TAGS. When any electrical equipment is to be repaired or replaced, the main supply switches or cutout switches in each circuit from which power could possibly be fed should be secured in the open position and tagged.
The tag should read, "This circuit is open for repairs". After work has been completed the technician performing the work should remove the tag(s).
SAFETY DEVICES
The covers of fuse boxes and junction boxes should be kept securely closed except while being serviced; safety devices such as interlocks, overload relays, and fuses should never be altered or disconnected except for replacements.
The interlock switch is usually wired in series with the power-line leads to the power supply unit, and is installed on the lid or door of the enclosure to break the circuit when the lid or door is opened. (See animation below.)
A true interlock switch is entirely automatic in action. Multiple interlock switches, connected in a series, may be used for increased safety.
One switch may be installed on the access door of a device, and another on the cover of the power-supply section. If panel is opened for service always use a voltmeter to check for power.
High Voltage Safety Precautions
Death can be the direct result of violating a basic safety practice and indirectly an individuals lack of equipment knowledge.
Many pieces of electrical equipment employ voltages which are dangerous and may be fatal if contacted. Practical safety precautions are incorporated into most electrical systems:
Basic Rules for High Voltage Precautions
These rules are your greatest safeguard and may prevent serious injury or even death.
Working on Energized Circuits
Repair work on energized circuits should not be undertaken whenever practicable. Only experienced personnel should do essential repairs and every safety precaution should be utilized.
The technician should be insulated from ground with some suitable non-conducting material, such as dry canvas, dry wood, rubber-soled shoes, or a rubber mat of approved construction.
The technician should use only one hand in doing repairs if possible and an assistant should be near the main switch so the equipment can be de-energized immediately in case of an emergency.
Personnel should be trained in first aid for electric shock.
Grounding of Equipment and Tools
A poor safety ground, or one that is wired incorrectly, is more dangerous than no ground at all. The poor ground is dangerous because it does not offer full protection.
The incorrectly wired ground is a hazard because if one of the line wires and the safety ground are transposed, it makes the shell of the tool "hot" the instant the plug is connected.
The old method of using a separate external grounding wire has been discontinued.
Instead, a 3-wire, standard, color-coded cord with a polarized plug and ground pin is required. In this manner, the safety ground is made a part of the connecting cord and plug.
Since the polarized plug can be connected only to a mating receptacle, the user has no choice but to use the safety ground.
All new tools, properly connected, use the green wire as the safety ground.
This wire is attached to the metal case of the tool at one end and to the polarized grounding pin in the connector at the other end. It normally carries no current, but is used only when the tool insulation fails, in which case it short circuits the electricity around the user to the ground and protects them from shock.
The green lead must never be mixed with the black or white leads that are the true current-carrying conductors.
Check the resistance of the grounding system with a low reading ohmmeter to be certain that the grounding is adequate (less than 1 ohm is acceptable).
If the resistance indicates greater than 1 ohm, use a separate ground strap. Some old installations are not equipped with receptacles that will accept the grounding plug. In this event, use one of the following methods:
1. Use an adapter fitting.
2. Use the old type plug and bring the green ground wire out separately.
3. Connect an independent safety ground line.
When using the adapter, be sure to connect the ground lead extension to a good ground. (Do not use the center screw that holds the cover plate on the receptacle.)
Where the separate safety ground leads are externally connected to a ground, be certain to first connect the ground and then plug in the tool.
Likewise, when disconnecting the tool, first remove the line plug and then disconnect the safety ground. The safety ground is always connected first and removed last.
Safety Precautions When Using Hand Tools
All tools being used should be maintained.
Any damaged or non-working tools should be repaired or discarded.
Screwdrivers, and other hand tools, should have non-conducting handles.
Portable Power Tools
All power tools should be inspected before being used, especially check cords to be sure they are free of any defects, and cords should not be run over, kinked, or patched with tape.
