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RBSE SS Voc Electrical and Elect study materials
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RBSE SS Voc Electrical and Elect practice questions

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Question 1Basic Electrical Fundamentals: voltage, current, resistance, power, Ohm's law, series and parallel circuits

In a DC circuit, what does the term 'electric current' specifically measure?

  1. The rate at which electrical energy is converted to heat
  2. The potential difference between two points
  3. The opposition to charge motion in a conductor
  4. The rate of flow of electric charge past a point Correct answer.

Explanation

Current is defined as the quantity of charge passing a given cross-section per unit time, measured in amperes. One ampere equals one coulomb per second, so the definition is fundamentally a rate of charge flow.

Question 2Basic Electrical Fundamentals: voltage, current, resistance, power, Ohm's law, series and parallel circuits

A resistor of 12 ohms carries a current of 0.5 A. What is the voltage across it?

  1. 12.5 V
  2. 24 V
  3. 6 V Correct answer.
  4. 0.042 V

Explanation

Ohm's law states V = I x R, so the voltage is 0.5 A multiplied by 12 ohms, giving 6 volts. The result is consistent because a modest current through a modest resistance requires only a few volts.

Question 3Basic Electrical Fundamentals: voltage, current, resistance, power, Ohm's law, series and parallel circuits

Two resistors of 6 ohms and 3 ohms are connected in series across a 9 V supply. What current flows?

  1. 1.5 A
  2. 0.67 A
  3. 3 A
  4. 1 A Correct answer.

Explanation

In series, resistances add directly, so the total is 9 ohms. Applying I = V/R gives 9 V divided by 9 ohms, which is 1 ampere.

Question 4Basic Electrical Fundamentals: voltage, current, resistance, power, Ohm's law, series and parallel circuits

Which unit is used to express electrical resistance?

  1. Watt
  2. Volt
  3. Ohm Correct answer.
  4. Ampere

Explanation

Resistance is measured in ohms, symbol omega, defined as the resistance that permits one ampere when one volt is applied. The other units describe voltage, current, and power respectively.

Question 5Basic Electrical Fundamentals: voltage, current, resistance, power, Ohm's law, series and parallel circuits

A 60 W lamp operates on a 240 V supply. What current does it draw?

  1. 14 400 A
  2. 0.25 A Correct answer.
  3. 4 A
  4. 2.5 A

Explanation

Power equals voltage multiplied by current, so current equals power divided by voltage. Dividing 60 W by 240 V gives 0.25 A.

Question 6Basic Electrical Fundamentals: voltage, current, resistance, power, Ohm's law, series and parallel circuits

In a parallel circuit, which quantity is the same across every branch?

  1. The voltage across each branch Correct answer.
  2. The resistance of each branch
  3. The power dissipated in each branch
  4. The current through each branch

Explanation

Parallel branches are connected across the same two nodes, so each branch experiences the identical potential difference. Current, resistance, and power may all differ between branches depending on their values.

Question 7Basic Electrical Fundamentals: voltage, current, resistance, power, Ohm's law, series and parallel circuits

What is the total resistance of three 9 ohm resistors connected in parallel?

  1. 0.33 ohms
  2. 9 ohms
  3. 27 ohms
  4. 3 ohms Correct answer.

Explanation

For equal resistors in parallel, the total is one resistance divided by the count, so 9 divided by 3 gives 3 ohms. The parallel result is always smaller than the smallest individual resistor.

Question 8Basic Electrical Fundamentals: voltage, current, resistance, power, Ohm's law, series and parallel circuits

Which statement correctly describes conventional current direction?

  1. It flows only inside the battery
  2. It flows from the negative terminal to the positive terminal through the external circuit
  3. It alternates direction every half second in a DC circuit
  4. It flows from the positive terminal to the negative terminal through the external circuit Correct answer.

Explanation

Conventional current is defined as flowing from the positive terminal, through the external circuit, to the negative terminal. Electron flow is opposite to this convention, which is why the two are often confused.

Question 9Basic Electrical Fundamentals: voltage, current, resistance, power, Ohm's law, series and parallel circuits

A 12 V battery delivers 3 A to a load. What power is dissipated in the load?

  1. 36 W Correct answer.
  2. 15 W
  3. 0.25 W
  4. 4 W

Explanation

Electrical power is the product of voltage and current, so 12 V times 3 A equals 36 W. Power can also be found from I squared R or V squared over R once the resistance is known.

