Working of MCB practical

Working of MCB practical
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1 Answer

Answer :

Practical Significance

The switches and MCBs are the most important components of an electrical installation. They provide the safety & ease in smooth operation of the installation. It is essential to know the working of these components in an electrical installation.


Minimum Theoretical Background

Switches: All the switches are specified in accordance with their function, location , type of mounting, current capacity and working voltage.

Various types of switches according to their function and place of use are :

Single pole-one way switch, Single pole-two way switch, Intermediate switch, Bell push or push-button switch, Pull or ceiling switch, Double pole switch (D.P. Switches), Double pole - iron clad (D.P.I.C.) switch, Three pole- iron clad (T.P.I.C.) switch.


Miniature Circuit Breaker (MCB): MCB switches “OFF” the electrical circuit during overload and short circuits. MCBs have cuirent ratings ranging from 0.5 A to 100 A and, as their name implies, they have a compact size. There are three main types of MCBs, classified according to the current range at which they trip instantly.


  •  Type B - Trips at 3 to 5 times rated current, suitable for resistive or slightly inductive loads.
  •  Type C - Trips at 5 to 10 times rated current, suitable for moderate inductive loads.
  • Type D - Trips a t 10 to 20 times rated current, suitable for loads with a high inductive component.

Practical set-up / Circuit diagram / Work Situation
1. Test working of single pole switch and MCB.
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2. Working of MCB on short circuit
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Resources Required
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Precautions to be followed

1. Please read carefully the following precautions before operating Digital Multi-meter (DMM).

i. Check test leads for broken insulation before working.

ii. Don’t use ohmmeter section on live system.

iii. Don’t use ammeter section parallel to voltage source.

iv. Don’t touch the test lead terminals while connecting live circuits; otherwise it will give a shock.

v. Disconnect the test leads immediately after the testing.

2. While testing MCB proper ratings of fuses should be used in ICDP.


Procedure

1 Test the continuity of switches and MCB using DMM

(Read the users manual provided with DMM before using it)

a) Test the continuity of single pole one-way switch.

i. Turn the dial to Continuity Test mode. (If required, press the continuity button.)

ii. Insert the black test lead into the COM jack.

iii. Insert the red lead into the V Ohm jack. (When finished, remove the leads in reverse order: red first, then black.)

iv. Keep switch in “OFF’ position.

v. Connect the test leads across then terminals of SP switch.

vi. Observe the DMM.

1. If DMM will not beep, the circuit is open.

2. If DMM beeps, a complete path (continuity) is detected, means switch is short.

vii. Keep switch in “ON’ position.

viii. Observe the DMM.

1. If DMM will not beep, the circuit is open, means switch is faulty.

2. If DMM beeps, a complete path (continuity) is detected.

ix. Turn the DMM OFF.


b) Test the continuity of single pole two way switch.

Repeat the above procedure, on both two-way terminals of single pole two way switch.

c) Test the continuity of single pole MCB.

Repeat the procedure given in (a).


2 Test working of single pole switch and MCB.

i. Connect ICDP, variac, MCB, load and SP. switch as shown in the Fig. 1.

ii. Connect DMM across variac.

iii. Switch on the supply.

iv. Increase voltage in steps by varying variac and set at voltage rating of MCB.

v. Observe Ammeter.

vi. Switch on one of switch of load.

vii. Observe Ammeter.

viii. Switch on single pole switch.

ix. Observe Ammeter.

x. Increase the load up to 3 times current rating of MCB.

xi. Observe Ammeter

xii. Wait for some time.

xiii. Record the tripping time of MCB and ammeter reading in observation table.

xiv. Switch on MCB.

xv. Increase the load up to 4 and 5 time’s current rating of MCB.

xvi. Record the tripping time of MCB and ammeter readings in observation table.


3 Working of MCB on short circuit.

i. Connect ICDP, MCB, and knife switch (should be open) as shown in the Fig.2.

ii. Switch on the supply.

iii. Close the knife switch.

iv. Observe the working of MCB.


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