Almost everyone is aware that electrical shock can be a hazard to life. Many people, however, have experienced minor shocks with no dire consequences. This tends to make people somewhat complacent around electricity.
What most people don't know is that approximately half of the serious electrical injuries involve burns. Electrical burns include not only burns from contact, but also radiation burns from the fierce fire of electric arcs that result from short circuits due to poor electrical contact or insulation failure.
The electric arc between metals is, next to the laser, the hottest thing on earth. It is about four times as hot as the sun's surface.
Where high arc currents are involved, burns from such arcs can be fatal, even when the victim is some distance from the arc. Serious or fatal burns can occur at distances of more than 304 cm (10 ft) from the source of a flash.
In addition to burns from the flash itself, clothing is often ignited. Fatal burns can result because the clothing cannot be removed or extinguished quickly enough to prevent serious burns over much of the body.
Thus, even at what a person thinks to be a large distance, serious or fatal injuries can occur to a person's bare skin or skin covered with flammable clothing as a result of a severe power arc. Electrical workers are frequently in the vicinity of energized parts.
It is only the relative infrequency of such arcs that has limited the number of injuries. Examples of exposure are working on open panelboards or switchboards, hook stick operation of medium-voltage fuses, testing of cable terminals, grounding before testing, or working in manholes near still-energized cables.
Several studies, tests, and technical papers are being written on the subject of the flash hazard. Safety standards and procedures are being developed to recognize the fact that arcs can cause serious injuries at significant distances from energized sources.
Equally important in these new safety standards is the fact that, in many cases, only trained people with arc protective equipment should approach exposed energized electrical equipment. Spectators should stay away because, even though they think they are far enough away, they generally don't have an understanding of what is a safe approach distance.
Depending upon the fault energy available, spectators can be seriously hurt at large distances from the point of an arc.
NATURE OF ELECTRICAL ARCS BASIC INFORMATION AND TUTORIALS
What are arcing? What is the effect of electrical arcing?
Electrical arcing is the term that is applied to the passage of substantial electrical currents through what had previously been air. It is initiated by flashover or the introduction of some conductive material.
Current passage is through ionized air and the vapor of the arc terminal material, which is usually a conductive metal or carbon. In contrast to current flow through low-pressure gases such as neon, arcing involves high temperatures of up to, or beyond, 20 000 °K (35 000 °F) at the arc terminals.
No materials on earth can withstand these temperatures; all materials are not only melted, but vaporized. Actually, 20 000 °K (35 000 °F) is about four times as hot as the surface temperature of the sun.
The vapor of the terminal material has substantially higher resistance than solid metal, to the extent that the voltage drop in the arc ranges from 29.53 V/cm (75 V/in) to 39.37 V/cm (100 V/in), which is several thousand times the voltage drop in a solid conductor.
Since the inductance of the arc path is not appreciably different from that of a solid conductor of the same length, the arc current path is substantially resistive in nature, thus yielding unity power factor. Voltage drop in a faulted large solid or stranded conductor is about 0.016-0.033 V/cm (0.5-1 V/ft).
For low-voltage circuits, an arc length of 29.53-39.37 V/cm (75-100 V/in) consumes a substantial portion of the available voltage, leaving only the difference between source voltage and arc voltage to force the fault current through the total system impedance, including that of the arc. This is the reason for the "stabilization" of arc current on 480 Y/277 V circuits when the arc length is of the order of 10.16 cm (4 in), such as with bus spacing.
For higher voltages, the arc lengths can be substantially greater, e.g., 2.54 cm (1 in) per 100 V of supply, before the system impedance starts to regulate or limit the fault current. Note that the arc voltage drop and the source voltage drop add in quadrature, the former resistive, the latter substantially reactive.
The length or size of arcs in the higher voltage systems thus can be greater and can readily bridge the gap from energized parts to ground or other polarities with little drop in fault current.
The hazard of the arc is not only due to the level of voltage. Under some cases it is possible to generate a higher energy arc from a lower voltage than from a higher voltage.
The amount of arc energy generated is dependent upon the amount of short-circuit current available and the amount of time before the fault causing the arc is cleared (removed from the power source) by a circuit breaker or fuse.
Electrical arcing is the term that is applied to the passage of substantial electrical currents through what had previously been air. It is initiated by flashover or the introduction of some conductive material.
