Porcelain insulators (bell type) were first created in 1849 by Werner von Seimens. Porcelain Insulators was originally used for insulation of telephone lines. Since then, porcelain insulators have evolved and has found vast usage especially in the transmission lines system acting as insulators.

Porcelain Insulators have two basic purposes on transmission lines:
1.To support conductors and attache them to structures
2.To electrically isolate conductors from other components on a transmission line
The second purpose is very important to operation since without some form of insulating material, electrical circuit cannot operate.

To be able to isolate conductors, insulators must be made of materials that offer a great deal of resistance to the flow of electricity. Porcelain is one of the most highly used insulator type along with glass and other synthetic materials.

Porcelain is a multiphase ceramic material that is obtained by heating aluminum silicates until a mullite phase is formed. Mullite us porous, its surface must be glazed with a high melting point glass to render its smooth and impervious for use in overhead line insulators.

Porcelain insulators, like any insulators come in a variety of different shapes and sizes, to accommodate the insulator rating as well as its usage.
Insulators are rated in terms of its electrical and mechanical handling capabilities.

Voltage Level
Low Frequency Dry and Wet Flashover
Positive and Negative Impulse Flashover

Cantilever Strength
Tensile Strength
Combined M&E Strength
Maximum Working Load

As for its usage, there are three types of insulators typically found on transmission lines. They are pin insulators, post insulators, and disc insulators.

Pin Insulators

Are generally designed for use on lower range of transmission voltages. Pin insulators are mounted on poles or cross arms using an insulator pin, made up of metal or wood. Pin insulators are always designed to support a conductor upright or vertical on top.

Post Insulators

Post insulators can be mounted vertically on the tops of poles or cross arms or horizontally on the side of the pole.

Disc Insulators

Disc insulators can be connected together in strings to accommodate the requirements of any transmission voltage. They are usually bell shaped, and have mechanisms on the top and bottom for connecting.


The Right of Way
Before we are going to discuss the classification of power lines, it is good to be acquainted with the concept of the Right-of-Way. Transmission lines are normally constructed on a tract of land known as right-of-way. In most cases, only one type of structure is used on a right-of-way. But there are also instances wherein different types of structures and different types of lines are being put.

A right-of-way may follow a straight path, or may change direction in order to avoid obstacles, which happen in many cases. This change in direction causes strain on structures, and the need to compensate from these strains causes the emergence of much type of Transmission Line Structures.

The general types of structures used on transmission lines are wooden poles, concrete poles, metal or steel poles, and lattice towers. These structures can be classified as tangent, angled, or dead-end structure, depending on how it is used in a line.

Dead-End Structures
Dead-end structures, or strain-termination structures, are used wherever a transmission line ends. It is specifically designed to withstand relative greater deal of stress and strain. Dead-end structures at the end of a transmission line are generally identified by insulator strings in the strain insulators.
Dead-end construction may also be found within transmission lines at any point where excess strain is placed on the structures or its components. Example of these is double dead-end, wherein it supports strain of each phase of a line in two directions.

Angle Structures
This structure type is used at points where a transmission line undergoes a significant change in direction. Angle structures are specially reinforced to withstand the strain placed on them by changes in direction.

Tangent Structures
This type is the most commonly used structure type on a transmission line. It is also called as straight-through/ along the line construction. It is generally located on relatively straight portions of a right-of-way.
Tangent structures must be capable of supporting each phase of a transmission line as it passes from one structure to the next. In most cases, the insulator strings on the tangent structures are mounted in the suspension position.

Factors Affecting Transmission Line Design
All of those transmission line structure types can be found on virtually any transmission line. Where and how particular structures are used depends on many factors. Some the more important factors are the following:

1. Ground Clearance
2. Load Requirements
3. Type of Terrain
4. Span Length Conductors
5. Weather Conditions


Power Cable Capacitance (C) Formula
Single Conductor Shielded Cable
        C = 0.024113 x e/ [log (d2/d1)]   microfarad/ kilometer
e = dielectric constant for XLPE = 2.3, PVC = 5.0-7.0
d2 = diameter under insulation
d1 = diameter over the insulation

Power Cable Insulation Resistance (IR) Formula
According to ICEA Specification
IR @ 15.6 degrees C = K log (d2/d1)    Megaohm - 1000ft

According to JIS Specification
IR @ 20.0 degrees C = 3.665 x 10^-12 x p x log (d2/d1)    Megaohm - km

d2 = diameter under insulation
d1 = diameter over the insulation
K = constant (XLPE = 20,000; PVC = 500)
p = volume density (ohm-cm) ; XLPE = 2.5 x 10^15, PVC = 1 x 10^13

Power Cable Inductance (L) Formula
Multiple conductor cable or single conductor cable arranged in parallel and three single conductor arranged in triangular
          L = 0.46 log (S/d) + 0.19    mH/km
d = diameter of conductor
S = distance between conductor

Power Cable Charging Current (Ic) Formula
          Ic = 2 x pi x fC x v/ 1.73    Amp/km
C = capacitance (F/km)
V = rated line to line voltage (Volt)
f = frequency (Hz)

Power Cable Potential Gradient Formula
         E = (v/1.73)/ X ln (d2/d1)    kV/mm
X = distance from center of the conductor (mm)
V = rated line to line voltage
d2 = diameter under insulation
d1 = diameter over the insulation
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