N2 Electrical Trade Theory Module 6 Protection

CAMPUS: ITEMOHELENG SUBJECT: ELECTRICAL TRADE THEORY LEVEL: N2 MODULE 6: PROTECTION LECTURER: MOFOKENG AD Mr Downloaded from hlayiso.com

MODULE 6: PROTECTION LEARNING OBJECTIVES/LEARNING OUTCOMES Student should be able to apply knowledge, and understand how to protect electrical installation using the following protective devices and measures: 6.1 Earth leakage protection 6.2 Core balance earth leakage relay 6.3 Phase imbalance 6.4 Surge protection 6.5 Earth current compensation 6.6 Overload relay 6.8 Fuses Downloaded from hlayiso.com

Module 6: Protection EARTH LEAKAGE PROTECTION • The purpose of an earth leakage unit is to detect an earth fault current and to automatically disconnect an installation or circuit from the supply, within a specified time, when it exceeds a specified or predetermined value.  Or Is to detect an earth fault current an automatically disconnect the supply within a specified time, when fault current exceeds the rated value. Downloaded from hlayiso.com

Module 6: Protection (continued) CORE BALANCE EARTH LEAKAGE RELAY Core balance earth leakage relays are basically current transformers which operate tripping devices when they detect an earth fault current which exceeds a predetermined or rated value, which is 30mA Downloaded from hlayiso.com

Module 6: Protection (continued) Core balance Earth leakage relay  CORE BALANCE EARTH LEAKAGE CONSIST OF THE FOLLOWING: 1. Test button- Use to simulates an earth fault current and to test the effectiveness of an earth leakage unit. 2. Relay energised by current. 3. Current transformer 4. Live phase 5. Neutral phase 6. Tripping mechanism Downloaded from hlayiso.com

Module 6: Protection (continued) PHASE IMBALANCE (Connection should be 3-Phase supply) In a multiphase installation, the circuits shall be so arranged that the total load is, as nearly as is practically possible, balanced over the three phases. With a rigidly balanced three-phase load such as a three-phase motor, any imbalance in the supply current will indicate a fault. Single- phasing is a severe case of phase imbalance. Motor will not operate if single phase occurs. 3-phase overload relays it will protect circuit against single phasing by tripping all the phases/lines if one current is excessive in one of the phases. Downloaded from hlayiso.com

Module 6: Protection (continued) WHEN THERE IS NO FAULT: No imbalance in an installation The current through live and neutral conductors are the same in size and direction, the phases are balanced.  When there is an earth fault current: Imbalance occurs when one current is larger than the other, an earth fault current greater than 30mA will cause the circuit to trip/earth leakage to trip.  THREE-PHASE EARTH LEAKAGE RELAYS AND THREE-PHASE CIRCUIT BREAKERS:  3-phase overload relays protect circuit against single phasing by tripping all the phases/lines if one current is excessive in one of the phases.  What are four faults that can lead to phase imbalance in a three-phase system? 1. Difference in size of current 2. Occur of single phasing 3. Open circuit 4. Short circuit Downloaded from hlayiso.com

Module 6: Protection (continued) SURGE PROTECTION Lightning is a major cause of short-circuits on overhead transmission and distribution lines. Transient voltages on the lines caused by lightning strikes may exceed the insulation level of the line. This would cause a flashover between a phase and earth or between phases. The function of a lightning arrester is to provide a path by which these surges are discharged to earth before a flashover on the line can occur. Downloaded from hlayiso.com

Module 6: Protection (continued) EARTH CURRENT COMPENSATION If the system is earthed at more than one point, a proportion of the earth fault current will return by a path other than that at which the relays are located. Earth compensation relays are required to correct this. Downloaded from hlayiso.com

Module 6: Protection (continued) OVERLOAD RELAYS An overload (overcurrent) relay is a protective device designed to protect a motor from overload during starting and running. Downloaded from hlayiso.com

Module 6: Protection (continued) FUSES The function of a fuse is to detect overcurrent or short-circuit currents and to then automatically disconnect the faulty equipment or circuit from the supply. Rating of a fuse: Fuse has two current ratings: namely , 1. an overcurrent 2. and a short-circuit rating. Downloaded from hlayiso.com

TYPES OF FUSES 1. High rupturing capacity (HRC) 2. Rewireable fuses 3. Edison screw fuse 4. Fast-blow fuse 5. Slow-blow fuse 6. Cartridge fuse Downloaded from hlayiso.com

SUMMARY  Know how protect an electrical installation using protective device and know its regulations as stipulated on SANS 10142.  Know wiring of earth leakage unit and its purpose.  The use of core balance earth leakage relay.  How to correct phase imbalance and what cause it.  The use of surge protection for low voltage.  Effect of severe starting can have on our protective device Do all exercises appear on module 6: Protection and also use previous question papers. Use college website: www.malutitvet.co.za Question should be post on WhatsApp Group: Electrical Trade theory N2 Downloaded from hlayiso.com Presentation by: Mr AD Mofokeng Thank you very much God Bless.