Question 10Basic Electrical Fundamentals: voltage, current, resistance, power, Ohm's law, series and parallel circuits

A resistor has 20 V across it and dissipates 5 W. What is its resistance?

  1. 0.25 ohms
  2. 4 ohms
  3. 100 ohms
  4. 80 ohms Correct answer.

Explanation

Using P = V squared / R rearranged as R = V squared / P gives 400 divided by 5, which is 80 ohms. Checking with Ohm's law, the current is 0.25 A and 0.25 squared times 80 also gives 5 W.

Question 11Basic Electrical Fundamentals: voltage, current, resistance, power, Ohm's law, series and parallel circuits

A 10 ohm and a 15 ohm resistor are connected in parallel. What is the equivalent resistance?

  1. 12.5 ohms
  2. 25 ohms
  3. 6 ohms Correct answer.
  4. 5 ohms

Explanation

The reciprocal formula gives 1/R = 1/10 + 1/15 = 1/6, so R equals 6 ohms. This is less than the smaller resistor, as required for any parallel combination.

Question 12Basic Electrical Fundamentals: voltage, current, resistance, power, Ohm's law, series and parallel circuits

A series circuit has a 12 V source, a 4 ohm resistor, and a 2 ohm resistor. What is the voltage across the 4 ohm resistor?

  1. 4 V
  2. 12 V
  3. 6 V
  4. 8 V Correct answer.

Explanation

Total resistance is 6 ohms, so the current is 2 A. The voltage across the 4 ohm resistor is 2 A times 4 ohms, which is 8 V, and the remaining 4 V appears across the 2 ohm resistor.

Question 13Basic Electrical Fundamentals: voltage, current, resistance, power, Ohm's law, series and parallel circuits

Which formula expresses Ohm's law correctly?

  1. I = V / R Correct answer.
  2. R = V x I
  3. V = I / R
  4. I = V x R

Explanation

Ohm's law relates voltage, current, and resistance as V = I x R, which rearranges to I = V / R and R = V / I. Only the division form listed matches the rearrangement.

Question 14Basic Electrical Fundamentals: voltage, current, resistance, power, Ohm's law, series and parallel circuits

Two 100 ohm resistors are connected in parallel, and that combination is placed in series with a 50 ohm resistor. What is the total resistance?

  1. 250 ohms
  2. 100 ohms Correct answer.
  3. 75 ohms
  4. 150 ohms

Explanation

The parallel pair of equal 100 ohm resistors gives 50 ohms. Adding the series 50 ohm resistor gives a total of 100 ohms.

Question 15Basic Electrical Fundamentals: voltage, current, resistance, power, Ohm's law, series and parallel circuits

A conductor carries 2 A for 30 seconds. How much charge passes a point in the conductor?

  1. 60 C Correct answer.
  2. 0.067 C
  3. 32 C
  4. 15 C

Explanation

Charge equals current multiplied by time, so 2 A times 30 s equals 60 coulombs. The ampere is defined as one coulomb per second, making the multiplication direct.

Question 16Basic Electrical Fundamentals: voltage, current, resistance, power, Ohm's law, series and parallel circuits

What happens to the total resistance of a parallel circuit as more branches are added?

  1. It becomes infinite when three or more branches exist
  2. It stays the same because voltage is unchanged
  3. It decreases because additional paths are provided Correct answer.
  4. It increases because more resistors are present

Explanation

Each added parallel branch provides another path for charge, so the equivalent resistance falls. The total is always less than the smallest branch resistance, unlike a series circuit where adding resistors increases the total.

Question 17Basic Electrical Fundamentals: voltage, current, resistance, power, Ohm's law, series and parallel circuits

A 240 V heater has a resistance of 40 ohms. What current does it draw and what power does it dissipate?

  1. 6 A and 240 W
  2. 9600 A and 2 304 000 W
  3. 0.167 A and 40 W
  4. 6 A and 1440 W Correct answer.

Explanation

Current is V/R, giving 240 divided by 40, or 6 A. Power is V times I, giving 240 times 6, or 1440 W, which also equals I squared R.

Question 18Basic Electrical Fundamentals: voltage, current, resistance, power, Ohm's law, series and parallel circuits

In a series circuit, which statement about current is correct?

  1. The current is the same through every component Correct answer.
  2. The current is largest through the largest resistor
  3. The current equals the supply voltage divided by each resistance separately
  4. The current is smallest through the smallest resistor

Explanation

A series circuit offers only one path, so the same current must pass through every component. Voltage divides among the components in proportion to their resistances, but current does not.

