Zone : Physiological Effects

Zone 1: Usually no reaction effects.

Zone 2: Usually no harmful physiological effects.

Zone 3: Usually no organic damage to be expected. Likelihood of muscular contraction and difficulty in breathing, reversible disturbances of formation and conduction of impulse in the heart and transient cardiac arrest without ventricular fibrillation increases with current magnitude and time.

Zone 4: In addition to the effects of Zone 3, probability if ventricular fibrillation increased upto 5% (curve c2) upto 50% (curve c3) and above 50% beyond curve c3. It increases with magnitude and time, and path-physiological effects such as cardiac arrest, breathing arrest and heavy burns may occur.

1. Protection Against Electrocution: The use of exposed, substandard, badly wired, wrongly connected or damaged equipment as well as frayed cables or bad cable joints reduces the safety of an installation and increases the risk to persons receiving electric shock. Fuses and MCBs, whilst provide protection against over-currents and indirect contact, do not give any protection to any person, who may come into contact with live parts causing current to flow through the body to earth. A correctly chosen RCCB can detect such small currents flowing to earth and breaks the circuit to greatly reduce the risk of electrocution to human and domestic cattle at shock hazard voltages.

2. Protection Against Indirect Contact; In case of circuits with high earth fault loop impedance, the use of over-current protective devices is disqualified by wiring regulations. Please refer to the method of calculation of earth fault loop impedance for more details.

3. Protection Against Fire: The majority of fires occur as a result of faulty wiring, which results in a fault current flowing to the earth. Firs can be started by fault currents of this nature of less than 1 Amp. The normal domestic overload protective device, such as a fuse or an MCB, cannot detect such a small current. A correctly chosen RCCB shall detect this fault current, interrupt the supply, and greatly reduce the risk of fire.

4. Energy Conservation: Detection of small leakage currents and fault repair can result in a considerable saving in energy loss.

EXERCISE

1.    Explain with diagram the working of MCB.

2.    What is ELCB? Explain the brief its working.

3.    State the advantages/disadvantages of ELCB.

4.    Compare fuse with MCB.

5.    What protections are provided by RCCB?

6.    What are the different circuit interrupting devices? Explain any one of it.

7.    Give the types of HRC fuse. Explain any one type with constructional details.

8.    State the advantages and disadvantages of HRC fuse.

9.    Give the applications of HRC fuse.

10.  Compare fuse with circuit breaker.

11.  What is an isolator? Where it is used?

12.  Give the classification of isolators.

13.  Explain any one type of isolator.

14.  What is a circuit breaker? State its importance in the protective circuits.

15.  What are the different methods of Arc-interruption?

16.  What is RRRV? State the formula of re-striking voltage.

17.  Compare isolator with circuit breaker.

18.  Give the classification of circuit breaker.

19.  What are the different operating mechanisms for circuit breakers.

20.  State the properties of oil in CB.

21.  State the properties and drawbacks of SF₆ gas.

22.  What are the advantages and disadvantages of minimum oil CB?

23.  Compare bulk oil CB with air blast CB.

24.  Explain puffer type SF₆ CB.

25.  Explain vacuum CB.

26.  Explain ABCD.

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