Showing posts with label of. Show all posts
Showing posts with label of. Show all posts
Sunday, May 26, 2013
Hazards of Electricity
The primary hazards associated with electricity and its use are:
1- SHOCK. Electric shock occurs when the human body becomes part of a path through which electrons can flow. The resulting effect on the body can be either direct or indirect.
- Direct. Injury or death can occur whenever electric current flows through the human body. Currents of less than 30 mA can result in death. A thorough coverage of the effects of electricity on the human body is contained in the section of this module entitled Effects of Electricity on the Human Body.
- Indirect. Although the electric current through the human body may be well below the values required to cause noticeable injury, human reaction can result in falls from ladders or scaffolds, or movement into operating machinery
2- BURNS. Burns can result when a person touches electrical wiring or equipment that is improperly used or maintained. Typically, such burn injuries occur on the hands.
3- ARC-BLAST. Arc-blasts occur from high-amperage currents arcing through air. This abnormal current flow (arc-blast) is initiated by contact between two energized points. This contact can be caused by persons who have an accident while working on energized components, or by equipment failure due to fatigue or abuse. Temperatures as high as 35,000oF have been recorded in arc-blast research. The three primary hazards associated with an arc-blast are:
- Thermal Radiation. In most cases, the radiated thermal energy is only part of the total energy available from the arc. Numerous factors, including skin color, area of skin exposed, type of clothing have an effect on the degree of injury. Proper clothing, work distances and overcurrent protection can improve the chances of curable burns.
- Pressure Wave. A high-energy arcing fault can produce a considerable pressure wave. Research has shown that a person 2 feet away from a 25 kA arc would experience a force of approximately 480 pounds on the front of their body. In addition, such a pressure wave can cause serious ear damage and memory loss due to mild concussions. In some instances, the pressure wave may propel the victim away from the arc-blast, reducing the exposure to the thermal energy. However, such rapid movement could also cause serious physical injury.
- Projectiles. The pressure wave can propel relatively large objects over a considerable distance. In some cases, the pressure wave has sufficient force to snap the heads of 3/8 inch steel bolts and knock over ordinary
construction walls. The high-energy arc also causes many of the copper and aluminum components in the electrical equipment to become molten. These "droplets" of molten metal can be propelled great distances by the pressure wave. Although these droplets cool rapidly, they can still be above temperatures capable of causing serious burns or igniting ordinary clothing at distances of 10 feet or more. In many cases, the burning effect is much worse than the injury from shrapnel effects of the droplets.
4- EXPLOSIONS Explosions occur when electricity provides a source of ignition for an explosive mixture in the atmosphere. Ignition can be due to overheated conductors or equipment, or normal arcing (sparking) at switch contacts. OSHA standards, the National Electrical Code and related safety standards have preciserequirements for electrical systems and equipment when applied in such areas.
5- FIRES Electricity is one of the most common causes of fire both in the home and workplace. Defective or misused electrical equipment is a major cause, with high resistance connections being one of the primary sources of ignition. High resistanceconnections occur where wires are improperly spliced or connected to other components such as receptacle outlets and switches. This was the primary cause of fires associated with the use of aluminum wire in buildings during the 1960s and 1970s. Heat is developed in an electrical conductor by the flow of current at the rate I2R. The heat thus released elevates the temperature of the conductor material. A typical use of this formula illustrates a common electrical hazard. If there is a bad connection at a receptacle, resulting in a resistance of 2 ohms, and a current of 10 amperes flows through that resistance, the rate of heat produced (W) would be:
W = I ^2R = 10^2 x 2 = 200 watts
If you have ever touched an energized 200 watt light bulb, you will realize that this is a lot of heat to be concentrated in the confined space of a receptacle. Situations similar to this can contribute to electrical fires.
Continue reading...
1- SHOCK. Electric shock occurs when the human body becomes part of a path through which electrons can flow. The resulting effect on the body can be either direct or indirect.
