Installation, Earthing & Safety MCQ – 31 previous year questions with solutions

IS 732 provides a code of practice for electrical wiring installations. It covers design, selection/erection and safety-related practices for building/low-voltage electrical installations under its applicable edition.

Under the IS 732 rule referenced by the paper, installation leakage current should not exceed 1/5000 of the full-load current. Limiting leakage is important for shock protection, insulation health and avoiding unwanted protective-device operation.

B — Suitably rated protective devices such as a fuse or MCB, which disconnect the circuit when current exceeds the safe value.

For a large power station, earth resistance should be extremely low so high fault currents can return safely and protective devices operate quickly. The exam's IS 3043-based target is ideally not exceeding about 0.5 Ω.

IS 3043 is the Indian Standard code of practice for earthing. It covers principles and practices for system earthing and protective/equipment earthing, including electrode arrangements and safety considerations.

A — CEA Regulations, 2010. The safety provisions of the Indian Electricity Rules, 1956 were replaced by the CEA (Measures relating to Safety and Electric Supply) Regulations, 2010.

The 2023 edition of India's National Electrical Code includes stronger/current provisions for modern electrical installations, including electric-vehicle charging infrastructure. That emerging installation area is the key focus among the options.

The protective earth conductor is identified by green in the convention used by this paper/IS 732 question. The distinctive earth-conductor colour prevents accidental connection as a live or neutral conductor.

More properly spaced earth electrodes, bonded together, give fault current more paths into the soil and lower the overall earth resistance.

Multiple or larger electrodes increase electrode–soil contact and current-spreading area, lowering earth resistance.

Equipment earthing provides a low-impedance path for leakage/fault current from exposed conductive parts to earth/protective conductor so protective devices can operate rapidly and dangerous touch voltage is reduced.

C — Electric shock. Protective earthing bonds exposed conductive parts to a fault-current path and works with automatic disconnection to reduce shock risk.

C — 0.5 Ω. This is the large-power-station earth-resistance value expected in the supplied examination.

Protective earthing provides a low-impedance path for fault current and stabilizes exposed conductive parts/neutral references. Earth resistance should be low enough to meet protection requirements and is checked periodically because soil/moisture/corrosion can change electrode performance. Key relation: Touch voltage and disconnection time depend on earthing and protective-device coordination.

Exam focus: Low earth resistance is desirable—not high resistance. Common trap: Never state that high earth resistance improves safety.

B — Resistivity decreases with increase in salt content. Dissolved salts add mobile ions to moist soil and increase its electrical conductivity.

C — The expected simultaneous use of the total load. Not all connected loads operate together; diversity factor = sum of individual maximum demands / maximum demand of the whole system (≥ 1).

A — Flexible copper wire. An appropriately rated, insulated flexible copper conductor is the suitable choice among the given options for the fan’s final supply connection.

D — Green. This is the traditional earthing-wire colour intended by the supplied options; modern protective-conductor identification commonly uses green-and-yellow.

C — PVC conduit does not conduct electricity. PVC is an electrical insulator, whereas metallic conduit is conductive (and must be earthed).

C — Two two-way switches in a crossover arrangement. Their strappers interconnect, so either switch can complete or break the lamp circuit, letting one lamp be controlled from two places.

D — Electrical layout. It shows the positions of switches, socket outlets, light points and other electrical accessories.

C — Copper. It is the standard practical conductor for residential wiring because of its high conductivity, ductility and reasonable cost (silver is costlier).

An RCCB/RCD detects imbalance between live and neutral currents caused by leakage to earth and is used for shock protection. An MCB protects conductors against overload and short circuit. Key relation: Typical additional personal protection uses 30 mA RCD/RCCB.

Exam focus: RCCB → leakage; MCB → overcurrent; SPD → transient surge. Common trap: Do not expect an RCCB alone to provide overload protection.

B — Phase. A protective fuse is placed in the phase (live) conductor so that a fault disconnects the supply to the load side.

An RCCB/RCD detects imbalance between live and neutral currents caused by leakage to earth and is used for shock protection. An MCB protects conductors against overload and short circuit. Key relation: Typical additional personal protection uses 30 mA RCD/RCCB.

Exam focus: RCCB → leakage; MCB → overcurrent; SPD → transient surge. Common trap: Do not expect an RCCB alone to provide overload protection.

An RCCB protects people against dangerous earth-leakage/residual current by comparing the currents in live and neutral conductors. If the imbalance exceeds its trip threshold, it disconnects the circuit rapidly.

Industrial cable selection must satisfy current-carrying capacity, voltage drop, installation conditions and short-circuit withstand. XLPE insulation generally permits a higher continuous conductor temperature than PVC. Key relation: Voltage drop≈I(R cosφ+X sinφ)L; adiabatic short-circuit relation I²t≈k²S².

Exam focus: Check ampacity, voltage drop and short-circuit withstand together. Common trap: Cable short-circuit rating is not independent of insulation temperature limits.

A splice makes a mechanically secure, electrically continuous joint between two conductor ends; insulation is then restored over it.

C — Zinc. Galvanizing protects iron or steel with a zinc coating that acts as a barrier and provides sacrificial corrosion protection.

The cone-of-protection method estimates the zone shielded by a lightning air terminal/lightning rod. A geometric protective zone is drawn below/around the rod so equipment or building portions inside the zone are considered protected under that simplified method.

Preventive maintenance aims to detect deterioration before failure. Electrical programs combine thermal inspection, insulation testing, earth-continuity/resistance checks, contact tightening, protection testing and oil diagnostics where applicable. Key relation: Thermal principle: abnormal I²R heating reveals high-resistance connections. Dielectric-strength tests determine the electric stress at which insulation breaks down under the specified test method.

Exam focus: Thermography = hot spots; earth tester = grounding health; BDV test = transformer oil insulation quality. Common trap: Believing dielectric testing applies only to solid cable insulation.