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MEngC PE Electrical Building Services study materials
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MEngC PE Electrical Building Services practice questions

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Question 1Electrical power systems and distribution design for buildings

In a TN-S earthing arrangement for a commercial building, how are the protective earth and neutral conductors configured from the transformer to the final circuits?

  1. Separate PE and N conductors with multiple earth-neutral links at every distribution board
  2. PE and N combined in the supply but separate within the installation downstream of the meter
  3. Separate PE and N conductors throughout, joined only at the origin of the installation Correct answer.
  4. Combined PEN conductor throughout with separation only at each final distribution board

Explanation

TN-S means the supply provides separate neutral and protective conductors all the way back to the source, and they are bonded together only at the origin (transformer star point and the main earthing terminal). Multiple links would create parallel neutral return paths and stray earth currents.

Question 2Electrical power systems and distribution design for buildings

A balanced three-phase 400 V load draws 50 A per phase at a power factor of 0.85. What is the approximate active power drawn?

  1. 58.9 kW
  2. 34.0 kW
  3. 17.0 kW
  4. 29.4 kW Correct answer.

Explanation

Active power for a balanced three-phase load is P = √3 × V_L × I_L × cosφ. Substituting gives 1.732 × 400 × 50 × 0.85 ≈ 29.4 kW.

Question 3Electrical power systems and distribution design for buildings

Why is discrimination (selectivity) between protective devices important in a building's distribution design?

  1. To ensure only the device nearest the fault operates, limiting disruption to healthy circuits Correct answer.
  2. To reduce the total number of protective devices required in the installation
  3. To guarantee that the main incoming breaker trips first to protect the transformer
  4. To lower the prospective short-circuit current at the origin of the installation

Explanation

Discrimination ensures that a fault is cleared by the protective device immediately upstream of the fault, so unaffected parts of the installation remain energised. This minimises downtime and limits the extent of supply loss.

Question 4Electrical power systems and distribution design for buildings

In a building with a 400 V three-phase, 50 Hz supply, what is the line-to-neutral voltage approximately?

  1. 115 V
  2. 690 V
  3. 230 V Correct answer.
  4. 400 V

Explanation

For a star-connected system, the line-to-neutral voltage equals the line-to-line voltage divided by √3. Thus 400 / 1.732 ≈ 231 V, conventionally taken as 230 V.

Question 5Electrical power systems and distribution design for buildings

What is the primary purpose of a residual current device (RCD) in a final circuit?

  1. To detect imbalance between live and neutral currents caused by earth leakage Correct answer.
  2. To limit the prospective short-circuit current during a phase-to-phase fault
  3. To correct the power factor of inductive loads on the circuit
  4. To interrupt overload currents that exceed the cable current-carrying capacity

Explanation

An RCD monitors the vector sum of live and neutral currents; any residual imbalance indicates current flowing to earth, and it trips at its rated residual operating current. It provides additional protection against earth fault and electric shock.

Question 6Electrical power systems and distribution design for buildings

A distribution board feeds a 30 kW balanced load at 400 V with a power factor of 0.8. What approximate line current flows?

  1. 31 A
  2. 108 A
  3. 43 A
  4. 54 A Correct answer.

Explanation

Line current is I = P / (√3 × V × cosφ). Substituting gives 30000 / (1.732 × 400 × 0.8) ≈ 54 A.

Question 7Electrical power systems and distribution design for buildings

Which standard cable insulation material is most commonly specified for general fixed wiring in buildings because of its good dielectric strength and cost?

  1. Silicone rubber
  2. Impregnated paper
  3. Cross-linked polyethylene (XLPE) Correct answer.
  4. Polytetrafluoroethylene (PTFE)

Explanation

XLPE is widely used for building wiring because it offers high dielectric strength, good thermal performance, and reasonable cost. Its higher operating temperature also allows smaller conductor sizes for a given current.

Question 8Electrical power systems and distribution design for buildings

What does the term 'prospective short-circuit current' refer to at a point in an installation?

  1. The rated current of the protective device installed at that point
  2. The maximum current that would flow if a bolted short circuit occurred at that point Correct answer.
  3. The residual current that an RCD is designed to detect
  4. The current that flows during normal full-load operation of the circuit

Explanation

Prospective short-circuit current is the maximum fault current that would flow into a zero-impedance fault at a given point, determined by source impedance and circuit impedance. Protective devices must have a breaking capacity exceeding this value.

