✓
4. D current entering and leaving the protected zone ✓

Buchholz relay is installed on transformers to detect

1. A overflux due to high voltage only ✓
2. B differential current only on lines ✓
3. C distance to fault on cable ✓
4. D incipient faults producing gas in the oil ✓

Restricted earth fault (REF) protection is sensitive to

1. A phase faults on long transmission lines only ✓
2. B earth faults within the transformer differential zone near the star point ✓
3. C motor starting inrush only ✓
4. D Ferranti effect only ✓

Distance relay measures apparent impedance to fault as

1. A V/I seen by the relay ✓
2. B only current magnitude without voltage ✓
3. C only frequency ✓
4. D only zero-sequence voltage only ✓

Plug setting multiplier (PSM) equals

1. A time setting only ✓
2. B CT ratio only ✓
3. C breaking capacity only ✓
4. D fault current divided by relay pick-up current ✓

Time setting multiplier (TSM) on an IDMT relay adjusts

1. A CT primary turns only ✓
2. B the time dial to shift the inverse time curve ✓
3. C circuit breaker arc voltage only ✓
4. D motor slip only ✓

Relay coordination on radial feeders ensures

1. A all relays trip simultaneously always ✓
2. B only backup relay ever operates ✓
3. C the relay nearest the fault operates first ✓
4. D fuse never blows ✓

Numerical protection relays offer

1. A only mechanical induction disc operation ✓
2. B programmable logic, self-test and event recording ✓
3. C no communication capability ever ✓
4. D mandatory oil immersion ✓

Electromechanical induction disc relay operating time depends on

1. A only ambient temperature without current ✓
2. B only line length in km ✓
3. C only SIL of line ✓
4. D current magnitude and time dial setting ✓

CT burden on relay must be kept within rating to avoid

1. A increase of SIL ✓
2. B Ferranti voltage rise ✓
3. C saturation and loss of accuracy in protection ✓
4. D motor pull-out torque change ✓

Overflux protection on transformers responds to

1. A only differential current magnitude ✓
2. B only distance to fault ✓
3. C only negative-sequence current only on lines ✓
4. D V/f ratio exceeding allowable limit ✓

High-set instantaneous element is used to

1. A replace all time-delayed elements always ✓
2. B accelerate clearance of close-in high-current faults ✓
3. C measure power factor only ✓
4. D control excitation of alternator only ✓

Stability of differential relay during external faults requires

1. A opening of all circuit breakers ✓
2. B CTs of adequate ratio and knee-point to avoid false differential current ✓
3. C zero load current always ✓
4. D leading power factor at receiving end only ✓

SF6 circuit breaker uses sulfur hexafluoride primarily because

1. A it has high dielectric strength and excellent arc quenching capability ✓
2. B it is lighter than air only without arc benefit ✓
3. C it eliminates all fault current instantly without arc ✓
4. D it replaces the need for contacts ✓

Vacuum circuit breaker extinguishes arc by

1. A rapid dielectric recovery in high vacuum between contacts ✓
2. B oil spray on contacts only ✓
3. C compressed air blast only ✓
4. D long arc running in open air only ✓

Air blast circuit breaker interrupts current by

1. A SF6 liquefaction only ✓
2. B vacuum pumping only ✓
3. C high-pressure air jet axially along the arc ✓
4. D oil decomposition only ✓

Oil circuit breaker relies on

1. A vacuum alone without oil ✓
2. B arc decomposition of oil generating gas flow to extinguish arc ✓
3. C pure SF6 without tank ✓
4. D no arc interruption medium ✓

Making capacity of a circuit breaker refers to

1. A only load current at unity pf ✓
2. B maximum peak current it can close onto without excessive damage ✓
3. C only charging current of line only ✓
4. D only excitation current of alternator ✓

Breaking capacity of a circuit breaker must

1. A equal only full-load current ✓
2. B be less than load current always ✓
3. C exceed prospective fault level at the installation point ✓
4. D ignore short-circuit MVA always ✓

Restriking voltage after current zero is the transient voltage appearing

1. A only at transformer no-load ✓
2. B only during motor starting ✓
3. C across breaker contacts during interruption ✓
4. D only on DC systems without arc ✓

