42 Chapter 2 • Cable joints and switchgear


Exercise 2.6


1. Name the two types of low tension domestic circuit-breakers with which you are familiar.


2. Explain how the time delay in each of the circuit-breakers that you named in question 1 above, is obtained.


3. Which one of the circuit-breakers mentioned in question 1 above, trips thermally and which one trips magnetically, on overload?


4. By what means do the circuit-breakers mentioned in question 1, trip under short-circuit conditions?


5. With the aid of a neat sketch, explain the operation of a thermal magnetic circuit-breaker.


6. With regard to circuit-breakers, what is time delay and why is it necessary? 7. What do you understand by inverse time delay characteristics?


8. Why are the three contacts of a three-phase circuit-breaker mechanically coupled?


9. What is the main advantage of having the three contacts of a three-phase circuit-breaker mechanically coupled?


10. What do understand by the term single-phasing and what does SANS 10142 state with regard to it?


11. What is the function of interpole barriers on three-phase circuit-breakers?


12. What are the special design features incorporated in the switching mechanism of a circuit-breaker?


13. How does the ambient temperature affect the operation of circuit-breakers?


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