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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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What is used as a protective device in an electric machine?
Fuse
Overload protection
Temperature meter
All of above
All of above
Quick Summary: Protective devices in electrical machines are essential components designed to isolate the machine from abnormal operating conditions such as overcurrent, short circuits, or excessive heating. While a fuse provides simple overcurrent protection, overload relays and thermal sensors provide more sophisticated, time-dependent protection to prevent damage to insulation and mechanical parts.
Protective devices in electrical machines are essential components designed to isolate the machine from abnormal operating conditions such as overcurrent, short circuits, or excessive heating. While a fuse provides simple overcurrent protection, overload relays and thermal sensors provide more sophisticated, time-dependent protection to prevent damage to insulation and mechanical parts.
Ifuse≈1.25×IFLC — Minimum fuse rating for motor startup
Q=I2Rt — Principle of thermal heating used in overload protection
Fuses provide high-speed protection against short circuits.
Overload protection prevents long-term thermal degradation of insulation.
Temperature sensors are vital for large machines where external current monitoring may not detect internal localized faults.
Electrical machine maintenance standards like IS 9000 recommend integrated protective schemes.
Prevents catastrophic machine failure
Reduces downtime costs
Ensures personnel safety
Adds cost and complexity to the machine control circuit
Requires periodic calibration and testing
Electric motors (AC and DC)
Power transformers
Industrial generators
The choice of device depends on the machine rating and the nature of the expected fault.
Option C, while sometimes considered a 'meter', acts as part of a protective scheme (e.g., thermal trip) in modern systems.
D is correct — Fuse, overload protection, and temperature meters are all integral parts of a coordinated protection system for electric machines.
In competitive exams, always look for 'All of the above' if the other options are distinct elements of the same system category (like protection or control).