Earth leakege circuit breaker

EARTH LEAKEGE CIRCUIT BREAKER

1. Topic : earth leakage circuit breaker (ELCB) : MINIATURE CIRCUIT BREAKER (MCB) MADE BY: ID NO. En NO..

2. ABOUT ELCB: The earth leakage circuit breaker (ELCB) is one such device which senses the extremely small leakage currents and thus prevents death , injury or fire arising from the electric shocks . The device , also called a residual current circuit breaker (RCCB) cut off the supply before any real danger arises.

3. The ELCB or the RCCB operate very quickly with extremely small leakage currents i.e. the time in which they operate is far less as compared to that of the fuse or the ( MCB). These breakers are extensively used in the residential , commercial and industrial premises.

4. Working of an ELCB: The ELCB consists of an operating coil and a trim mechanism which in turn trips the supply to the circuit. It will operate if a live terminal or conductor is touched by an earthed object or person provided the leakage current exceeds the rated operating current of the ELCB .

5. Constuction of ELCB: The phase and neutral wires of the circuit being protected , are led through one winding of a transformer . A second winding contols the tripping mechanism . Under normal condition , when there is no leakage current the tripping mechanism is not energized and the relay contacts are kept closed using a spring mechanism .

6. When the earth leakage occurs from either the phase or the neutral side of the circuit , the currents are unbalanced by an amount equal to the leakage current and if this current reaches the rated tripping current of the ELCB , the tripping mechanism will operate opening he ELCB contacts . A test button is included in the ELCB for testing its trip mechanism . This test should be carried out periodically to ensure its satisfactory working . The ELCB can be reset manually after its operation . But it cannot be reset , as long as the leakage persists .

7. The ELCB is provided near the main switch board to protect the whole of the residence installation from leakage in any circuit or appliances , by switching off the supply .

8. MINIATURE CIRCUIT BREAKER (MCB) The miniature circuit breaker is an electro – mechanical device that operates and disconnect the circuit when the current reaches a certain pre determined value . The MCB is the most modern substitute for a conventional rewirable fuse and provides protection against overloads and short circuits .

9. The MCB operates at 125% of the rated currents , E.g. a 10 amp .MCB will operates at 11.5 amp as compared to a rewirable fuse of the same rating which will operates at 20 amp.

10. Construction of an MCB :

11. Components of MCB 1) Silver Graphite contacts 2)Arc Chutes & Horns 3)MCB Housing 4)Inner Mechanism 5)Angular vents 6)Simultaneous tripping of all phase in multiple MCB’s

12. 1) Silver Graphite contacts:: While breaking very high short circuit currents , the breaker contacts separate. These contacts remain closed under normal condition . If ordinary silver alloy contacts are used , the contacts normally bounce , resulting in contact welding . For this purpose silver – grafite is used for the contacts since it is a non melting material.

13. 2) Arc Chutes or Horns : :As the contacts separate , an arc is drawn between them . The arc is nothing but a self sustained discharge of electricity that is produced when the contacts separates . Under normal condition , since the currents carried by the contacts is less , the stength of the magnetic field produced is also less . However during short- circuit or over loads , since the current in the circuit increases , the strength of the magnetic field produced also increases .

14. 3) MCB Housing :: The body of the MCB is made of plastic material having high melting point , high dielectric strength and minimum water absorption. 4) Inner Mechanism :: The moving contacts of the MCB are closed in a sheet metal casing and not inside plastic . This is because plastic will tend to charge its shape and accuracy under continuous stresses of load and consistent tripping .

15. 5) Angular vents :: are provided to provided to prevent the dust from entering into the casing . Thus dust free perfect electrical contacts are obtained . 6)Simultaneous tr ipping of all phases in multiple MCB’s:: The tripping design is so done that in case of any over load or short circuit in one phase , all the phases trip simultaneously . Thus , the damage done due to single phasing is reduced .

16. WORKING OF MCB There are two main operation of the MCB . (1) The first operation is thermal , resulting in the inverse time current characteristics . This operation is obtained with the help of the bimetallic strip . Here , as the current flowing through the bi- mettalic strip increases during overloads , it deflects or bends and in the process causes the fixed and moving contacts to separate . The inverse time current characteristics mean , higher the current , smaller will be the time in which the MCB operates and vise -versa .

17. (2) During short circuits , as the current rises to very high levels very rapidly , the rising current energizes the solenoid . This in turn operates the plunger to strike the trip level . This causes the fixed and moving contacts to separate .

18. Thus an MCB instantly switches off the supply automatically , when there is a short circuit or an overloads .In this way , damage to the equipments and wiring is prevented . The supply is re-started after tripping by simply switching on the MCB switch . The MCB does not show wear and tear with time . It does not require any maintenance or repairs.

19. MCB’s are used in residences , offices , shops , in the complete wiring of building etc. Though they are expensive as compared to fuses , there is no cost involved in resetting them after operation . We simply have to switch on the MCB . In case of the fuse , it will have to be re-wired once it has blown out . MCB’s are being used extensively these days and have virtually replaced the re- wirable fuses .