MEngC P.E. Part I — Electrical (B.S) Engineering (Myanmar)

In a balanced three-phase four-wire star (Y) connected electrical supply system with a line-to-neutral voltage of 230 V RMS, what is the nominal line-to-line RMS voltage and the phase angle displacement between line and phase voltages?

• A400 V RMS, with line voltage leading the corresponding phase voltage by 30 degrees
• B400 V RMS, with line voltage lagging the corresponding phase voltage by 30 degrees
• C325 V RMS, with line voltage in phase with the corresponding phase voltage
• D230 V RMS, with line voltage leading the corresponding phase voltage by 60 degrees

A balanced delta-connected three-phase load consumes a total active power of 36 kW from a 400 V, 50 Hz supply at a lagging power factor of 0.80. What are the phase current (I_ph) in each load branch and the line current (I_L) supplied by the mains?

• APhase current I_ph = 64.95 A, Line current I_L = 37.5 A
• BPhase current I_ph = 37.5 A, Line current I_L = 64.95 A
• CPhase current I_ph = 21.65 A, Line current I_L = 37.5 A
• DPhase current I_ph = 30.0 A, Line current I_L = 51.96 A

A 400 V balanced three-phase star-connected distribution feeder supplies a load drawing 50 A per line at a power factor of 0.866 lagging. What are the total apparent power (S), active power (P), and reactive power (Q) supplied by the feeder?

• AS = 34.64 kVA, P = 17.32 kW, Q = 30.00 kVAR
• BS = 20.00 kVA, P = 17.32 kW, Q = 10.00 kVAR
• CS = 34.64 kVA, P = 30.00 kW, Q = 17.32 kVAR
• DS = 60.00 kVA, P = 51.96 kW, Q = 30.00 kVAR

In a three-phase four-wire low-voltage building distribution system supplying perfectly balanced linear loads, what is the magnitude of the steady-state current flowing through the neutral conductor?

• AEqual to one-third of the total connected load current
• BEqual to the line current I_L, because the neutral conductor carries the common return cu…
• CEqual to 1.732 times the line current, due to constructive interference between phase cur…
• D0 A, because the phasor sum of the three balanced line currents displaced by 120 degrees…

A three-phase four-wire 400/230 V star-connected distribution board supplies single-phase resistive lighting loads. The line currents measured at fundamental frequency are I_A = 60 A at 0 deg, I_B = 40 A at -120 deg, and I_C = 40 A at +120 deg. What is the magnitude of the resulting neutral current I_N?

• A20 A
• B140 A
• C46.7 A
• D0 A

What is the standard procedure to reverse the direction of rotation of a three-phase squirrel-cage induction motor operating on a building services ventilation system?

• AInterchange the incoming neutral conductor with any one of the phase conductors
• BInterchange any two of the three incoming supply phase conductors at the motor terminal b…
• CReverse the polarity of all three phase conductors simultaneously
• DInstall a series resistor in one phase winding to shift the magnetic axis

In symmetrical component analysis (Fortescue theorem) used for power system fault calculations, what are the sequence components present during a single line-to-ground (L-G) fault on an unloaded generator?

• APositive and negative sequence currents are equal in magnitude but opposite in phase; zer…
• BOnly zero sequence current is present; positive and negative sequence currents remain zero
• CPositive, negative, and zero sequence currents are all equal in magnitude and in phase at…
• DPositive sequence current is double the negative sequence current, and zero sequence curr…

Before paralleling an incoming diesel generator with a live building main switchboard busbar, which set of conditions must be strictly satisfied at the moment the breaker is closed?

• AIdentical governor droop percentage, zero reactive power output, and equal neutral ground…
• BEqual active power rating, equal power factor, identical frequency, and 90-degree phase l…
• CEqual terminal voltage magnitude, identical short-circuit ratio, and 180-degree phase opp…
• DEqual terminal voltage magnitude, identical frequency, identical phase sequence, and zero…

What is the synchronous speed of a 4-pole three-phase AC induction motor connected to a standard 50 Hz public distribution supply?

• A1,500 rpm
• B3,000 rpm
• C1,440 rpm
• D1,000 rpm

A 4-pole, 50 Hz three-phase induction motor driving a chilled water circulating pump operates at a full-load rotor speed of 1,425 rpm. What is the operational per-unit slip of the motor at this loading condition?

