By L. Hewitson, et al.,
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Additional info for Practical Power System Protection
That is one more reason for using solid earthing in utility distribution transformers. 1 Electric shock and sensitive earth leakage protection There are four major factors, which determine the seriousness of an electric shock: 1. 2. 3. 4. Path taken by the electric current through the body Amount of current Time the current is flowing The body’s electrical resistance. 8). Persons are not normally electrocuted between phases or phase-to-neutral, almost all accidents are phase-to-earth. 9 shows the four stages of the effect of a current flow through the body: 1.
6. Transient overvoltages limited – prevents stressing of insulation, breaker restrikes. 1. 1 Solid earthing of power transformer Advantages • Neutral held effectively at earth potential. • Phase-to-ground faults of same magnitude as phase-to-phase faults; so no need for special sensitive relays. • Cost of current limiting device is eliminated. • Grading insulation towards neutral point N reduces size and cost of transformers. 28 Practical Power Systems Protection Disadvantages • As most system faults are phase-to-ground, severe shocks are more considerable than with resistance earthing.
Disadvantages 1. Open to abuse due to incorrect rating of replacement elements hence affording incorrect protection. 2. Deterioration of element as it is open to the atmosphere. 3 Cartridge type Silver element, specially shaped, enclosed in a barrel of insulating material, filled with quartz. 1). 1 Sectional view of a typical class – GP type 5 Cartridge fuse – link Advantages 1. Correct rating and characteristic fuse always fitted to a circuit-not open to abuse as rewireable type. 2. Arc and fault energy contained within insulating tube-prevents damage.