Question 9Electrical power systems and distribution design for buildings

A 100 m run of single-core copper cable has a resistance of 0.02 Ω per 100 m per conductor. For a two-wire circuit, what is the approximate voltage drop when carrying 40 A?

  1. 0.4 V
  2. 1.6 V Correct answer.
  3. 0.8 V
  4. 3.2 V

Explanation

Voltage drop equals current times the total loop resistance. With 0.02 Ω in each of the two conductors, the loop resistance is 0.04 Ω, so the drop is 40 × 0.04 = 1.6 V.

Question 10Electrical power systems and distribution design for buildings

In a TN-C-S system, what is the term for the combined protective and neutral conductor in the supply portion?

  1. PE conductor
  2. PEN conductor Correct answer.
  3. N conductor
  4. CPC conductor

Explanation

In TN-C-S the supply uses a combined protective earth and neutral conductor called the PEN conductor, which is separated into distinct PE and N conductors at the origin of the installation.

Question 11Electrical power systems and distribution design for buildings

Why might a designer specify a 4-pole changeover switch for a building with a TN-S supply and a standby generator?

  1. To provide additional short-circuit protection during generator operation
  2. To switch the neutral as well as the three phases, preventing parallel neutral paths and stray currents Correct answer.
  3. To allow the generator to run in parallel with the utility supply
  4. To reduce the generator's fuel consumption during changeover

Explanation

Switching the neutral with the phases prevents the generator neutral from being connected to the utility neutral, which would create parallel neutral paths and stray earth currents. A 4-pole switch isolates all live conductors including neutral.

Question 12Electrical power systems and distribution design for buildings

A submain is protected by a 100 A MCCB with a breaking capacity of 25 kA. The prospective fault current at its terminals is calculated as 30 kA. What is the consequence?

  1. The MCCB is acceptable provided discrimination with upstream devices is achieved
  2. The MCCB is inadequate and a device with a higher breaking capacity is required Correct answer.
  3. The MCCB is acceptable because the fault current will be limited by cable impedance downstream
  4. The MCCB is acceptable because breaking capacity applies only to earth faults

Explanation

The device's rated breaking capacity must equal or exceed the prospective fault current at its point of installation. With 30 kA available and only 25 kA capacity, the device could fail to safely interrupt the fault, so a higher-rated device is needed.

Question 13Electrical power systems and distribution design for buildings

What is the main reason for installing a main equipotential bonding conductor in a building?

  1. To replace the circuit protective conductors of final circuits
  2. To carry the full load current of the installation under normal conditions
  3. To provide the sole earth electrode for the installation
  4. To keep metallic services at substantially the same potential and limit touch voltage during a fault Correct answer.

Explanation

Main equipotential bonding connects incoming metallic services to the main earthing terminal so they remain at similar potential, reducing touch voltages during faults. It does not normally carry load current.

Question 14Electrical power systems and distribution design for buildings

A designer needs to reduce voltage drop on a long submain without increasing conductor cross-section. Which measure is most effective?

  1. Reducing the power factor of the connected load
  2. Using a longer route for the submain
  3. Increasing the load current on the submain
  4. Raising the distribution voltage at which the power is transmitted Correct answer.

Explanation

Voltage drop is proportional to current and resistance; for a given power, raising the transmission voltage reduces the current and hence the drop. This is why higher distribution voltages are used for long runs.

Question 15Electrical power systems and distribution design for buildings

Which characteristic makes an MCB Type C trip curve suitable for a circuit supplying a motor with high inrush current?

  1. It trips between 5 and 10 times rated current, tolerating brief motor starting surges Correct answer.
  2. It has no magnetic trip element and relies solely on thermal operation
  3. It trips between 3 and 5 times rated current, giving fast magnetic operation
  4. It trips instantly at any current above its rating

Explanation

Type C devices have a magnetic trip threshold of roughly 5 to 10 times rated current, which allows motor starting inrush without nuisance tripping while still clearing faults. Type B trips at 3 to 5 times and is less tolerant of inrush.

Question 16Electrical power systems and distribution design for buildings

A building's calculated maximum demand is 250 kVA at 400 V. What approximate current must the main incoming switchgear carry?