Rate of rise of restriking voltage (RRRV) affects

1. A successful arc extinction in oil and air break CBs ✓
2. B only cable charging current magnitude ✓
3. C only PID integral gain ✓
4. D only Buchholz gas volume only ✓

Current chopping in vacuum CB on low inductive current can cause

1. A high transient overvoltages due to rapid current interruption ✓
2. B reduced insulation stress always ✓
3. C zero restriking voltage always ✓
4. D automatic reclosing failure only on fuses ✓

Auto-reclosing on transmission lines attempts

1. A never used on overhead lines ✓
2. B only on underground cables always without blocking ✓
3. C automatic closure after brief outage to restore permanent faults selectively ✓
4. D replacement of all relays ✓

Arc chutes in air break low-voltage breakers

1. A increase fault level ✓
2. B lengthen and cool the arc for extinction ✓
3. C replace CT cores ✓
4. D measure distance to fault ✓

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

1. A using smaller quantity of oil in interrupting chamber only ✓
2. B using no oil at all ✓
3. C operating only at EHV ✓
4. D having no moving contacts ✓

SF6 breaker contact separation is accompanied by

1. A high-pressure gas blast through the arc region ✓
2. B only vacuum creation in main tank ✓
3. C only oil jet without gas ✓
4. D only fuse wire melting ✓

Circuit breaker rated voltage must match

1. A only load current in amperes ✓
2. B only motor slip ✓
3. C only cable length in metres only ✓
4. D highest system voltage including insulation coordination margins ✓

Operating mechanism of a CB may be

1. A only manual fuse wire replacement ✓
2. B spring charged, pneumatic or hydraulic depending on design ✓
3. C only permanent magnet only without motion ✓
4. D only Buchholz float movement ✓

Successful interruption requires current to reach

1. A maximum peak always without zero ✓
2. B natural or forced zero with adequate dielectric recovery across contacts ✓
3. C DC offset only without decay ✓
4. D infinite frequency always ✓

HRC fuse stands for

1. A high reactive compensation ✓
2. B high rupturing capacity fuse ✓
3. C high resistance conductor ✓
4. D high relay coordination only ✓

Cartridge type HRC fuse contains

1. A only rewirable wire in open air ✓
2. B only oil without element ✓
3. C only vacuum interrupter ✓
4. D silver or copper element enclosed in silica sand filled tube ✓

Cut-off current of a current-limiting fuse is

1. A maximum instantaneous current reached before fuse clears ✓
2. B steady-state load current only ✓
3. C transformer magnetizing current only ✓
4. D motor full-load current only ✓

Inverse time characteristic of a fuse means

1. A time fixed for all currents ✓
2. B fuse never blows on overload ✓
3. C blow time increases with fault current ✓
4. D higher overcurrent clears in shorter time ✓

Rewirable Kit-Kat fuse is considered obsolete because

1. A it has highest rupturing capacity ✓
2. B it has lower breaking capacity and poor discrimination ✓
3. C it is used on 400 kV lines ✓
4. D it replaces numerical relays ✓

Expulsion fuse used in outdoor distribution

1. A uses arc energy to expel gases that extinguish the arc ✓
2. B requires SF6 tank always ✓
3. C operates only on DC ✓
4. D cannot be used outdoors ✓

Fuse coordination ensures

1. A all fuses blow together always ✓
2. B breaker never operates ✓
3. C relay never picks up ✓
4. D fault is cleared by the smallest protective device upstream of the fault ✓

Current-limiting fuse reduces

1. A line SIL to zero ✓
2. B excitation reactance of alternator ✓
3. C Bode gain margin only ✓
4. D I²t let-through energy seen by downstream equipment ✓

Fuse rating in amperes indicates

1. A maximum current the fuse can carry continuously without melting ✓
2. B minimum fault MVA only ✓
3. C distance relay reach only ✓
4. D PID proportional gain only ✓

Unlike a circuit breaker, a fuse

1. A must be replaced after one fault operation ✓
2. B can reclose automatically always without inspection ✓
3. C measures impedance to fault ✓
4. D provides differential protection alone ✓