Electrical Hazards
Each person who works with electrical equipment must make it their responsibility to be alert to electrical hazards, be familiar with safety procedures, and identify and eliminate unsafe conditions.
How to Handle Electrical Shock and What to Do
Electric shock is a jarring, shaking sensation resulting from contact with electric circuits or lightning.
The Molecule
The molecule is the smallest particle to which a substance can be reduced and still be called the same name.
The Atom
The atom is the smallest particle that makes up the type of material called an element. The atom is broken into smaller pieces called electrons, protons, and neutrons.
Protons and neutrons are in the center of the atom with the electrons circling around the outside like planets around the sun. The orbit that the electron moves in is called a shell.
The electrical charge associated with an electron is negative and the proton is considered positive and the neutron neutral.
The element is the basic building block from which all matter is made and still retains its characteristics when subdivided into atoms.
A compound is composed of two or more elements.
Valence Shell
The outer shell of the atom is called the valence shell and the electrons are called valence electrons.
The valence electrons are very important in electronics because they can be easily freed and used to perform work. Electrons are arranged in shells and the number of shells depends on the element.
Two conditions that exist that cause an element to be a good conductor are:
How many electrons are in the valence shell, and
How far the valence shell is from the nucleus.
If an element has 8 electrons in its valence shell it is said to be stable, or a poor conductor, (an atom never contains more than 8 electron in it’s valence shell). If an element has one electron in the valence shell it is a good conductor because that electron can be easily dislodged.
Because the proton in the nucleus is positively charged and the electron is negatively charged they are attracted to each other.
Centrifugal force created by the electron moving in an orbit around the nucleus is what keeps them apart. The greater the distance between the electron and the nucleus the weaker the bond is and the easier it is to dislodge the electron.
The shell closest to the nucleus has up to two electrons.
The second shell contains up to 8 electrons, the third shell up to 18 electrons, and the fourth shell up to 32 electrons.
Elements such as copper, gold and silver each have one valence electron therefore these electrons are easily removed and considered good conductors.
Conductors
Are substances that permit the free motion of a large number of electrons.
Copper is considered a good conductor because it has many free electrons that migrate from atom to atom inside the conductor; electrical energy is transferred through conductors by the movement of the free electrons.
Some examples: Conductors
Silver
Gold
Copper
Aluminum
Zinc
Brass
Iron
Insulators
Are the poorest conductors as insulators they prevent the current from being diverted from the wires.
Insulators have five or more electrons in their valence shells.
Some examples: Insulators
Dry air
Glass
Mica
Rubber
Asbestos
Bakelite
Semi-conductor
Is neither a good conductor nor insulator made of germanium and silicon, which has four electrons in its valence shell, sometimes acts as a conductor and sometimes as an insulator.
Charged Bodies
One of the fundamental laws of electricity is that LIKE CHARGES REPEL EACH OTHER and UNLIKE CHARGES ATTRACT EACH OTHER.
A positive charge and a negative charge, since they are not alike, move toward each other.
Coulomb’s Law of Charges
CHARGED BODIES ATTRACT OR REPEL EACH OTHER WITH A FORCE THAT IS DIRECTLY PROPORTIONAL TO THE PRODUCT OF THEIR CHARGES, AND IS INVERSELY PROPORTIONAL TO THE SQUARE OF THE DISTANCE BETWEEN THEM.
The amount of attracting or repelling force which acts between two electrically charged bodies in free space depends on two things:
Their charge and
2. The distance between them.
Electric Fields
Electric fields are defined as the space between and around charged bodies.
Imaginary lines referred to as electrostatic lines of force usually represent this field.
A positive charge is always shown leaving the charge and a negative charge is shown as entering.
Magnetism
A magnet attracts substances such as iron steel, nickel, or cobalt. Magnetic poles are the points of maximum attraction.
All magnets have two poles, the north pole and the south pole.
Magnets are divided into three groups:
Natural magnets found in a mineral called magnetite.