  1. 361 A Correct answer.
  2. 250 A
  3. 625 A
  4. 181 A

Explanation

For a three-phase supply, I = S / (√3 × V). Substituting gives 250000 / (1.732 × 400) ≈ 361 A.

Question 17Electrical power systems and distribution design for buildings

Which factor most directly determines the current-carrying capacity of a cable installed in a thermally insulated wall?

  1. The colour of the cable sheath
  2. The nominal voltage of the system
  3. The length of the cable run alone
  4. The ability of the installation to dissipate heat, expressed through derating factors Correct answer.

Explanation

Current-carrying capacity is limited by the conductor's ability to dissipate heat. Enclosing a cable in thermal insulation restricts heat loss, so tabulated ratings are reduced by derating factors.

Question 18Electrical power systems and distribution design for buildings

In a building with both a TN-S supply and a generator, why is it important that the generator's neutral earthing arrangement be considered separately?

  1. The generator neutral must be left floating to prevent fault current
  2. The generator does not require any earthing because it is a standby source
  3. The generator may form a separate earth reference, and incorrect bonding can cause circulating currents or loss of protection Correct answer.
  4. The generator neutral must always be solidly bonded to the utility neutral to ensure protection operates

Explanation

A standby generator may establish its own earth reference, especially when operating as a separately derived source. Incorrect bonding between generator and utility neutrals can cause circulating currents or impair protective device operation, so the earthing design must be coordinated.

Question 19Electrical power systems and distribution design for buildings

What is the purpose of a surge protective device (SPD) installed at the main distribution board?

  1. To correct the power factor of the installation
  2. To divert transient overvoltages to earth and limit the voltage seen by downstream equipment Correct answer.
  3. To provide overload protection for the incoming supply cable
  4. To detect earth leakage current and disconnect the supply

Explanation

An SPD limits transient overvoltages from lightning or switching by diverting surge current to earth, clamping the voltage to a level downstream equipment can withstand. It does not provide overload or earth-leakage protection.

Question 20Electrical power systems and distribution design for buildings

A 400 V three-phase submain has a design current of 80 A and a measured voltage drop of 12 V at full load. What percentage voltage drop does this represent relative to the line-to-line voltage?

  1. 3.0 % Correct answer.
  2. 5.2 %
  3. 12 %
  4. 1.5 %

Explanation

Percentage drop is the drop divided by the reference voltage, so 12 / 400 × 100 = 3.0 %. This is within commonly applied limits for submains.

Question 21Electrical power systems and distribution design for buildings

Why is a TT earthing system often used where the supply is overhead and the earth impedance of the supply neutral cannot be relied upon?

  1. The installation shares the supply PEN conductor for both protection and neutral
  2. The installation omits the need for any earth electrode
  3. The installation relies on the supply transformer star point for protective earthing
  4. The installation uses its own local earth electrode, so protection does not depend on the supply neutral earth path Correct answer.

Explanation

In a TT system the installation's exposed conductive parts are earthed via a local electrode independent of the supply neutral. Because the loop impedance is relatively high, RCDs are typically required for fault protection.

Question 22Electrical power systems and distribution design for buildings

A designer is coordinating an upstream 400 A ACB with a downstream 100 A MCCB. Which approach best demonstrates correct discrimination?

  1. Relying solely on the breaking capacities of both devices being equal
  2. Comparing time-current curves so the downstream device clears faults before the upstream device operates Correct answer.
  3. Ensuring the upstream device has the lower current rating
  4. Selecting both devices with identical trip settings to simplify coordination

Explanation

Discrimination is verified by comparing the time-current characteristics of the two devices, ensuring the downstream device operates first for faults within its zone. Identical settings or equal breaking capacities do not by themselves provide selectivity.

Question 23Electrical power systems and distribution design for buildings

In a high-rise building, a rising main supplies multiple floors. Which consideration most strongly influences the choice of a busbar trunking system over cable?

  1. Elimination of the need for protective devices on each floor
  2. High current capacity with tap-off facilities at each floor and reduced installation space Correct answer.
  3. Lower initial material cost than any cable option
  4. Removal of the requirement for earthing of the rising main

Explanation

Busbar trunking offers high current capacity in a compact profile and allows tap-off units at each floor, simplifying distribution in tall buildings. It does not remove the need for floor protective devices or earthing.

