Question

a) A copper strip 100 mm x 5 mm, 25 m long is buried at a depth of 1.5 m, in soil having a resistivity of 100 ?m Calculate the resistance to ground. (4 marks) a) Determine the minimum size of ground conductor considering a substation grid made of Zn coated steel having the following parameters and in keeping with IEEE-80 standard 419, 20.1H (10 marks CTTL b) A power distribution system having the main incorning feeder rated for 400 A, and the outgoing feeders rated up to 200 A. If the system is protected through HRC Fuses with aluminum used as grounding conductor with r.m.s current density of 126 assuming the final tempcraturc to bc at 325°C Calculate i. Ground Fault Current (2 marks) ii. Interrupting time of 400A HRC Fuscs using charactoristic curves provided in figure 2 (2 marks) iii. Main ground conductor size (2 marks) 20 EC 4 of5 542 05 ??5

Answer 1

Copper Strip area = 100 mm times 5 mm A = 500 mm^2 = 500 times times 10^-6 m^2 length = 25 m,out of which the strip us only at of 1.5 m thus l = 1.5 m of soil = rho = 100 Ohm m Thus resistence = rho l/A = 100 Ohm m times 1.5/500 times 10^-6 = 1.5 times 1000 times 1000/50 = 300 KOhm