• A0.025 (2.5%)
• B0.05 (5.0%)
• C0.075 (7.5%)
• D0.015 (1.5%)

If a 6-pole, 50 Hz three-phase induction motor operates at a slip of 4% (0.04), what is the frequency of the induced electromotive force (EMF) and current in the rotor windings?

• A48.0 Hz
• B50.0 Hz
• C2.0 Hz
• D0.08 Hz

In the standard steady-state torque-speed characteristic curve of a three-phase squirrel-cage induction motor, how does increasing the rotor circuit resistance affect the starting torque and the maximum (breakdown) torque?

• AMaximum breakdown torque decreases significantly, while starting torque remains constant
• BBoth starting torque and maximum breakdown torque increase proportionally
• CStarting torque decreases, while maximum breakdown torque increases
• DStarting torque increases (up to the point where slip for maximum torque equals 1.0), whi…

A three-phase induction motor receives 50 kW of air-gap power (P_ag) across the stator-rotor interface. If the motor is operating at a slip of 0.04 (4%), what are the total rotor copper loss (P_rcu) and the gross mechanical power developed (P_m)?

• ARotor copper loss P_rcu = 2.0 kW, Developed mechanical power P_m = 48.0 kW
• BRotor copper loss P_rcu = 4.0 kW, Developed mechanical power P_m = 46.0 kW
• CRotor copper loss P_rcu = 1.0 kW, Developed mechanical power P_m = 49.0 kW
• DRotor copper loss P_rcu = 2.0 kW, Developed mechanical power P_m = 52.0 kW

Why is a single-phase induction motor with only a single main stator winding inherently incapable of producing starting torque at standstill?

• AThe single-phase stator winding produces no magnetic flux whatsoever until the rotor is m…
• BThe pulsating single-phase magnetic field resolves into two equal and opposite revolving…
• CThe rotor bars in a single-phase motor are insulated and prevent induced current flow at…
• DThe voltage across the squirrel-cage rotor is exactly zero due to infinite reactance at 5…

Which type of single-phase motor provides the highest starting torque per ampere of starting current, making it ideal for refrigeration compressors in building air-conditioning units?

• ASplit-phase (resistance-start) motor
• BShaded-pole motor
• CCapacitor-start, induction-run motor
• DPermanent split-capacitor (PSC) motor

When controlling an induction motor using a variable frequency drive (VFD) below base rated speed, why is the stator voltage reduced in direct proportion to the output frequency (constant V/f control)?

• ATo eliminate all acoustic noise generated by the inverter PWM switching frequencies
• BTo increase the rotor winding resistance as frequency decreases
• CTo keep the rotor slip speed constant regardless of applied mechanical load
• DTo maintain constant magnetic flux in the core and prevent magnetic saturation and excess…

In an ideal single-phase step-down transformer having a primary-to-secondary turns ratio of N_1 / N_2 = 10:1, what are the secondary voltage and secondary current when the primary is connected to a 2,300 V supply and draws 5 A of primary load current?

• ASecondary voltage V_2 = 230 V, Secondary current I_2 = 50 A
• BSecondary voltage V_2 = 23,000 V, Secondary current I_2 = 0.5 A
• CSecondary voltage V_2 = 230 V, Secondary current I_2 = 0.5 A
• DSecondary voltage V_2 = 2,300 V, Secondary current I_2 = 50 A

Which internal transformer losses and equivalent circuit parameters are primarily determined by conducting an open-circuit (no-load) test at rated voltage on the low-voltage side of a power transformer?

• AFull-load winding copper losses (I^2 * R) and equivalent leakage reactance X_eq
• BCore (iron) losses (hysteresis and eddy current), magnetizing reactance X_m, and core-los…
• CDielectric breakdown voltage of transformer oil and winding insulation resistance
• DStray load losses and inter-turn capacitance under lightning impulse surges

A short-circuit test is performed on the high-voltage side of a 100 kVA, 11,000/400 V distribution transformer with the low-voltage terminals short-circuited. The instruments record V_sc = 550 V, I_sc = 9.09 A (rated HV current), and P_sc = 1,200 W. What are the equivalent resistance R_eq and equivalent reactance X_eq referred to the high-voltage winding?

