# Electrical Power & Machines “Electrical Engineering Dept” Prepared By: Dr. Sahar Abd El Moneim Moussa 1.

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Electrical Power & Machines “Electrical Engineering Dept” Prepared By: Dr. Sahar Abd El Moneim Moussa 1

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 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 Moussa3

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 Moussa4

- 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 Moussa5

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 Moussa6

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. I 2 t Value: It is a measure of the heat capacity of the fuse.

Dr. Sahar Abd El Moneim Moussa8  Major Types of Fuses: semi-enclosed Fuse 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 Moussa9 Low voltage Fuses

Dr. Sahar Abd El Moneim Moussa10

Dr. Sahar Abd El Moneim Moussa11 High voltage Fuses

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PROTECTIVE DEVICES (b) Circuit Breakers Dr. Sahar Abd El Moneim Moussa13

 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 Moussa14

Dr. Sahar Abd El Moneim Moussa15 Low voltage Circuit-Breakers

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Dr. Sahar Abd El Moneim Moussa17  Major Types of LVCB: Miniature 1- Miniature-type Circuit Breaker (MCB) Molded Case 2- Molded Case-type Circuit Breaker (MCCB)

Prof Mah El-Gammal18 1- Miniature 1- Miniature-type Circuit Breaker (MCB) Type: 4-poles.Type: 4-poles. Overload protection I r : is set fixed at rated current value I n.Overload protection I r : is set fixed at rated current value I n. Short-circuit protection I m : is set fixed at standard setting of 5I n  I m  10I nShort-circuit protection I m : is set fixed at standard setting of 5I n  I m  10I n Application: domestic use and illumination systems in commercial or industrial use.Application: domestic use and illumination systems in commercial or industrial use. Standard current ratings I n : 15, 20, 30, 40, 65, 80 and 100 A.Standard current ratings I n : 15, 20, 30, 40, 65, 80 and 100 A. Standard short-circuit protection I m : 4-kA (low-setting), 10-kA (standard setting) and 15-kA (high setting).Standard short-circuit protection I m : 4-kA (low-setting), 10-kA (standard setting) and 15-kA (high setting).

Prof Mah El-Gammal19 2- Molded Case 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.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.Type of protective relay: thermal- magnetic or electronic. Overload & short-circuit protection mode of operation: fixed or adjustable.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.Auxiliary: can be equipped with shunt tripping coil to receive tripping signals from other relaying devices.

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