Pharos University EE-385

Dr. Sahar Abd El Moneim Moussa 1

Pharos UniversityEE-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.

• 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


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


- 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


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


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.


Major Types of Fuses:

1- Rewirable semi-enclosed Fuse2- Quartz sand-filled Cartridge HRCF3- HRC fuse type4- Expulsion fuse type5- Current Limiting Fuse


Low voltage Fuses


High voltage Fuses


PROTECTIVE DEVICES

(b) Circuit Breakers


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.


Low voltage Circuit-Breakers


Major Types of LVCB:

1- Miniature-type Circuit Breaker (MCB)

2- Molded Case-type Circuit Breaker (MCCB)

Prof Mah El-Gammal 18

1- Miniature-type Circuit Breaker (MCB)

• Type: 4-poles.• Overload protection Ir : is set fixed at rated current

value In.• Short-circuit protection Im : is set fixed at standard

setting of 5InIm10In

• Application: domestic use and illumination systems in commercial or industrial use.

• Standard current 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.

Pharos University EE-385

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