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Pharos University EE-385

Pharos University EE-385. Electrical Power & Machines “Electrical Engineering Dept” Prepared By: Dr. Sahar Abd El Moneim Moussa. PROTECTION. IMPORTANCE OF PROTECTION Any electric circuit is subjected all the time to up normal conditions such as short-circuit.

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Pharos University EE-385

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  1. Pharos UniversityEE-385 Electrical Power & Machines “Electrical Engineering Dept” Prepared By: Dr. SaharAbd El MoneimMoussa Dr. Sahar Abd El Moneim Moussa

  2. PROTECTION Dr. Sahar Abd El Moneim Moussa

  3. IMPORTANCE OF PROTECTION • Any electric circuit is subjected all the time to up normal conditions such as short-circuit. • Hence, any circuit needs protection against these up normal conditions, otherwise, it would fail • The function of any protection system is to sense the fault and to clear it through disconnect the faulted section. • Many devices and systems are found such fuses and circuit-breakers Dr. Sahar Abd El Moneim Moussa

  4. ARCING PHENOMENON • When protection devices disconnect the circuit at any point, an arc is generated between the separation points due to the discharged energy stored in the network. • This arc ionizes the separating medium between the two points of separation due to its extreme high temperature and this means that the S.C. current still found. • Hence, the task of the disconnecting device is to disconnect the faulted section and extinguish the generated arc Dr. Sahar Abd El Moneim Moussa

  5. The engineer must select the suitable disconnecting device which can perform this task properly and safely • The selection depends on the value of the S.C. current and the system voltage at the separation point Dr. Sahar Abd El Moneim Moussa

  6. PROTECTIVE DEVICES(a)FUSES • A fuse is a current interrupting device which opens the circuit by a fusing element (it is the part which is designed to melt when the fuse operates) when the current in the circuit exceeds a certain value Dr. Sahar Abd El Moneim Moussa

  7. Important Definitions: • Rated Current: It is the maximum r.m.s. current at the rated frequency which can pass through the fuse without increasing its temperature. • Rated Voltage: It is the maximum r.m.s. voltage for the fuse to put on. The operating voltage must be equal or less than the rated voltage. • Intterrupting rating: It is the maximum r.m.s. current that the fuse can interrupt safely under rated voltage . • I2t Value: It is a measure of the heat capacity of the fuse. Dr. Sahar Abd El Moneim Moussa

  8. Major Types of Fuses: 1- Rewirable semi-enclosed Fuse 2- Quartz sand-filled Cartridge HRCF 3- HRC fuse type 4- Expulsion fuse type 5- Current Limiting Fuse Dr. Sahar Abd El Moneim Moussa

  9. Low voltage Fuses Dr. Sahar Abd El Moneim Moussa

  10. Dr. Sahar Abd El Moneim Moussa

  11. High voltage Fuses Dr. Sahar Abd El Moneim Moussa

  12. Dr. Sahar Abd El Moneim Moussa

  13. PROTECTIVE DEVICES(b) Circuit Breakers Dr. Sahar Abd El Moneim Moussa

  14. Any LVCB has the following principal parts: • Trip Unit: This unit is responsible for sensing the fault and giving signal to the latching . This unit may be adjustable or unadjustable in time or current or both. It has a time-current C/C’s which determine the operation of the CB. This unit may be mechanical or electronic. • Latching Mechanism: This is a mechanical part which receives order from the trip unit and separates the two poles of the CB which results in opening the metallic parts of circuit. • Arc Extinguish Chamber: This is a chamber provided with means to put out the generated arc. Dr. Sahar Abd El Moneim Moussa

  15. Low voltage Circuit-Breakers Dr. Sahar Abd El Moneim Moussa

  16. Dr. Sahar Abd El Moneim Moussa

  17. Major Types of LVCB: 1- Miniature-type Circuit Breaker (MCB) 2-Molded Case-type Circuit Breaker (MCCB) Dr. Sahar Abd El Moneim Moussa

  18. 1- Miniature-type Circuit Breaker (MCB) • Type: 4-poles. • Overload protection Ir: is setfixedat rated current value In. • Short-circuit protection Im: is setfixedat standard setting of 5InIm10In • Application: domestic use and illumination systems in commercial or industrial use. • Standardcurrent ratings In: 15, 20, 30, 40, 65, 80 and 100 A. • Standard short-circuit protection Im: 4-kA (low-setting), 10-kA (standard setting) and 15-kA (high setting). Prof Mah El-Gammal

  19. 2- Molded Case-type Circuit Breaker (MCCB) • Characteristics: heavy-duty industrial circuit breakers of large current ratings up to 1200A and breaking short circuit capacity up to 75kA. • Type of protective relay: thermal-magnetic or electronic. • Overload & short-circuit protection mode of operation: fixed or adjustable. • Auxiliary: can be equipped with shunt tripping coil to receive tripping signals from other relaying devices. Prof Mah El-Gammal

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