Question

(b) A 220 V single-phase electrical installation is wired up by 2 nos. of 25 mm² single-core PVC cu. cables from distribution board to a load. A 50 A Type B MCB is used for the earth fault protection. 25mm single-core PVC Copper Cable SOA Type BMcB Z Metallic Enclosure 220V 50Hz Load Metallic Parts fault N 45m "H E Z 6mm single-core PVC Copper Cable Figure B4(b) Given that: • the cable length measured from distribution board to load is 45 m. • the resistance of the 25 mm² cable is 0.96 /km. • the resistance of the 6 mm? circuit protective conductor (spc) is 4.07 2/km. • k factor of PVC copper cable is 115. • the earth fault loop impedance external to the installation is 0.25 2. • relevant information is attached in Appendix (4). Determine: (1) the earth fault loop impedance; (ii) the earth fault current; (iii) the tripping time of the 50 A Type B MCB and comment on its value; (iv) whether the circuit protective conductor (CPC) is thermally protected; (v) whether the touch voltage is acceptable and suggest the improvement measures (in case the touch voltage is not acceptable). (3 marks) (1 mark) (2 marks) (4 marks) (4 marks)

Answer 1

Angweo Oversize Cable is waste of mone and ander Size calle Can Cause short circuit er) Fire. Also yoll have to Consider the type of Cable Csingle cores multi Corse) what ever is sveitable for your application. Last but not least is the way you lay the cable, you must must take into consideration that the cable is in a foremking (oof on Cable trays i) Earth fault loop impedance (zu) is the in Ohms of the Giscuit Parthe Farmed during a fault between earthed metal work and live Parts. It includes the external impedance Circuit Czes and the seesstance of the Phase and Circuit Protective Conductors Crithed. An earth fault Current is a cursent that flows the magnitude that de fondo on 7) to earth and has CScanned with CamScanner

methelio of system Carthing. In solidly earthed and low impedance earthed bystery high level of earth Facelt Govent result... In effectively earthed systems, Booth Fault Gressents may be higher than three-Phase Fault Currents. Miniature Circuit Breaker (MCBS = A miniature Circuit breaker is an electxo magnetic device that carries a complete molded insulating material. The Primary function of This device is to switch the Biscuit. This meang to alltomatically open the Circuit which has been Gonnected to it) when the current fasting uksough the Circaert goes beyond a Bet value 100) Simist. CS Scanned with CamScanner

. his device can be manually switched oN m) OFF just like normal Switches whenever necessary, Tripping mechanism in Miniature Ciscecit Bazaxer. As explained above section, an MCB has two types of tripping mechanism 1. Thermal tripping 2. Magnetic tripping the thosmal trip unit protects against Gerload Gussents the magnetic työp Unit Psetects against a short circuit Topping time; Type Tripping Cusyent ofesating time Type B 370 5 times the Full load Cussent 0.04 to 13 sec. Scanned with CamScanner

in) Ciscoeit Protective Conductors © the comit Pootective Conductors Cincseasinglly Called the ICRC) is a system of Conductors joining together all exported Conductive Parts and Competing them to main earthing terminal the toom includes the Courth earthing Conductor as well as the equipotential bonding Condiectors. so res it is thermally protected. y Requirements for touch voltage Accsoding to Section 411.5.3 of 357671; 2008 Amendment 3, touch voltage must not exceeded exceeded. for at any location on an installation. Some special locations, such as Seimming feeds, sequire that the # touch voltage does not exceeded 250, CS Scanned with CamScanner