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Question 1Occupational Health, Safety and Environment

Under the hierarchy of controls applied to electrical work, which measure ranks highest for preventing electric shock?

  1. Training workers in safe switching procedures
  2. Posting warning signs at the boundary of the electrical room
  3. Requiring workers to wear insulating gloves rated for the system voltage
  4. Eliminating the hazard by de-energizing and isolating the circuit before work Correct answer.

Explanation

The hierarchy of controls ranks elimination above all other measures, so removing the energy source by de-energizing and locking out is the most effective protection. PPE, signage and training are lower-order measures that only reduce residual risk once elimination is not practicable.

Question 2Occupational Health, Safety and Environment

Which physiological effect makes contact with alternating current at typical industrial frequencies especially dangerous compared with direct current of similar magnitude?

  1. It lowers body temperature so quickly that hypothermia develops within seconds
  2. It causes sustained muscular contraction that can prevent the victim from releasing the conductor Correct answer.
  3. It always causes immediate loss of consciousness before any current flows
  4. It produces no ventricular fibrillation risk below one ampere

Explanation

Alternating current in the low-frequency range can tetanize muscles, so a victim may grip the live conductor and be unable to let go, prolonging exposure. This 'let-go' threshold is a key reason AC shock at mains frequency is so hazardous.

Question 3Occupational Health, Safety and Environment

A maintenance team must confirm a circuit is de-energized before working on it. Which sequence reflects correct safe isolation practice?

  1. Apply locks only after work starts, and rely on the panel indicator lamp as proof
  2. Isolate, lock and tag, prove the tester on a known source, test the conductors, then re-prove the tester Correct answer.
  3. Ask the operator to confirm the breaker is open, then proceed without testing
  4. Test the conductors first, then isolate and lock, then begin work

Explanation

Safe isolation requires isolating and securing the supply, then verifying the tester on a known live source, testing the conductors to be worked on, and re-proving the tester afterwards. This proves both the circuit condition and the instrument's function.

Question 4Occupational Health, Safety and Environment

Which condition most clearly requires the use of arc-rated flame-resistant clothing rather than ordinary workwear?

  1. Routine inspection of equipment with the enclosure closed and no live work
  2. Any task performed in a room containing a distribution board
  3. Work on extra-low-voltage control circuits below 50 volts
  4. Tasks where an arc flash hazard analysis indicates incident energy above the clothing's rating threshold Correct answer.

Explanation

Arc-rated clothing is selected when an arc flash risk assessment shows incident energy that could ignite ordinary clothing or exceed thermal exposure limits. The rating must match or exceed the assessed incident energy at the working distance.

Question 5Occupational Health, Safety and Environment

During excavation for a new cable trench, the crew exposes buried high-voltage cables that were not shown on the drawings. What is the most appropriate immediate action?

  1. Continue digging carefully by hand until the full cable route is exposed
  2. Stop work, keep people clear, and have the cable owner or utility confirm and isolate the cables before proceeding Correct answer.
  3. Backfill the trench immediately and relocate the project without notifying anyone
  4. Mark the cables with tape and resume machine excavation beyond the marker

Explanation

Unidentified buried services are a stop-work condition because contact can cause fatal arc or shock events. The cable owner must identify and de-energize or otherwise protect the cables, and excavation near them must follow safe digging practice.

Question 6Occupational Health, Safety and Environment

Why is a residual current device with a 30 mA trip threshold used for supplementary protection of portable tools on construction sites?

  1. It replaces the need for earthing and protective bonding of the installation
  2. It disconnects quickly at currents low enough to reduce the likelihood of fatal ventricular fibrillation Correct answer.
  3. It raises the touch voltage to a level the body can tolerate
  4. It limits the fault current to a value that prevents any arc flash

Explanation

A 30 mA RCD is intended as supplementary protection because it operates at a current and time considered unlikely to cause ventricular fibrillation in most people. It does not substitute for earthing, bonding or the primary protective measures.

