Page 124 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

I.E. Rules regarding - Voltage drop concept:
1 I.E. Rule 48: The insulation resistance between the wiring of an installation and earth should be of such a
value that the leakage current may not exceed 1/50000 the part or 0.02 percent of the F.L. current.
2 The permissible voltage drop in a lighting circuit is 2% of the supply voltage plus one volt.
3 The maximum permissible voltage drop in a power industrial circuit should not be more than 5% of the declared
supply voltage.
4 The insulation resistance of any wiring installation should not be less than 1M .
5 The earth resistance should not exceed the value of one ohm.
I.E. Rules regarding to power wiring:
1 In a power sub circuit the load is normally restricted to 3000 watts and number of outlets to two in each sub
circuit.
2 All equipment used in power wiring shall be iron clad construction and wiring shall be of the armoured cable
or conduit type.
3 The length of flexible conduit used for connections between the terminal boxes of motors and starters, switches
and motors shall not exceed 1.25 meters

4 Every motor, regardless of its size shall be provided with a switch fuse placed near it.
5 The minimum cross-sectional area of conductor, that can be used for power mining of 1.25 mm for copper
conductor cables and 1.50 mm for Aluminium conductor cables (refer ISI recommendations). Hence VIR or
PVC cables of size lower than 3/0.915 mm copper or 1/1.80 mm Aluminium can not be used for motor wiring.

Circuit Breaker (CB) - Miniature Circuit Breaker (MCB)-

Moulded Case Circuit Breaker (MCCB)

Objectives: At the end of this lesson you shall be able to:
• explain the types, working principle and parts of a miniature circuit breaker.
• state the advantages and disadvantages of MCB
• state the categories and applications of MCBs
• state the application, advantage and disadvantage of MCCBs.

Circuit Breaker
A circuit breaker is a mechanical switching device capable of making, carrying and breaking the currents under
normal condition and breaking the currents under abnormal conditions like a short circuit.
Miniature circuit breaker (MCB)
A miniature circuit breaker is a compact mechanical device for making and breaking a circuit both in normal
condition and in abnormal conditions such as those of over current and short circuit.
Types of MCB’s
MCBs are manufactured with three different principles of operation namely
a Thermal Magnetic
b Magnetic hydraulic and

c Assisted bimetallic
Out of three MCB’s thermal magnetic MCB is discussed below

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CITS : Power - Electrician & Wireman - Lesson 20-25 CITS : Power - Electrician & Wireman - Lesson 20-25