2. Permanent magnets, which are bars of hardened steel.
3. Electromagnets are a soft iron core, which are wound by coils of insulated wire. When current flows through the coil the core becomes magnetized.
Nature of Magnetism
Weber’s theory considers the molecular alignment of the material. The theory assumes all magnetic substances are composed of molecular magnets.
A magnetized material will have most of its molecular magnets lined up so that the north pole of each molecule points in one direction and the south pole faces the opposite direction.
This theory is supported by the fact that if the magnet is split in half the pieces each has a north and south pole.
Magnetic Fields and Lines of Force
The poles of the magnet are where the attractive force is the greatest.
The magnetic lines of force are the lines in which the needle of a compass aligns itself.
A magnetic field is the space surrounding the magnet.
Characteristics of lines of force are described as follows:
1. Magnetic lines of force are continuous and always form closed loops.
2. Magnetic lines of force never cross one another.
3. Parallel magnetic lines of force traveling in the same direction repel each other. Parallel lines of force traveling in the opposite direction tend to unite and form into single lines.
4. Magnetic lines of force tend to shorten themselves.
5. Magnetic lines of force pass through all materials.
Laws of Attraction and Repulsion
1. Like magnetic poles repel each other.
2. Unlike magnetic poles attract each other.
3. The force of attraction or repulsion existing between two magnets decreases when the distance increases
Difference in Potential
As discussed earlier an atom has positively charged protons and negatively charged electrons.
If the atom has the same number of electrons as it has protons it is said to have a neutral charge-neither negative nor positive.
If an electron is removed from an atom it will have more protons (positive) than electrons (negative) and will therefore be a positively charged atom.
If an atom has more electrons than protons it will be negatively charged.
An atom will always try to be in balance (neutral), so if it is positively charged it will attract an electron from another atom. If it is negatively charged it will reject an electron to a neighboring atom.
Electrons move freely between atoms in a random fashion constantly, but when we cause them to all move in the same direction we create “current flow” or what we call “electricity”.
The force that causes free electrons to move in a conductor as an electric current are referred to as follows:
1. EMF electromotive force.
2. Voltage.
Difference in potential.
Potential difference is another name for electrical force.
The potential for producing current exists even though no current is presently flowing.
A difference in potential exists between two charged bodies that are connected by a conductor, has electrons that flow along the conductor, this flow is from the negatively charged body to the positively charged body until the two charges are equalized and the potential difference no longer exists.
The fundamental law of current electricity is the current is directly proportional to the applied voltage that is, if the voltage is increased the current is increased.
Transversely if the voltage is decreased the current is decreased.
friction
Friction - voltage produced by rubbing two materials together.
Voltage produced by friction.
The least common used. For example when you walk across a carpet and get an unpleasant shock referred to as static electricity, which can also interfere with radio communications.
The main use is for Van de Graf generators used in laboratories to produce high voltages.
Pressure
Pressure- voltage produced by squeezing crystals of certain substances together.
Voltage produced by pressure.
This is referred to as piezoelectricity. Compressing or decompressing crystals of certain substances produces voltage.
Crystals are defined as molecules arranged in an orderly and uniform manner. They are useful because of their extreme sensitivity to changes of mechanical force or change in temperature.
They are primarily used in communication equipment.
heat
Heat-voltage produced by heating the joint where two unlike metals are joined.
Voltage produced by heat.
Voltage produced by heat is referred to as thermoelectricity.
When a metal such as a copper is heated at one end, electrons move away from the hot end to the cooler end.
This is true of most metals however, in iron, the opposite takes place and electrons move toward the hot end.
The negative charges (electrons) are moving through copper away from the heat and through the iron toward the heat. They cross at the hot junction this is known as a thermocouple.
This is used in heat-sensing devices such as automatic temperature control equipment.
light
Light-voltage produced by light striking photosensitive substances.
Voltage produced by light.
In photoelectricity light strikes a surface of a substance and dislodges electrons from their orbits, this occurs because light has energy.
The photosensitive material most commonly used to produce voltage is silver oxide or copper oxide.
