Protection Devices For Electrical Machines & Control

Topics: Switch, Circuit breaker, Electric motor Pages: 4 (698 words) Published: September 28, 2013
Chapter 14

Protection Devices

LESSON PLAN
ECT0313
Electrical Devices and Control
Location
R203/R209
Chapter
Protection Devices

14

Prepared by
Kamarudin
Objectives
1.
Observe and practice safety measures in electrical control.
2.
Practice proper protection of electric motor
References
Media
1.
Handbook of electrical motor control system, U.S. Eswar, Tata McGraw Hill Publishing Company Limited 1.
OHP
2.
Electrical Control 1,Theoretical Part1, Voest-Alpine, Austria. 2
LCD
CHAPTER 14PROTECTION DEVICES

14.0 Protection Earth PE – Earth-Leakage Protection

All chassis and enclosures for the electric equipment must be grounded. Neutral and safety ground must be connected at only one point and must be at the main service entrance. Improper earth protection will lead to certain hazards such as:

A) Hazard due to stray impedance.

B) Hazard due to direct contact

14.1 Fuse and Circuit Breaker

A) Fuse

Fuse is used to protect electrical devices and conductors from excessive current which arises from short circuits or overload current. Its enclosure is made from certain type of material. Fibre is popularly used because of its tough resistance to arcs, its mechanical strength and its excellent insulating properties. Higher operating voltage needs long lasting fuses.

14.2 Motor Protection with Fuses

Fuse alone does not protect electrical motors efficiently. It only serves as a short -circuit protection. In most cases, time-lag fuses are applied to release current of which depends upon the value of the starting current of the machine.

Fuse does not provide any protection in the following cases:-

a) Overload due to continuous operation.
b) High operating frequency of the machine.
c) Failure of an outer conductor in three-phase system.
d) Under voltage or over voltage.
e) Variation of frequency.
f) External heating, e.g....
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