ARC Flashcards
Is a complex, safety-related procedure, and should be done only under the direction of experienced engineers
The Calculation of Actual Values for Specific Field Conditions
ANSI
American National Standards Institute
IEEE
Institute of Electrical and Electronics Engineers
“A discharge of electricity through a gas, normally characterized by a voltage drop in the immediate vicinity of the cathode approximately equal to the ionization potential of the gas.”
ARC
The heat and light energy release that is caused by the electrical breakdown of and subsequent electrical discharge through an electrical insulator such as air.
ARC
Occurs when a substantial amount of electric current flows through what previously had been air
ELECTRIC ARCING
air is a poor ____
CONDUCTOR
Since air is a poor conductor, most of the current flow is actually occurring through the vapor of the arc terminal material and the ionized particles of air. This mixture of super-heated, ionized materials, through which the arc current flows, is called a ____.
PLASMA
Arc energy is released in at least three forms
Light
Heat
Mechanical
Also note that mechanical injuries are usually categorized as ___, even though the ultimate cause is the _____.
BLAST INJURIES, ELECTRIC ARC
Principally eye injuries, although severe burns can also be caused if the ultra violet component is strong enough and lasts long enough
LIGHT
Severe burns caused by radiation and/or impact of hot objects such as molten metal
HEAT
Flying objects as well as concussion pressures
MECHANICAL
Are the subject of many on-going studies
The quantitative effects of electric arc
Is a function of the total arc energy, the distance of the subject from the arc, and the cross-sectional area of the individual exposed to the arc
An individual’s exposure to arc energy
Several major factors determine the amount of energy created and/or delivered by an electric arc. (12)
DISTANCE TEMPERATURE ABSORPTION COEFFICIENT TIME ARC LENGTH CROSS-SECTIONAL AREA OF BODY EXPOSED TO THE ARC ANGLE INCIDENCE OF THE ARC ENERGY ARC INPUT ARC VOLTAGE ARC SURFACE AREA INCIDENT ENERGY
Ralph Lee has predicted that the heat energy received by an object (or worker) can be calculated
The Lee Method
The amount of damage done to the recipient diminishes by approximately square the distance of the arc. Twice as far means one-fourth the damage.
Distance
The amount of energy received is proportional to the difference between the fourth power of the arc temperature and the body temperature.
Temperature
The ratio of energy received to the energy absorbed by the body.
Absorption Coefficient
Energy received is proportional to the amount of time that the arc is present.
Time
The amount of energy transmitted is a function of the arc length. For example, a zero will transmit zero energy. Note that for any given system, there will be an optimum arc length for energy transfer.
Arc Length
The greater the area exposed, the greater the amount of energy received.
Cross-Sectional Area of Body Exposed to the Arc
Energy is proportional to the sine of the angle of incidence. Thus, energy impinging at 90-degree is maximum.
Angle of Incidence of the Arc Energy
The energy supplied to an electric arc by the electrical system
Arc Input
Research has shown that electric arcs are rarely perfect _____
Sinusoids
is somewhat more difficult to determine. Values used in power system protection calculations vary from highs of 700 V/ft (214.4 V/m) to as low as 300 V/ft (91.4 V/m).
Arc Voltage
Arc voltages start __ and tend to ___. Periodically, the arc voltage will drop if the arc lasts long enough.
low, rise
Arc voltage is _____ to arc length. Therefore, arc power and energy are proportional to arc length.
proportional
The single most important of all arc energy calculations is the one that determines the energy transfer from the arc to the nearby body.
Incident Energy
This information can be used to determine the necessary level of protective clothing required, and can also be used in the performance of a risk analysis.
Incident Energy
_____ causes painful trauma to the outer layers of the skin. Little permanent damage results from a first-degree burn because all the growth areas survive. Healing is usually prompt and leaves no scarring.
First Degree Burning/Burn
_____ result in relatively severe tissue damage and blistering. If the burn is to the skin, the entire outer layer will be destroyed. Healing occurs from the sweat glands and/or hair follicles.
Second Degree Burning/Burn
_____ result in complete destruction of the growth centers. If the burn is small, healing may occur from the edges of the damaged area; however, extensive third-degree burns require skin grafting.
Third Degree Burning/Burn
Has yielded a slightly different result based primarily on empirical results. Using an experimental setup, the researchers measured energy received from an electric arc at various distances. The arc was created using a 600-V source, and different configurations were used to simulate a completely open-air arc versus the so-called “arc-in-a-box.” Using these experiments, they developed two equations to model the amount of energy received.
Research by Bingham and others
While the actual shape of an electrical arc may vary, all classic, realistic solutions start by assuming that an arc causes an approximately cylindrical plasma cloud with length L and radius r. This cylindrical structure will have a lateral surface area equal to 2πrL.
Arc Surface Area