- Direct. Injury or death can occur whenever electric current flows through the human body. Currents of less than 30 mA can result in death. A thorough coverage of the effects of electricity on the human body is contained in the section of this module entitled Effects of Electricity on the Human Body.
- Indirect. Although the electric current through the human body may be well below the values required to cause noticeable injury, human reaction can result in falls from ladders or scaffolds, or movement into operating machinery
2- BURNS. Burns can result when a person touches electrical wiring or equipment that is improperly used or maintained. Typically, such burn injuries occur on the hands.
3- ARC-BLAST. Arc-blasts occur from high-amperage currents arcing through air. This abnormal current flow (arc-blast) is initiated by contact between two energized points. This contact can be caused by persons who have an accident while working on energized components, or by equipment failure due to fatigue or abuse. Temperatures as high as 35,000oF have been recorded in arc-blast research. The three primary hazards associated with an arc-blast are:
- Thermal Radiation. In most cases, the radiated thermal energy is only part of the total energy available from the arc. Numerous factors, including skin color, area of skin exposed, type of clothing have an effect on the degree of injury. Proper clothing, work distances and overcurrent protection can improve the chances of curable burns.
- Pressure Wave. A high-energy arcing fault can produce a considerable pressure wave. Research has shown that a person 2 feet away from a 25 kA arc would experience a force of approximately 480 pounds on the front of their body. In addition, such a pressure wave can cause serious ear damage and memory loss due to mild concussions. In some instances, the pressure wave may propel the victim away from the arc-blast, reducing the exposure to the thermal energy. However, such rapid movement could also cause serious physical injury.
- Projectiles. The pressure wave can propel relatively large objects over a considerable distance. In some cases, the pressure wave has sufficient force to snap the heads of 3/8 inch steel bolts and knock over ordinary
construction walls. The high-energy arc also causes many of the copper and aluminum components in the electrical equipment to become molten. These "droplets" of molten metal can be propelled great distances by the pressure wave. Although these droplets cool rapidly, they can still be above temperatures capable of causing serious burns or igniting ordinary clothing at distances of 10 feet or more. In many cases, the burning effect is much worse than the injury from shrapnel effects of the droplets.
4- EXPLOSIONS Explosions occur when electricity provides a source of ignition for an explosive mixture in the atmosphere. Ignition can be due to overheated conductors or equipment, or normal arcing (sparking) at switch contacts. OSHA standards, the National Electrical Code and related safety standards have preciserequirements for electrical systems and equipment when applied in such areas.
5- FIRES Electricity is one of the most common causes of fire both in the home and workplace. Defective or misused electrical equipment is a major cause, with high resistance connections being one of the primary sources of ignition. High resistanceconnections occur where wires are improperly spliced or connected to other components such as receptacle outlets and switches. This was the primary cause of fires associated with the use of aluminum wire in buildings during the 1960s and 1970s. Heat is developed in an electrical conductor by the flow of current at the rate I2R. The heat thus released elevates the temperature of the conductor material. A typical use of this formula illustrates a common electrical hazard. If there is a bad connection at a receptacle, resulting in a resistance of 2 ohms, and a current of 10 amperes flows through that resistance, the rate of heat produced (W) would be:
W = I ^2R = 10^2 x 2 = 200 watts
If you have ever touched an energized 200 watt light bulb, you will realize that this is a lot of heat to be concentrated in the confined space of a receptacle. Situations similar to this can contribute to electrical fires.
Tuesday, May 14, 2013
Calculation of Fault Current Using Impedance Diagram
Let us first illustrate the calculation of the fault current using the impedance diagram with the help of the following examples.
Example
Consider the power system of Fig. 6.8 in which a synchronous generator supplies a synchronous motor. The motor is operating at rated voltage and rated MVA while drawing a load current at a power factor of 0.9 (lagging) when a three phase symmetrical short circuit occurs at its terminals. We shall calculate the fault current that flow from both the generator and the motor.