• AR_eq = 1.20 ohms, X_eq = 5.50 ohms
• BR_eq = 60.50 ohms, X_eq = 14.52 ohms
• CR_eq = 14.52 ohms, X_eq = 58.74 ohms
• DR_eq = 14.52 ohms, X_eq = 0 ohms

A distribution transformer has an equivalent resistance of 1.5% (0.015 pu) and an equivalent leakage reactance of 5.0% (0.05 pu). Using the approximate voltage regulation formula, what is the percentage voltage regulation at full load with a power factor of 0.80 lagging?

• A-1.8%
• B+1.8%
• C+6.5%
• D+4.2%

A 500 kVA distribution transformer has a constant core loss of 2,000 W and a full-load copper loss of 8,000 W. At what percentage of full-load kVA does the transformer operate at its maximum efficiency?

• A50.0% of full load (250 kVA)
• B25.0% of full load (125 kVA)
• C70.7% of full load (353.5 kVA)
• D100.0% of full load (500 kVA)

What is the primary electrical difference between an autotransformer and a conventional two-winding isolation transformer?

• AAn autotransformer contains no magnetic core and relies exclusively on electrostatic coup…
• BAn autotransformer has a single continuous winding shared between primary and secondary,…
• CAn autotransformer can only step voltage up, whereas two-winding transformers can only st…
• DAn autotransformer operates exclusively on direct current (DC) circuits

A standard two-winding 10 kVA, 2,200/220 V transformer is reconnected as a step-up autotransformer to supply 2,420 V from a 2,200 V building distribution source. What is the new transformed kVA power handling rating of this autotransformer connection?

• A12 kVA
• B10 kVA
• C110 kVA
• D55 kVA

Why is a Delta-Star (Dyn11) vector group commonly selected for secondary distribution step-down transformers (e.g. 11 kV to 400/230 V) in building services?

• AThe connection avoids any phase angle displacement between primary and secondary voltages
• BThe Dyn11 connection eliminates all line-to-line fault currents on the secondary side
• CThe Dyn11 configuration allows secondary voltage to be varied continuously without tap ch…
• DThe delta primary traps triplen (3rd) harmonic currents, while the star secondary provide…

Two 11 kV / 400 V distribution transformers are to be operated in parallel to supply a shared low-voltage switchboard. Which condition must be satisfied to prevent continuous circulating currents between the two transformers under zero-load conditions?

• AIdentical primary and secondary voltage ratings, identical voltage transformation ratios,…
• BIdentical physical tank dimensions and oil volume
• CIdentical kVA ratings and identical cooling methods (ONAN)
• DCompletely identical winding ohmic resistance without regard to reactance

What is the primary physical cause of magnetizing inrush current when an unloaded power transformer is energized, and which harmonic component is commonly used in protective relays to restrain false differential tripping?

• AHigh dielectric capacitance between windings; restrained using the 3rd harmonic component
• BCore magnetic flux doubling driving the iron into severe saturation; restrained using the…
• CRapid thermal expansion of copper conductors; restrained using the 5th harmonic component
• DReverse power flow from the open secondary; restrained using the 7th harmonic component

In medium-voltage (11 kV or 22 kV) distribution for a critical healthcare or commercial building complex, what is the primary operational advantage of a ring-main arrangement compared with a simple radial feeder?

• AProtection coordination requires only simple non-directional instantaneous fuses at all n…
• BThe initial capital cost of switchgear and cabling is significantly lower than a radial s…
• CAn alternative supply path allows a faulted cable section to be isolated while unaffected…
• DFault currents are reduced to zero due to loop impedance cancellation

What is the standard functional configuration of a typical three-way Ring Main Unit (RMU) installed in a secondary distribution substation?

• ATwo fuse-switches for main incoming cables and one solid busbar link for the transformer
• BThree air-break disconnectors without arc-quenching or fault-making capability
• CThree motorized draw-out vacuum circuit breakers with busbar differential protection
• DTwo incoming/outgoing load-break fault-make ring switches and one tee-off circuit breaker…

A 400 V (line-to-line), 50 Hz three-phase 4-wire radial feeder supplies two balanced three-phase loads: Load 1 is located 100 m from the source drawing 50 A at unity power factor; Load 2 is located at the feeder end, 200 m from the source, drawing 50 A at unity power factor. The cable resistance is 0.20 ohms per km per phase (reactance is negligible). What is the total line-to-neutral voltage drop at the end of the feeder?

