Practical Power System Protection
Providing both the underpinning knowledge and basic calculations needed to understand, specify, use and maintain power protection systems, this book also covers the practical techniques required on a daily basis.
TABLE OF CONTENTS Practical Power System Protection
Chapter 11: Low-Voltage Networks
11.1 Introduction
The low-voltage network is a very important component of a power system as it is at this level that much of the power is distributed and utilized by the end consumer. Essential loads such as lighting, heating, ventilation, refrigeration, air-conditioning and so on are generally fed at voltages such as 380, 400, 415, 480, 500, 525 V, three-phase 3 wire and three-phase 4 wire.
In mining industry, heavy motor loads often require voltages as high as 1000 V. Because of the diverse nature of the loads coupled with the large number of items requiring power, it is usual to find a bulk in-feed to an LV switchboard, followed by numerous outgoing circuits of varying current ratings, in contrast to the limited number of circuits at the medium-voltage level. Large frame (high current rated) air circuit breakers are therefore specified as incomers from the supply transformer and moulded case circuit breakers (MCCBs) for all outgoing feeders. The downstream network generally consists of MCCBs of varying current ratings and as the current levels drop miniature circuit breakers (MCBs) are used for compactness and cost saving.
11.2 Air Circuit Breakers
These breakers are available in frame sizes ranging from 630 to 6000 A in 3 and 4 pole versions and are generally insulated for 1000 V. Rated breaking capacities of up to 100 kA rms symmetric to IEC947-2 are claimed at rated voltage of 660 V.
Fixed and draw-out models are available and each unit invariably comes complete with a protection...
Copyright IDC Technologies 2004 under license agreement with Books24x7
