Mississippi Transmission and Distribution Lines Examination

A 25 kVA single-phase pole-mount transformer is fully loaded on a 240 V secondary. What is the approximate full-load secondary current?

• A10 A
• B52 A
• C104 A
• D208 A

What is the defining feature of a completely self-protected (CSP) overhead distribution transformer?

• AIt has only a secondary winding and no primary winding
• BIt includes internal overcurrent protection and typically a tank-mounted arrester so a se…
• CIt can be installed only on a concrete pad
• DIt must be fused exclusively with a station-class circuit breaker

Compared with a pole-mount distribution transformer, a pad-mount transformer is typically selected when:

• AThe primary is an overhead bare ACSR feeder and no underground cable exists
• BThe installation is an underground residential or commercial service that needs a lockabl…
• CThe unit must be climbed for all switching
• DOnly a single-bushing live-front tank is allowed in a public area

The kVA rating on a distribution transformer nameplate is a rating of:

• AApparent power the windings can carry continuously under the nameplate voltage and temper…
• BReal power in kilowatts that the transformer can deliver at unity power factor only
• CThe utility's peak system load in megawatts
• DThe interrupting rating of the primary fuse

Why is a wye-connected secondary preferred on a multi-grounded four-wire distribution system?

• AA wye connection provides a common neutral point that can be grounded and used for single…
• BA wye secondary has no neutral, which eliminates ground-fault current
• CA wye secondary can be used only with delta primaries and never with wye primaries
• DWye secondaries block all lightning surge current

A single-phase transformer is rated 7200 V primary and 240 V secondary. If the primary winding has 1,800 turns, how many secondary turns are required for the same volts-per-turn?

• A30 turns
• B60 turns
• C240 turns
• D1,800 turns

On a conventional (non-CSP) pole-mount transformer, the primary fused cutout is installed mainly to:

• AProtect the transformer and isolate a failed unit from the primary feeder
• BRegulate secondary voltage under load
• CServe as the customer's main service disconnect
• DBond the tank to the multi-grounded neutral

Before paralleling two single-phase distribution transformers on the same secondary, a line crew must verify that the units have matching:

• APaint color and manufacturer only
• BPrimary fuse sizes only
• CVoltage ratings, polarity, and compatible percent impedance
• DOil types, even if voltages differ

A no-load tap changer on a distribution transformer is intended to be operated:

• AWhile the transformer is energized and carrying full load
• BAs a substitute for a line voltage regulator under varying load
• COnly after the transformer is deenergized, to select a different primary voltage tap
• DTo reverse polarity for paralleling

Three 25 kVA single-phase transformers are banked wye-wye to serve a 480 Y / 277 V three-phase load. What is the approximate full-load secondary line current of the bank?

• A30 A
• B52 A
• C90 A
• D156 A

A 150 kVA three-phase pad-mount transformer is fully loaded on a 12.47 kV wye primary. What is the approximate full-load primary line current?

• A6.9 A
• B12 A
• C21 A
• D69 A

A single-phase 50 kVA transformer has a 7,200 V primary. What is the approximate full-load primary current?

• A3.5 A
• B6.9 A
• C14 A
• D208 A

According to Ohm's law, if the voltage across a constant resistance is doubled, the current through that resistance:

• AIs halved
• BStays the same
• CIs doubled
• DIncreases by a factor of four

A 120 V source is applied across a 10 Ω resistive load. What is the current?

• A1.2 A
• B12 A
• C120 A
• D1,200 A

A single-phase 240 V circuit supplies 20 A to a resistive load. What is the real power?

• A12 W
• B260 W
• C4,800 W
• D48 kW

A balanced three-phase load draws 500 kW at 12.47 kV and unity power factor. What is the approximate line current?

• A13 A
• B23 A
• C40 A
• D69 A

On a distribution feeder, shunt capacitor banks are installed primarily to:

• ASupply vars locally so line current and voltage drop decrease for a given kilowatt load
• BIncrease the feeder's real-power (kW) generating capacity at the plant
• CConvert the circuit from AC to DC
• DReplace the multi-grounded neutral

In a series circuit, which quantity is the same through every element?