• A3.0 V
• B2.0 V
• C6.0 V
• D4.5 V

A single-phase ring distribution feeder ABCA has a total resistance of R_AB = 0.05 ohms, R_BC = 0.10 ohms, and R_CA = 0.05 ohms. An infeed at point A supplies two tapping loads: 30 A at B and 50 A at C. What is the current flowing from infeed A towards point B (I_AB)?

• A45 A
• B35 A
• C30 A
• D50 A

Which voltage and frequency combination best reflects Myanmar distribution infrastructure documented by the Ministry of Electric Power?

• AMedium voltage: 6.6 kV; Low voltage: 380 V three-phase / 220 V single-phase (60 Hz)
• BMedium voltage: 4.16 kV; Low voltage: 480 V three-phase / 277 V single-phase (60 Hz)
• CMedium-voltage distribution at 11 kV and 33 kV; low-voltage distribution at about 0.4 kV,…
• DMedium voltage: 22 kV; Low voltage: 208 V three-phase / 120 V single-phase (50 Hz)

Which design approach best addresses the principal fire, oil-containment, ventilation, and escape risks of an indoor oil-immersed transformer room?

• ADirect open architectural connectivity to public escape corridors for natural ventilation
• BWaterproof vinyl flooring with floor drainage directly connected to the municipal stormwa…
• CUnventilated airtight acoustic walls without pressure relief dampers
• DProvide code-sized oil containment, fire-rated separation and doors, adequate heat-remova…

A 1,600 kVA, 11 kV / 400 V three-phase distribution transformer has an impedance voltage of 6.0% (Z% = 0.06 pu). Assuming the 11 kV primary grid has infinite short-circuit capacity, what is the prospective symmetrical three-phase short-circuit fault current (I_sc) at the transformer 400 V secondary terminals?

• A38.49 kA
• B2.31 kA
• C64.15 kA
• D26.67 kA

Why is a sandwich-type compact busbar trunking system (busduct) preferred over multiple parallel single-core cables for vertical electrical power risers in high-rise commercial towers?

• AComplete immunity to three-phase short-circuit electromagnetic forces without needing bra…
• BLower physical space requirement in vertical service shafts, lower voltage drop due to lo…
• CAbility to bend around sharp corners without specialized elbow fittings
• DLower conductor operating temperatures without requiring fire barriers between floor pene…

When designing an Automatic Transfer Switch (ATS) scheme between a utility grid supply and a standalone standby diesel generator for a building with multiple earthed neutral systems, why must a 4-pole switch be used instead of a 3-pole switch?

• ATo completely isolate the generator neutral from the utility neutral, preventing circulat…
• BTo allow the generator to operate at a different frequency than the utility grid
• CTo reduce the short-circuit breaking capacity requirement of the transfer switch
• DTo allow single-phase loads to remain energized when all three phases are disconnected

In a commercial office building supplying substantial non-linear electronic IT loads, why can the neutral conductor current exceed the phase conductor currents, and what harmonic order is primarily responsible?

• AEven harmonics (2nd, 4th, 6th) cancel in the phase conductors and divert exclusively to e…
• BTriplen harmonics (3rd, 9th, 15th) are zero-sequence components that add in-phase arithme…
• CThe 5th and 7th harmonics rotate backward and force fundamental current into the neutral…
• DSkin effect in the phase conductors increases neutral conductivity at 50 Hz

What is the primary objective of protective device coordination (discrimination / selectivity) in an electrical distribution network?

• AReducing the total electrical energy consumption of the building switchboard during norma…
• BEnsuring that all upstream and downstream circuit breakers trip simultaneously during any…
• CEnsuring that only the protective device immediately upstream of a fault trips to clear t…
• DEqualizing the operating wear and contact erosion across all switchboard breakers

An IDMT overcurrent relay conforms to the standard IEC 60255 Normal Inverse curve: t = [0.14 / ((I / I_s)^0.02 - 1)] * TMS. The relay has a current pickup setting of I_s = 400 A and a Time Multiplier Setting of TMS = 0.20. What is the calculated operating trip time when a fault current of I = 4,000 A passes through the circuit?