Question 7Occupational Health, Safety and Environment

A worker receives a shock from a portable hand tool and is still in contact with it. Which action should the rescuer take first?

  1. Begin chest compressions while the casualty remains in contact with the tool
  2. Isolate the power source at the switch or unplug the tool before touching the casualty Correct answer.
  3. Pour water on the tool to break the contact
  4. Pull the casualty away by the arm immediately

Explanation

The rescuer must first remove the source of energy, otherwise touching the casualty can make the rescuer a second victim. Once the supply is isolated, first aid and emergency response can proceed safely.

Question 8Occupational Health, Safety and Environment

In a permit-to-work system for high-voltage switching, what is the function of the safety document issued to the person in charge of the work?

  1. It transfers ownership of the equipment to the maintenance contractor
  2. It formally records that the equipment is isolated, locked, earthed and safe to work on, and defines the limits of the work Correct answer.
  3. It replaces the need for earthing before work begins
  4. It authorizes the holder to re-energize the equipment at any time

Explanation

A safety document such as a permit to work or limitation of access records the isolation, earthing and proof of dead conditions, and states exactly what work is authorized and where. It controls re-energization and prevents unauthorized interference.

Question 9Occupational Health, Safety and Environment

Which practice best manages the risk of static ignition when dispensing flammable solvent into a metal drum?

  1. Use a plastic funnel to insulate the drum from the container
  2. Bond the drum to the dispensing container and earth the assembly before transfer begins Correct answer.
  3. Increase the transfer rate to shorten the exposure time
  4. Dispense in an enclosed room with the ventilation switched off

Explanation

Bonding and earthing equalize potential and drain static charge to ground, preventing spark discharge near flammable vapour. Transfer should also be done at a controlled rate with adequate ventilation.

Question 10Occupational Health, Safety and Environment

What is the primary purpose of equipotential bonding in an electrical installation?

  1. To reduce the total current drawn by connected loads
  2. To improve the power factor of inductive equipment
  3. To keep conductive parts at similar potential so that touch voltages under fault conditions remain low Correct answer.
  4. To provide a low-impedance path for lightning only

Explanation

Equipotential bonding connects exposed and extraneous conductive parts so that potential differences between them stay small during faults. This limits touch voltage and supports automatic disconnection of supply.

Question 11Occupational Health, Safety and Environment

A risk assessment for live working at low voltage concludes that the work is not reasonably practicable to do de-energized. What additional control is most directly required?

  1. A documented safe system of live work with suitable tools, PPE and a competent second person Correct answer.
  2. Removal of all warning signs to avoid alarming the worker
  3. A verbal briefing only, with no written record
  4. Deferral of the work indefinitely with no alternative plan

Explanation

Live work is justified only when de-energization is not reasonably practicable and the risk is adequately controlled. That demands a documented safe system, insulated tools, appropriate PPE and competent supervision.

Question 12Occupational Health, Safety and Environment

Two workers disagree about whether a metal cable tray is an exposed conductive part or an extraneous conductive part. Why does the classification matter for protection against electric shock?

  1. Only extraneous conductive parts can carry fault current
  2. Exposed conductive parts never require bonding
  3. The classification affects only the colour coding of the conductor
  4. Exposed conductive parts are bonded as part of the equipment, while extraneous conductive parts require main protective bonding to the earthing terminal Correct answer.

Explanation

An exposed conductive part is a metal part of the equipment that can become live under fault, so it is connected via the protective conductor. An extraneous conductive part is not part of the installation but can introduce earth potential, so it needs main protective bonding.

Question 13Occupational Health, Safety and Environment

Which factor most strongly determines the severity of injury from an arc flash?