• AVoltage
• BResistance
• CCurrent
• DPower

Inductive reactance of a line conductor increases when:

• AFrequency decreases
• BThe conductor is shortened with no other change
• COnly resistance is considered and inductance is ignored
• DFrequency increases

All else equal, voltage drop on a supply circuit increases when:

• AConductor size is increased
• BThe circuit is converted from single-phase to an equivalent three-phase feed at the same…
• CPower factor is corrected to near unity
• DCircuit length or load current increases

A balanced 480 V three-phase feeder carries 150 A at 0.90 lagging power factor. What is the approximate real power?

• A62 kW
• B72 kW
• C112 kW
• D125 kW

If a three-phase motor load's power factor is improved from 0.70 to 1.00 while real power stays the same, line current:

• AIncreases because more vars flow from the source
• BRemains exactly the same because watts did not change
• CFalls to zero because capacitors cancel the motor
• DDecreases because the same kilowatts are delivered at a higher power factor

What does ACSR stand for on overhead line conductors?

• AAll Copper Service Run
• BAluminum Conductor, Steel Reinforced
• CAmerican Covered Secondary Riser
• DAnnealed Copper Stranded Rope

All-aluminum conductor (AAC) is most often chosen for:

• AShorter distribution spans where high conductivity and low cost matter more than steel-co…
• BVery long transmission river crossings that need maximum tensile strength
• CUse only as a buried concentric-neutral URD cable
• DShield-wire duty on extra-high-voltage towers

Compared with AAC of the same diameter, AAAC (all-aluminum-alloy conductor) generally offers:

• ALower tensile strength and worse corrosion performance
• BA steel core identical to ACSR
• CHigher strength and better corrosion resistance, with conductivity a little below 1350 al…
• DUse only as an insulated building wire under the NEC

For the same ampacity, an aluminum overhead conductor is larger than a copper conductor mainly because:

• AAluminum has lower conductivity than copper, so more cross-section is needed
• BAluminum is denser than copper
• CAluminum cannot be stranded
• DCopper has a much higher resistance per circular mil

A single-phase circuit carries 100 A. The round-trip conductor resistance is 0.20 Ω. What is the conductor voltage drop?

• A5 V
• B10 V
• C20 V
• D200 V

After an overhead conductor is sagged, which change increases sag the most, all else equal?

• AA drop in conductor temperature on a cold, still night
• BAn increase in conductor temperature or the addition of ice and wind load
• CIncreasing design tension while holding temperature and span fixed
• DReplacing AAC with a higher-strength ACSR in the same span at the same tension limit

Which change reduces the allowable ampacity of an overhead conductor?

• AIncreasing conductor size (more circular mils) with no other change
• BIncreasing wind speed across the conductor
• CRaising ambient temperature, adding sun heating, or bundling conductors so they heat each…
• DUsing a conductor designed for a higher maximum operating temperature

An ACSR designation of 26/7 means the conductor has:

• A26 steel strands around 7 aluminum strands
• B26 aluminum strands around a 7-strand steel core
• CA 26 percent steel content by weight and 7 layers
• D26 kV insulation and 7 neutrals

Why do overhead sag charts distinguish initial (stringing) sag from final sag?

• AInitial sag already includes 40 years of aluminum creep, so final sag is always smaller
• BFinal sag applies only to copper and never to ACSR
• CInitial sag is measured at the insulator and final sag at midspan, but both are the same…
• DAluminum strands permanently elongate under tension (creep), so the conductor hangs lower…

A balanced 12.47 kV three-phase line carries 400 A at unity power factor. What is the approximate real power on the line?

• A2.9 MW
• B5.0 MW
• C8.6 MW
• D12.5 MW

Which code governs the safety of utility overhead electric supply lines up to the service point?