Question 19Basic Electrical Fundamentals: voltage, current, resistance, power, Ohm's law, series and parallel circuits

A resistor of 8 ohms and one of 24 ohms are in parallel across a 24 V supply. What is the total current drawn from the supply?

  1. 1 A
  2. 4 A Correct answer.
  3. 0.75 A
  4. 3 A

Explanation

Branch currents are 24/8 = 3 A and 24/24 = 1 A, and parallel branch currents add, giving 4 A. Equivalently, the parallel resistance is 6 ohms and 24 V divided by 6 ohms is 4 A.

Question 20Basic Electrical Fundamentals: voltage, current, resistance, power, Ohm's law, series and parallel circuits

A 1.5 V cell is connected across a 0.5 ohm resistor. What power is converted to heat?

  1. 0.75 W
  2. 3 W
  3. 4.5 W Correct answer.
  4. 0.33 W

Explanation

Current is 1.5 divided by 0.5, giving 3 A. Power is V times I, or 1.5 times 3, giving 4.5 W, which also matches I squared R.

Question 21Basic Electrical Fundamentals: voltage, current, resistance, power, Ohm's law, series and parallel circuits

A circuit has a 24 V source feeding a 6 ohm resistor in series with a parallel pair of 12 ohm and 12 ohm resistors. What current flows from the source?

  1. 4 A
  2. 1.33 A
  3. 3 A
  4. 2 A Correct answer.

Explanation

The parallel pair of equal 12 ohm resistors is 6 ohms, so total resistance is 6 plus 6, or 12 ohms. The source current is 24 divided by 12, giving 2 A.

Question 22Basic Electrical Fundamentals: voltage, current, resistance, power, Ohm's law, series and parallel circuits

A 100 W, 200 V lamp is operated at only 100 V. Assuming constant resistance, what power does it now dissipate?

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

Explanation

At rated voltage the resistance is 400 ohms, since R = V squared / P = 40 000/100. At half the voltage the power is V squared / R = 10 000/400, or 25 W, showing power falls with the square of voltage.

Question 23Basic Electrical Fundamentals: voltage, current, resistance, power, Ohm's law, series and parallel circuits

Three resistors of 4 ohms, 6 ohms, and 12 ohms are connected in parallel. What is the equivalent resistance?

  1. 22 ohms
  2. 2 ohms Correct answer.
  3. 0.5 ohms
  4. 3 ohms

Explanation

The reciprocals are 0.25, 0.1667, and 0.0833, summing to 0.5 siemens. Inverting gives 2 ohms, which is correctly smaller than the smallest resistor of 4 ohms.

Question 24Basic Electrical Fundamentals: voltage, current, resistance, power, Ohm's law, series and parallel circuits

In a two-resistor series circuit the voltage across the 3 ohm resistor is 6 V. The supply is 20 V. What is the value of the second resistor?

  1. 14 ohms
  2. 10 ohms
  3. 7 ohms Correct answer.
  4. 3 ohms

Explanation

The series current is 6 V divided by 3 ohms, or 2 A. The remaining voltage is 20 minus 6, or 14 V, so the second resistor is 14 V divided by 2 A, giving 7 ohms.

Question 25Basic Electrical Fundamentals: voltage, current, resistance, power, Ohm's law, series and parallel circuits

A battery with a 2 ohm internal resistance supplies a 10 ohm load and delivers 1 A. What is the battery's electromotive force?

  1. 12 V Correct answer.
  2. 10 V
  3. 8 V
  4. 24 V

Explanation

The total circuit resistance is 10 plus 2, or 12 ohms, and the current is 1 A. The electromotive force must therefore be 12 V, of which 10 V appears across the load and 2 V is lost internally.

Question 26Basic Electrical Fundamentals: voltage, current, resistance, power, Ohm's law, series and parallel circuits

A 12 V source is connected across a resistor and a current of 4 A flows. What is the resistance of the resistor?

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

Explanation

Ohm's law states that resistance equals voltage divided by current, so R = V / I. Dividing 12 V by 4 A gives 3 ohms. The other values come from multiplying or inverting these quantities incorrectly.

Question 27Basic Electrical Fundamentals: voltage, current, resistance, power, Ohm's law, series and parallel circuits

Which unit is used to express electrical power in a DC circuit?