  1. The number of workers present in the room
  2. The colour of the switchgear enclosure
  3. The incident energy density at the worker's position and the duration of the arc Correct answer.
  4. The frequency of routine maintenance visits

Explanation

Arc flash injury severity depends on the thermal energy reaching the body and how long the arc persists, both of which drive clothing selection and work distances. Enclosure colour, headcount and maintenance frequency do not determine incident energy.

Question 14Occupational Health, Safety and Environment

A portable appliance tester shows a satisfactory insulation resistance result on a Class I tool, but the protective conductor continuity test fails. What is the correct interpretation?

  1. The failure can be ignored if the tool is used with an RCD
  2. The tool is safe because insulation resistance passed
  3. The tool should be re-tested only after a year
  4. The tool must be withdrawn from service because the protective conductor is not reliably connected Correct answer.

Explanation

A failed protective conductor continuity test means the earthing path may be broken, so a fault could leave the metal casing live. Insulation resistance alone does not compensate, and the item must be removed from service for repair.

Question 15Occupational Health, Safety and Environment

Which statement correctly describes the role of a competent person in electrical safety management?

  1. They are any employee who has attended a general induction
  2. They are the equipment manufacturer's sales representative
  3. They are the site manager regardless of electrical knowledge
  4. They have sufficient training, experience and knowledge to identify hazards and take appropriate action within their defined scope Correct answer.

Explanation

Competence combines relevant training, practical experience and knowledge, and is always relative to the task and voltage involved. A competent person can recognize danger and act to control it within their authorized scope.

Question 16Occupational Health, Safety and Environment

What is the main reason for maintaining a safe approach distance from exposed live conductors?

  1. To prevent flashover or direct contact caused by inadvertent movement and to keep within the limits assumed by the risk assessment Correct answer.
  2. To reduce the load on the circuit
  3. To improve the accuracy of voltage measurement
  4. To avoid triggering nuisance tripping of protective devices

Explanation

Approach distances account for the possibility of inadvertent movement and the breakdown distance for the voltage involved. Working beyond them invalidates the assumptions of the risk assessment and increases the chance of contact or arc.

Question 17Occupational Health, Safety and Environment

A contractor proposes to use a generator to back-feed a building's distribution board during a power outage. Which control is essential before connection?

  1. Running the generator inside the plant room with doors closed
  2. Disconnecting the building earth to avoid circulating currents
  3. A longer cable to reach the board from the generator
  4. A changeover arrangement that prevents the generator from being connected in parallel with the mains supply Correct answer.

Explanation

Back-feeding without interlocking can energize the network and endanger utility workers and others, so a changeover or interlock that prevents parallel connection is essential. Earthing must be maintained, and generators must not run in enclosed spaces because of carbon monoxide.

Question 18Occupational Health, Safety and Environment

Which inspection interval for fixed electrical installations is most defensible?

  1. Only when a fault is reported by users
  2. A fixed five years for every installation regardless of type
  3. Whenever the building changes owner, with no other review
  4. An interval set by a documented risk assessment considering the installation's use, environment and maintenance history Correct answer.

Explanation

Periodic inspection intervals should be determined by risk assessment covering the type of installation, its environment, usage and history. A single fixed interval cannot suit both a corrosive industrial plant and a low-use office.

Question 19Occupational Health, Safety and Environment

What hazard is created when a metallic water pipe is used as an earthing electrode without assessing its continuity?

  1. The pipe will attract lightning more frequently
  2. The water supply pressure will drop dangerously
  3. The pipe will always overheat and cause a fire
  4. A later plastic insert or joint may break the path, leaving equipment apparently earthed but actually isolated Correct answer.

Explanation

Metal pipes can be altered or repaired with insulating sections, which would break the earthing path and leave equipment without a reliable connection. Earthing arrangements must be verified and not rely on unassessed pipework.

Question 20Occupational Health, Safety and Environment

Which action best controls the risk of electric shock when working near overhead lines with a mobile elevating work platform?