The cell has a curved light sensitive surface focused on the central anode. Light energy is converted to a flow of electrons and a usable current is developed.
This type of power is small but successfully used in television cameras, door openers, burglar alarms, and any photoelectric cell devices.
chemical
Chemical action- voltage produced by chemical reaction in a battery cell.
Voltage produced by chemical action.
When molecules of a substance are altered this action is referred to as chemical action.
This means the molecules have been combined with another substance or gives up atoms of its own. If atoms are added to or taken away from the molecules of a substance this chemical change will cause the substance to take on an electrical charge.
This process is used in batteries.
magnetism
Magnetism- voltage produced in a conductor when the conductor moves through a magnetic field, or a magnetic field moves through a conductor in such a manner as to cut the magnetic lines of the force of the field.
Voltage produced by magnetism.
Three conditions must exist before voltage can be produced by magnetism.
1. There must be a conductor in which voltage will be produced.
There must be a magnetic field in the conductor’s vicinity.
There must be relative motion between the field and the conductor.
With these conditions present when a conductor(s) moves across a magnetic field and cuts lines of force, electrons within the conductor are impelled in one direction or another; thus, an electric force or voltage is created.
This is used to produce vast quantities of electric power from mechanical sources. The mechanical power may be provided by different power such as gasoline, diesel, water, or steam turbines.
Electric Current - electron flow
The flow of electrons through a conductor is called an electric current or electron flow.
The movement of electrons will be from a negative potential to a positive potential and is called by various terms, current, current flow, electron flow, and electron current.
There are two types of current, direct current and alternating current.
Direct current flows in the same direction whereas an alternating current periodically changes direction.
Electric Current - AMPERE
Ampere is the term used to define the unit of measurement for current the abbreviation is amp. The amp is abbreviated A.
Electric Current - COULOMB
Coulomb the unit of measurement for electrical charge.
The COULOMB is a unit quantity of electricity moved through an electric current when 1 ampere of current flows for 1 second of time, this is equivalent to 6.28 x 10 to the power of 18 electrons.
The symbol for coulomb is “ Q “. The coulomb is to electricity as the gallon is to water.
Resistance
Every material offers resistance. Good conductors offer little resistance such as copper, silver, aluminum, poor conductors are glass, wood, paper, which offer high resistance to current flow.
The size and type of material of wires are chosen to keep electrical resistance as low as possible.
The larger the diameter of the wires the lower will be their resistance (opposition) to the flow of current through them.
Resistance is expressed by the symbol “ R “ and is measured by OHMS. The Greek letter omega right side up W represents resistance.
Conductance
The term that is the opposite of resistance is conductance (G).
The symbol for conductance is the Greek letter omega upside down.
CONDUCTANCE is the ability of a material to pass electrons.
The formula is written as follows:
G (conductance) = 1/R (resistance)
Cell
A device that transforms chemical energy into electrical energy.
The cell is the fundamental unit of the battery; the simple cell consists of two strips or electrodes, placed in a container that holds the electrolyte.
Battery
A battery consists of two or more cells placed in a common container. The cells are connected in series, in parallel, or a combination of both depending on the amount of voltage required of the battery.
Batteries are used in direct current electrical energy automobiles, boats, aircraft, ships, and portable electronics equipment.
Battery Chemistry
If a conductor is connected externally to the electrodes of a cell, electrons will flow under the influence of a difference in potential across the electrodes from the zinc (negative) through the external conductor to the carbon (positive), returning within the electrolyte solution to the zinc.
After a short period of time, the zinc will begin to burn (waste away) because of the electrolyte. If zinc is surrounded by oxygen, it will burn as a fuel.
Energy is released by the zinc and is transformed into electrical energy rather than heat energy.Series connected cells
When the negative electrode of the first cell is connected to the positive electrode of the second cell, the negative electrode of the second cell is connected to the positive of the third cell this is called series connected cells.