Current passage is through ionized air and the vapor of the arc terminal material, which is usually a conductive metal or carbon. In contrast to current flow through low-pressure gases such as neon, arcing involves high temperatures of up to, or beyond, 20 000 °K (35 000 °F) at the arc terminals.
No materials on earth can withstand these temperatures; all materials are not only melted, but vaporized. Actually, 20 000 °K (35 000 °F) is about four times as hot as the surface temperature of the sun.
The vapor of the terminal material has substantially higher resistance than solid metal, to the extent that the voltage drop in the arc ranges from 29.53 V/cm (75 V/in) to 39.37 V/cm (100 V/in), which is several thousand times the voltage drop in a solid conductor.
Since the inductance of the arc path is not appreciably different from that of a solid conductor of the same length, the arc current path is substantially resistive in nature, thus yielding unity power factor. Voltage drop in a faulted large solid or stranded conductor is about 0.016-0.033 V/cm (0.5-1 V/ft).
For low-voltage circuits, an arc length of 29.53-39.37 V/cm (75-100 V/in) consumes a substantial portion of the available voltage, leaving only the difference between source voltage and arc voltage to force the fault current through the total system impedance, including that of the arc. This is the reason for the "stabilization" of arc current on 480 Y/277 V circuits when the arc length is of the order of 10.16 cm (4 in), such as with bus spacing.
For higher voltages, the arc lengths can be substantially greater, e.g., 2.54 cm (1 in) per 100 V of supply, before the system impedance starts to regulate or limit the fault current. Note that the arc voltage drop and the source voltage drop add in quadrature, the former resistive, the latter substantially reactive.
The length or size of arcs in the higher voltage systems thus can be greater and can readily bridge the gap from energized parts to ground or other polarities with little drop in fault current.
The hazard of the arc is not only due to the level of voltage. Under some cases it is possible to generate a higher energy arc from a lower voltage than from a higher voltage.
The amount of arc energy generated is dependent upon the amount of short-circuit current available and the amount of time before the fault causing the arc is cleared (removed from the power source) by a circuit breaker or fuse.
OIL TESTING OF POWER EQUIPMENT BASIC INFORMATION AND TUTORIALS
Why do we conduct oil testing of our power equipment?
Many medium- and high-voltage transformers and circuit breakers utilize different types of oils for insulation. Chemical testing of the oil has proven to be a very dependable method of locating existing or potential problems.
Only a brief overview of some of the common tests can be provided here.
One of the most obvious problems that would significantly reduce the insulation value of the oil is contamination such as moisture or, for circuit breakers and load top changers, carbon.
This can be tested on-site by measuring the voltage at which dielectric breakdown occurs with a special test set that is designed for this purpose.
Oil samples may be sent to a testing laboratory for a series of tests. Measurements of the acidity give an indication of how much oxidation or contamination the oil has experienced.
Interfacial tension, the force that is required to rupture the surface tension at an oil-water interface, is also an indication of possible oxidation or contamination.
One of the most successful tests in determining if a transformer winding has experienced hot spots, corona discharge, or arcing is the dissolved gas analysis test. An oil sample is taken with a special cylinder that is air tight, and the gases that are dissolved in the oil are analyzed.
By determining the type and amount of gas that has been dissolved in the oil, predictions can be made about the internal integrity of the transformer winding.
Many medium- and high-voltage transformers and circuit breakers utilize different types of oils for insulation. Chemical testing of the oil has proven to be a very dependable method of locating existing or potential problems.
Only a brief overview of some of the common tests can be provided here.
One of the most obvious problems that would significantly reduce the insulation value of the oil is contamination such as moisture or, for circuit breakers and load top changers, carbon.
This can be tested on-site by measuring the voltage at which dielectric breakdown occurs with a special test set that is designed for this purpose.
Oil samples may be sent to a testing laboratory for a series of tests. Measurements of the acidity give an indication of how much oxidation or contamination the oil has experienced.
Interfacial tension, the force that is required to rupture the surface tension at an oil-water interface, is also an indication of possible oxidation or contamination.
One of the most successful tests in determining if a transformer winding has experienced hot spots, corona discharge, or arcing is the dissolved gas analysis test. An oil sample is taken with a special cylinder that is air tight, and the gases that are dissolved in the oil are analyzed.
By determining the type and amount of gas that has been dissolved in the oil, predictions can be made about the internal integrity of the transformer winding.
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