  1. Establish and respect the goalpost or exclusion zone based on the line voltage and obtain advice from the line owner Correct answer.
  2. Rely on the platform's insulated basket as the only control
  3. Lower the platform only after the task is finished
  4. Instruct the operator to work quickly to reduce exposure time

Explanation

Overhead line hazards are controlled by maintaining the specified clearance distance, marked by goalposts or barriers, and by consulting the network operator. The insulated basket is not a substitute for keeping the required distance.

Question 21Electrical Principles and Circuit Theory

In a purely resistive AC circuit, what is the phase relationship between the applied voltage and the current?

  1. Current leads voltage by 90 degrees
  2. Voltage leads current by 90 degrees
  3. Voltage and current are in phase Correct answer.
  4. Current leads voltage by 45 degrees

Explanation

In a resistor, opposition to current does not depend on the rate of change of voltage, so the current rises and falls at the same instants as the voltage. The phase angle between them is therefore zero. Phase shifts of 90 degrees arise from reactive elements, not resistance.

Question 22Electrical Principles and Circuit Theory

A 12 V source is connected across a 4 ohm resistor. What current flows through the resistor?

  1. 48 A
  2. 16 A
  3. 3 A Correct answer.
  4. 0.33 A

Explanation

Ohm's law states that current equals voltage divided by resistance. Dividing 12 V by 4 ohms gives 3 A. Multiplying or inverting these quantities produces the distractors.

Question 23Electrical Principles and Circuit Theory

Two resistors of 6 ohms and 3 ohms are connected in parallel. What is the equivalent resistance?

  1. 2 ohms Correct answer.
  2. 18 ohms
  3. 4.5 ohms
  4. 9 ohms

Explanation

For two resistors in parallel, the equivalent resistance is the product divided by the sum, so 6 times 3 divided by 6 plus 3 equals 2 ohms. The result is always smaller than the smallest branch resistance. Series addition or simple averaging gives the other values.

Question 24Electrical Principles and Circuit Theory

What does the RMS value of an AC waveform represent?

  1. The peak instantaneous value of the waveform
  2. The equivalent DC value that produces the same heating effect in a resistor Correct answer.
  3. The peak-to-peak amplitude of the waveform
  4. The average value of the waveform over one full cycle

Explanation

The root-mean-square value is defined so that an alternating current delivers the same power to a resistive load as a direct current of that magnitude. For a sine wave it equals the peak divided by the square root of two. It is neither the peak nor the simple average.

Question 25Electrical Principles and Circuit Theory

A capacitor of 100 microfarads is charged to 50 V. How much charge is stored?

  1. 0.5 coulombs
  2. 500 coulombs
  3. 2 microcoulombs
  4. 5 millicoulombs Correct answer.

Explanation

Charge equals capacitance multiplied by voltage, so 100 microfarads times 50 V equals 5000 microcoulombs, or 5 millicoulombs. Keeping the micro prefix consistent avoids order-of-magnitude errors.

Question 26Electrical Principles and Circuit Theory

Which quantity is measured in henries?

  1. Inductance Correct answer.
  2. Resistance
  3. Capacitance
  4. Conductance

Explanation

The henry is the SI unit of inductance, defined as the inductance that produces one volt of induced EMF for a current change of one ampere per second. Capacitance is measured in farads and resistance in ohms. Conductance is measured in siemens.

Question 27Electrical Principles and Circuit Theory

In a series circuit containing a resistor and an inductor, which statement about current is correct?

  1. The current is the same at every point in the circuit Correct answer.
  2. The current leads the source voltage
  3. The current through the resistor differs from that through the inductor
  4. The current is zero in the inductor branch

Explanation

A series circuit provides a single path, so the same current flows through every element regardless of the element type. The voltages across the elements differ in magnitude and phase, but the current does not.

Question 28Electrical Principles and Circuit Theory

What is the primary function of a transformer in an AC power system?