The positive electrode of the first cell and the negative electrode of the last cell then serve as the power takeoff terminals of the battery.
Parallel connected cells
Parallel connected cells are used when more power is needed but using the same voltage.
In a parallel connection all positive connections are connected to one line and all negative are connected to one line.
Series-Parallel Connected Cells
In the graphic below a group of three cells are connected in a series. Then two series groups of three are connected in parallel.
This is used when higher voltage and increased current capacity are needed.
Simple Electric Circuit
An electric circuit is a completed conducting pathway, consisting not only of the conductor but also including the path through the voltage source.
Current flows from the positive terminal through the source and emerges at the negative terminal.
Ohm’s Law
A relationship exists between current, voltage, and resistance.
The current in a circuit is directly proportional to the applied voltage and inversely proportional to the circuit resistance. This law is written as follows:
I=E/ R
Where:
I = current in amperes
E= potential in volts
R = resistance in ohms
An ohm is defined as a unit of electrical resistance.
(Represented by the Greek symbol omega W .)
Applying OHM’S Law
To determine the unknown quantity in an equation cover that quantity with your finger, the remaining letters will be the mathematical operation to be performed. For example to find I cover I, the uncovered letters are E divided by R.
OHMS LAW IN DIAGRAM FORM
See Image Below:
OHMS LAW IN THREE DIFFERENT EQUATIONS
I (current)= E (voltage) / R (resistance)
E= IR (I x R)
R= (E / I )
I = (E / R)
Electric Power and Energy Power
Power, whether electrical or mechanical, is the rate of work being done.
The basic unit of power is measured in watts and is equal to the voltage across a circuit multiplied by current through the circuit.
P indicates the symbol for power the basic power formula is P= E x I, E is the voltage across and I is the current through the resistor.
Forms of energy are light, heat, and motion. Work is done whenever a force causes motion.
P = I E (I x E)
I = (P / E)
E = (P / I)
P = E x I
Where E = 2 volts and I = 2 amps
P = 2 x 2
P = 4 watts
Rating of Electrical Devices by Power
Lamps, fans, and motors are electrical devices rated in watts.
The wattage rating of a device indicates the rate at which the device converts electrical energy into another energy such as light, heat and motion.
If the normal wattage rating is exceeded the equipment will overheat and may cause damage.
Rating of Electrical Devices by Power
Lamps, fans, and motors are electrical devices rated in watts.
The wattage rating of a device indicates the rate at which the device converts electrical energy into another energy such as light, heat and motion.
If the normal wattage rating is exceeded the equipment will overheat and may cause damage.
Power Capacity of Electrical Devices
Wattage rating of a device indicates the device’s operating limits; the limits are given as maximum or minimum safe voltages that a device can be subjected to.
If a device is not limited to any specific voltage its limits are given in watts.
Resistors
A resistor is defined as a circuit element whose basic characteristic is resistance, it is used to oppose the flow of current this device is used in circuits with widely different voltages
Fuses
Fuses are metal resistors with low resistance values. They are designed to blow out and open a circuit when current exceeds the fuse’s rated value.
Resistance
The total circuit resistance (Rt) is equal to the sum of the individual resistances.
For example the current in a series circuit in completing its electrical path must flow through each lamp, each lamp offers added resistance.
As an equation:
Rt = R1 + R2 + R3………Rn
Voltage
Voltage is the measure of electromotive force or EMF. Volt is the unit of measurement for electromotive force.
In a series circuit the total voltage is equal to the applied voltage.
Power
Power is the rate of doing work in an electrical circuit or rate of expending energy the symbol for power is P and measured in watts.
In a series circuit the total power is equal to the SUM of the powers dissipated by the individual resistors.
Total power is PT=p1+p2+p3+……Pn.
Rules for Series DC Circuits
Factors governing the operation of a series circuit.
The same current flows through each part of a series circuit.
The total resistance of a series circuit is equal to the sum of the individual resistors.
The total voltage across a series circuits is equal to the sum of the individual voltage.
