Circuit Brakers

Introduction

 The primary function of an electrical circuit breakers is to provide opening and closing the current carrying contacts. Although it is seems to be very simple . But we should remember , that one circuit breaker remains at its closed position for maximum period of its life span. Very occasionally it is required to operate a circuit breaker for opening and closing contact. Hence , circuit breaker operation must be very reliable  without any delay or sluggishness. For achieving this reliability the circuit breaker operating mechanism becames more complex than it was first thought.

Opening and closing distance, as well as stroke between contacts and velocity of moving contacts during operation, are the most important parameters to be considered during designing circuit breaker. Contact gap, traveling distance of moving contacts and their velocity are determined by types of arc quenching medium, current and  voltage rating of a circuit breaker.

Circuit Breaker opening operation requirement 

1. The circuit breaker is desired to be at open position as fast as possible . It is because of limiting contacts erosion and to interrupt faulty current as rapidly as possible. But total travel distance of the moving contact is not determined only by necessity of interruption of  faulty current, but rather the contacts gap needed to withstand the  normal dielectric stresses and lighting impulse voltage appears across the contacts when the CB is at open position.

2. The need for carrying the continuous current and for withstanding a period of arc in CB, make it necessary to use two sets of contacts in parallel one the primary contact which is always made of high conductive material such as copper and  the other is arcing contact , made of arc resistance material such as tungstan or molybdenum , which has much lower conductivity than primary contacts. During opening circuit breaker operation, the primary contacts open before the arcing contact.

3.However, due to the difference in the electrical resistance and the inductor of the electrical paths of the primary and arcing contacts, a finite time is required to attain total current commutation, from main contact to arcing contact.

Harmonic Distortion in electrical system

A pure sinusoidal voltage is a conceptual quantity produced by an ideal AC generator built with finely distributed stator and windings that operated in a uniform magnetic field. Since neither the winding distribution nor the magnetic field are uniform in a working AC machine, voltage waveform distortions are created,and the voltage- time relationship deviates from the pure sine function. The distortion at the point of generation is very small (about 1% or 2%) , but nonetheless it exists. Because this is a deviation from a pure sine wave, the deviation is in the form of a periodic function, and by definition, the voltage distortion contains harmonics.

When a sinusoidal voltage is applied to a certain type of load, the current drawn by the load is proportional to the voltage and impedance and follows the envelope of the voltage waveform. These loads are referred to as linearloads(loads where the voltage and current  follows one another without any distortion to their pure sine waves). Examples of linear loads are resistive heaters, incandescent lamps, and constant speed induction and synchronous motors.


In contrast, some loads cause the current to vary disproportionately with the voltage during each half cycle. These loads are classified as nonlinear loads, and current and voltage have waveform has numerous additional waveform superimposed upon it, creating multiple  frequencies within the normal 60 Hz sine wave. The multiple frequencies are harmonics of the fundamental frequency.

Which is more dangerous voltage or current!!!!

A common phrase heard in reference to electrical safety goes something like this "It's not voltage that kills, its current !". While there is an element of truth to this, there's more to understand about shock hazard than this simple adage. If voltage presented no danger, no one would ever print an display signs saying: DANGER -HIGH VOLTAGE!

The principle that "current kills" is essentially correct . It is electric current that burns tissue, freezes muscles, and fibrillates hearts. However , electric current doesn't just occur on its own: there must be voltage available to motivate electrons to flow through a victim. A person's body also presents resistance to current, which must be taken into account.

Taking ohm's law for voltage, current, and resistance, and expressing it in terms of current for a give n voltage and resistance , we have this equation:


I  = V/R

Current = Voltage / Resistance 


The amount of current through a body is equal to the amount of voltage applied between two points on that body, divided by the electrical resistance offered by the body between those two points. Obviously, the more voltage available to cause electrons to flow, the easier they will flow through any given amount of resistance. Hence, the danger of high voltage : high voltage means potential for large amount of current through your body, which will injure or kill you. The more resistance a body offers to current , the slower electrons will flow for any given amount of  voltage. just how much voltage is dangerous depends on how much total resistance is in the circuit to oppose the flow of electrons.


Body resistance is not fixed quantity. It varies from person to person and from time to time.
There's even a body fat measurement technique based on a measurement of electrical resistance between a person's toes and fingers. Different percentage of body fat give provide different resistance : just one variable affecting electrical resistance in human body. In order for the technique to work accurately , the person must regulate their fluid intake for several hours prior to the test, indicating that body hydration is another factor impacting the body's electrical resistance.