Electrical Measurements and Instrumentation MCQs 2026

Resolution of a digital measuring instrument is

1. Amaximum full-scale value only✓
2. Baccuracy class percentage only✓
3. Csmallest change in input that produces a discernible change in reading✓
4. Dhysteresis loop width on analog meters✓

Calibration traceability ensures

1. Ainstruments never drift after calibration✓
2. Bmeasurement chain links to recognized national or international standards✓
3. Crandom errors are eliminated permanently✓
4. DCT secondary can be left open safely✓

Percentage error in a measurement equals

1. Ameasured value divided by full scale only without true reference✓
2. Btrue value minus measured only without percentage✓
3. C(measured value − true value) / true value × 100✓
4. Drandom scatter standard deviation only✓

Loading effect of an ammeter is minimized by using

1. Ahigh series resistance in the coil✓
2. Bconnection across the load terminals✓
3. Copen-circuit secondary of CT✓
4. Dvery low voltage drop (low impedance) across the meter✓

Loading effect of a voltmeter is minimized by using

1. Avery low input impedance✓
2. Bseries connection in the line current path✓
3. Cvery high input impedance✓
4. Dmoving iron movement without springs✓

Random measurement errors are reduced by

1. Ausing a lower accuracy class meter✓
2. Bopening CT secondary during test✓
3. Ctaking multiple readings and averaging✓
4. Dincreasing loading effect of the voltmeter✓

Systematic measurement error is

1. Arandom scatter around mean only✓
2. Berror eliminated by averaging infinite readings without removing bias✓
3. Calways caused by quantization only✓
4. Dconsistent deviation repeating under the same conditions✓

Accuracy class 0.5 on an indicating instrument means

1. Amaximum error is ±0.5% of full-scale value at reference conditions✓
2. B±0.5% of reading at all points always without qualification✓
3. C±5% of full scale✓
4. Dzero error at half scale only✓

Turns ratio of a CT is approximately inverse to

1. Aburden volt-ampere rating only✓
2. Bwattmeter pressure coil resistance✓
3. Ccurrent transformation ratio between primary and secondary✓
4. Denergy meter registration constant✓

Protection CTs are designed to

1. Ameasure kWh only at light load✓
2. Boperate with secondary always open✓
3. Creplace fuses on distribution feeders✓
4. Dmaintain defined performance up to high fault currents without excessive saturation✓

Instrument CT accuracy class applies at

1. Aany burden without limit always✓
2. Brated burden and between stated percentage of rated current✓
3. Cprimary voltage only✓
4. DDC primary current only✓

Knee-point voltage of a protection CT indicates

1. Arated secondary current only✓
2. Bmeter accuracy class 0.2 only✓
3. Cpoint where CT begins to saturate significantly with increased voltage✓
4. Dprimary conductor cross-section✓

Ring-type (window) current transformer is installed by

1. Abreaking the bus to insert series primary winding always✓
2. Bpassing the primary conductor through the CT window✓
3. Cconnecting only to PT secondary✓
4. Dmounting on the energy meter disc spindle✓

Burden of a current transformer is

1. Aprimary line impedance only✓
2. Bratio error in per unit only✓
3. Cmagnetizing reactance of core alone✓
4. Dtotal impedance of secondary load including leads and relays✓

Potential transformer (VT) is used to

1. Astep up current for ammeters✓
2. Bmeasure frequency without voltage✓
3. Creplace energy meter disc✓
4. Dstep down high system voltage to standard low voltage for instruments✓

CT ratio is defined as

1. Asecondary voltage to primary voltage✓
2. Bratio of primary current to secondary current✓
3. Cprimary turns to secondary turns squared✓
4. Dburden impedance to secondary current✓

Current transformer secondary must never be left open while primary carries current because

1. Adangerous high voltage can appear across secondary terminals✓
2. Bprimary current becomes zero automatically✓
3. Cmeter reads high resistance only✓
4. Ddisc creep increases on energy meters✓

Power factor of load affects induction energy meter accuracy mainly through

1. Aphase angle between voltage and current fluxes✓
2. Bfrequency alone without phase✓
3. Cbraking magnet strength only✓
4. Ddisc material resistivity only✓

Maximum demand indicator associated with energy metering records

1. Ainstantaneous peak current without averaging✓
2. Bhighest average demand over a defined interval✓
3. Ctotal kWh only without demand✓
4. Dpower factor minimum over a year only✓

Electronic static energy meter computes energy by

1. Arotating aluminium disc without sensors✓
2. Bmultiplying instantaneous voltage and current samples and integrating✓
3. Cmoving iron attraction only✓
4. DFourier series of voltage without current✓

