Journeyman Electrician: The Basics of Electrical Equipment and Documentation

Switches
A switch is a device for easily making and breaking electrical contact or for changing the configuration of connections.  The purpose of a switch is to maintain a low resistance (usually no more than a few tenths of an ohm) when it is closed (connection made) and a high resistance
(usually more than 1,000,000 ohms) when it is open (no connection made).
Switches are classified according to the action that operates them.  The most common designs are toggle, rocker, push button, rotary, slide, key, knife, and leaf.
Switches are also classified by their number of parallel independent connections (called poles) and the number of positions possible for each pole (called throws).
Switches can be continuous or momentary. In momentary switches, the connection or disconnection is made only while force is applied. 
A spring in the switch returns the switch to its original position when the operator releases it.

Common Designs of Switches
A toggle switch is actuated by the movement of an actuator that extends from the body of the switch.  Mercury toggle switches don’t wear out as quickly as the standard type and they operate silently.  Instead of a snap as the contacts are made, liquid mercury flows onto the contacts and makes the electrical connection.
A rocker switch is similar to a toggle switch but operates by rocking the actuator, where one side of the switch is raised while the other side is depressed.
Slide switches are actuated by moving an actuator from side to side or up and down.

Rotary switches are actuated through the rotation of a shaft that moves various contacts to make the connections.  A key switch is a special case of a rotary switch with a key being required to change the position.

A leaf switch is composed of metal leaves.  The action of moving one leaf causes it to bend and make contact with the other leaf.
Pushbutton switches are actuated by pressing in on a button.  Some ON-OFF types automatically change from being extended to depressed each time the switch is pressed.
The knife switch is so named because it has a thin metal blade that is hinged and rotates into and out of position to make or break contact.

Common Uses of Switches
Almost all switch types may be found mounted in panels, whether they are electrical distribution panels or control panels for equipment and appliances.  Key switches are used for added security, on machines and computers for example. Toggle and rocker switches are commonly used for control of power to loads such as lights.  The common wall switch is usually either a toggle or rocker switch.  Knife switches are used for the application of high current applications where arcing might occur.  The simple design of the knife switch actuator limits the effects from this arcing and extends the life of the switch.  Leaf switches are often used as limit switches to control mechanical devices.  A limit switch is usually a leaf switch that is used on machinery to open a circuit when travel has reached a preset limit.

Configurations of Poles and Throws in Common Switches
The number of parallel independent connections (poles) is usually one or two, but can range much higher.  The number of positions (throws) possible for each pole is also usually one or two but can be higher.  A rotary switch may have dozens of possible positions for each of several poles that are ganged together. The following abbreviations are used for the most common combinations of poles and throws:
SPST – single pole single throw: a single make-or-break switch (ON-OFF).
SPDT – single pole double throw: a single selection switch where the common lead can be connected to either of two other leads.
DPST – double pole single throw: two independent SPSTs ganged together.
DPDT – double pole double throw: two independent DPSTs ganged together.

Switches with higher numbers of poles and throws are abbreviated using numbers, such as 3PDT for a three pole double throw switch.

Special Considerations in Operating a Knife Switch
Knife switches are used in high current applications where arcing might occur.  To minimize arcing, the switch must be opened and closed quickly.  When closing the switch, the operator must push the knife actuator firmly and rapidly into the contacts so there is no intermittent contact.  When opening the switch, the operator should jerk the actuator out and away from the contacts quickly to avoid or interrupt any arc.
Single throw knife switches that are mounted vertically must have their blade hinge at the bottom.
Knife switches have open frames that expose the electrical contacts so their use is limited.  Often there is an insulated handle that the operator uses while the contacts are concealed inside an enclosure.  An enclosed knife switch that requires the switch to be closed before the housing door can be opened is known as a safety switch.

Configurations for Wiring Wall Switches
Simple wall switches to turn lights on and off are usually SPST toggle or rocker switches.  They switch the ungrounded (black) conductor while the grounded (white) conductor is connected directly to the load.
SPDT switches can be used for three-way connections where two switches can be used to control a single light.  This is shown below:


The light will be energized only when both switches are in the same position, either up or down.  If the switches are in opposite positions as shown the light will be off.  The wires that connect the two switches together are called traveler wires.

Operation and Use of a 4-Way Switch
The four- way switch has two common inputs that can be switched to either of two outputs as shown:


Four-way switches are used in combination with three way switches to provide more than two control points for a circuit.  Shown below is the configuration for three switches to control a load.  For higher numbers of control points, additional 4-way switches are added between the 3-way switches at each end of the string.

Types of Switches
Toggle: panel mount switch, wall switch



Rocker: panel mount switch, wall switch



Key:



Slide:



Microswitch:


Knife: single pole double throw, double pole single throw



Button:



Precautions Before Approaching a Job
Electricians must know the code rules and regulations so they can perform jobs correctly and safely, and must also use sound judgment when working.  If heavy objects are moved, use your legs and arms and keep your back as upright as possible while spreading your feet to provide a solid base. 
The most important safety tip, especially in hazardous locations, is to be alert.
The electrician’s tools and equipment must be appropriate for the job and be in good repair.  Defective or altered tools can cause accidents.  A ladder that is painted might be unsafe because the paint could conceal damage to the rungs or rails.  Rubber gloves for use with high voltage should always be tested with high voltage.  Sponge rubber soled shoes can be punctured by nails.
A proper fire extinguisher should always be accessible. 
Chemical extinguishers should be used in case of an electrical fire as water on an electrical fire may shock the user.  The best electrical fire extinguisher type uses CO2.
Circuits must be considered hot unless known to be otherwise, and power should be removed when this is practical. 

When working on a motor, always remove its fuses or otherwise de-energize the circuit to avoid accidental starting. 

Two people should be present when working on high voltage so that one can provide help if an accident occurs.

Safety Features in Tools and Equipment
All branch circuits are protected by fuses or circuit breakers, and in some cases GFCIs.  These are installed primarily to protect the electrical system and users, but they also offer some protection to the electrician.
Some equipment may contain an interlock which is a device that when actuated causes other devices associated with it to be disabled.  An example of an electrical interlock is a switch on a heating system that prevents operation of the heater or fan when the cover is removed.  Some panels are mechanically prevented from being opened unless the power switch is moved to the off position.  For maximum safety, magnetic contactors should always be electrically and mechanically interlocked.
Power tools are either double insulated or have a grounded frame to prevent them from receiving a stray voltage and becoming live.
If a live wire is touched accidentally, the severity of a shock is directly determined by the contact resistance between operator and the power source.  Insulating tools and clothing are therefore helpful in reducing the risk of injury from shocks.

