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Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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If a live wire comes in contact with the metal casing, the excess current will move to
power house
casing
dynamo
transformer
casing
Quick Summary: When a live wire comes into contact with a metal casing (chassis) of an appliance, the fault current flows through the casing. Proper electrical safety protocols require that this casing be connected to a protective earth (ground) wire to provide a low-resistance path for the fault current, which then trips the protective device (fuse or MCB).
When a live wire comes into contact with a metal casing (chassis) of an appliance, the fault current flows through the casing. Proper electrical safety protocols require that this casing be connected to a protective earth (ground) wire to provide a low-resistance path for the fault current, which then trips the protective device (fuse or MCB).
Ifault=ZfaultVphase — Current flowing through the fault path
Vtouch=Ifault⋅Rcasing — Voltage experienced on the casing
The metal casing acts as a conductive surface. In the event of insulation failure of the live wire, the potential of the metal casing rises to the potential of the live wire. By connecting the casing to an earth wire, the high current path is diverted to the ground, preventing the exterior of the device from remaining energized at high voltage.
The metal casing becomes live due to insulation breakdown (Direct contact).
Earthing provides the lowest resistance path for the leakage current.
The primary goal of grounding is personal safety against electric shock.
Without earthing, the metal casing remains at line voltage, posing a lethal risk.
Prevents electric shock to users
Facilitates fast operation of protective circuit breakers
Requires additional grounding infrastructure
Increased installation complexity
Domestic appliances (refrigerators, washing machines)
Industrial motor enclosures
Switchgear housings
The Earth wire should have significantly lower resistance than the human body to ensure safety.
Option A, C, and D are incorrect as the current immediately seeks the path of least resistance through the nearest grounding conductor or the structure itself, rather than flowing back to power generation sources like dynamos or transformers.
B is correct — If a live wire contacts a metal casing, the casing becomes live and the excess fault current will flow through the casing to the earthing conductor.
Remember that the fault current always follows Kirchhoff's Current Law; it will divert into any available path, specifically targeting the lowest impedance path to ground.