The voltage drop across a resistor in a series circuit is proportional to the size of the resistor.
The total power dissipated in a series circuit is equal to the sum of the individual power dissipations.
Ground
The earth ground is said to be zero potential, the term GROUND is used to denote a common electrical point of zero potential.
Open Circuits
A circuit is open when a break exists in a complete conducting pathway. An open occurs any time a switch is thrown to de-energize a circuit.
Short Circuit
A short circuit is an accidental path of low resistance that would have basically the same effect, however the short will cause an increase in current and the possibility of component damage.
Parallel Circuit Characteristics
A parallel circuit is defined as one having more than one current path connected to a common voltage source.
Voltage
In a parallel circuit the same voltage is present across all the resistors of a parallel group.
Current Division
The current in a circuit is inversely proportional to the circuit resistance.
The current divides in a parallel circuit among the available paths in relation to the value of the resistors.
Parallel Resistance
The total resistance in a parallel circuit must be less than any individual resistor in the circuit.
PARALLEL RESISTANCE FORMULA
Product over sum is a convenient formula for finding the equivalent resistance of two parallel resistors.
The formula for finding the equivalent resistance of two parallel resistors is as follows: R t is circuit resistance.
R t = 1 / ( (1/R1 ) + ( 1/R2) )
Effects of Source Resistance
Every known source has resistance. Internal source resistance affects the total power dissipated and also affects the transfer of power. The presence of internal resistance results in:
Less voltage supplied to the components.
Decrease in total current.
Increase in total resistance.
Series-Parallel Circuit
The majority of circuits is a combination of series and parallel elements and is referred to as series-parallel circuits.
An electrical circuit where both series and parallel components are used the series components feel the total current for the circuit, while the parallel components feel only that current which flows through that particular branch.
Total resistance for the series-parallel circuit is determined by combining both series and the parallel methods of calculating resistance.
Specific Resistance or Resistivity
Specific resistance or resistivity is the resistance in ohms offered by unit volume of a substance to the flow of electric current.
Resistivity is the reciprocal of conductivity.
Relation Between Wire Sizes
Wires are manufactured in sizes on a table known as the American wire gage (AWG).
Wire diameters become smaller as the gage numbers become larger.
A wire gage is the instrument used to measure the diameter of wire.
Stranded Wires and Cables
A wire is a slender rod or filament of drawn metal. If a wire is covered with insulation it is called an insulated wire.
A conductor is a wire or combination of wires not insulated from one another.
A stranded conductor is a conductor composed of a group of wires these wires are usually twisted together.
A cable is either a stranded conductor or a combination of conductors insulated from one another. In general the word cable usually applies to larger conductors.
Factors governing the selection of wire size
(Allowance must be made for the influence of external heat.)
Allowable power loss in line, the loss represents electrical energy converted into heat.
Voltage drops in the line.
Current carrying ability of the line.
Current carrying capacities (in amperes) of single copper conductors at ambient temperature of below 30 ยบ C see chart below.
Conductor Insulation
Insulation is coating over the wires so that they do not come into contact with one another, other equipment, or people.
Two properties of insulation are insulation resistance and dielectric strength.
Insulation resistance
Is the resistance to current leakage through the insulation.
Dielectric strength
Is the ability of the insulator to withstand potential difference. To test this raise the voltage in a sample until the insulation breaks down.
Rubber
The most common type of insulation is rubber. The thicker the insulation the higher the voltage.
A thin coating of tin or cotton must be used between the wire and the rubber, or the rubber will become soft and gummy.
Plastic
Is another good type of insulation it has flexibility and is moisture resistant.
Varnished cambric
Is made of cotton cloth coated by varnish. It can stand much higher temperatures than rubber.
Paper
Serves as a satisfactory insulator when impregnated by high-grade mineral oil. Paper works well on high voltage cable.
Silk and Cotton
This is used in communications circuits the threads are wrapped around individual conductors, in reverse directions and coated with a special wax compound.