Reasons for Grounding Electrical Equipment
Electrical devices have their external metal surfaces grounded to limit excess voltage to ground caused by lightning or unintended exposure to circuit voltages.
The grounded conductor of a lighting circuit is always connected to a screw on the device to reduce the possibility of electrical shock.  A grounding wire is used for the same reason on any metal cabinet.
The earth ground is developed at the utility pole or the service panel and requires a metal rod to be inserted into the earth.  To ensure a low resistance path to this earth ground, sometimes multiple rods or treatment of the soil are required.

Worker Safety
The most important first step for the safety of a worker who has been injured and for the personnel attempting to help is to remove the hazard.  Never approach a victim of an electrical injury until the power is off or isolated.  If a victim is still in contact with a live wire, use a dry rope or stick to remove the wire.
Sometimes it is not obvious that a significant injury has occurred.  Normal signals of electrical injury include erratic pulse, being dazed or confused, or unconsciousness.
Burns can be disabling, disfiguring, or even life threatening.  When a person is burned, cover and cool the injured area while taking steps to prevent shock and infection.  First aid for acid burns is to flush with water, then apply petroleum jelly to keep air from the wound.  When a chemical is splashed into the eyes, always flush them with clean water.
If someone has been injured and is going into shock, the most important step is to keep them warm.  If a victim is not breathing, it is also critical to start artificial respiration immediately.
An accident report probably will be required after an injury occurs.  It is important to determine and report the cause of the accident in order to prevent it from happening again.

Switches
Two switches in one box under one faceplate are called a 2-gang switch.  There are also 3-, and 4-gang switches. 

This is a faceplate for a 2-gang switch.


Switches are combined with outlets in a combination switch/outlet:


Dimmer (rheostat) switches – rotary and slide


A handy box used for splices or to install switches and outlets



Motors
Motors convert electrical energy to rotational mechanical energy, which is the opposite of generators, which convert mechanical to electrical energy.  The design and construction of motors is in fact very similar to that of generators.  The stationary part of a motor is known as the stator and the moving portion is the rotor.  These contain the field windings, which set up one magnetic field, and the armature windings, which produce a second field.  The principle of operation for a motor is attraction and repulsion created by sets of magnetic fields in the field and armature windings of the motor.
Motors are classified according to the type of voltage and current on which they run: alternating current (AC), direct current (DC), and universal types that can run on either AC or DC.
The efficiency of a motor is a measure of how well it converts electrical energy to mechanical energy, and it equals the mechanical power output (usually in horsepower) divided by the electrical power input (usually given in Watts, then converted to horsepower).

Safety Features Incorporated in Motors
The frame of a motor is always grounded to prevent against electrical shock from stray voltages.  Overload circuits are often used with motors.  For single-phase motors, an overload device may be placed in the ungrounded conductor.  The overload device is used to protect the motor from sustained overcurrent and overheating due to excess mechanical loading.  An overload is defined as operation beyond the normal full load rating or ampacity of the motor.
Thermal overload devices operate in any of a number of ways. 
A temperature control relay may respond directly to the temperature of a probe in the motor windings.  Other devices react to heat generated within them by the current drawn by the motor.  Solid state relays may be used to electronically monitor the current and open the circuit when the current and time limits have been exceeded.
A motor enclosure that totally contains sparks and flashes is referred to as explosion proof and is used in hazardous locations.

DC Motor
Motors rotate to align the magnetic fields from their armatures and fields.  In a DC motor, the direction of the torque must constantly be reversed as the motor rotates in order to keep it moving.  A commutator and brushes are used to reverse the current applied to the armature.  In a DC motor, the rotor is the armature and the stator contains the field coils.
DC motors have a wide speed range that is controlled by the current to the field windings.  To change the direction of rotation of DC motors, the polarity of the voltage is reversed. DC motors have a high starting torque.  They have very low armature resistance, so starting current must be limited to prevent damage to the motor.

AC Motor
Most common alternating current (AC) motors fall into two types: synchronous motors and induction motors.  Field coils are contained in the rotor and armature coils are in the stator.
Synchronous motors operate at a fixed speed regardless of the load or applied voltage.  Synchronous motors will stall if the applied current is not sufficient to provide the torque necessary to maintain synchronous operation.  The synchronous speed is determined by the applied power line frequency and the number of magnetic pole pairs (sets of N and S magnetic poles).  Slip rings or brushes are used to provide the field current to the rotor.
Induction motors contain no brushes or slip rings; the current in their rotors is induced from current in the stators.  Similar to synchronous motors, induction motors are near synchronous; that is, they operate slightly below synchronous speed.

Series Motor and Repulsion Motor
In a series AC motor, also called a series wound motor, the armature coil is connected in series with the field coil and receives current through a commutator. Series motors were once used extensively where variable speed was required before the advent of modern electronic speed controls.  They have very high starting torques and must be loaded during start up to prevent excessive speed that could damage them.
The repulsion motor is a special case of an induction motor. 
There are brushes and a commutator connected to the armature, but the brushes are shorted together and the armature receives power by induction.

Induction Motor
Induction motors have no commutator.  Power to the armature is received by induction from the field windings.  The armature may be constructed of wound coils or it may be made of solid bars, in the case of the squirrel cage motor. Induction motors usually run slightly below synchronous speed.  If an induction motor runs slow, it may be because of worn bearings.
Squirrel cage induction motors have low starting torque and can draw large currents on start up.  Large squirrel cage motors are started at lower voltages to avoid this excessive current.
Common designs for wound rotor induction motors are split phase and shaded pole.  Split phase motors have a secondary starting field winding that is displaced in phase.  After start up, this winding is disconnected and the primary winding is used.  Split phase motors generate considerable torque for use in appliances like washing machines.
Shaded pole motors are used for low torque applications like electric fans.  A shading coil produces a moving magnetic field that causes rotation.

Ohmmeter
An ohmmeter is basically a sensitive voltmeter connected in series with an internal DC current source.  The resistance of an external circuit determines the voltage resulting from the connection to the unknown resistance and therefore it is used to measure this resistance.
Analog ohmmeters normally read backwards: the zero ohm mark, which corresponds to the maximum voltage read, is located at the far right of the meter. The infinite resistance mark corresponds to no voltage and is located on the left. There is usually an adjustment knob to zero the meter.
Ohmmeters are very useful in detecting short or open circuits and can be relatively accurate when used over the proper range.  For resistances lower than 10 ohms, the operator may want to subtract the resistance of the test leads to get an accurate reading.  Measurement of resistances higher than 100,000 ohms will normally be made with a megger, a special device made for this purpose.
Sources of error in ohmmeter measurements include using the wrong range, and the presence of an external voltage between the points under test.