Question 24Electrical power systems and distribution design for buildings

A building has a large number of non-linear loads such as LED drivers and variable-speed drives. Why might the designer specify a neutral conductor with a larger cross-section than the phase conductors in a four-wire distribution circuit?

  1. Larger neutrals reduce the prospective short-circuit current
  2. The neutral must always be larger than the phase conductors by regulation
  3. Triplen harmonic currents can accumulate in the neutral, potentially exceeding the phase current Correct answer.
  4. Larger neutrals eliminate the need for an RCD

Explanation

Triplen harmonics (3rd, 9th, 15th) from non-linear loads add in phase in the neutral, so the neutral current can exceed the phase current. Oversizing the neutral prevents overheating in such installations.

Question 25Electrical power systems and distribution design for buildings

When calculating the earth fault loop impedance for a TN system, which path must be considered to verify disconnection times?

  1. Only the resistance of the earth electrode at the building
  2. The impedance of the source, the phase conductor, and the protective conductor back to the source Correct answer.
  3. Only the impedance of the phase conductor to the fault point
  4. The impedance of the neutral conductor alone under balanced load

Explanation

Earth fault loop impedance comprises the source impedance plus the phase conductor and the protective conductor forming the return path. The measured or calculated value must be low enough for the protective device to disconnect within the required time.

Question 26Electrical power systems and distribution design for buildings

In a TN-S installation, a final circuit is protected by a 32 A Type B MCB. Which condition must be met to confirm that automatic disconnection occurs within the required time for a 230 V line-to-neutral fault?

  1. The prospective short-circuit current must be at least the rated current of the MCB
  2. The earth fault loop impedance must not exceed the maximum value stated by the device manufacturer for that device rating and disconnection time Correct answer.
  3. The circuit protective conductor must have a cross-section equal to the phase conductor
  4. The insulation resistance of the circuit must exceed 1 MΩ at 500 V test voltage

Explanation

Disconnection in a TN system relies on the fault loop impedance being low enough that the fault current trips the protective device within the disconnection time. The maximum permissible loop impedance is obtained from the device manufacturer's time-current data for the specified disconnection time. Insulation resistance, rated current and CPC sizing are separate requirements and do not by themselves confirm disconnection.

Question 27Electrical power systems and distribution design for buildings

A main distribution board is supplied by a 1250 A transformer through busbar trunking. Which parameter of the incoming protective device is the primary concern when the prospective fault current at the board is 40 kA?

  1. The frame size, which must be at least the transformer rating in kVA
  2. The rated operational voltage, which must be at least twice the system voltage
  3. The rated breaking capacity, which must be at least equal to the prospective fault current at the point of installation Correct answer.
  4. The number of poles, which must match the number of outgoing circuits

Explanation

The breaking capacity of a protective device must be at least the prospective short-circuit current at its point of installation, otherwise the device may fail to clear the fault safely. Voltage rating must match or exceed the system voltage, not double it. Frame size and pole count do not address fault current interruption.

Question 28Electrical power systems and distribution design for buildings

A design team is selecting the earthing arrangement for a rural building supplied by an overhead line where the supply neutral has a high earth impedance. Which arrangement is most appropriate, and why?

  1. TN-C-S, because the combined neutral and earth conductor removes the need for a local earth electrode
  2. IT, because the supply is unearthed and no earth electrode is required at the building
  3. TN-S, because a separate protective conductor from the supply guarantees a low fault loop impedance
  4. TT, because the installation uses its own earth electrode and RCD protection provides disconnection independent of the supply neutral Correct answer.

Explanation

In a TT system the installation's earth electrode and the supply neutral earth are separate, so a high supply neutral impedance does not compromise fault clearance. Disconnection is normally achieved by RCDs because the loop impedance is too high for overcurrent devices alone. TN variants depend on a reliable metallic return path to the supply earth, which is exactly what is unreliable here.

Question 29Electrical power systems and distribution design for buildings

A three-phase submain carries a balanced load of 60 kW at 400 V with a power factor of 0.85. What is the approximate line current?

  1. 153 A
  2. 88 A
  3. 120 A
  4. 102 A Correct answer.

Explanation

Line current equals power divided by (√3 × line voltage × power factor). Here 60 000 / (1.732 × 400 × 0.85) ≈ 102 A. Using the apparent power of about 70.6 kVA and dividing by √3 × 400 V gives the same result.