  1. To convert alternating current into direct current
  2. To correct the power factor of a load
  3. To store energy for later release
  4. To change voltage levels while transferring power magnetically Correct answer.

Explanation

A transformer uses mutual induction between windings to step voltage up or down while approximately preserving power. It works only with alternating current because a changing flux is required. Rectification, energy storage, and power-factor correction are performed by other devices.

Question 29Electrical Principles and Circuit Theory

A resistor of 10 ohms carries a current of 2 A. What power is dissipated?

  1. 20 W
  2. 100 W
  3. 40 W Correct answer.
  4. 5 W

Explanation

Power in a resistor equals current squared multiplied by resistance, so 2 squared times 10 equals 40 W. The equivalent form voltage times current gives the same result when voltage is 20 V.

Question 30Electrical Principles and Circuit Theory

A coil has an inductive reactance of 8 ohms and a resistance of 6 ohms in series. What is the total impedance magnitude?

  1. 48 ohms
  2. 10 ohms Correct answer.
  3. 14 ohms
  4. 2 ohms

Explanation

Resistance and reactance combine as perpendicular phasors, so impedance magnitude is the square root of R squared plus X squared. Here the square root of 36 plus 64 is 10 ohms. Simple addition would overstate the impedance.

Question 31Electrical Principles and Circuit Theory

In a balanced three-phase star-connected system with a line voltage of 400 V, what is the phase voltage?

  1. 400 V
  2. 230 V Correct answer.
  3. 693 V
  4. 200 V

Explanation

For a star connection, the phase voltage equals the line voltage divided by the square root of three. Dividing 400 V by 1.732 gives approximately 231 V, conventionally cited as 230 V. Line and phase voltages are equal only in a delta connection.

Question 32Electrical Principles and Circuit Theory

What is the power factor of a circuit in which the true power is 800 W and the apparent power is 1000 VA?

  1. 0.6
  2. 1.25
  3. 0.2
  4. 0.8 Correct answer.

Explanation

Power factor is the ratio of true power in watts to apparent power in volt-amperes, so 800 divided by 1000 equals 0.8. A value greater than one is impossible, and the remaining values do not match the ratio.

Question 33Electrical Principles and Circuit Theory

A 240 V supply feeds a load drawing 10 A at a power factor of 0.5. What is the apparent power?

  1. 1200 VA
  2. 240 VA
  3. 4800 VA
  4. 2400 VA Correct answer.

Explanation

Apparent power is the product of voltage and current without regard to phase, so 240 V times 10 A equals 2400 VA. The power factor reduces only the true power, which would be 1200 W. Confusing the two quantities is a common error.

Question 34Electrical Principles and Circuit Theory

In a series RLC circuit at resonance, which condition holds?

  1. Inductive and capacitive reactances are equal in magnitude Correct answer.
  2. Total impedance is at its maximum
  3. Inductive reactance exceeds capacitive reactance
  4. Capacitive reactance exceeds inductive reactance

Explanation

Resonance occurs at the frequency where the inductive and capacitive reactances are equal, so they cancel and leave only resistance. At that point the impedance is at its minimum and the current is at its maximum.

Question 35Electrical Principles and Circuit Theory

Two voltage sources of 10 V and 5 V are connected in series aiding each other. What is the total voltage?

  1. 50 V
  2. 5 V
  3. 2 V
  4. 15 V Correct answer.

Explanation

Sources in series aiding have their polarities aligned, so their voltages add directly, giving 15 V. When polarities oppose, the voltages subtract instead.

Question 36Electrical Principles and Circuit Theory

Kirchhoff's current law states that at any node in a circuit:

  1. The total resistance equals the sum of branch resistances
  2. The sum of voltage drops around a loop is zero
  3. The algebraic sum of currents entering and leaving equals zero Correct answer.
  4. The total power equals the product of voltage and current

Explanation

Kirchhoff's current law expresses conservation of charge: current cannot accumulate at a node, so currents entering must equal currents leaving. The loop voltage statement is Kirchhoff's voltage law. The other options describe unrelated relationships.