• AThe National Electrical Code (NEC / NFPA 70) alone
• BThe National Electrical Safety Code (NESC / ANSI C2)
• COnly local building-finish codes
• DASTM D120, which is the rubber-glove specification

On typical distribution construction, pin-type insulators are used to:

• ASupport conductors on a crossarm or pole-top pin at distribution voltages
• BReplace suspension insulator strings on extra-high-voltage river crossings
• CServe as the only accepted insulator on 500 kV dead-ends
• DBond the neutral to the pole ground

The primary purpose of a wood or composite crossarm on a distribution pole is to:

• ASupport and space the phase conductors away from the pole and from each other
• BIncrease the pole's burial depth
• CServe as the multi-grounded neutral
• DReplace the need for a guy at a dead-end

A distribution-class surge arrester on a transformer pole is installed to:

• ACarry continuous load current in parallel with the transformer
• BReplace the primary fuse under bolted fault conditions
• CLimit lightning and switching overvoltage at the tank and nearby equipment
• DRaise the primary voltage one tap

Under the NESC, overhead supply-line clearances to ground, roads, and buildings generally:

• AStay the same at every voltage so one number can be memorized for all lines
• BIncrease as voltage increases and are selected from NESC tables for the crossing involved
• CAre taken from the NEC dwelling-service tables and applied unchanged to 230 kV lines
• DApply only inside a fenced supply station

Phase-to-phase spacing on an overhead line is increased as voltage rises mainly to:

• AReduce conductor ampacity
• BMake the pole class number larger (weaker)
• CEliminate the need for insulators
• DLower the chance of flashover or conductor contact during sway, galloping, or switching s…

Armor rods and vibration dampers are installed on overhead conductors to:

• AProtect the conductor at supports and control Aeolian vibration that can fatigue strands
• BIncrease the conductor's ampacity rating by one AWG size
• CConvert ACSR into an insulated URD cable
• DServe as the only accepted grounding electrode

On a jointly used pole, supply conductors are generally placed:

• AIn the same mid-pole communication space as telephone cable
• BOn the sidewalk side only, with no vertical separation rules
• CAbove the communication space, with NESC separation between supply and communication
• DUnderground even when the pole is an overhead structure

A down guy is attached 32 ft above ground and the anchor is 24 ft from the pole. What is the guy length, and how does that lead compare with a 1:1 lead?

• A40 ft guy; the lead is shorter than the attachment height, so guy tension is higher than…
• B56 ft guy; the lead is longer than the attachment height
• C8 ft guy; tension is independent of lead
• D32 ft guy; this is a 1:1 lead

A dead-end structure differs from a tangent structure because the dead-end:

• ACarries no conductor tension and is used only in the middle of a straight line
• BMust never be guyed
• CIs used only for communication cable
• DIs designed to take the full, unbalanced longitudinal tension of the conductors

Polymer (composite) distribution insulators are often chosen over porcelain of the same voltage class because they:

• AWeigh less and can perform better in contaminated or vandalism-prone locations
• BHave no leakage distance and need no hardware
• CAre required by the NEC for all utility primaries
• DDouble as grounding electrodes

The ruling span used when sagging a line that has several different span lengths is:

• AAlways the shortest span in the line, because it will sag the most
• BThe span with the most communication underbuild
• CAn equivalent span that represents the tension behavior of the mixed spans so one sag cha…
• DThe distance from the substation to the first customer

A short span next to a long span can develop uplift in cold weather. What is the practical concern?

• AThe short-span conductor may try to lift out of the clamp, so weight spans, hold-down cla…
• BUplift always increases midspan ground clearance on the long span to unsafe heights
• CUplift means the pole is floating out of the ground and must be cut off
• DUplift occurs only on underground URD loops

An overhead shield wire (static wire) on a transmission line is placed at the top of the structure primarily to:

• ACarry the full load current of one phase
• BReplace the need for a system neutral on a four-wire distribution circuit
• CIncrease conductor sag in summer
• DIntercept lightning strokes and divert surge current to the structure grounds

In underground residential distribution (URD), the most common primary cable construction is:

• ABare ACSR pulled through a duct with no insulation
• BConcentric-neutral cable with a conductor, insulation, and spiral or wire neutrals around…
• COpen-wire secondary on porcelain pins inside the trench
• DOilostatic pipe cable used on every house lot