Question 30Electrical power systems and distribution design for buildings

A designer is choosing between a 4-pole and a 3-pole automatic transfer switch for a TN-S installation with a standby generator. Which consideration drives the choice of a 4-pole device?

  1. The switch must provide a fourth pole for a separate fire alarm supply
  2. The generator must supply more current than the mains, requiring an extra pole for capacity
  3. A 4-pole device is required whenever the supply voltage exceeds 230 V
  4. The generator neutral must be switched to prevent parallel neutral paths and circulating currents between the two earth references Correct answer.

Explanation

With a TN-S supply and a generator with its own neutral-earth link, leaving both neutrals connected creates parallel return paths and possible circulating currents. A 4-pole transfer switch isolates the neutral as well as the phases, preserving the intended earthing arrangement. Voltage level and load capacity do not determine pole count in this way.

Question 31Electrical power systems and distribution design for buildings

Which cable characteristic is used to correct tabulated current-carrying capacity when several circuits are bunched together in a single conduit?

  1. The thermal insulation factor, applied because conduit acts as thermal insulation
  2. The grouping factor, applied because adjacent loaded cables restrict heat dissipation Correct answer.
  3. The soil thermal resistivity factor, applied because conduit is buried
  4. The ambient temperature factor, applied because conduit raises the surrounding air temperature

Explanation

When circuits are grouped, mutual heating reduces the heat each cable can dissipate, so tabulated capacities are multiplied by a grouping factor. Ambient temperature, thermal insulation and soil conditions are separate correction factors applied where those conditions exist. Conduit grouping is specifically addressed by the grouping factor.

Question 32Electrical power systems and distribution design for buildings

A 400 V three-phase feeder has a design current of 150 A and a calculated voltage drop of 9 V line-to-line at full load. What is the percentage voltage drop?

  1. 2.25 % Correct answer.
  2. 3.9 %
  3. 1.1 %
  4. 5.6 %

Explanation

Percentage voltage drop is the drop divided by the nominal voltage, multiplied by 100. Here 9 / 400 × 100 = 2.25 %. Voltage drop limits for feeders are typically set well below this value, so the design would normally be acceptable.

Question 33Electrical power systems and distribution design for buildings

An upstream 400 A air circuit breaker must coordinate with a downstream 100 A MCCB. Which approach best demonstrates correct discrimination?

  1. Assuming discrimination is assured because the upstream rating is four times the downstream rating
  2. Selecting the upstream device with a lower breaking capacity than the downstream device
  3. Checking that the upstream device's time-current curve lies above the downstream curve for all fault currents up to the downstream breaking capacity Correct answer.
  4. Setting both devices to the same instantaneous trip threshold so they operate together

Explanation

Discrimination is demonstrated by comparing time-current curves so the downstream device clears faults in its zone before the upstream device responds. The comparison must extend up to the prospective fault current at the downstream device. A simple ratio of ratings is not evidence of coordination, and matching trip thresholds defeats selectivity.

Question 34Electrical power systems and distribution design for buildings

A building has a high concentration of LED drivers and variable-speed drives. Why might the designer specify a derated neutral or a larger neutral conductor in the distribution system?

  1. Harmonics increase the neutral voltage, which requires a larger conductor cross-section
  2. Non-linear loads reduce the neutral current, so the neutral must be sized to avoid overvoltage
  3. The neutral carries only unbalanced fundamental current, which is increased by harmonics
  4. Triplen harmonic currents from non-linear loads add in the neutral, so the neutral current can exceed the phase current Correct answer.

Explanation

Single-phase non-linear loads produce triplen harmonics (3rd, 9th, 15th) whose currents are in phase in the neutral and therefore sum arithmetically rather than cancelling. In a four-wire system with balanced fundamental loading the neutral current can exceed the phase current. Oversizing or derating the neutral, and considering harmonic-rated protective devices, mitigates this.

Question 35Electrical power systems and distribution design for buildings

Which measure most effectively limits the prospective short-circuit current at the outgoing ways of a main low-voltage distribution board supplied by a large transformer?

  1. Increasing the cross-section of the outgoing submain cables
  2. Adding more outgoing circuits to share the load
  3. Installing current-limiting devices or a busbar system with a higher short-circuit withstand rating Correct answer.
  4. Reducing the transformer tap setting to lower the secondary voltage

Explanation

The prospective fault current at a board is largely set by the transformer impedance and the impedance of the conductors between transformer and board. Where the fault level exceeds the rating of downstream equipment, current-limiting protective devices or higher-rated busbar and switchgear are used. Larger outgoing cables reduce impedance and would slightly increase fault current, while load sharing does not affect it.