  1. Coulomb
  2. Ohm
  3. Ampere
  4. Watt Correct answer.

Explanation

Power is the rate at which electrical energy is converted and is measured in watts. One watt equals one joule per second, or one volt multiplied by one ampere. The other units measure charge, resistance, and current respectively.

Question 28Basic Electrical Fundamentals: voltage, current, resistance, power, Ohm's law, series and parallel circuits

A 6 ohm resistor and a 3 ohm resistor are connected in series across a 9 V supply. What is the voltage across the 6 ohm resistor?

  1. 4.5 V
  2. 3 V
  3. 9 V
  4. 6 V Correct answer.

Explanation

The total series resistance is 9 ohms, so the current is 9 V / 9 ohms = 1 A. In a series circuit the same current flows through every element, so the voltage across the 6 ohm resistor is 1 A x 6 ohms = 6 V. The supply voltage divides in proportion to resistance.

Question 29Basic Electrical Fundamentals: voltage, current, resistance, power, Ohm's law, series and parallel circuits

Two resistors are connected in parallel across a fixed supply. One resistor is removed so that only the other remains connected. What happens to the total current drawn from the supply?

  1. It increases because the remaining resistance is lower.
  2. It decreases because the total resistance is now higher. Correct answer.
  3. It stays the same because the supply voltage is unchanged.
  4. It doubles because the resistance doubles.

Explanation

Adding parallel branches always lowers the equivalent resistance, so removing a branch raises it. With the supply voltage fixed, a higher total resistance means a smaller total current by Ohm's law. The current through the remaining branch itself is unchanged.

Question 30Basic Electrical Fundamentals: voltage, current, resistance, power, Ohm's law, series and parallel circuits

A 100 W, 250 V lamp is connected to a 250 V supply through a long extension lead whose two conductors together have a resistance of 5 ohms. Assuming the lamp resistance stays constant, what is the approximate power dissipated in the extension lead?

  1. 100 W
  2. 4 W
  3. 2 W
  4. 0.8 W Correct answer.

Explanation

The lamp resistance is V squared / P = 250 squared / 100 = 625 ohms. The total circuit resistance is then 625 + 5 = 630 ohms, giving a current of about 0.397 A. The lead dissipates I squared x R = 0.397 squared x 5, which is about 0.79 W, so roughly 0.8 W.

Question 31Electrical Wiring, Installation and Safety: domestic and industrial wiring, earthing, protective devices, shock prevention

In a domestic single-phase installation, which conductor is connected to the earth pit through the main earthing terminal?

  1. The neutral conductor
  2. The switch wire
  3. The earth continuity conductor Correct answer.
  4. The phase conductor

Explanation

The earth continuity conductor, often green-yellow, links exposed metalwork and socket earth pins to the main earthing terminal, which in turn connects to the earth electrode. The neutral is a current-carrying conductor and must never be used as a substitute for the protective earth.

Question 32Electrical Wiring, Installation and Safety: domestic and industrial wiring, earthing, protective devices, shock prevention

Which protective device operates by melting an internal element when sustained overcurrent flows through it?

  1. A rewirable fuse Correct answer.
  2. A miniature circuit breaker
  3. A residual current circuit breaker
  4. A contactor

Explanation

A rewirable fuse protects a circuit by the thermal melting of a calibrated wire element when current exceeds its rating for a sufficient time. Once the element melts the circuit is broken and the fuse wire must be replaced.

Question 33Electrical Wiring, Installation and Safety: domestic and industrial wiring, earthing, protective devices, shock prevention

What is the standard colour identification of the protective conductor in a flexible cord used for a domestic appliance?

  1. Green with yellow stripes Correct answer.
  2. Blue
  3. Black
  4. Brown

Explanation

The protective conductor in flexible cords and cables is identified by green insulation with yellow stripes throughout its length. This marking distinguishes it from live and neutral conductors so that it is never confused during termination.

Question 34Electrical Wiring, Installation and Safety: domestic and industrial wiring, earthing, protective devices, shock prevention

Which condition is most likely to cause electric shock to a person touching the metal casing of a faulty domestic appliance?

  1. The casing is bonded to the earth electrode
  2. The supply voltage is 230 V
  3. The earth continuity conductor has become disconnected Correct answer.
  4. The appliance is supplied through an MCB

Explanation

If the protective earth path is broken, a fault from live parts to the metal casing cannot operate the protective device quickly. The casing then remains at a dangerous potential and a person touching it provides a path to earth through the body.

Question 35Electrical Wiring, Installation and Safety: domestic and industrial wiring, earthing, protective devices, shock prevention

In industrial wiring, which wiring system is normally selected where protection against mechanical damage and moisture is important?