Drop-out fuse cutout on distribution poles allows

1. A visible open isolation when fuse carrier drops after operation ✓
2. B measurement of positive-sequence impedance only ✓
3. C automatic PID tuning ✓
4. D distance protection zone extension ✓

Striker fuse operates an indicator or tripping mechanism by

1. A measuring V/f on transformer only ✓
2. B computing symmetrical components only ✓
3. C generating PWM for inverter only ✓
4. D mechanical movement when fuse element blows ✓

Selectivity between series fuses requires

1. A identical ratings always ✓
2. B downstream fuse always slower ✓
3. C no coordination study ✓
4. D upstream fuse has higher rating or slower characteristic than downstream ✓

Sand filling in HRC fuse cartridge helps

1. A increase fault current magnitude ✓
2. B quench the arc and absorb energy during interruption ✓
3. C reduce breaking capacity ✓
4. D eliminate the need for element ✓

Fuse element material is often silver because

1. A it never melts ✓
2. B it increases fault level ✓
3. C it has stable melting characteristics and good conductivity ✓
4. D it replaces CT saturation curve ✓

Fortescue theorem states that

1. A any unbalanced three-phase phasors can be resolved into three balanced sets ✓
2. B only positive sequence exists always ✓
3. C zero sequence requires no ground path ✓
4. D negative sequence rotates same as positive ✓

Positive sequence network is used for

1. A only double line-to-ground faults without line ✓
2. B only DC circuits ✓
3. C only single-phase motors without neutral ✓
4. D balanced three-phase faults and normal balanced operation ✓

Negative sequence components produce

1. A reverse rotating field harmful to rotating machines ✓
2. B no effect on induction motors ✓
3. C only DC offset in transformers ✓
4. D only Ferranti voltage rise ✓

Zero sequence current flows only when

1. A system is perfectly balanced always ✓
2. B delta winding carries zero sequence freely always ✓
3. C no ground exists anywhere ✓
4. D a return path exists for the zero sequence network (usually ground/neutral) ✓

Delta-connected transformer winding blocks

1. A positive sequence only ✓
2. B zero sequence current from passing through the delta ✓
3. C all sequence networks equally ✓
4. D negative sequence only always on all designs ✓

Single line-to-ground (SLG) fault involves

1. A only positive sequence alone ✓
2. B all three sequence networks connected in series at fault point ✓
3. C only negative sequence alone ✓
4. D no zero sequence ever ✓

Line-to-line (LL) fault involves

1. A zero sequence only ✓
2. B positive sequence only ✓
3. C positive and negative sequence networks in series ✓
4. D no sequence analysis needed ✓

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

1. A Z0 = 0 always ✓
2. B Z1 = Z2 and Z0 > Z1 for overhead lines with ground return ✓
3. C Z1 = Z0 always without ground wires ✓
4. D Z2 = infinity always ✓

Unbalanced fault analysis using symmetrical components requires

1. A only per unit change of base ✓
2. B only Bode plot of open-loop system ✓
3. C only SCR latching current test only ✓
4. D interconnection of sequence networks according to fault type ✓

Negative sequence heating in synchronous machines is severe because

1. A no current flows in rotor ✓
2. B only stator heats without rotor effect ✓
3. C rotor sees double-frequency induced currents relative to forward field ✓
4. D machine runs as induction generator only ✓

Conversion from abc phase quantities to symmetrical components uses

1. A only Laplace transform of DC signal ✓
2. B only Fourier series of square wave without phase ✓
3. C only PID derivative term ✓
4. D linear transformation with operator a = e^(j120°) ✓

Double line-to-ground (LLG) fault uses

1. A positive sequence only ✓
2. B no zero sequence if ungrounded system only in some cases ✓
3. C DC component only ✓
4. D positive, negative and zero sequence networks interconnected ✓

Balanced three-phase load carries

1. A equal negative and zero sequence always ✓
2. B only positive sequence current under normal conditions ✓
3. C only zero sequence ✓
4. D unbalanced sequence always ✓