Question 36Electrical power systems and distribution design for buildings

What is the primary function of a main equipotential bonding conductor connected to the incoming metallic water and gas services?

  1. To carry the full load current of the installation back to the supply transformer
  2. To ensure that simultaneously accessible metallic parts are at substantially the same potential, limiting touch voltage during a fault Correct answer.
  3. To provide an alternative return path for earth fault current in a TT system
  4. To protect the metallic services from lightning-induced surges

Explanation

Main equipotential bonding connects extraneous-conductive-parts to the main earthing terminal so that no dangerous potential difference develops between them and exposed-conductive-parts during a fault. It is not intended to carry load current or to serve as a fault return path. Surge protection is a separate function.

Question 37Building services electrical loads, demand and energy management

In an electrical building services design, the connected load is 850 kW and the designer applies a diversity (demand) factor of 0.65. What is the resulting maximum demand used for sizing the incoming supply transformer capacity?

  1. 1307.7 kW
  2. 552.5 kW Correct answer.
  3. 650 kW
  4. 850 kW

Explanation

Maximum demand is obtained by multiplying the connected (installed) load by the diversity factor: 850 × 0.65 = 552.5 kW. This reduced figure reflects that not all loads operate simultaneously at full rating and is the basis for sizing supply equipment.

Question 38Building services electrical loads, demand and energy management

Which quantity does the load factor of a building's electrical installation express?

  1. The ratio of reactive power to apparent power at the intake
  2. The ratio of average demand over a period to the maximum demand in that period Correct answer.
  3. The ratio of maximum demand to the connected load
  4. The ratio of energy exported to energy imported over a year

Explanation

Load factor is the average load divided by the peak load over the same period. A high load factor indicates steady consumption and more efficient use of supply capacity, whereas a low value signals peaky, under-utilised demand.

Question 39Building services electrical loads, demand and energy management

A building has a monthly energy consumption of 216,000 kWh and a recorded maximum demand of 600 kW. What is the load factor for that month (taking 720 hours in the month)?

  1. 0.72
  2. 1.20
  3. 0.50 Correct answer.
  4. 0.30

Explanation

Average demand is 216,000 kWh ÷ 720 h = 300 kW. Load factor = average demand ÷ maximum demand = 300 ÷ 600 = 0.50, meaning the installation peaks at twice its average draw.

Question 40Building services electrical loads, demand and energy management

A tenant's metered maximum demand is charged on a tariff with a capacity (demand) charge of $12 per kVA of billing demand per month. If the recorded maximum demand is 450 kVA, what is the monthly capacity charge before any power-factor adjustment?

  1. $540
  2. $54,000
  3. $5,400 Correct answer.
  4. $3,750

Explanation

The capacity charge equals billing demand multiplied by the unit rate: 450 kVA × $12/kVA = $5,400 per month. Demand charges recover the cost of providing capacity that must be available even if energy use is low.

Question 41Building services electrical loads, demand and energy management

Which measure most directly reduces a building's billed maximum demand without reducing the total energy consumed?

  1. Installing a standby generator for emergency loads
  2. Staggering the starting times of large chillers and motors Correct answer.
  3. Replacing tungsten lamps with LED luminaires
  4. Improving the maintenance schedule for air filters

Explanation

Staggering start times prevents large loads from coinciding, lowering the coincident peak that sets the demand charge while total kWh stays the same. Energy-efficiency measures reduce kWh but may not shift the peak.

Question 42Building services electrical loads, demand and energy management

A building's power factor is corrected from 0.80 to 0.95 lagging at the same active power. What is the effect on the apparent power drawn from the supply?

  1. It increases because more capacitors draw current
  2. It decreases because the reactive component is reduced Correct answer.
  3. It stays the same because active power is unchanged
  4. It doubles because the power factor has more than halved

Explanation

Apparent power S = P / power factor. Raising the power factor toward unity reduces S for the same active power because the reactive component is partly supplied locally by capacitors, relieving the supply and reducing line losses.

