cables

CONTENTS
CONTENTS
264

Principles of Power System

CHAPTER


Underground Cables
11.1
11.2
11.3
11.4
11.5
11.6
11.7
11.8
11.9
11.10
11.11
11.12
11.13
11.14
11.15
11.16
11.17
11.18
11.19
11.20
11.21
11.22
11.23

Underground Cables
Construction of Cables
Insulating Materials for Cables
Classification of Cables
Cables for 3-Phase Service
Laying of Underground Cables
Insulation Resistance of a Single-Core
Cable
Capacitance of a Single-Core Cable
Dielectric Stress in a Single-Core Cable
Most Economical Conductor Size in a
Cable
Grading of Cables
Capacitance Grading
Intersheath Grading
Capacitance of 3-Core Cables
Measurements of Ce and Cc
Current-Carrying Capacity of Underground Cables
Thermal Resistance
Thermal Resistance of Dielectric of a
Single-Core Cable
Permissible Current Loading
Types of Cable Faults
Loop Tests for Location of Faults in
Underground Cables
Murray Loop Test
Varley Loop Test

Introduction
Introduction

E

lectric power can be transmitted or dis tributed either by overhead system or by underground cables. The underground cables have serveral advantages such as less liable to damage through storms or lightning, low maintenance cost, less chances of faults, smaller voltage drop and better general appearance.
However, their major drawback is that they have greater installation cost and introduce insulation problems at high voltages compared with the equivalent overhead system. For this reason, underground cables are employed where it is impracticable to use overhead lines. Such locations may be thickly populated areas where municipal authorities prohibit overhead lines for reasons of safety, or around plants and substations or where maintenance conditions do not permit the use of overhead construction.
The chief use of underground cables for many years has been for distribution of electric power in congested urban areas at comparatively low or moderate voltages.