What is the device to protect equipments from the electric shock?

Question

UnlockIAS · Accepted Answer

Correct — A, Fuse. Read the object of the sentence before anything else: the question asks for the device that protects EQUIPMENT — उपकरणों, in the Hindi column — from an electrical surge, and that is exactly the fuse's job. A fuse is a deliberately weak link built into a circuit. It is a short piece of wire made of a metal or alloy of suitably low melting point, which NCERT lists as aluminium, copper, iron or lead, connected IN SERIES with the appliance so that the entire current has to pass through it, and usually encased in a porcelain cartridge with metal end-caps. The physics is Joule's law of heating, H = I²Rt. Because the fuse wire is thinner than the rest of the wiring it has the highest resistance per unit length in the loop, so it heats up faster than anything else; and because heating rises with the SQUARE of the current, a modest overcurrent produces a disproportionate temperature rise. Once the current exceeds the rated value the fuse wire melts, the circuit is broken, and the appliance and the house wiring are saved from the fire or damage that a short circuit or an overload would otherwise cause. Domestic fuses are rated 1 A, 2 A, 3 A, 5 A and 10 A, and the rating is chosen from the load: a 1 kW electric iron on a 220 V supply draws 1000/220 = 4.54 A, so a 5 A fuse is the correct one. Now the caveat that matters more than the mark, because it is a genuine safety misconception and this question's loose English invites it. A fuse does NOT protect a human being from an electric shock. It protects the circuit and the appliances from an unduly high current. A current of only a few tens of milliamperes across the chest can be lethal, which is thousands of times below the several amperes a domestic fuse must reach before it melts — the fuse would never even notice. What protects a person is the earth wire, green-insulated and run to a metal plate buried deep near the house, which keeps the metal body of an appliance at earth potential so, in NCERT's own words, the user may not get a severe electric shock; and, in modern installations, an ELCB or RCCB, which compares the current leaving on the live wire with the current returning on the neutral and trips within milliseconds when a small residual current — typically about 30 mA on devices sold for personal protection — is escaping somewhere else, such as through a person. The stem's wording, 'protect equipments', is precisely what makes Fuse the right answer here.

Why the other options are wrong — (b) A generator is a source, not a safeguard. It converts mechanical energy into electrical energy by electromagnetic induction — a coil rotated in a magnetic field, with slip rings in the AC version and a split-ring commutator in the DC version. It is the thing that pushes current into a circuit in the first place; asking a generator to protect equipment from excess current is asking the supply to protect you from itself. Nothing in its construction senses or interrupts an over-current.  (c) The exact converse device: a motor converts electrical energy into mechanical energy, using the force experienced by a current-carrying conductor placed in a magnetic field, and it is what drives fans, pumps, mixers and water coolers. A motor is a LOAD — one of the appliances that itself needs protecting, and one that draws a heavy starting current. Older NCERT editions set 'an electric motor converts mechanical energy into electrical energy' as a true-or-false item, and the answer is false; that is the generator. The statement was dropped in the rationalised text, so do not look for it in the book currently in print.  (d) The intelligent distractor, because it names the right job in the wrong sense. A rheostat or variable resistance does control current, by inserting resistance and letting Ohm's law do the rest, and it is used to dim a lamp or regulate a fan. But regulating current during normal operation is not the same as breaking the circuit when the current turns dangerous: a rheostat cannot disconnect the supply, and at short-circuit currents it would burn out itself. 'Current controller' is not the name of any standard protective device — those are the fuse and the miniature circuit breaker.

Concept: A domestic electrical installation faces three different hazards and answers each with a different device, and confusing them is the commonest and most dangerous gap in a candidate's knowledge. The first hazard is OVERCURRENT: either an overload, when too many appliances are drawn from one circuit or the supply voltage rises accidentally, or a short circuit, when the live and neutral wires touch directly and the resistance of the path collapses. The device for this is the fuse, or its reusable modern equivalent the miniature circuit breaker, placed in series in the live wire so that all the current passes through it; the fuse melts and the MCB trips, and in both cases the circuit is broken before the wiring can catch fire. The second hazard is LEAKAGE to the metal casing of an appliance such as an iron, toaster or refrigerator, and the device for that is the earth wire with green insulation, connected to a metal plate buried deep in the ground near the house, which offers a low-resistance path and holds the casing at earth potential. The third hazard is a residual current actually flowing through a human body, and the device for that is an ELCB or RCCB, which continuously compares the current going out on the live wire with the current coming back on the neutral and cuts the supply the instant the two fail to match. Fuse for the equipment, earthing and RCCB for the person: that separation is the whole of domestic electrical safety in one line.

State PCS 2025 Q130 — Science & Technology PYQ with Answer

State PCS 2025 Q130 — Science & Technology PYQ with Answer