Earthing, Safety and Pakistan Grid Practices MCQs 2026

Minimum size of main earthing conductor in many codes depends on

1. A only lamp wattage ✓
2. B largest incoming service conductor and fault level ✓
3. C only motor color code ✓
4. D only cable brand name ✓

Earth Leakage Circuit Breaker (ELCB) detects

1. A only overvoltage on lighting circuit ✓
2. B imbalance indicating leakage current to earth ✓
3. C only motor rpm ✓
4. D only transformer flux density ✓

Residual Current Device (RCD) operates when

1. A load power factor is unity only ✓
2. B voltage is exactly 230 V ✓
3. C motor is underloaded ✓
4. D vector sum of line currents deviates from zero (leakage to earth) ✓

Standard residential RCD sensitivity is often

1. A 300 A ✓
2. B 30 A ✓
3. C 30 mA for additional protection against shock ✓
4. D 3 A ✓

RCD does not replace

1. A overcurrent protection (MCB/fuse) for short circuit and overload ✓
2. B need for earthing ✓
3. C insulation on conductors ✓
4. D correct wiring polarity checks ✓

Voltage-operated ELCB is largely superseded because

1. A it is always faster than 1 ms ✓
2. B it depends on rise of earth voltage and may be less reliable ✓
3. C it eliminates need for earthing ✓
4. D it measures only DC traction current ✓

RCD nuisance tripping may be caused by

1. A perfect insulation with zero leakage always ✓
2. B unity power factor alone ✓
3. C correct TN-S wiring only ✓
4. D cumulative leakage from filters, capacitors and damp equipment ✓

Selective (time-graded) RCD coordination uses

1. A all 30 mA instant on main incomer only ✓
2. B no earthing on sub-circuits ✓
3. C higher sensitivity downstream and delayed upstream devices ✓
4. D only fuse without any RCD ✓

RCD test button creates

1. A permanent short of phase to neutral for load ✓
2. B simulated imbalance to verify mechanical trip function ✓
3. C open-circuit of earth electrode ✓
4. D increase of supply voltage to 440 V ✓

IEC 60439 / modern switchgear may use RCBO which combines

1. A overcurrent protection and residual current protection in one device ✓
2. B only lighting dimmer ✓
3. C only surge arrester without MCB ✓
4. D only ammeter shunt ✓

30 mA RCD may not detect

1. A line-to-line fault if no earth path involved (unlikely on earthed systems) ✓
2. B line-to-earth fault through person ✓
3. C leakage via damaged cable to wet floor ✓
4. D appliance frame fault to PE ✓

Type AC RCD is suitable for

1. A pure DC leakage only without AC component ✓
2. B high-frequency PWM without Type A/B consideration ✓
3. C sinusoidal AC residual currents ✓
4. D only lightning impulse without evaluation ✓

ELCB/RCD must be connected such that

1. A neutral bypasses sensor always ✓
2. B earth wire carries load current intentionally ✓
3. C all live conductors pass through the toroidal sensor ✓
4. D phase is left out of sensor ✓

In TT installations RCD is often mandatory because

1. A earth resistance is always zero ✓
2. B earth fault loop impedance may be too high for fast MCB disconnection ✓
3. C neutral is solidly earthed at consumer ✓
4. D no touch voltage can exist ✓

RCD rated residual operating current IΔn is

1. A the load current rating In only ✓
2. B the leakage current at which the device must trip ✓
3. C the breaking capacity in kA only ✓
4. D the lamp lumen output ✓

Fire protection RCDs may be rated

1. A 300 mA with time delay for discrimination ✓
2. B 30 mA on entire factory incomer without grading ✓
3. C 3 A residual only ✓
4. D without any earthing ✓

Periodic inspection of RCD includes

1. A trip time test at rated residual current per IEC 61557 ✓
2. B only painting enclosure ✓
3. C only tightening lamp holder ✓
4. D only measuring lux level ✓

NEPRA in Pakistan regulates

1. A only telecommunications spectrum ✓
2. B only water supply tariffs ✓
3. C electric power sector including licensing and consumer protection standards ✓
4. D only aviation fuel pricing ✓

NTDC operates

1. A only domestic house wiring ✓
2. B only diesel generator retail sales ✓
3. C only street lighting design ✓
4. D national transmission grid and bulk power dispatch in Pakistan ✓

Grid Code issued under NEPRA framework defines

1. A technical and safety requirements for grid-connected parties ✓
2. B only color of insulators without electrical limits ✓
3. C only welding electrode sizes ✓
4. D only lamp CRI values ✓

Permissible voltage variation at consumer terminals in many standards is approximately

1. A ±6% or as per applicable regulation ✓
2. B ±50% without limit ✓
3. C exactly 0% always ✓
4. D ±30% for all loads ✓

Safety clearance from live overhead lines for workers is governed by

1. A only luminaire spacing tables ✓
2. B NTDC/utility safety rules and IE Rules aligned practices ✓
3. C only motor nameplate only ✓
4. D only cable color mnemonic ✓

Consumer earthing inspection under distribution safety rules checks

1. A only decorative wiring ✓
2. B only highest lamp wattage ✓
3. C only motor paint color ✓
4. D earth electrode resistance, bonding and RCD operation ✓

Working near exposed busbars in substation requires

1. A conductive jewelry for convenience ✓
2. B wet cotton gloves only ✓
3. C arc-rated clothing and insulated tools per hazard assessment ✓
4. D no barricades ever ✓

Grid frequency in Pakistan is nominally

1. A 60 Hz always without exception ✓
2. B 25 Hz for all transmission ✓
3. C 400 Hz for distribution ✓
4. D 50 Hz with operational limits per grid code ✓