Voltage element of an induction energy meter is connected

1. Aacross the supply voltage✓
2. Bin series with line current only✓
3. Cbetween CT secondary terminals only✓
4. Donly to the braking magnet poles✓

Current element of an induction energy meter is connected

1. Aacross supply voltage only✓
2. Bin series with the line carrying load current✓
3. Cin the braking magnet circuit only✓
4. Donly to the potential coil without series path✓

Registration constant of an energy meter specifies

1. Anumber of disc revolutions per unit energy (kWh)✓
2. Bwatts per revolution only✓
3. Cmaximum demand in kVA only✓
4. Dpower factor at full load✓

Light-load adjustment in an energy meter is intended to

1. Aincrease creep for testing✓
2. Bdisable the braking magnet✓
3. Cimprove accuracy at small fractions of rated current✓
4. Draise rated voltage of the pressure coil✓

Lag adjustment in an energy meter compensates for

1. Acreep at zero load only✓
2. Bfrequency variation above 60 Hz only✓
3. CCT polarity reversal on three-phase✓
4. Derror due to lagging power factor of the load✓

Creep in an energy meter refers to

1. Afast rotation at full load only✓
2. Bcorrect registration at unity power factor✓
3. Cdisc stopping instantaneously at zero power✓
4. Dunwanted slow disc rotation with no load applied✓

Braking magnet in an energy meter provides

1. Adamping torque proportional to disc speed✓
2. Badditional driving torque at low power factor✓
3. Cseries connection to the voltage coil✓
4. Dcalibration of CT ratio only✓

The rotating aluminium disc in a single-phase induction energy meter is driven by

1. Apermanent magnet rotation without flux✓
2. Bspring control torque only✓
3. Cpressure coil inductance alone✓
4. Dinteraction of eddy currents in the disc with crossing fluxes✓

An induction-type energy meter is essentially

1. Aan integrating wattmeter registering energy in kWh✓
2. Ba peak power indicator only✓
3. Ca frequency meter with drag magnets✓
4. Da DC-only coulomb counter✓

UPF error in electrodynamometer wattmeter is reduced by

1. Ashorting the current coil for tests✓
2. Bremoving the compensating winding✓
3. Cusing moving iron instead of dynamometer coils✓
4. Dmaking pressure coil resistance large compared with its reactance✓

Phantom loading during wattmeter calibration allows

1. Afull rated voltage and current simultaneously always✓
2. Btesting at rated current with reduced voltage power✓
3. Cmeasurement without any coils energized✓
4. Ddirect connection to PT secondary only✓

Pressure coil of a wattmeter should have

1. Avery low resistance to maximize coil current✓
2. Bsame turns as current coil always✓
3. Chigh resistance to limit current drawn from the circuit✓
4. Dzero inductance without any resistance✓

One wattmeter in the two-wattmeter method reads zero when

1. Apower factor is unity always for both meters✓
2. Bpower factor angle is 60° lagging for balanced load✓
3. Cload is open circuit on all phases always✓
4. Dboth wattmeters always read equal on lagging load✓

For balanced lagging load, power factor angle φ is obtained from two wattmeters by

1. Acos φ = W1 × W2 only✓
2. Bsin φ = W1 + W2 only✓
3. Ctan φ = √3 (W1 − W2) / (W1 + W2)✓
4. Dφ = 0 whenever W1 equals W2 always✓

Two-wattmeter method is applicable to

1. Aonly single-phase loads✓
2. Bonly DC distribution✓
3. Cthree-phase three-wire circuits balanced or unbalanced✓
4. Dfour-wire systems without any modification ever✓

In the two-wattmeter method on a three-phase three-wire system, total power equals

1. Aproduct of the two readings✓
2. Bdifference only regardless of power factor✓
3. Czero always for balanced load✓
4. Dalgebraic sum of the two wattmeter readings✓

Electrodynamometer wattmeter can measure power on

1. ADC only like PMMC ammeters✓
2. BAC only without DC✓
3. Cthree-phase reactive power without any connection rule✓
4. Dboth AC and DC circuits✓

A low power factor wattmeter is designed with

1. Ahigher pressure coil inductance for lagging loads✓
2. Bno pressure coil at all✓
3. Clow inductance pressure coil and higher resistance✓
4. Dmoving iron vanes instead of coils✓

Compensating winding in a wattmeter is provided to

1. Areduce error caused by pressure coil inductance✓
2. Bincrease creep on energy meters✓
3. Creplace the braking magnet✓
4. Dmeasure reactive power directly✓

