Cork Institute of Technology
Bachelor of Engineering in Building Services Engineering – Award
(National Diploma in Engineering in Building Services Engineering – Award)
(NFQ – Level 7)
Summer 2005
Building Services (Electrical) Paper 2
(Time: 3 Hours)
Instructions
Answer FIVE questions.
All questions carry equal marks.
Examiners: Mr. J. C. Duggan
Mr. T. Dunne
Dr. N.J. Hewitt
Q1. (a) Two transformers with secondary voltages of 3.3kV are connected in parallel to supply
an industrial load of 140kW at a power factor of 0.85 (lag). Supply frequency is 50 Hz.
The effective resistance and reactance of transformer T1 are 3.0Ω and 2.5 Ω respectively.
Corresponding valves for T2 and are 2.0 Ω and 5.0 Ω. Determine:
(i) The power share of each transformer.
(ii) The power factor at which each transformer operates. (15 marks)
(b) With the aid of a neat labelled sketch explain the operating principles of a Bucholz Relay.
(5 marks)
Q2. A three phase 400V, 50Hz supply system has the following loads connected.
Single phase loads:
L
1 (R) Phase – 10 kW @ P.F. of 0.9 Lagging
L
2 (S) Phase – 12kW @ P.F. of 0.75 Lagging
L
3 (T) Phase - 8kW @ P.F. of 0.85 Lagging
Three Phase:
One, Balanced, 15kW Load at Unity P.F.
(i) State if the supply system is obtained from a star or delta connect source.
Briefly explain your answer. (5 marks)
(ii) Calculate the total current in each phase. (5 marks)
(iii) Determine the neutral current. (5 marks)
(iv) Find the total power. (5 marks)
