Hazards of electricity

 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.

Power Factor basic

 Power Factor Fundamentals

Most Industrial loads require both Real power and Reactive power to produce useful work

You pay for BOTH types of power

Capacitors can supply the REACTIVE power thus the utility doesn’t need to

Capacitors save you money!

Why Apply PFC’s?

Power Factor Correction Saves Money!

Reduces Power Bills

Reduces I2R losses in conductors

Reduces loading on transformers

Improves voltage drop

What is PF ?

Introduction:

Most plant loads are Inductive and require a magnetic field to operate:

- Motors

- Transformers

- Florescent lighting

The magnetic field is necessary, but produces no useful work

The utility must supply the power to produce the magnetic field and the power to produce the useful work: You pay for all of it!

These two types of current are the ACTIVE and REACTIVE components

The Basics:

The Power Triangle:

You pay for fuel for the VERTICAL portion of flight, as well as the fuel for the HORIZONTAL portion of flight

Similarly, motors require REACTIVE power to set up the magnetic field while the ACTIVE power produces the useful work (shaft horsepower). Total Power is the vector sum of the two & represents what you pay for:

The Power Triangle:

Power Factor is the ratio of Active Power to Total Power:

Power Factor = Active (Real) Power /Total Power

                      = kW / kVA

                      =  cos(φ)

 Power Factor is a measure of efficiency (Output/Input)

Why do we Install Capacitors?

Capacitors supply, for free, the reactive energy required by inductive loads.

You only have to pay for the capacitor !

Since the utility doesn’t supply it (kVAR), you don’t pay for it!

Utility Supplies Reactive Current

Capacitor Supplies Reactive Current

Other Benefits:

1- Released system capacity:

The effect of PF on current drawn is shown below:

Decreasing size of conductors required to carry the same 100kW load at P.F. ranging from 70% to 100%

2- Reduced Power Losses:

- As current flows through conductors, the conductors heat. This heating is power loss.

- Power loss is proportional to current squared (PLoss=I2 R).

- Current is proportional to P.F.

- Conductor loss can account for as much as 2-5% of total load

3- Capacitors can reduce losses by 1-2% of the total load

4- Voltage Improvement:

- When capacitors are added, voltage will increase

- Typically only a few percent

- Not a significant economic or system benefit

Severe over-correction (P.F.>1) will cause a voltage rise that can damage insulation & equipment; or result
in utility surcharges!

- Usually a result of large fixed capacitors at mains

Summary of Benefits:

1- Reduced Power Costs:

- Since Capacitors supply reactive power, you don’t pay the utility for it

- You can calculate the savings

2- Off-load transformers

- Defer buying a larger transformer when adding loads

3- Reduce voltage drop at loads

- Only if capacitors are applied at loads

- (minimal benefit at best)

What we learned : 

1- Most Industrial loads (i.e. motors)are Inductive and draw REACTIVE power

2- The Utility supplies this energy therefore you pay for it

3- Power Factor Capacitors supply REACTIVE energy thus the utility doesn’t need to

4- Power Factor Capacitors save money

5- There are other benefits to correcting power factor,

6- reduced heating in cables

7- reduced heating in transformer(s)

8- frees up system capacity

AC resistor circuits

AC resistor circuits :

Pure resistive AC circuit: resistor voltage and current are in phase.

If we were to plot the current and voltage for a very simple AC circuit consisting of a source
and a resistor it would look something like this.

Voltage and current “in phase” for resistive circuit..

Because the resistor simply and directly resists the flow of electrons at all periods of time, the waveform for the voltage drop across the resistor is exactly in phase with the waveform for the current through it. We can look at any point in time along the horizontal axis of the plot and compare those values of current and voltage with each other (any “snapshot” look at the values of a wave are referred to as instantaneous values, meaning the values at that instant in time). When the instantaneous value for current is zero, the instantaneous voltage across the resistor is also zero. Likewise, at the moment in time where the current through the resistor is at its positive peak, the voltage across the resistor is also at its positive peak, and so on. At any given point in time along the waves, Ohm’s Law holds true for the instantaneous values of voltage and current.

We can also calculate the power dissipated by this resistor, and plot those values on the
same graph

Instantaneous AC power in a pure resistive circuit is always positive.

Note that the power is never a negative value. When the current is positive (above the line), the voltage is also positive, resulting in a power (p=ie) of a positive value. Conversely, when the current is negative (below the line), the voltage is also negative, which results in a positive value for power (a negative number multiplied by a negative number equals a positive number). This consistent “polarity” of power tells us that the resistor is always dissipating power, taking it from the source and releasing it in the form of heat energy. Whether the current is positive or negative, a resistor still dissipates energy.

reference :
Lessons In Electric Circuits, Volume II – AC By Tony R. Kuphaldt

Armstrong oscillator

 Armstrong oscillator :

The Armstrong oscillator (also known as Meissner oscillator ) is named after the electrical engineer Edwin Armstrong, its inventor. It is sometimes called a tickler oscillator because the feedback needed to produce oscillations is provided using a tickler coil (T in the circuit diagram) via magnetic coupling between coil L and coil T. Assuming the coupling is weak, but sufficient to sustain oscillation, the frequency is determined primarily by the tank circuit (L and C in the illustration) and is approximately given by . In a practical circuit, the actual oscillation frequency will be slightly different from the value provided by this formula because of stray capacitance and inductance, internal losses (resistance), and the loading of the tank circuit by the tickler coil.

This circuit is the basis of the regenerative receiver for amplitude modulated radio signals. In that application, an antenna is attached to an additional tickler coil, and the feedback is reduced, for example, by slightly increasing the distance between coils T and L, so the circuit is just short of oscillation. The result is a narrow-band radio-frequency filter and amplifier. The non-linear characteristic of the transistor or tube provides the demodulated audio signal.

The circuit diagram shown is a modern implementation, using a field-effect transistor as the amplifying element. Armstrong's original design used a vacuum tube triode.

Note that in the Meissner variant, the LC resonant (tank) circuit is exchanged with the feedback coil, i.e. in the output path (Anode, Drain, Collector) of the amplifier, e.g. Grebennikov, Fig.  Many publications, however, embrace both variants with either name; apparently the English speakers using Armstrong, and the German speakers Meibner.

reference : 

http://en.wikipedia.org/wiki/Armstrong_oscillator