• A0.14 seconds
• B1.45 seconds
• C2.97 seconds
• D0.59 seconds

How does a Core Balance Current Transformer (CBCT) detect low-magnitude earth fault currents in a three-phase four-wire building distribution circuit?

• AAll three phase conductors and the neutral conductor pass through the single circular mag…
• BEach phase conductor passes through its own separate CBCT core and outputs are multiplied…
• CThe protective earth conductor alone is wrapped around the CBCT core to measure direct le…
• DThe CBCT measures high-frequency voltage oscillations between phase and neutral conductors

In a biased (percentage) differential protection scheme for a 10 MVA distribution transformer, what is the engineering purpose of incorporating a bias (restraint) winding in the relay?

• ATo artificially increase the sensitivity of the relay for high-impedance internal turn-to…
• BTo prevent spurious relay tripping during heavy external through-faults caused by CT satu…
• CTo eliminate the need for primary current transformers entirely
• DTo advance the phase angle of the tripping coil current by 90 degrees

What is the technical meaning of the specification '5P20, 30 VA' marked on a protection current transformer (CT) nameplate under IEC 61869-2?

• AA CT capable of operating for 5 periods of 20 cycles at 30 kA fault level
• BA metering CT with 5% ratio error at 20% of rated load current and 30 V output
• CA protection class CT having a composite error not exceeding 5% at 20 times rated symmetr…
• DA Class 0.5 instrument transformer operating up to 20 kV with 30 VA burden

Why is it an extreme safety hazard to open-circuit the secondary terminals of an energized current transformer (CT) while primary load current is flowing?

• AThe CT core will become permanently demagnetized and lose all magnetic permeability
• BThe secondary winding will immediately melt due to excessive secondary current flow
• CThe primary circuit will automatically trip on reverse power
• DWith zero secondary demagnetizing ampere-turns, the entire primary current acts as magnet…

What destructive non-linear resonance phenomenon can occur in an ungrounded or high-impedance grounded medium-voltage substation when voltage transformers (VTs) interact with system ground capacitance during switching transients?

• AFerroresonance, resulting in severe overvoltages, core overheating, and blown primary fus…
• BSubsynchronous torsional oscillation in generator shafts
• CAcoustic cavity resonance inside busbar ducts
• DCorona discharge causing instantaneous vacuum breaker failure

How does an optical arc-flash protection system initiate a rapid trip in low-voltage and medium-voltage switchboards?

• ABy detecting ultrasonic acoustic waves generated by the air expansion during an arc
• BBy combining point or fiber-optic light detection with a current criterion and sending a…
• CBy measuring switchboard enclosure ambient temperature rise over a 10-minute window
• DBy using infrared cameras to detect gradual cable insulation degradation

Under IEC 60529, what level of environmental protection is provided by an electrical distribution enclosure rated IP65 installed in a building mechanical plant room?

• AProtection against dust deposits and protection against continuous submersion under water…
• BProtection against solid objects larger than 12.5 mm and protection against vertically fa…
• CComplete protection against dust ingress (dust-tight) and protection against low-pressure…
• DExplosion-proof enclosure for hazardous gas environments

Under Singapore CP 5 / BS 7671 standards for temporary electrical installations on building construction sites, what is the maximum rated residual operating current (I_delta_n) mandated for RCD protection on all final socket-outlet circuits up to 32 A?

• A500 mA, with adjustable time delay
• B100 mA, with time delay
• C300 mA, for fire protection
• D30 mA, with instantaneous disconnection

Why must the RCD type for a circuit supplying a VFD or solar inverter be selected from the equipment's possible residual-current waveform and the manufacturer's instructions, rather than defaulting to Type AC?

• APower-electronic equipment can produce pulsating or smooth DC and mixed-frequency residua…
• BType AC RCDs operate only on pure DC circuits and will instantly trip on 50 Hz AC supply
• CType AC RCDs cannot handle voltages above 110 V RMS
• DType AC RCDs lack an internal test pushbutton