Question 43Building services electrical loads, demand and energy management

An installation draws 400 kW at a power factor of 0.75 lagging. What reactive power (in kvar) must be supplied by capacitors to raise the power factor to 0.95 lagging?

  1. 400 kvar
  2. 88 kvar
  3. 133 kvar
  4. 221 kvar Correct answer.

Explanation

Initial reactive power = 400 × tan(41.41°) ≈ 352.8 kvar; final reactive power = 400 × tan(18.19°) ≈ 131.5 kvar. The difference, about 221 kvar, is the capacitor rating required.

Question 44Building services electrical loads, demand and energy management

Which load characteristic makes a capacitor bank unsuitable as the sole power-factor correction for a supply dominated by large variable-speed drives?

  1. The drives draw purely capacitive reactive power
  2. The drives cannot tolerate any supply voltage
  3. The drives inject harmonic currents that can resonate with the capacitors Correct answer.
  4. The drives operate only at unity power factor

Explanation

Non-linear drive loads produce harmonic currents, and plain capacitors can form a resonant circuit with supply inductance, amplifying harmonics and risking overload. Detuned reactors or active filters are used instead.

Question 45Building services electrical loads, demand and energy management

In a building-services design, which factor is used to convert the total connected load into the maximum demand for sizing the main switchboard?

  1. The power factor
  2. The load factor
  3. The diversity (demand) factor Correct answer.
  4. The utilisation factor of the transformer

Explanation

The diversity or demand factor accounts for the fact that not all connected loads operate simultaneously, giving the coincident maximum demand. This reduced value is used to size switchgear and supply capacity.

Question 46Building services electrical loads, demand and energy management

Which of these building loads is the most likely candidate for demand-side load shifting to reduce peak demand?

  1. Fire alarm control panel
  2. A thermal storage chiller charging at night Correct answer.
  3. Corridor emergency lighting
  4. Lift motor during morning rush

Explanation

Thermal storage chillers can charge during off-peak hours and discharge during peak periods, shifting energy use away from the maximum-demand window. Safety-critical and occupant-driven loads cannot be rescheduled.

Question 47Building services electrical loads, demand and energy management

A building services engineer is asked to estimate annual energy use for a new office. Which approach gives the most defensible estimate at concept design stage?

  1. Reading the maximum demand from the incoming supply and multiplying by 8760 hours
  2. Assuming all connected loads run continuously at full rating
  3. Using the transformer nameplate rating as the annual consumption
  4. Applying a benchmark energy intensity per square metre and adjusting for building services Correct answer.

Explanation

Benchmark energy intensity (kWh/m²·yr) adjusted for the specific services gives a realistic early estimate. Multiplying peak demand by all hours assumes the peak occurs continuously and grossly overestimates consumption.

Question 48Building services electrical loads, demand and energy management

What is the primary purpose of a maximum demand controller in a building's electrical installation?

  1. To increase the transformer's rated capacity automatically
  2. To shed or defer non-essential loads before the demand reaches a preset threshold Correct answer.
  3. To correct the power factor of the installation
  4. To record individual tenant energy consumption for billing

Explanation

A maximum demand controller monitors demand and automatically sheds or defers non-essential loads to keep the peak below a set target, avoiding demand-charge penalties. It does not change capacity or correct power factor.

Question 49Building services electrical loads, demand and energy management

Which statement correctly describes the relationship between load factor and the cost-effectiveness of a supply capacity?

  1. A higher load factor increases the demand charge proportionally
  2. A lower load factor always reduces total energy consumption
  3. A higher load factor spreads fixed capacity costs over more energy, lowering unit cost Correct answer.
  4. Load factor has no bearing on the economics of supply capacity

Explanation

With a high load factor, the fixed cost of providing capacity is recovered over a larger energy throughput, reducing the cost per kWh. Low load factors mean capacity sits idle much of the time, raising unit costs.

Question 50Building services electrical loads, demand and energy management

In a building with a predominantly inductive load, what is the effect of adding shunt capacitors on the current drawn from the utility supply?

  1. The supply current increases because capacitors add load
  2. The supply current stays unchanged because active power is constant
  3. The supply current reverses direction through the meter
  4. The supply current decreases because reactive current is supplied locally Correct answer.

Explanation

Shunt capacitors supply the reactive current the inductive load needs, so less reactive current flows from the utility, reducing total supply current and losses. Active power is essentially unchanged.