We shall choose a base of 50 MVA, 20 kV in the circuit of the generator. Then the motor synchronous reactance is given by
 per unit |
Also the base impedance in the circuit of the transmission line is
 Ω |
 |
Fig. 1 generator supplying a motor load though a transmission line.
Therefore the impedance of the transmission line is
 per unit |
The impedance diagram for the circuit is shown in Fig. 6.9 in which the switch S indicates the fault.
 |
Fig. 2 Impedance diagram of the circuit of Fig. 6.8.
The motor draws a load current at rated voltage and rated MVA with 0.9 lagging power factor. Therefore
 per unit |
Then the subtransient voltages of the motor and the generator are
 per unit  per unit |
Hence the subtransient fault currents fed by the motor and the generator are
 per unit  per unit |
and the total current flowing to the fault is
 per unit |
Note that the base current in the circuit of the motor is
 A |
Therefore while the load current was 1603.8 A, the fault current is 7124.7 A.
Sunday, March 31, 2013
Basics of Schmitt Trigger Circuits – Part 2
A Schmitt trigger is a simple concept, but it was not invented until 1934, while an American scientist by the name of Otto H. Schmittwas still a graduate student. He was not an electrical engineer, as his studies were focused on biological engineering and biophysics.
He came up with the idea of a Schmitt trigger as he was trying to engineer a device that would replicate the mechanism of neural impulse propagation in squid nerves. His thesis describes a “thermionic trigger” that allows an analog signal to be converted to a digital signal, which is either full on or off (‘1’ or ‘0’).
He came up with the idea of a Schmitt trigger as he was trying to engineer a device that would replicate the mechanism of neural impulse propagation in squid nerves. His thesis describes a “thermionic trigger” that allows an analog signal to be converted to a digital signal, which is either full on or off (‘1’ or ‘0’).
Little did he know that major electronics companies like Microsoft, Texas Instruments, and NXP Semiconductors could not exist as they are today without this unique invention. The Schmitt trigger turned out to be such an important invention that it is used in the input mechanisms of virtually every digital electronic device on the market.
The concept of a Schmitt trigger is based around the idea of positive feedback, and the fact that any active circuit or device can be made to act like a Schmitt trigger by applying the positive feedback such that the loop gain is greater than one. The output voltage of the active device is attenuated by a determined amount and applied as positive feedback to the input, which effectively adds the input signal to the attenuated output voltage.
This creates a hysteresis action with upper and lower input voltage threshold values. Most of the standard buffers, inverters, and comparators use only one threshold value. The output changes state as soon as the input waveform crosses this threshold in either direction. A noisy input signal or a signal with a slow waveform would appear on the output as a series of noise pulses. A Schmitt trigger cleans this is up - after the output changes state as its input crosses a threshold, the threshold itself also changes, so now the input voltage has to move farther in the opposite direction to change state again. Noise or interference on the input would not appear on the output unless its amplitude happens to be greater than the difference between the two threshold values. Any analog signal, such a sinusoidal waveforms or audio signals, can be translated into a series of ON-OFF pulses with fast, clean edge transitions.
There are three methods of implementing the positive feedback to form a Schmitt trigger circuit. In the first configuration, the feedback is added directly to the input voltage, so the voltage has to shift by a greater amount in the opposite direction to cause another change in output. This is commonly known as parallel positive feedback. In the second configuration, the feedback is subtracted from the threshold voltage, which has the same effect as adding feedback to the input voltage. This forms a series positive feedback circuit, and is sometimes called a dynamic threshold circuit. A resistor-divider network usually sets the threshold voltage, which is part of the input stage. The first two circuits can easily be implemented via use of a single op amp or two transistors along with a few resistors. The third technique is a little more complex, and is different in that it doesn’t have any feedback to any part of the input stage. This method uses two separate comparators for the two threshold limit values and a flip-flop as a 1 bit memory element. There is no positive feedback applied to the comparators, as they are contained within the memory element. Each of these three methods is explained in more detail in my next article.
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