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2008 Feb Exam QUESTION 4
(a) An electrical circuit may develop either an overload or a short-circuit condition. Briefly explain the differences between these two conditions.
(b) Figure 4 shows part of the single-line diagram of a distribution board (DB):
Figure 4: Part of the single-line diagram of a DB
(i) Determine the operating time of MCB 1 if circuit 1 draws a constant current of 25 A.
(ii) If a line to earth fault of 200 A occurs at circuit 2, which of the protective device will trip first and what is the tripping time of this protective device?
(iii) Determine the operating times of MCB 3 and MCB 4 if circuit 3 develops a live-to-neutral fault and draws a current of 120A. Has “discrimination” been achieved between MCB 3 and MCB 4? Justify your answer with an explanation.
2007 Feb Exam – Question 1
(c) With reference to the time/current characteristic curve of a 50 A Type B circuit breaker;
i) determine the minimum operating current. (ii) what is the tripping time of a current of 100 A flowing through the circuit breaker?
2007 Aug Exam – Question 1
(b) Based on your understanding of the time current characteristics of an MCB, sketch a typical 50 A Type B time/current curve and state the current levels for the following operation of the MCB: (i) Will definitely trip instantaneously? (ii) May or May not trip instantaneously? (iii) Will trip in less than 1 hour after a time delay?
2007 Aug Exam – Question 2
(b) Describe briefly the meaning of the following abbreviated descriptions:- 400AF/250AT 25kA TP MCCB
2007 Aug Exam – Question 4
(a) If In = MCB’s nominal current rating, I2 = MCB’s minimum tripping current, Ib = design current of a load, Iz = current carrying capacity of a cable.
Indicate In, I2, Ib, Iz in a simple
(a) An electrical circuit may develop either an overload or a short-circuit condition. Briefly explain the differences between these two conditions.
(b) Figure 4 shows part of the single-line diagram of a distribution board (DB):
Figure 4: Part of the single-line diagram of a DB
(i) Determine the operating time of MCB 1 if circuit 1 draws a constant current of 25 A.
(ii) If a line to earth fault of 200 A occurs at circuit 2, which of the protective device will trip first and what is the tripping time of this protective device?
(iii) Determine the operating times of MCB 3 and MCB 4 if circuit 3 develops a live-to-neutral fault and draws a current of 120A. Has “discrimination” been achieved between MCB 3 and MCB 4? Justify your answer with an explanation.
2007 Feb Exam – Question 1
(c) With reference to the time/current characteristic curve of a 50 A Type B circuit breaker;
i) determine the minimum operating current. (ii) what is the tripping time of a current of 100 A flowing through the circuit breaker?
2007 Aug Exam – Question 1
(b) Based on your understanding of the time current characteristics of an MCB, sketch a typical 50 A Type B time/current curve and state the current levels for the following operation of the MCB: (i) Will definitely trip instantaneously? (ii) May or May not trip instantaneously? (iii) Will trip in less than 1 hour after a time delay?
2007 Aug Exam – Question 2
(b) Describe briefly the meaning of the following abbreviated descriptions:- 400AF/250AT 25kA TP MCCB
2007 Aug Exam – Question 4
(a) If In = MCB’s nominal current rating, I2 = MCB’s minimum tripping current, Ib = design current of a load, Iz = current carrying capacity of a cable.
Indicate In, I2, Ib, Iz in a simple