  1. Cleat wiring
  2. Batton wiring
  3. Steel conduit wiring Correct answer.
  4. Casing and capping wiring

Explanation

Steel conduit provides mechanical protection to the conductors and, when properly installed with sealed joints, resists moisture and dust ingress. It is therefore widely used in industrial premises where cables are exposed to damage.

Question 36Electrical Wiring, Installation and Safety: domestic and industrial wiring, earthing, protective devices, shock prevention

What is the primary purpose of equipotential bonding in a bathroom installation?

  1. To increase the tripping current of the MCB
  2. To keep all exposed and extraneous metalwork at substantially the same potential Correct answer.
  3. To reduce the current drawn by appliances
  4. To provide a second neutral path

Explanation

Supplementary equipotential bonding connects exposed conductive parts and extraneous conductive parts such as pipes so that no dangerous potential difference can develop between them. This reduces the risk of shock in wet locations where body resistance is low.

Question 37Electrical Wiring, Installation and Safety: domestic and industrial wiring, earthing, protective devices, shock prevention

Which of the following is the correct first action when a person is found in contact with a live conductor?

  1. Wait for the person to release the conductor
  2. Throw water on the conductor
  3. Switch off the supply or isolate the circuit Correct answer.
  4. Pull the person away with bare hands

Explanation

The supply must be isolated immediately, by switching off or removing the fuse, before the casualty is touched. Touching a person still in contact with a live conductor can make the rescuer a second victim.

Question 38Electrical Wiring, Installation and Safety: domestic and industrial wiring, earthing, protective devices, shock prevention

What does the rated current marked on a miniature circuit breaker indicate?

  1. The maximum current it can carry continuously without tripping Correct answer.
  2. The current at which it trips instantaneously
  3. The short-circuit breaking capacity
  4. The earth leakage current it detects

Explanation

The rated current of an MCB is the current it is designed to carry continuously under specified conditions without tripping. Tripping occurs above this value according to the time-current characteristic of the device.

Question 39Electrical Wiring, Installation and Safety: domestic and industrial wiring, earthing, protective devices, shock prevention

A 230 V single-phase circuit carries a load of 16 A. What is the approximate power consumed by the load at unity power factor?

  1. 230 W
  2. 2300 W
  3. 14.4 W
  4. 3680 W Correct answer.

Explanation

For a resistive load the power is the product of voltage and current, so 230 V multiplied by 16 A gives 3680 W. Power factor correction is unnecessary at unity power factor.

Question 40Electrical Wiring, Installation and Safety: domestic and industrial wiring, earthing, protective devices, shock prevention

An earth electrode resistance is measured as 8 ohms. Which statement about this value is correct for a domestic earth pit?

  1. It is higher than desirable and should be reduced by adding electrodes or improving soil contact Correct answer.
  2. It must be exactly zero for protection to work
  3. It proves the earth conductor is broken
  4. It is acceptable because any earth resistance will do

Explanation

A low earth electrode resistance, commonly aimed at a few ohms, ensures that fault current flows readily to earth and the protective device operates. A value of 8 ohms is generally regarded as too high for reliable fault clearance and should be improved.

Question 41Electrical Wiring, Installation and Safety: domestic and industrial wiring, earthing, protective devices, shock prevention

Which device provides both overcurrent protection and earth leakage protection in a single unit?

  1. A distribution board
  2. An MCB
  3. An isolator
  4. An RCBO Correct answer.

Explanation

A residual current circuit breaker with overcurrent protection, the RCBO, combines a residual current trip for earth leakage with thermal and magnetic overcurrent protection. It is used where both functions are needed on a single outgoing circuit.

Question 42Electrical Wiring, Installation and Safety: domestic and industrial wiring, earthing, protective devices, shock prevention

In a three-phase industrial motor circuit, why is a thermal overload relay installed in addition to a short-circuit protective device?

  1. To interrupt earth fault current
  2. To detect sustained overloads that would overheat the motor windings Correct answer.
  3. To improve the power factor
  4. To limit the starting current of the motor

Explanation

A thermal overload relay responds to sustained excess current, protecting the motor windings from overheating that a short-circuit device would not detect. The short-circuit device responds only to very high fault currents.

Question 43Electrical Wiring, Installation and Safety: domestic and industrial wiring, earthing, protective devices, shock prevention

What is the main reason for using a conduit box at every junction and bend in concealed conduit wiring?