Multimeter
The multimeter is a device for measuring multiple electrical values, most often AC and DC voltage and current as well as resistance.
For voltage measurements, the device’s internal meter is wired directly to the test leads through internal resistors that are selected by the range switch on the meter.  The impedance of the instrument is normally fairly high and its leads can be connected at most any point in a circuit without affecting the circuit.  The only consideration is using the proper range to protect the meter from damage.
For current measurements, an internal shunt (low resistance element) is switched in.  The meter must be connected in series with the circuit being tested. Care must be taken to make these connections correctly to avoid damaging the meter or affecting the performance of the circuit.  For DC, the connections from the circuit to the meter are + to + and – to –.

Clamp-on Ammeter
A clamp-on ammeter or current sensor can be used for readings of AC or DC current that are slightly less accurate than a standard current meter.  The device consists of a coil that is clamped around a conductor and either has its own meter or connects to a multimeter to read the current.  Use of this device does not require breaking connections that a standard ammeter requires.  The device measures the current indirectly from the magnetic field surrounding the conductor.  It should be clamped around a single conductor without interference from other wires and must be held so the coil is perpendicular to the conductor being measured.

Wattmeter
The wattmeter measures either AC or DC power.  It is essentially both a voltmeter and an ammeter that determines the power by multiplying these two measurements together.  It measures real power (the in-phase result of the product of V and I).  The meter is connected in both series and parallel in order to obtain the readings.

Watt-Hour Meter
The watt-hour meter, or kilowatt hour meter, is an instrument used to measure electrical energy, or the product of power and time.  It also must be connected in both series and parallel to read the voltage and current.  The watt-hour meter is a recording instrument rather than an indicating one because it reads the energy consumed over some period of time.
To measure the phase relationship between voltage and current a device called a power factor meter is used.

Testing Devices
The simplest test device is a test lamp and battery, and it can be used to verify continuity (low resistance) such as for a ground check. 
Similar to this device is a bell and battery set that will ring or buzz when continuity is present.
Slightly more sophisticated is the neon glow tester, which is a neon lamp with leads that can determine whether an AC or DC circuit is alive, and check the polarity of DC voltages.

Wheatstone Bridge
For DC resistance readings more accurate than what can be obtained with an ohmmeter, a Wheatstone bridge can be used. It is a precision instrument that is used to measure low and medium resistance with great accuracy.

Growler
A growler is a device for finding short circuits in motors. 
It consists of a coil and a metal feeler.  If the stator of the motor is shorted, the growler coil will induce a localized magnetic field that is indicated by vibration in the feeler.

Continuity and Resistance Measurements
Simple continuity, or the absence of it, can be detected with a test lamp.  If it lights, then ere is continuity.  If not, then there is not. This can be useful in detecting short or open circuits in condensers and coils.
A better indication is provided by the ohmmeter.  When using an ohmmeter be sure to disconnect or remove any external voltages from the circuit being tested; otherwise a number of bad things could occur.  Incorrect readings will result if external voltages are present, and in the worst case damage to the meter may result.
When the needle of a volt-ohm meter will not align with the zero mark for resistance measurements but there is no problem when it is used for other functions, the battery that is used for the ohmmeter function is probably weak.

Electrical Current Measurements
When connecting the ammeter, the operator must open the circuit under test and connect the meter in series to complete the circuit. 
Care must be taken not to damage the circuit or the meter.
Because of the high voltage that can be developed from the inductance of transformers when disconnecting an ammeter from an in service transformer the secondary terminals must be shorted.  Before removing an ammeter from the current transformer, its secondary must be short circuited to avoid damage from the induced voltages as mentioned above.
To increase the range of an AC ammeter, a current transformer is sometimes used.

Voltage Measurements
Voltage measurements can usually be made without affecting the circuits under test, from almost any point to any other.  Voltmeters are connected across any source or load to read the voltage or IR drop.  Allow the test circuit to remain as is and connect the meter in parallel across two points.
There are some precautions that must be taken with voltage measurements. When measuring an unknown voltage, always start with highest range on the meter.  When measuring DC voltages with analog meters, reversed polarity can damage the meter.  Be prepared to quickly check the meter for the proper response and disconnect the leads if the polarity is not as expected.

Atoms and Electrical Conductivity
All matter is composed of atoms, which are the smallest independent units of matter.  Atoms contain electrons in their orbits and the electrons on the outer shell of each atom are called valence electrons.  The valence electrons determine the chemical and electrical characteristics of the material including electrical conductivity.  These are also known as free electrons because they are easily displaced and travel through the material as electrical current.
Electrons have a negative charge so the direction they travel is opposite to the direction of the current they produce.
Conductive materials have sufficient free electrons to conduct electric current when a voltage (also known as an electromotive force, or EMF) is applied. Insulators or dielectrics do not have many free electrons and so do not conduct current.

Common Conductive and Non-Conductive Materials
Most metals are electrically conductive because their atoms have free electrons. Here is a list of some common conductive materials, all metals and alloys, in order of their conductivity (highest to lowest):
Silver, copper, gold, aluminum, iron, brass, platinum, lead, mercury, carbon.
Copper is the most commonly used because its conductivity is good and it is considerably cheaper than the metals ahead of it on the list.

Here is a list of commonly used insulating materials:
Porcelain, rubber, plastic, poly vinyl chloride (PVC), Teflon, mica, varnish, glass, bakelite, oil, air, paper, wood.
Pure water is theoretically non-conductive but the presence of ions, especially in seawater, makes it slightly conductive.

Electrical Charges Production and Storage
Electrical charges are produced in a variety of ways but all involve the transfer of electrons.
Ions are formed from molecules that have more or fewer electrons than normal. They can have either – or + charges depending on the balance of electrons.
Insulating materials can be charged by being rubbed against other suitable materials that transfer electrons from one to another.
All charges interact with other charges.  Charges of the same polarity repel each other while opposite charges attract.
Electrical charge can be stored in a condenser.  This device consists of metal plates that are separated by an insulator.  The plates do not have to be flat; they may be rolled into any shape.  The most common condensers, also called capacitors, consist of many layers of foil separated by waxed paper and rolled into a cylinder with an electrical connection at each end.  The value of capacitance increases with the plate area and with closer spacing of the plates.

Electric and Magnetic Fields
An electric field is a region in which a charged particle or body experiences electrostatic force.  The magnitude of the field is directly proportional to the strength of the charge and inversely proportional to the distance from the charge and the permittivity of the material containing the field.  Permittivity is the characteristic that classifies materials as conductors or insulators.
A magnetic field is the region in which a moving charged particle or body experiences force due to an external magnetic pole.  Magnetic fields are produced by electrical current, and the strength of the field is proportional to the magnitude of the current.