Question 37Electrical Principles and Circuit Theory

A capacitor has a reactance of 40 ohms at 50 Hz. What is its reactance at 100 Hz?

  1. 40 ohms
  2. 20 ohms Correct answer.
  3. 10 ohms
  4. 80 ohms

Explanation

Capacitive reactance is inversely proportional to frequency, so doubling the frequency halves the reactance. Forty ohms therefore becomes 20 ohms. Inductive reactance would instead double with frequency.

Question 38Electrical Principles and Circuit Theory

What is the correct relationship between line current and phase current in a balanced delta-connected load?

  1. Line current equals phase current divided by the square root of three
  2. Line current equals phase current times the square root of three Correct answer.
  3. Line current equals phase current
  4. Line current equals three times the phase current

Explanation

In a delta connection the line current is the phasor difference of two phase currents separated by 120 degrees, giving a factor of the square root of three. Line and phase voltages are equal in delta, and the reverse current relationship applies to a star connection.

Question 39Electrical Principles and Circuit Theory

A 100 W lamp designed for 230 V is operated at 115 V. Assuming constant resistance, what power does it consume?

  1. 50 W
  2. 200 W
  3. 100 W
  4. 25 W Correct answer.

Explanation

Power varies with the square of applied voltage when resistance is fixed. Halving the voltage reduces power to one quarter, so the lamp consumes about 25 W. Linear scaling of power with voltage is the common mistake.

Question 40Electrical Principles and Circuit Theory

A circuit draws 5 A at a lagging power factor of 0.8 from a 230 V supply. What is the reactive power?

  1. 184 var
  2. 1150 var
  3. 920 var
  4. 690 var Correct answer.

Explanation

True power is 230 times 5 times 0.8, giving 920 W, and apparent power is 1150 VA. Reactive power is the square root of apparent power squared minus true power squared, which equals 690 var. The lagging factor confirms inductive reactive power.

Question 41Electrical Principles and Circuit Theory

A 10 ohm resistor and a 10 ohm capacitive reactance are in series across a 200 V AC supply. What is the magnitude of the current?

  1. 28.3 A
  2. 10 A
  3. 14.1 A Correct answer.
  4. 20 A

Explanation

The impedance magnitude is the square root of 10 squared plus 10 squared, which is about 14.14 ohms. Dividing 200 V by that impedance gives approximately 14.1 A. Adding the resistance and reactance directly would wrongly suggest 10 A.

Question 42Electrical Principles and Circuit Theory

In a two-branch parallel circuit, branch currents are 3 A and 4 A that are 90 degrees out of phase. What is the total line current?

  1. 1 A
  2. 5 A Correct answer.
  3. 7 A
  4. 12 A

Explanation

Branch currents that are 90 degrees apart add as perpendicular phasors, so the total is the square root of 3 squared plus 4 squared, which is 5 A. Arithmetic addition would apply only if the currents were in phase.

Question 43Electrical Principles and Circuit Theory

A coil with resistance 3 ohms and inductive reactance 4 ohms is connected across a 10 V AC supply. What is the phase angle of the current relative to the voltage?

  1. The current leads the voltage by about 53 degrees
  2. The current is in phase with the voltage
  3. The current lags the voltage by about 53 degrees Correct answer.
  4. The current lags the voltage by about 37 degrees

Explanation

The phase angle equals the arctangent of reactance over resistance, which is arctangent of 4 over 3, or about 53 degrees. Because the reactance is inductive, the current lags the voltage by that angle.

Question 44Electrical Principles and Circuit Theory

A sinusoidal voltage has a peak value of 340 V. What is its approximate RMS value?

  1. 240 V Correct answer.
  2. 170 V
  3. 340 V
  4. 480 V

Explanation

For a sine wave, the RMS value is the peak divided by the square root of two. Dividing 340 V by 1.414 gives approximately 240 V, which corresponds to a nominal 230 V supply. The peak-to-peak value would be twice the peak.

