Ensamblador

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  • Publicado : 29 de noviembre de 2011
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1
ELECTRIC ARC FUNDAMENTALS
1.0 INTRODUCTION

From the time when the existence of the electric current flow was first established and even before the basic thermal, mechanical, and chemical effects produced by such current were determined, it had become clear that there was a need for inventing a device capable of initiating and stopping the flow of the current..

Fundamentally, thereare two ways by whch the flow of current can be stopped; one is to reduce the driving potential to zero, and the other is to physically separate a pair of contacts to create an open gap between the conductor that is carrying the current. Historically, the later method has been the one most commonly used to achieve current interruption.

Hanz Christian Oersted, Andre-Marie Ampere, and MichaelFaraday are among the first known users of circuit breakers, and according to recorded history those early circuit breakers are known to have been a mercury switch that simply consisted of a set of conducting rods that were immersed in a pool of mercury.

Later as the current switching technology evolved, the mercury switch was replaced by a knife blade switch design, which is still widely used forsome basic low-voltage, low-power applications. Today, under the present state of the art in current interruption technology, the interruption process begins at the very instant
when a pair of electric contacts separate. It continues as the contacts recede from each other and as plasma bridges the newly created gap. The interruption process is completed when the conducting plasma is deprived ofits conductivity.

By recognizing that the conducting plasma is nothing more than the core of an electric arc, it becomes quite evident that inherently the electric arc constitutes a basic, indispensable, and active element in the process of current interruption.

Based on this simple knowledge, it follows that the process of extinguishing the electric arc constitutes the foundation upon whchcurrent interruption is predicated. It is rather obvious then that a reasonable knowledge of the fundamentals of arc theory is essential to the proper understanding of the interrupting process. It is intended that the following basic review describing the phenomena of electric discharges will serve to establish the foundation of the work that will be presented later dealing with currentinterruption.

1.1 BASIC THEORY OF ELECTRICAL DISCHARGES

The principles that govern the conduction of electricity through either a gas or a metal vapor, are based on the fact that such vapors always contain positive and negative charge carriers and that all types of discharges always involve the very fundamental processes of production, movement and final absorption of the charge carriers as the means ofconveying the electric current between the electrodes.

For the sake of convenience and in order to facilitate the review of the gas discharge phenomena, the subject will be divided into the following three very broad categories:

(a) The non-self-sustaining discharge
(b) The self-sustaining discharge and
(c) The electric arc.

2.1.1 Non-Self-sustaining Discharges
When voltageis applied across two electrodes a force proportional to the electric field strength acts upon the charge carriers. This force establishes a motion of the ions toward the cathode and of the electrons toward the anode. When the moving charges strike the electrodes they give up their charges thus producing an electric current through the gaseous medium. A continuous flow of current can take placeonly if the carriers whose charges are absorbed by the electrodes are continuously replaced. The replacement of the charge carriers can be made by a number of ionizing processes such as photoelectric, or thermionic emissions.

Initially, the discharge current is very small; however, as the voltage is increased it is observed that the current increases in direct proportion to the voltage applied...
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