Magnetic Characteristics of Materials
All materials contain magnetic regions at the atomic level and exhibit some form of magnetism according to their permeability.  Magnetic permeability, similar to electrical permittivity, is a measure of the reaction of a material to magnetic fields.
Materials whose permeabilities are high are known as magnetic, or more correctly ferromagnetic.  The internal magnetic regions of these materials align strongly with any external magnetic field.  The alignment can result in residual magnetism where permanent magnets can be created by the application of a strong enough field.  These materials also magnify the effects of the external magnetic field while it is applied.  Materials that are ferromagnetic, listed in order of their permeability (magnetic potential), include iron, steel, nickel, and cobalt.
Common electrical conductors that are nonmagnetic include copper and aluminum.

Battery Cells and Voltage
Battery cells e a specific voltage, which is dependent on the chemistry of the cell.  The voltage of an individual dry cell is approximately 1.5 volts while that of a lead-acid car battery is 2 volts.  In order to increase the voltage output from a battery, cells are connected in series, which causes the voltages to add.  Six dry cells are added to make a 9 volt battery while the same number of lead-acid cells produce the 12 volts a car electrical system uses.  The current capacity for this configuration of cells is the same as a single cell.  By connecting the cells in parallel, the current capacity can be multiplied in the same way.  A series-parallel combination is sometimes used to achieve the required voltage and current outputs.

Capacitors and Inductors
All conductors have some amount of capacitance and inductance, but they are concentrated and purposely used in devices called capacitors and inductors. Capacitance and inductance store energy.  Capacitors, also called condensers, store it in the form of electric fields, while inductors store energy in the form of magnetic fields.
Capacitors block DC current but offer a low impedance to high frequencies and they are used in electronics for these purposes.  In power applications, they are used to compensate for the unwanted effect of inductance.
Inductors pass DC current but offer a high impedance to high frequencies.  They are used in electronics for these purposes.  In power applications, inductance is the byproduct of devices that convert electric current into mechanical force such as motors and solenoids and the usually unwanted effect from transformers.
In DC circuits, capacitance and inductance have no long-term effect and simply block or pass the DC current.  The short-term effect on the voltages and currents when DC is first applied is set by a time constant which is the product of the inductance or capacitance and the resistance of the circuit.

Construction of Resistors
Resistors with lower power ratings are usually packaged in a cylindrical form with the ratings printed on the side or shown in the form of a color code.  Power resistors are packaged in a metal case that can be mounted to a metal surface and with ridges, both of which act to dissipate heat.
The power rating of a resistor is given in watts; it has the ability to absorb heat without damage.  Also associated with resistors is a tolerance in percent that describes how accurately the resistance is required to match its nominal value. Normally, this is 5% except in precision resistors where it is 1% or lower.

Contact Resistance
Contact resistance is the electrical resistance from the contact of two conductors or of a conductor to a conductive surface.  With a good connection, this resistance is usually some small fraction of an ohm. 
With a poor connection, this resistance can become significant and cause problems in the circuit.  As the contact resistance rises so does the circuit resistance, which is the sum of the resistances of the load, the conductors, and all the contact resistances.  Higher contact resistance means energy converted to heat and less current delivered to the load.  If the measured resistance of circuit varies, it is most likely caused by a loose connection.

Cross-Sectional Areas
The common cross-sectional areas that an electrician might need to calculate are those of a circle and a rectangle.  These are the shapes of conduit and raceways as well as wires.

The area of a circle is given as:
A = (pi) r2
(pi) is the ever-present value from geometry, approximately
3.14. r is the radius of the circle, which is one half the diameter.

The area of a rectangle is simply the product of two adjacent sides:
A = hw where h is the height and w is the width.

In order to determine how many wires (circular cross-sections) will fit in a larger circular or rectangular space, a packing factor for circles can be used.  This factor varies for different shapes and number of conductors, but will not generally be greater than π/4, or about ¾. 
This means that only about 30 conductors with an area of 1 square inch will fit into a space with a cross section of 40 square inches.  The circles just can’t be packed any tighter.  Keep in mind that there may be other factors limiting the number of wires that can be enclosed in a confined space such as heat buildup from high current in the wires.

Drill Bit Sizes
Drill bits are sized in a number of ways. In the US the sizes are mostly by fractions or decimals of an inch (1/4” or .250 for example).  There are also lettered and numbered sizes that range from smaller diameters with higher numbers up to #1 followed by letter designations from A to Z as the size continues to increase.  The #80 drill is .0135”, smaller than the smallest fraction designation of 1/64”.  The size Z drill is .4130”, which is slightly larger than 13/32”.

Metric sizes are also used extensively and they are in given in millimeters with decimals (6.25 mm for example).

Machine Screw Sizes
Machine screws are designated with the diameter of the screw body and the fineness of the threads.  In the US, the diameters are given by a numerical size from 0 to 12 with the size #0 screw at .060” and the #12 at .216”.  These are followed by fractions of an inch starting with ¼”.  The thread fineness is in threads per inch, and for each size there are more than one thread designations that are sometimes referred to as fine, standard, and coarse.
Metric screws are designated with an “M” followed by the diameter of the screw body in millimeters, an “X”, and then the screw pitch in millimeters.  The pitch is the distance between adjacent threads.
The length of the screw is usually also given after the diameter and thread data. Length is measured from the underside of the head to the tip of the screw.

Head Types for Machine Screws
Profiles of common screws used in electrical applications are shown below:


On the left is the panhead screw, which has a flattened profile due to its chamfered outer edge.
Next are the round head, button head or dome head screws. 
Each of these is dome shaped but has a slightly different profile.  The round head is the most common.
Last is the flathead or countersunk screw.  The head is flat on the top with sloped sides that are recessed into a material to provide a smooth finish to the fastened surface.

Types of Drives Used for Machine Screws
Each of the screw profiles can have one of several different types of drives that determine the tool required to use them. The most common are shown below:
 

Slotted:
Phillips:
Hex:
Posidrive:
Torx:

Techniques for Bonding of Electrical Conductors
Secure electrical connections between different conductors are commonly accomplished using screws such as on terminal strips or other electrical devices, soldering conductors together, crimping the wires, or using solderless connectors (also known as wire nuts).
Screw connections are very secure and reliable, but require a special device such as a terminal strip, or a device that has screws or threaded holes.  Wing nuts can be used where frequent removal is required. 
With screw connections, wires should encircle the screw in the same direction that tightens them to maintain a secure connection when the screw is tightened.
Soldered connections are also secure if made correctly but require special tools, techniques, and materials and must be properly insulated afterwards.  Crimping also requires a special tool and must be performed with care for a secure permanent connection.
Wire nuts provide a quick and easy alternative.  They have the additional advantages of being resistant to heat and of instantly insulating the bonded joint.