Grounding system design affects primarily

1. A zero sequence impedance and fault current magnitude for earth faults ✓
2. B only positive sequence only on delta systems without ground ✓
3. C only motor slip under load ✓
4. D only inverter switching frequency only ✓

Symmetrical components simplify fault studies because

1. A they eliminate the need for per unit ✓
2. B they remove all resistances ✓
3. C sequence networks are often uncoupled except at fault point ✓
4. D they set all voltages equal ✓

Phase-a to ground voltage during SLG fault is

1. A always equal for all phases ✓
2. B always maximum on faulted phase only without ground ✓
3. C approximately zero at fault location while other phases remain finite ✓
4. D undefined without sequence networks ✓

Impedance relay measures

1. A only real power flow direction without voltage ✓
2. B only third harmonic only ✓
3. C only DC resistance of tower footing only ✓
4. D apparent impedance V/I to the fault point ✓

Mho relay characteristic on R-X diagram is

1. A a circle passing through origin oriented to forward faults ✓
2. B a vertical line only at all settings ✓
3. C a horizontal line at negative reactance only always ✓
4. D unrelated to impedance plane ✓

Zone 1 distance protection typically covers

1. A entire adjacent line without delay always ✓
2. B only transformer winding only ✓
3. C about 80% of protected line length instantaneously ✓
4. D only busbar differential zone ✓

Zone 2 of distance relay provides

1. A backup coverage beyond line end with intentional time delay ✓
2. B instantaneous tripping for all faults on adjacent lines without coordination ✓
3. C only reverse power protection ✓
4. D only REF on transformer ✓

Zone 3 distance element acts as

1. A instantaneous primary only without settings ✓
2. B far backup with longest time delay ✓
3. C motor stall protection only ✓
4. D Buchholz gas detection only ✓

Power swing blocking in distance relays prevents

1. A all internal transformer faults ✓
2. B all SLG faults always ✓
3. C fuse blowing on overload ✓
4. D tripping during stable power swings without fault ✓

Carrier aided distance schemes use

1. A only DC injection in rotor ✓
2. B only cable Murray loop only ✓
3. C communication between line ends for permissive or blocking tripping ✓
4. D only PID anti-windup only ✓

Reach setting of distance relay is specified in

1. A only amperes of full-load current ✓
2. B only motor RPM only ✓
3. C ohms or percent of line positive-sequence impedance ✓
4. D only per unit sag only ✓

Fault resistance can cause distance relay to

1. A always overreach without exception ✓
2. B ignore all faults ✓
3. C measure only zero sequence ✓
4. D underreach compared to metallic fault assumption ✓

Blinders on distance relays exclude

1. A internal differential current only ✓
2. B transformer magnetizing inrush only without harmonic restraint ✓
3. C load encroachment into tripping characteristic ✓
4. D Ferranti effect on open end only ✓

Quadrilateral distance characteristic allows

1. A only circular mho without resistance coverage adjustment ✓
2. B only instantaneous overcurrent without voltage ✓
3. C independent control of resistive and reactive reach ✓
4. D only fuse time-current curve only ✓

Permissive overreach transfer trip (POTT) uses

1. A only local zone 3 without communication ✓
2. B only Buchholz relay contacts ✓
3. C received permissive signal from remote end to trip on internal fault ✓
4. D only open-loop PID step response ✓

Step distance scheme applies

1. A multiple impedance zones with increasing time delays ✓
2. B single instantaneous zone only always ✓
3. C only DC machine commutation only ✓
4. D only cable capacitance grading only ✓

Distance relay is primarily applied on

1. A low-voltage lighting circuits only ✓
2. B only battery rooms without CT ✓
3. C transmission lines and EHV feeders ✓
4. D only open-loop water level control only ✓

Voltage memory in distance relays helps

1. A increasing Ferranti rise ✓
2. B maintaining directional and impedance measurement during close-in faults with voltage collapse ✓
3. C measuring motor slip only ✓
4. D replacing symmetrical components entirely ✓

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

1. A blocking of all zone 1 always ✓
2. B fast tripping when breaker closes onto a fault ✓
3. C disabling carrier channel permanently ✓
4. D measuring only third harmonic in transformer only without restraint ✓