Electrical Safety and Earthing Questions and Answers

Answer: B

Miniature circuit breakers are made in different series, with different tripping characteristics for different loads. The key gives the G series for the protection of air conditioners, whose compressor motors take a high starting current. The DC series is meant for direct current circuits and so does not suit an AC load, and the L series is for ordinary lighting and resistive loads with little inrush, as the key reads it. The answer is G series MCBs, option (B), and this is a recall item from manufacturers' ranges.

Answer: C

If a person is in contact with a live electrical supply, the first thing to do is to break the contact by turning off the power supply at once, because the current will keep flowing through the victim as long as the supply is on. Pushing the victim with the bare hand would pass the current to the rescuer, and calling the ambulance or informing neighbours may be done afterwards but would not stop the current. The answer is Break the contact by turning off the power supply, option (C).

Answer: D

A voltage-operated earth leakage circuit breaker has a coil connected between the metal work of the installation and a separate earth electrode, and it trips when the potential of the metal work above earth reaches a set value. The key gives that value as 24 V for the purpose of safety, so option (D) is the answer. The values 2 V, 6 V and 12 V are lower than the setting, and the breaker would then trip for small harmless voltages. The item is a recall question and the key is followed.

Answer: D

An HRC fuse, a high rupturing capacity fuse, is built to break fault currents of very high value without damage to its surroundings, and its rupturing capacity is given as above 80 kA. It has a ceramic body filled with a quartz sand filler, which quenches the arc. The values above 10 kA, 20 kA and 40 kA are the capacities of ordinary fuses and do not describe a high rupturing capacity fuse. The answer is above 80 kA, option (D).

Answer: A

The alternating flux in an AC electromagnetic relay falls to zero twice in each cycle, so the pull on the armature varies and the contacts chatter. To reduce this noisy chatter, a shading coil, a short-circuited copper ring, is placed near the tip of the pole face; it gives a second flux out of phase with the main flux so that the pull never falls to zero. A restraining spring, holes in the disc and a brake magnet do not remove chatter from an AC relay. The answer is option (A).

Answer: D

In the Indian classification of fires, class A is solid material, class B is liquids, class C is gases and class D is metals. For a fire involving gases, the dry powder (dry chemical) extinguisher is used, so the answer is Dry powder type, option (D). A liquid type is for class A fires, carbon dioxide is for electrical and liquid fires, and a halon type is another gaseous agent, which the key does not choose for the gas fire.

Answer: C

In rod earthing the earth electrode is a rod driven into the ground, and a copper rod needs a minimum diameter so that it is strong enough to be driven and carries the fault current without corroding away. The key gives the minimum diameter as 12.5 mm, option (C). The larger sizes of 18.3 mm and 22 mm would be allowed but are not the minimum, and 5.6 mm is too thin to be driven into the ground and would soon corrode.

Answer: B

A sign in the form of a triangle with a thick border and a pictorial symbol, here a child running with a bag and the word SCHOOL, is a cautionary (warning) sign. It warns drivers that there is a school near the road. Mandatory signs are circular and tell what must be done, informatory signs are rectangular, and a police sign is not a standard category of road sign. The answer is cautionary sign, option (B). Road signs are of three kinds: mandatory signs which command, cautionary signs which warn and informatory signs which give information, and this sign belongs to the second.

Answer: A

When the victim has a chest injury from electric shock, the methods that press on the chest or back, such as the Holger Nielsen method and the Schafer method, would hurt the victim more, so they cannot be used. The mouth-to-mouth method gives air directly into the lungs without any pressure on the chest, so it is the one to adopt, option (A). The Nelson method is one of the pressure methods, and a hand-to-hand method is not a resuscitation method.

Answer: C

A fuse made of several strands in parallel does not carry the full sum of the strand ratings, because of the effect of heating of neighbouring strands, and the paralleling factor takes this into account.

\[ I = 3 \times 5 \times 0.8 = 12\ \mathrm{A} \]

So the overall current rating is 12 A, option (C). The value 15 A would be the rating without the paralleling factor, and 22 A and 8.3 A do not follow from the data. The paralleling factor is less than one because the strands do not share the current equally, and so the total is less than the sum of the ratings.