Domestic Wiring and Illumination Questions and Answers

Answer: A

The official key marks both statements as true. Statement B matches the usual rule for domestic wiring that a sub-circuit should not carry more than about 800 W of load, and that each sub-circuit is protected by its own fuse or miniature circuit breaker at the distribution board, so that a fault on one circuit does not put the whole installation out. Statement A concerns the mounting height of the energy meter, a figure of 1.75 m above the floor, with the meter placed above the switchboard. I have not been able to verify the exact height in a published code, so this explanation follows the key. The options saying both are false, or only one is true, are rejected by the key.

Answer: D

A carpenter's hammer, such as the claw hammer, is specified by the weight of its head and the shape of the pein (the peen), which is the end of the head opposite the face, for instance the ball pein or cross pein. It is not specified by the length and width of the hammer, by the shape of the eye hole, which is only the hole in the head for the handle, or by the cross section of the cheek. The answer is weight and shape of the pein, option (D).

Answer: D

The Indian Electricity Rules require that the insulation resistance of a wiring installation, measured with a megger between conductors and earth and between conductors, should not fall below a stated minimum; the key gives that minimum as 1 megohm. Values of 2, 5 and 8 megohm are higher than the minimum the key gives, so they are not the limit. The answer is 1 M ohm, option (D). The question is a recall item of the rules, and the key is followed.

Answer: A

The Indian Electricity Rules limit the voltage drop in a circuit so that equipment gets close to its rated voltage, and for a power industrial circuit the key gives a drop of not more than 5 percent of the declared supply voltage. The larger values, 10, 25 and 35 percent, would let motors and other equipment run well below their rating, with higher currents and heating, so they are not the limit. The answer is 5%, option (A).

Answer: A

In a public supply wiring installation the conductors must be large enough that the voltage drop from the consumer's terminals to any point of the installation does not exceed 3 percent of the voltage at the consumer's terminals, as the key gives. The larger values of 13, 10 and 8 percent would let appliances get a much lower voltage than their rating and would waste energy as heat in the wires. The answer is 3%, option (A).

Answer: B

In an electric kettle the pressure plate holds the heating element firmly against its seat. If the plate is improperly or loosely fitted, the element is not held well and it moves as it heats and cools, so repeated expansion and contraction can damage the element. The other results are not caused by the plate: contact of the kettle body with the element would come from broken insulation, heat loss from the bottom is a matter of design, and scale forms from hard water. The answer is damage of element due to expansion and contraction, option (B).

Answer: A

In a workshop with large machines and obstructions, direct lighting from a few fittings would cast sharp shadows behind the machines and give glare. Indirect lighting sends the light to the ceiling and walls, from which it is reflected, so it is soft, spread out and nearly shadow free, which is what the key gives as the preferred scheme, option (A). Direct lighting is the one that casts shadows, and optical lighting is not a standard scheme; diffusing systems still send much light downward.

Answer: D

The diagram shows four lamps L1 to L4, each switched by its own switch S1 to S4, with the lamps and switches wired in the zig-zag manner between the neutral N and live L lines along a long run. The key gives this as tunnel wiring, in which a row of lamps along a tunnel is controlled in this way. It is not a godown lighting circuit or jail wiring, and corridor wiring uses two-way switches at the two ends. The answer is option (D); the item depends on recognising the diagram and the key is followed.

Answer: D

A sodium vapour lamp needs a high voltage to start the discharge and then a limit on the current while it runs. The leak transformer, which has a high leakage reactance, provides the high open-circuit voltage for ignition and also limits the current once the lamp has struck, since its leakage flux makes the output drop as the current rises. It does not prevent leakage of sodium, it is not the means of improving the power factor, and limiting the starting current is only part of its job. The answer is provide ignition and current limitation, option (D).

Answer: A

Luminous efficiency is the light output in lumens divided by the electrical power in watts, that is, the lumens given per watt.

\[ \eta = \frac{860\ \mathrm{lumen}}{60\ \mathrm{W}} \]

\[ \eta = 14.3\ \mathrm{lumen/watt} \]

So the luminous efficiency is 14.3 lumen per watt, option (A). The options in watt per lumen give the inverse quantity, which is not the luminous efficiency, and 6.97 is not the ratio of 860 to 60. Higher luminous efficiency means more light for each watt, and a filament lamp of 14.3 lumen per watt is in the usual range for a 60 W bulb, far below that of a fluorescent lamp.