NDA (II) 2018 General Ability Test · Question 92 of 146

Correct — C, red for live, green for earth, black for neutral. This is the domestic wiring convention that NCERT's Class 10 physics chapter on magnetic effects of electric current sets out, and the one this question is drawn from: in the three-core cable of a mains appliance the red wire is live, the black wire is neutral, and the green wire is the earth. The green core is the one that is easiest to fix in memory and hardest to get wrong in practice, because it is the safety conductor — it runs from the metal body of the appliance to a plate buried in the earth, so that if the live wire ever touches the casing the fault current runs to earth and blows the fuse instead of passing through whoever touches the appliance.

• (a)Red—live wire, Green—neutral wire, Black—ground wire — It gets the live wire right and then swaps earth with neutral. That swap is the dangerous one in real life — connecting an appliance body to neutral instead of to earth leaves the casing at the wrong potential if the neutral is ever broken.
• (b)Red—neutral wire, Green—ground wire, Black—live wire — The green earth is right but the other two are exchanged, making the black wire live and the red one neutral. The colour that carries the danger is red, and treating a red conductor as safe to touch is exactly the mistake the code exists to prevent.
• (d)Red—ground wire, Green—live wire, Black—neutral wire — Only the neutral is right. It puts the live supply on the green conductor, which is the one conventionally reserved for earth in this code, and demotes the red wire to earth. Every safety assumption built into the convention would be reversed.

A domestic mains supply reaches an appliance through three conductors with three separate jobs. The live wire carries the alternating supply at high potential and is the one the fuse and the switch must be placed in. The neutral wire is the return path and is held at or near earth potential at the supply end. The earth wire carries no current in normal working; it is bonded to the metal body of the appliance so that a fault current has a low-resistance path to ground, driving a large current that melts the fuse or trips the breaker and disconnects the supply. In this older Indian domestic code those three are coloured red, black and green respectively.

The reliable route through this item is the green wire. Green is universally the earth in this convention, so any option that puts anything but earth on green can be struck out at once — that removes (a) and (d) in one stroke. Between the two that remain, red against black for live, the memory hook is that red is the warning colour everywhere else in engineering and is the live conductor here too. Worth knowing but distinct: modern flexible cords sold under the international harmonised code use brown for live, blue for neutral and green-with-yellow stripes for earth. The jobs of the three conductors are identical; only the colours changed, and the earth conductor stayed green in both systems.

• In the Indian domestic code used by NCERT, red is the live wire, black the neutral and green the earth.
• The earth wire is connected to the metal body of the appliance and carries no current in normal operation.
• A fuse or a switch must always be placed in the live wire, so that breaking it isolates the appliance from the supply.
• The newer international harmonised colours are brown for live, blue for neutral and green-and-yellow for earth.

• Swapping earth and neutral. They sit at similar potentials in normal working but do completely different jobs during a fault.
• Applying the modern brown-blue-green/yellow colours to a question written around the older red-black-green code, or the reverse.
• Assuming the earth wire carries current all the time; in a healthy appliance it carries none.

NDA asks for the colour of a named conductor, for the purpose of the earth wire, or for which wire a fuse must be connected in.

• practice — not a real PYQ

  In a domestic electric circuit, the fuse should be connected in the

  • (a)earth wire
  • (b)neutral wire
  • (c)live wire
  • (d)any of the three

  Answer(c) live wire — only then does a blown fuse actually cut off the appliance from the high-potential supply.

• practice — not a real PYQ

  The earth wire in a household appliance is provided mainly to

  • (a)complete the circuit and carry the working current
  • (b)give a low-resistance path to ground if the live wire touches the metal body
  • (c)reduce the electricity bill
  • (d)increase the voltage available to the appliance

  Answer(b) give a low-resistance path to ground if the live wire touches the metal body — the resulting surge blows the fuse before anyone can receive a shock.