Protection Devices For Electrical Machines & Control

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 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. Fiber 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. ) Failure of an outer conductor in three-phase system. ) Under voltage or over voltage. E) Variation of frequency. F) External heating, e. G. Due to rise of bearing temperature. G) Increase in ambient temperature. H) Flow of impeded coolant. Fuse also offers a conditional protection for the following cases:- a) Continuous starting and braking cycles. B) Locked rotor. C) Starting of motor with blocked rotor. Fuses should only be used in connection with other motor protections. The function of circuit breaker is similar to fuses but with added property of setting.

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It is available for single and three phase power supplies. Figure 3. 2. 1 illustrates the list of circuit breakers and their symbols. 14. 3 Motor Protection Designation of operating equipment: F Symbols: As fuses do not provide reliable motor protection, motor protective switches of thermal overload trip with bimetallic strips are used. These will be heated by the motor current. If a motor consumes too much of current, the bimetallic strip bends and the control will be interrupted via a mechanical link.

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