Techniques Used in Soldering
Solder flux is used while heating to thoroughly clean the copper surfaces of conductors prior to bonding.  The flux material most commonly used is rosin. Acid has been used in some applications but it is generally not recommended due to corrosion.  This may lead to failure of the bonded joint in the future.
When soldering stranded wires, make sure to twist the strands together then clean the wires and use enough heat to thoroughly flow the solder.
Copper conductors that are not soldered are sometimes coated with solder (“tinned”) to hold strands together and prevent future chemical reaction of the copper that might weaken the material or cause the resistance to increase. Copper is used in the tip of a soldering iron because copper is a good conductor of heat, and tinning of the tip is also helpful in transferring the heat to the work.

Incandescent Lamp
Incandescent is the most common type of lighting for residential use, but it is the least efficient lighting source.  The filaments of incandescent lamps are made of tungsten and the bulbs contain either a vacuum or a gas.  The inert gas is used to increase the light output.  With incandescent lights, the lamp brightness is measured in lumens.
The hot resistance of an incandescent lamp is 10 times the cold resistance, because as the temperature of the filament is raised the resistance increases. This results in a large surge current when power is first applied and means that the bulb life is shortened if the power is cycled more often.
If an incandescent bulb is operated at a higher than rated voltage, the life of the bulb will also be shortened.

Fluorescent Lamp
The fluorescent light consists of a glass tube containing a partial vacuum with a small amount of mercury.  Electrical current through the mercury vapor results in light emission in the invisible ultraviolet range, but a phosphor coating in the tube then emits visible light.
The fluorescent fixture assembly also contains a device called a ballast that initially provides a peak voltage to fire the tube and then limits the current to the lamps.
The fluorescent fixture is more efficient than incandescent bulbs and fluorescent filaments seldom burn out.  Room temperature has an effect on the performance of the lamps, and like incandescent bulbs the life of the fixture assembly is affected by the frequency of starting and stopping.  A blinking fluorescent light can damage the ballast.
A mercury lamp requires a cooling off period before restarting.

Measuring Illumination
Light intensity is measured in foot-candles, equal to the intensity of a standard candle at a distance of one foot.  The metric unit is the lux, which is equal to about 10 foot-candles.
Illumination, or luminous flux, is the measure of the total light emitted from a device and is measured in lumens.  The light output is limited to the visible portion of the light spectrum.  The measured amount of illumination does not include infrared or ultraviolet light and is adjusted to compensate for the varying sensitivity to different types of light of the human eye.
The foot-candle is equal to one lumen per square foot.  A 100 watt incandescent light bulb produces an illumination of about 1700 lumens.  By comparison, a fluorescent lamp with the same power consumption may emit 5000 lumens.  The best illumination is provided by numerous lights evenly spaced.

Common Symbols Used in Lighting
 

Lighting and Outlet Symbols

Measuring Device Symbols

Switch Symbols

Electronic Circuit Symbols

Safety Device Symbols

Power Circuit Symbols

Schematic Diagram
A schematic diagram is one that shows the simplified layout of an electric circuit. It is a functional representation of the circuit consisting of graphical symbols for devices and their associated connections rather than a representation of their actual appearance.  The actual topology of the circuit is not represented and details that are not relevant to the functioning of the circuit are omitted.  The purpose of a schematic diagram is to provide a quick and simple functional understanding of the circuit and its connections.

Technical Drawing
Technical drawing or drafting provides physical or schematic representations of objects.  It is used widely in architecture and engineering to provide technical details for building structures, plumbing, ventilation, and electric circuits.
Architectural drawings typically show a plan view, which is a view from above showing the layout of rooms, and an elevation view, which shows the vertical details.  Mechanical engineering drawings typically use orthographic projections, which consist of views from two or three right angles.  Schematic drawings show the functional connections of electrical, pneumatic, hydraulic, or ventilation systems.
In mechanical drawings, visible edges are shown with solid lines and invisible edges are identified with broken lines.  Additional views called sections are sometimes used to show the hidden internal structure of components.

Transformer Design
Transformer efficiency is usually about 90% due to resistive losses in the windings, leakage inductance, and eddy current losses in the core.  Eddy currents are induced in the conductive core at right angles to the magnetic flux by the changing magnetic fields.  Steel or iron cores are used to increase the magnetic coupling between the windings and reduce the stray inductance, and these are always laminated to reduce eddy currents.
The higher current side of the transformer usually has larger gauge wires to reduce resistance losses.  Some transformers contain oil to provide electrical insulation and cooling.  Varnish is often used on the wires of the windings for insulation and to prevent oxidation.

Other Magnetic Devices
Devices other than motors, generators, and transformers include relays and solenoids.  A relay’s function is to apply and remove power from another device. It consists of a magnetic coil that controls a number of switches.  By applying and removing power from the coil, the switch(es) are opened and closed.  Relays are commonly used when the voltage available to operate the relay coil is different from the power that is applied by activating the relay.  The coil might be driven by AC that switches to DC power or vice versa.

A solenoid is an electromagnet that is used to initiate some mechanical action such as actuating a valve or unlocking a device.

A typical relay and solenoid are shown below:

Relay	Sealed tubular solenoid

Physical Measuring and Marking Devices

The most common physical measuring device an electrician uses is a tape measure.  Steel measuring tapes are undesirable because of possible entanglement in machinery as well as the short circuit and shock hazard if they are used around live circuits.  A plastic or wooden ruler or tape should be used to measure wherever there is live equipment.
To determine whether something is vertical, a spirit level is used.  A plumb bob is used to mark a point on the floor directly below a point on the ceiling.  To lay out a straight line, use a chalk line
A dial indicator is used to check motor shaft alignment. 
The instrument used to measure the diameter of a wire, conduit, drill bits, or other small objects to thousandths of an inch is a micrometer.

Cutting Devices
The knife is the basic cutting tool.  It can be sharpened with a carborundum stone.
The hacksaw is probably the most commonly used cutting device.  It can be used for conduit, raceways, large gauge wire, or any other material required to be cut to length.  When cutting with a hacksaw, apply pressure on the forward stroke only as this is when the teeth dig into the material.
For cutting conduit and raceway, a fine-toothed blade is recommended (something like 32 teeth per inch).  This is especially true for cutting stranded wire to avoid snagging or pulling the strands.  A hacksaw with fine teeth that is used specifically to cut raceways is known as a tube saw.
After cutting conduit, remove any burrs and rough edges with a file or a tapered reamer.  A pipe cutter can also be used for conduit and less reaming will be required.
An electric saber saw is often used to cut holes through plywood.

