CHAPTER 5 QUESTIONS

5.1       QUESTIONS

1.          What is the fundamental difference between breaking a d.c. and an a.c. circuit?

2.          If an a.c. circuit undergoes a sudden short-circuit, what is the nature of the current waveform which immediately follows?

3.          Make a sketch of the current waves which immediately follow a short-circuit on a 3-phase system.

4.          What do you understand by the term ‘bolted fault’?  How does such a fault differ from the more likely arcing fault?

5.          What is the approximate ratio of the peak value of a short-circuit current to the rms value of the symmetrical current at that time?

6.              What effect will there be on the current in a purely inductive circuit following a short-circuit which occurs at:

(a)        a voltage peak (positive or negative)

(b)        a voltage zero

(c)        a voltage between zero and peak?

7.          If a ‘bolted’ short-circuit occurs across all the phases of a 3-phase inductive circuit, what effect will it have on the three current waveforms?

8.          What do you understand by the term ‘a.c. component’ and ‘d.c. component’ of a current waveform?

9.          How are the breaking capacities of circuit-breakers normally given?

10.        What do you understand by ‘restriking voltage’ in a circuit-breaker?

11.        What are the three main types of high-voltage switchgear met with on onshore and on offshore installations?

12.        What are the essential differences in arc control methods used by the three types of Q.11?

13.        How does the nature of the restriking voltage differ between an oil circuit-breaker and an air-break circuit-breaker?

14.        Why is a ‘puffer’ often employed with an air-break circuit-breaker?

15.        What is the principal difference between a circuit-breaker and a contactor?

16.        Why is a contactor usually fitted with back-up fuses?

17.        What precautions are necessary before withdrawing a circuit-breaker truck from, and reinserting it into, its switchboard housing?

18.        How is safety ensured when withdrawing a voltage transformer from a high-voltage switchgear unit?

19.        What types of circuit-breaker operating mechanisms are used?  Describe each briefly.

20.        What do you understand by ‘Trip Circuit Supervision’?  Why is it necessary?

21.        Why is it necessary to earth down a feeder or a busbar when work is to be done on it?  Describe two methods of doing this.

22.        What type of main (incomer and bus-section) switchgear would you expect to find on a low-voltage switchboard?

23.        How is general distribution arranged from an LV switchboard?

24.        Describe briefly (a) a moulded-case circuit-breaker (MCCB) and (b) a miniature circuit-breaker (MCB).

25.        How is access gained to the inside of an MCC distribution cubicle of an LV switchboard?

26.        If you wish to test the operation of an LV motor starting contactor without operating the motor, how would you do it?

27.        Can you start and stop a motor from its MCC cubicle?

28.        Why are busbars strongly braced in switchboards?

29.        Why are hollow busbars sometimes used for very heavy currents?

30.        What tests would you make on a circuit-breaker to establish (or check) the over-current protective relay settings?  What are the two main methods of testing?

31.        What are the advantages and disadvantages of these two types of test?

32.       How would you test the soundness of busbar and similar heavy-current joints in a switchboard?  How are the readings compared?