Wattmeter error at low power factor is mainly due to

1. Aexcessive current coil resistance only✓
2. Bbraking magnet creep only✓
3. CCT saturation in the voltage circuit✓
4. Dinductance of the pressure coil carrying current✓

Wattmeter deflection indicates

1. Areactive power only✓
2. Bapparent power without angle✓
3. Caverage real power over the meter time constant✓
4. Dpeak instantaneous power only✓

Pressure coil of a wattmeter is connected

1. Ain series with load current only✓
2. Binside current coil only✓
3. Cto the watt-hour meter disc only✓
4. Dacross the load voltage✓

Current coil of a wattmeter is connected

1. Aacross the load voltage only✓
2. Bin parallel with supply bus only✓
3. Cin series with the load carrying line current✓
4. Din the pressure coil circuit only✓

An electrodynamometer wattmeter has

1. Aa fixed coil (current coil) and a moving coil (pressure coil)✓
2. Bonly moving iron vanes without coils✓
3. Conly one permanent-magnet coil✓
4. Donly a thermocouple junction✓

Electrodynamometer movement differs from PMMC in that it uses

1. Apermanent magnet stronger than any electromagnet always✓
2. Belectromagnetic field from fixed coils instead of a permanent magnet✓
3. Cmoving iron vanes only without coils✓
4. Dthermocouple heating without coils✓

Full-scale deflection current of a sensitive PMMC movement may be as low as

1. A50 A in standard panel meters always✓
2. B50 μA in laboratory galvanometers✓
3. C500 A without shunts always✓
4. D5 kA for moving coil only✓

PMMC meter cannot read AC mains directly because

1. Abidirectional torque averages to zero over a cycle✓
2. Bspring control fails on AC only✓
3. Cdamping vane blocks AC deflection✓
4. Dpermanent magnet demagnetizes on AC✓

Moving iron instruments generally consume

1. Aless power because iron is lighter✓
2. Bequal power at all frequencies by definition✓
3. Cmore power than PMMC instruments for comparable range✓
4. Dno power because deflection is mechanical only✓

Permanent magnet in PMMC provides

1. Adamping torque directly without eddy currents✓
2. Bcontrolling torque replacing springs✓
3. CAC rectification for the coil✓
4. Dstrong uniform radial magnetic field across the air gap✓

Thermocouple-type ammeter used for AC measurement relies on

1. Arectified PMMC torque on half cycles only✓
2. Bheating effect of current measured via thermocouple EMF✓
3. Cmoving iron attraction on DC component only✓
4. DWheatstone bridge unbalance without heat✓

Sensitivity of a PMMC galvanometer increases with

1. Amore turns on the moving coil and stronger permanent magnet✓
2. Bheavier pointer without coil change✓
3. Chigher control spring stiffness only✓
4. Dopen-circuit damping vane only✓

Repulsion-type moving iron meter compared with attraction type has

1. Amore uniform scale over a wider range✓
2. Bhigher sensitivity at very low current always✓
3. CDC-only operation✓
4. Dno iron vanes in the magnetic circuit✓

Attraction-type moving iron instrument has scale that is

1. Aperfectly linear throughout✓
2. Bcramped at the lower end and spread at the higher end✓
3. Cuniformly spaced at low current only✓
4. Dindependent of current magnitude✓

PMMC instrument scale is approximately linear because

1. Amagnetic field is radial and coil turns are in uniform field✓
2. Biron vane saturation shapes the scale✓
3. Cspring stiffness varies with angle only✓
4. Ddamping magnet controls linearity✓

Damping torque in a PMMC instrument is mainly produced by

1. Acontrol springs only✓
2. Beddy currents induced in the aluminum former✓
3. Cseries resistance only✓
4. Dpressure coil inductance✓

Controlling torque in PMMC and moving iron meters is provided by

1. Apermanent magnet only✓
2. Beddy current disc only✓
3. Cair friction vanes only✓
4. Dspiral springs attached to the moving system✓

Deflection torque in a PMMC movement is proportional to

1. Asquare of voltage only✓
2. Bcoil current in a uniform radial magnetic field✓
3. Cfrequency only✓
4. Dpower factor only✓

Moving iron instruments can measure

1. Aboth AC and DC✓
2. BDC only✓
3. CAC only without DC✓
4. Dfrequency without current✓

A PMMC instrument can be used to measure

1. ADC only✓
2. BAC only✓
3. Cboth AC and DC equally✓
4. Dfrequency only✓

Standard secondary current for many CTs is

1. A110 V always✓
2. B415 V line voltage✓
3. C5 A or 1 A depending on system design✓
4. D50 μA galvanometer level✓