Other Common Tools
For cutting holes in masonry, a star or masonry drill is used.  The electrician’s diagonal pliers are used to cut copper wire.  They should not be used on steel. The tool used by linemen to strip large gauge wire is the electricians’ knife.  PVC conduit may be cut in a tight space using a nylon string.
The tool used for bending small sized conduit is called a hickey.
Taps and dies are sometimes needed to add or clear threads. 
The tap tool is used to cut internal (female) threads and the die is used on external (male) threads.  Cutting fluid may be used as a lubricant to improve the finish of the threads.
There are many types of files classified by size, shape, and purpose.  The most common shapes are the rattail (small and round), flat, and half-round.  Mill (single cut) files have a single row of teeth compared to double cut files.  There are fine and coarse toothed files and bastard files with medium teeth.
A chain, strap, or Stillson wrench can be used to attach rigid metal conduit.  The Stillson wrench has adjustable jaws and looks somewhat like a pipe wrench.  The advantages of the chain and strap wrenches are that they can be used with one hand, in close spaces, and on all sizes of conduit.

Safety Issues with Using Tools
- When using a file, make sure it has a handle to prevent injury to your hand.
- If a drill bit breaks, it is due to too much pressure being applied, which could be due to a dull bit.  Make sure a bit is sharp before using it.  An - overheated extension cord is due to corroded terminals or a defective cord.  Always use tools that are in good repair.
- Never strike a hardened steel surface with a hardened steel hammer.
- Insulated grips on tools are helpful when working around live equipment, but should always be used with other insulating equipment. 
- Never rely on a wooden or plastic handle alone to avoid shock.
- When using compressed air to clear a finished task, use less than 50 PSI so that electrical insulating tape is not loosened by the spray.

Common Prefixes Used as Multipliers for Measurements

For units less than one:
Pico = 10-12 (one trillionth), commonly used for small capacitance (picofarads).
Nano = 10-9 (one billionth), used for very short time intervals (nanosecond).
Micro = 10-6 (one millionth), used for capacitance and time intervals.
Milli = 10-3 (one thousandth), commonly used for small currents (milliamps).

For units greater than one:
Kilo = 103 (one thousand), commonly used for high resistance (kilohms).
Mega = 106 (one million), used for high resistance and high power (megawatt).
One thousandth of an inch (.001”) is also known as a mil.

Fasteners
Machine screws are used for fastening to metal devices. 
They have cylindrical bodies with threads that match those in the hole in the material.  A bolt is a similar device, but it passes through the material and is held by a nut on the far side.  Lock nuts are sometimes used to keep the connection secure.  Wood screws are tapered and are tightened into unthreaded holes in wood.  There are screws made for specific materials like sheet metal screws and drywall screws.
Toggle bolts are used in hollow drywall.  A large hole is drilled and the nut on the bolt toggles after it is inserted.  The nut grips the inside of the drywall as the bolt is tightened.  Hollow wall anchors are also used for walls and doors.  They expand as the screws are tightened.  All these devices are more or less permanent.
Expansion bolts are used to fasten to existing concrete or masonry.  They consist of a shield that expands when the screw is threaded into it to tighten.

AC Power Distribution and Usage
Electric power is almost exclusively generated and distributed as 3-phase alternating current (AC) because this is more efficient.  The voltage and current are sinusoidal and reverse at regular intervals.  Each cycle requires 1/60 second to complete; ½ cycle takes 1/120 second.
The standard commercial electrical service for power and lighting is 60 hertz (cycles/sec) 3 phase 4 wire.  Each phase is separated by
120 degrees (1/3 of a cycle) from the others.
The standard residential service is 60 hertz 3 wire single-phase 240 volts where each line supplies 120 VAC to neutral.  This is also sometimes called 2-phase AC.  The full 240 volt potential is used for higher current devices like air conditioners and ovens.  Using both lines results in less current and therefore lower line voltage drops.

Effects of Resistance
Power is dissipated by the resistance in loads, wires, or components. In a resistive load such as a heater or light bulb, power in the form of heat and light is put to good use.  In the other cases, it is beneficial to minimize these losses. When current flows through a wire, voltage drops occur and the wire heats up. This lost energy is not available to the circuit loads.  The greater the resistance and the current the larger is the loss.  Voltage drops in conductors can be reduced by decreasing the resistance by using larger gauge wires.

Series and Parallel Load Connections
Typically, loads are connected in parallel so they can operate independently and at full voltage.  Electrical outlets and lighting circuits are always wired in this way.
In some special cases, devices are connected in series.  An electrical timer must be wired in series with the loads it supplies in order to control them.  The individual lamps of some Christmas lights are wired in series so each lamp receives the proper voltage (only a fraction of the 120 Volts).
The heating elements of a stove are wired in series parallel combinations that are controlled by a switch to provide varying current and heat to the burners.
Multiple start buttons on a motor start circuit are connected in parallel.

Fuses
Fuses operate on the principle that electrical current develops heat.  By using a fuse element of a specific resistance and a low melting point, only a specified current can flow through the fuse before it melts and opens.  Fuses work in both AC and DC circuits.
Fuses are used inside appliances, in automobile electrical systems, and in home and industrial power circuits.  Most present day power distribution systems now use circuit breakers instead of fuses.
In power distribution, there are cartridge fuses and plug fuses.  Fuses are also classified by the amount of time they will allow an overcurrent condition to exist. Those without a time delay react almost instantly to the overcurrent condition while time delay types will not open until a specific time period has elapsed.  The time delay allows for short duration high start up current of motors for instance.
If a fuse has blown, the user should first replace it and resume operation.  If the new fuse blows, check the circuit for a problem.

Fuses Used in Power Distribution
Cartridge fuses are the most common design.  A fuse puller should be used to replace cartridge fuses.  Some cartridge fuses are renewable and can be restored by replacing their fusible elements.  Clip clamps are used to insure good contact between the fuse terminals of cartridge fuses and the fuse clips in the fuse box.
If the ends of a cartridge fuse become warmer than normal, the fuse clips should be tightened.  If the spring tension on a cartridge fuse clip is weak, the fuse clips will become warm.  Discoloration at one end of a fuse indicates poor contact with the panel.
Older homes may have boxes that take plug fuses, which screw into a threaded base that resembles a light socket.  Some use rejection base adapters that take type S fuses which have a different base that prevents installation of fuses with the wrong rating.  These are also called fusestat fuses.