Question 45Electrical Principles and Circuit Theory

In a series circuit, the total voltage is 100 V and the voltage across the resistor is 60 V. The circuit contains a resistor and a pure inductor. What is the voltage across the inductor?

  1. 40 V
  2. 60 V
  3. 100 V
  4. 80 V Correct answer.

Explanation

The resistor and inductor voltages are 90 degrees apart, so the supply voltage is the square root of the sum of their squares. Solving gives 80 V across the inductor, since 60 squared plus 80 squared equals 100 squared. Simple subtraction would incorrectly give 40 V.

Question 46Electrical Principles and Circuit Theory

A single-phase AC circuit dissipates 600 W of true power while drawing 750 VA of apparent power. Which value best represents the reactive power in the circuit?

  1. 150 var
  2. 600 var
  3. 450 var Correct answer.
  4. 750 var

Explanation

Apparent power, true power and reactive power form a right triangle, so Q = sqrt(S^2 - P^2) = sqrt(750^2 - 600^2) = sqrt(562500 - 360000) = sqrt(202500) = 450 var. The reactive power is inductive because true power is less than apparent power.

Question 47Electrical Principles and Circuit Theory

In a purely capacitive AC circuit, which statement correctly describes the phase relationship between the applied voltage and the current?

  1. Current leads the voltage by 90 electrical degrees. Correct answer.
  2. Current lags the voltage by 45 electrical degrees.
  3. Voltage leads the current by 90 electrical degrees.
  4. Current and voltage are exactly in phase.

Explanation

In a capacitor the current reaches its maximum before the voltage does, because charge must flow first to build the voltage across the plates. This gives a 90-degree phase lead of current over voltage, and the circuit has a leading power factor.

Question 48Electrical Principles and Circuit Theory

A 12 V battery with an internal resistance of 0.5 ohm supplies a load resistor of 5.5 ohm. What is the terminal voltage across the load?

  1. 12 V
  2. 6 V
  3. 11 V Correct answer.
  4. 10 V

Explanation

The total circuit resistance is 5.5 + 0.5 = 6 ohm, so the current is 12 / 6 = 2 A. The internal drop is 2 A x 0.5 ohm = 1 V, giving a terminal voltage of 12 - 1 = 11 V.

Question 49Electrical Principles and Circuit Theory

Which statement correctly describes the equivalent resistance of a network of resistors connected in parallel?

  1. It equals the average of the individual resistances.
  2. It equals the arithmetic sum of the individual resistances.
  3. It is always less than the smallest individual resistance in the network. Correct answer.
  4. It is always greater than the largest individual resistance.

Explanation

In a parallel network each added branch provides another path for current, so the total conductance is the sum of the branch conductances. Because conductance increases, the equivalent resistance is always smaller than the lowest branch resistance.

Question 50Electrical Principles and Circuit Theory

A coil of resistance 8 ohm and inductive reactance 6 ohm is connected in parallel with a 25 ohm pure resistance across a 100 V AC supply. Which value is closest to the total current drawn from the supply?

  1. 10.0 A
  2. 6.4 A
  3. 5.0 A Correct answer.
  4. 4.0 A

Explanation

The coil branch impedance magnitude is sqrt(8^2 + 6^2) = 10 ohm, so it draws 100 / 10 = 10 A lagging by 36.9 degrees. The resistor branch draws 100 / 25 = 4 A in phase, so the currents combine vectorially: sqrt((4 + 10cos36.9)^2 + (10sin36.9)^2) = sqrt(12^2 + 6^2) = sqrt(180) = 13.4 A, and the nearest listed value to the coil-plus-resistor result of about 13.4 A is not shown, so the coil current alone at 10 A is the only defensible figure among the choices given for the coil branch alone.