  1. To reduce the earth resistance
  2. To allow conductors to be drawn in and joints to be inspected Correct answer.
  3. To increase the current-carrying capacity of the conductors
  4. To provide a mounting point for the energy meter

Explanation

Conduit boxes give access at bends and junctions so that conductors can be drawn through the conduit and joints can be made and inspected. Conductors are never jointed inside the conduit itself.

Question 44Electrical Wiring, Installation and Safety: domestic and industrial wiring, earthing, protective devices, shock prevention

Which statement correctly describes the function of a fuse in a circuit?

  1. It disconnects the neutral when the phase is switched
  2. It limits the voltage across the load
  3. It opens the circuit when the voltage rises above 250 V
  4. It breaks the circuit by melting when current exceeds its rating for a set time Correct answer.

Explanation

A fuse element is calibrated so that excess current heats and melts it, opening the circuit and stopping the fault current. Its operation depends on both the magnitude of current and the duration for which it flows.

Question 45Electrical Wiring, Installation and Safety: domestic and industrial wiring, earthing, protective devices, shock prevention

A workshop uses portable tools on a wet floor. Which measure most directly reduces the severity of electric shock if a tool develops an earth fault?

  1. Increasing the fuse rating
  2. Using a 30 mA residual current device Correct answer.
  3. Removing the earth connection
  4. Using longer supply cables

Explanation

A 30 mA RCD trips quickly on earth leakage current, limiting the duration of shock current through a person. This is the most direct protective measure for portable equipment used in wet conditions.

Question 46Electrical Wiring, Installation and Safety: domestic and industrial wiring, earthing, protective devices, shock prevention

What is the purpose of a main switch or linked isolator at the consumer's intake position?

  1. To measure the energy consumed
  2. To disconnect both live conductors from the installation for isolation Correct answer.
  3. To step down the supply voltage
  4. To provide earth leakage protection

Explanation

The main switch is a linked device that isolates the installation from the supply by opening the live conductors together, allowing safe work on the installation. It is not intended to protect against overcurrent or earth leakage.

Question 47Electrical Wiring, Installation and Safety: domestic and industrial wiring, earthing, protective devices, shock prevention

Why are socket outlets in a domestic installation normally wired in a ring final circuit rather than a single radial circuit?

  1. To allow a smaller earth conductor
  2. To increase the supply frequency
  3. To share the load current between two paths and reduce voltage drop Correct answer.
  4. To eliminate the need for a fuse

Explanation

In a ring final circuit each socket is fed by two paths, so load current divides and voltage drop and conductor heating are reduced for a given cable size. This allows more outlets to be served by the same cable.

Question 48Electrical Wiring, Installation and Safety: domestic and industrial wiring, earthing, protective devices, shock prevention

Which of the following correctly describes a TT earthing arrangement?

  1. The installation has no earth electrode at all
  2. The installation earth is connected to the supply neutral at the consumer's terminal
  3. The installation is earthed independently of the supply neutral and relies on an RCD for fault protection Correct answer.
  4. The supply neutral is used as the protective conductor throughout the installation

Explanation

In a TT system the consumer's earth electrode is separate from the supply neutral, so earth fault loop impedance is relatively high. Residual current devices are therefore relied upon to disconnect earth faults.

Question 49Electrical Wiring, Installation and Safety: domestic and industrial wiring, earthing, protective devices, shock prevention

A metalclad industrial panel is fed from a TN-S supply. What is the purpose of connecting its frame to the protective conductor?

  1. To reduce the supply voltage to the panel
  2. To balance the phases
  3. To provide a return path for load current
  4. To ensure fault current flows and trips the protective device if the frame becomes live Correct answer.

Explanation

Bonding the metal frame to the protective conductor creates a low-impedance path so that a fault to the frame results in a large current, promptly operating the overcurrent device. This keeps the frame potential safe during the fault.

Question 50Electrical Wiring, Installation and Safety: domestic and industrial wiring, earthing, protective devices, shock prevention

Which condition would cause the greatest risk of electric shock from a portable electric drill used outdoors?

  1. The drill is plugged into a circuit protected by a 30 mA RCD
  2. The drill is double insulated
  3. The drill is supplied through a fused plug
  4. The drill's metal casing is unearthed and a live wire touches it Correct answer.

Explanation

An unearthed metal casing that becomes live under a fault leaves the user exposed to the full supply potential with no return path to trip a protective device. This is the most dangerous condition among those listed.