Circuit Breakers
Like fuses, circuit breakers may also operate on the principle that electrical current develops heat.  In the case of the breaker though, a switch responds to the heat and is opened automatically when the preset current limit is exceeded. There may also be an electromagnetic actuator within the breaker.  Unlike a fuse, circuit breakers can be reset to resume operation.
Even though the circuit breaker is wired into only the ungrounded conductor, when a breaker trips it may be due to a short circuit or other fault in either the grounded or the ungrounded conductor.
A GFCI breaker is one that has additional protection from ground faults.  If the current flowing from the ungrounded line to the grounded line is unbalanced, the device opens.  This situation only occurs when there is a short circuit to ground, which could cause equipment damage or injury.
If a circuit breaker is mounted vertically, the on position must be up.

Switchgear
The assembly of devices used for interruption, control, and metering of power is called switchgear.  It may be located at the power generation source, at a distribution location such as a power station or substation, or in or near a house or commercial building.
At a generating plant or substation, the monitoring and control functions are sophisticated and typically use not only circuit breakers, switches, and meters but also communication equipment and remote sensing and control circuits.
At the service entrance for a building, containing the loads will typically be a meter with its feeder conductors and the panels containing overcurrent protection devices for the main service and all the branch circuits. In some commercial buildings (particularly those receiving high voltage service), these devices may be enclosed in a separate building.
The main purpose of lower voltage switchgear is protection, isolation, and control of power distributed within the building.

Energy and Electricity
Energy is the product of the power consumption and the length of time.  Power is expressed in watts and energy is given in watt-hours.  Energy is the product of the voltage, current and the length of the time interval.  For AC circuits, RMS values are used to calculate it.
Efficiency is a measure of how much energy or power is delivered to the load divided by the energy or power applied.  If there are no losses the efficiency is 100%, but this is never the case.  Resistive and other losses always reduce the efficiency to a lower value.

Insulation Used on Wire
The purpose of insulation on conductors is to prevent short-circuiting between the wires.  Any number of insulating materials may be chosen depending on the voltage, current of the application and the environmental conditions where the conductors are used.  The most common materials are:
- Plastic: used extensively for home and industrial power distribution, as it is relatively cheap and durable.  Plastic jacketed 2- and
3- conductor cables (trade name Romex) are the most common form of wiring with this type of insulation.
- Rubber: used extensively in appliance power cords; it is moisture and heat resistant but is damaged by oil and solvents.  For this reason, neoprene is sometimes used instead.
- PVC and Teflon: have superior durability but are usually limited to electronic applications because their cost is relatively high.
- Paper: once used extensively but its dielectric strength is lower than newer synthetic materials.
Overloading a conductor will generate excessive heat and may cause the insulation on the conductors to deteriorate.

Materials and Construction Used for Electrical Wire
The vast majority of electrical wire consists of a copper conductor and an insulator made from some form of synthetic material.  Silver is actually a better conductor than copper but is not used extensively because of its cost.  Silver is sometimes used in small doses on electrical contacts to improve conductivity.
Aluminum wire was used in house wiring in the 1970s, but was discontinued due to problems including corrosion.   Aluminum wire does have the advantage of lower weight.
Wire is sized by gauge and this refers to the conductor total cross sectional area, which relates to the current carrying capacity, also called ampacity.
The wire conductor may be a single wire or consist of many smaller strands twisted together.  Solid wire is often preferred to stranded wire, because it can be shaped better and it is easier to connect without omitting stray strands.  The total diameter of stranded wire is larger than solid wire of the same gauge.

Thermocouples
Thermocouples are precise temperature sensors that operate on the physical principle that junctions of dissimilar metals produce small voltages.  These voltages vary with different materials, and they also change with temperature. With the proper instrumentation, these small potentials (in the microvolt range) can be converted to precise temperature measurements.
There are a wide variety of thermocouples that cover different temperature ranges.  They are classified by type such as type K, E, J, N, etc.  Each type uses a unique set of alloys.
The difficulty in wiring thermocouples into the instruments is that the same materials must be used from the junctions all the way to the instrument.  This requires extension cables and connectors made of special materials such as chromel (nickel chromium), alumel (nickel aluminum), constantan (copper nickel), etc.
In less precise applications, compensating cables are sometimes used for these connections.  They are special cables made from a single alloy that introduce an acceptable error, but do not require the restrictive connections described above.

Conductivity of Wire Materials and Temperature
The resistance of a 10 gauge copper wire ten feet long is .01 ohm. For an aluminum wire the same size and length the resistance is less than .02 ohm.  For shorter wires, any of the common conductive materials perform about the same.
In most cases, the resistance of a conductor increases with temperature.  The amount of increase is generally about 0.2% per degree Fahrenheit. 
This means that if a copper conductor has a resistance of 1 ohm at 70 degrees, at 170 degrees its resistance is 100 x 0.2% = 20% higher, or 1.2 ohms.  Carbon is an exception as its resistance decreases with higher temperatures.  The resistance of carbon decreases about 0.03% per degree Fahrenheit, so if a carbon motor brush has a resistance of 1 ohm at 70 degrees, at 170 degrees its resistance is 100 x -0.03% = 3% lower, or 0.97 ohms.

Generator Output
The output voltage of an AC or DC generator is proportional to its magnetic field strength and the speed at which the rotor is driven.  The voltage is produced by induction as the armature cuts through the magnetic lines of force.  The number of lines per second that are crossed determines the output.  This rate can be increased by strengthening the field by increasing the field current.
The frequency of an AC generator’s output depends on the number of pole pairs built into it and the rotational speed of the armature. 
If there are 6 pole pairs (six N and six S magnetic poles) per revolution, then each rotation of the rotor produces three electrical cycles.

Measurements Performed on Generators
The outputs of generators can be monitored with voltmeters and frequency meters.  The output frequency will also be proportional to the speed, which can be monitored with a tachometer.
The most critical parameter of a generator’s performance is whether it is being overstressed, which can lead to overheating and damage to the device.  The instruments that may be used to indicate whether a generator may overheat are the wattmeter, ammeter, or a stator thermocouple.  The wattmeter and ammeter show whether the generator’s ratings are being exceeded, which would lead directly to overheating, while the thermocouple measurement shows directly when this occurs.

Common Electrical Materials
Carbon is a very soft conductive metal.  Carbon brushes are used in motors and generators, because it makes good contact on the moving surfaces while lubricating and polishing the commutator.  Copper pigtails are used to connect to the brushes to improve the contact.
Galvanized steel is plated or dipped with a coating of zinc.  This is done to slow down the formation of rust if the material comes in contact with water. Brass is an alloy of zinc and copper.  It is gold in color, is conductive, does not tarnish as easily as copper, and is more easily manipulated than either metal alone.
Insulated non-metallic boxes are usually made of PVC.  In the past they were typically made from bakelite or phenolic, which were formations of synthetic resin and wood products used extensively at that time.