Safety tagging on isolated NTDC circuits uses

1. A green tag meaning energize ✓
2. B no documentation ✓
3. C tags on unrelated feeders only ✓
4. D danger tags and earth tags after proven dead and earthed ✓

Incident energy study for NTDC/DISCO switchgear supports

1. A only lamp maintenance factor ✓
2. B selection of arc-flash PPE and safe working distances ✓
3. C only motor slip calculation ✓
4. D only cable charging MVAR ✓

Earthing of NTDC substation structures and equipment follows

1. A only single 1 ohm rod without study ✓
2. B engineered earth grid design for fault current and touch/step limits ✓
3. C floating earth without reference ✓
4. D connection to phase conductor ✓

Protection coordination on NTDC grid ensures

1. A all breakers trip on any fault simultaneously always ✓
2. B no relay settings ever changed ✓
3. C fault cleared by nearest breaker with backup discrimination ✓
4. D only fuse protection on 500 kV ✓

Authorization to work on NTDC substation equipment requires

1. A only verbal agreement ✓
2. B direct energization for test without lockout ✓
3. C permit-to-work and isolation with earth application procedures ✓
4. D bypassing interlocks routinely ✓

Lightning protection earth should be

1. A always completely isolated without bonding rules ✓
2. B connected only to phase conductor ✓
3. C bonded to power system earth where codes permit common electrode ✓
4. D left unconnected intentionally ✓

Earth mat in substations is provided to

1. A limit step and touch potentials during grid faults ✓
2. B increase fault current path impedance only ✓
3. C decorate switchyard only ✓
4. D replace insulation coordination ✓

Chemical earthing (maintenance-free) uses

1. A only dry sand without metal ✓
2. B conductive compounds around electrode to lower contact resistance ✓
3. C only insulating varnish on rod ✓
4. D only neon lamp gas ✓

Step potential hazard near earth electrode during fault is

1. A voltage across lamp filament only ✓
2. B flux in transformer core only ✓
3. C voltage difference between feet at different soil potentials ✓
4. D power factor of motor only ✓

TN-C-S (PME) combines neutral and earth on supply side with

1. A separate PE after split at consumer service ✓
2. B permanent combined PEN to all sockets without split ✓
3. C no earthing at transformer ✓
4. D only DC traction return ✓

TT system earthing has

1. A no earth electrode at all ✓
2. B phase and neutral shorted at load ✓
3. C only IT system without any earth ✓
4. D local earth electrode at consumer with utility neutral earthed separately ✓

Separate protective earth (PE) and neutral (N) must not be

1. A combined after the main earthing terminal except as defined by TN-S rules ✓
2. B bonded at every socket always incorrectly ✓
3. C left floating on consumer side ✓
4. D connected only to phase ✓

Ring earth electrode around building in IEC practice provides

1. A more uniform earth potential and lower resistance ✓
2. B higher touch voltage always ✓
3. C no connection to main earthing terminal ✓
4. D only lightning without power earthing ✓

Measurement of earth resistance commonly uses

1. A only voltmeter across open circuit ✓
2. B fall-of-potential (three-point) method ✓
3. C only wattmeter on lighting load ✓
4. D only oscilloscope on communication line ✓

Soil resistivity affects earth electrode resistance such that

1. A resistivity has no effect ✓
2. B dry rocky soil always gives lowest R ✓
3. C only pipe length color matters ✓
4. D higher resistivity soil gives higher electrode resistance ✓

Earth continuity conductor (ECC) ensures

1. A phase voltage appears on casing ✓
2. B metallic enclosures remain at earth potential during faults ✓
3. C neutral is never connected ✓
4. D fuses never blow ✓

IS/IEC practice requires main equipotential bonding to connect

1. A only lighting switches ✓
2. B incoming metal services and structural metal to earth terminal ✓
3. C only decorative paint ✓
4. D only plastic pipes without metal ✓

Plate earth electrode uses

1. A only overhead conductor ✓
2. B only fluorescent lamp ballast ✓
3. C buried metal plate (GI or copper) in a pit with alternating layers of coke and salt ✓
4. D only transformer oil only ✓

Pipe earth electrode is commonly made of

1. A PVC conduit only ✓
2. B wooden pole only ✓
3. C air gap only ✓
4. D galvanized iron or copper pipe driven vertically into soil ✓

Earth electrode resistance should generally be

1. A as high as possible ✓
2. B equal to load resistance ✓
3. C infinite for safety ✓
4. D as low as practicable per national code (often ≤1–5 Ω for small installations) ✓

Purpose of electrical earthing in installations is primarily to

1. A provide a low-impedance path for fault current and limit touch voltage ✓
2. B increase line voltage for efficiency ✓
3. C eliminate need for overcurrent protection ✓
4. D store energy in soil ✓

Distributed generator connecting to grid must meet NEPRA/NTDC requirements for

1. A uncontrolled energization during outage ✓
2. B no circuit breaker at POI ✓
3. C anti-islanding, protection, metering and power quality ✓
4. D reverse power flow without study ✓

Neutral earthing in transmission is often through

1. A floating without any monitoring ✓
2. B direct connection to street lighting circuit ✓
3. C only ELCB at 30 mA on EHV bus ✓
4. D substation transformer neutral reactor/resistor or solid ground per study ✓

Live-line maintenance on transmission requires

1. A any worker without PPE ✓
2. B shorting phase to earth without permit ✓
3. C ignoring weather limits always ✓
4. D trained crews, approved tools and safety distances per utility procedure ✓

Reporting of major grid disturbances to regulator supports

1. A reliability monitoring and corrective investment under NEPRA framework ✓
2. B only internal memo without data ✓
3. C deletion of event records ✓
4. D no protection review ✓