Magnetic Hysteresis
In magnetic materials such as iron, hysteresis is the physical effect of a lag between the applied magnetizing force and the resulting magnetic flux.  If these are plotted as shown below, the resulting curve is in the form of a loop with a different response depending on whether the field is increasing or decreasing (the arrows indicate the direction).


The advantage of this effect is that when an external magnetic field is applied to a ferromagnetic material, it absorbs some of the field so when the external field is removed the material remains magnetized.
The disadvantage of hysteresis is that energy is lost if a field is applied repeatedly such as in a coil or transformer with an iron core.

Types of Electrical Cable

These cables are generally constructed as 2 and 3 conductor cables for electrical power distribution.  The colors of the wires are black and white for 2 conductor cable (used for single-phase hot and neutral), black/white/green or black/white/uninsulated for 2 conductor with ground (the green or uninsulated wire is used for the non-conducting ground), and red/black/white for 3 conductor cable (red and black are used for the ungrounded conductors).

Types of Conduit
Metallic conduit is designated with the following letters:

Non-metallic varieties include:

Conduit in concrete is considered to be in a wet location. 
Conduit or raceways encased in concrete should be a minimum of 1” deep to prevent cracking of the concrete.   Cover is defined as the shortest distance between the top of a buried conductor or conduit and the finished grade.

Configuration of 125-Volt Receptacles and Plugs
Older homes may have ungrounded receptacles; lamps and other small current devices sometimes use ungrounded plugs.  They may be unpolarized with both prongs being the same size, or polarized with the wider pin being neutral.  These appear as shown:


Modern low power (15 Amp) receptacles contain a grounding pin and are configured as:


Higher power (20 Amp) plugs have a horizontal pin for the neutral, and 20 amp receptacles are configured to accept either 15 or 20 amp plugs.  It is acceptable to wire a 15 amp receptacle into a 20 amp circuit because any device that requires the higher current would have the horizontal neutral pin that wouldn’t fit the lower current receptacle. 

The configuration of the 20 amp devices is:


30 amp receptacles are sometimes used for even higher current 125 volt devices such as air conditioners.



Corona Effect
A corona occurs when there is a high potential difference between two transmission wires causing the discharge of electricity due to ionization of air from the high voltage.  It is characterized by a glowing region of air and a hissing sound. Several thousand volts may be required to produce this effect.  A very small current results because the ionized air has a relatively high resistance. Coronas occur most commonly at sharp points on the conductive surfaces that tend to magnify the electric field strength and where the air gap between the conductors is small.

Rigid Metal Conduit
The reason for installing wires in a conduit is to protect them from damage.  Rigid metal conduit resembles water pipe and can be threaded or unthreaded.  Pipe and conduit threads are tapered unlike machine screw threads that are straight. Unthreaded conduit requires fittings for connections to other pieces.
Lock nuts are used on threaded conduits when they are connected to junction or outlet boxes.  Rigid conduit attached to an outlet box should have a locknut on the outside and a bushing on the inside.
Installed conduits should not have low points between successive outlets, because this would allow water to collect there if it entered the conduit.
Wires are pulled through the finished conduit connections with a device called a fish tape.  The usual lubricant used on the wires is powdered soapstone. Condulet fittings are special devices that are used instead of bends in conduit. They have small plates on the side that when removed make pulling the wires easier.  A box with a blank cover can also be used to facilitate pulling the conductors, and this is called a coupling box.

Power Distribution Wiring Methods
Many years ago, an exposed wiring method was used that involved insulating knobs and tubes that contained the wires.  These were concealed in hollow spaces of walls and ceilings.  Care must be taken with any knob and tube wiring that still exists, as the tubes may be brittle and susceptible to damage.
Today, wires are either formed into multiple conductor cable that is strung along building supports or some form of conduit or tubing is used.  Tubing typically already contains the wires while conduit must have the wires added after the conduit is assembled.
Insulated multiple conductor cables come in a variety of sizes and are specified for any number of applications.  Romex is a common trade name for these cables.

Reversing the Direction of Electric Motors
The direction of rotation of direct current (DC) motors is determined by the polarity of the applied voltage, so they are reversed simply by interchanging the wire connections.
If a single-phase motor has starting windings, the direction of rotation can be reversed by reversing these connections.  By convention the rotation of induction motors is counterclockwise when facing the front of the motor.
To reverse the rotation of a 3-phase motor, reverse the order of any two of the leads.  This reverses the order of the phases applied and causes the motor to rotate in the opposite direction.
Most motors are designed to run in a specific direction and may either not achieve their ratings or be degraded by forcing them to run in the opposite direction.

Stripping Wires
The best tool for stripping most wires is a wire stripper, not a pair of diagonal cutters or a knife.  This pliers-type tool has sized stripping holes and will generally remove the insulation smoothly without nicking the wire.  You may need to rotate the tool to cut through all sides of the insulation before pulling it off the wire.
Smaller gauge wires, or aluminum conductors that are not sized the same as the standard copper wire sizes, may be pencil stripped to prevent nicking them.  This is done with a knife and uses a similar motion as sharpening a pencil with the knife.  Always cut away from your body.

Splicing and Insulating Wires
Splices are made by twisting two or more wires together, then soldering them, and finally insulating them with tape.  There are several different types of splices, but all are made by twisting the wires tightly together.
A staggered splice is recommended for two conductor cable. 
It requires positioning the two splices at slightly different spots to avoid making a single large joint once the insulating tape is applied.
After soldering, insulation is added with the use of tape. 
The traditional method is to apply latex rubber tape first, followed by friction tape.  For each layer of rubber tape that is applied, it is stretched tight.  When completed, the layers of tape blend into a solid rubber insulating cover.  Friction tape is then added for a protective covering.  It is made of cotton cloth that is impregnated with rubber.
Plastic electrical tape is often used instead of rubber and friction tape.  It is much thinner and withstands higher voltages.  Wire nuts are often used in place of a traditional splice.

Color Code for Wires Used in Power Distribution
The electrical code dictates only two colors for standard power system wiring, green and white.  There are other code requirements for special uses such as in medical facilities, and there are other common wire identification colors that are not specifically dictated by the code. 

Overall, the convention is as follows:
- White or grey, or black with white stripes along the length of the wire: neutral, grounded conductor.
- Black: hot, or ungrounded conductor.
- Red (in 3-wire cable): hot, or ungrounded conductor.
- Green, or bare uninsulated wire: Grounding wire connected to equipment frame.

For isolated power in health care facilities, two phase power uses orange and brown. For three phase systems yellow is also used. If the conductors are wired into a single-phase outlet, the orange wire is used for the grounded conductor.
For the special case of a delta wired system with the midpoint of one phase grounded, orange is used for the high leg.

Quartz Lamp
Tungsten halogen lamps are also called quartz lamps.  They were introduced as an improvement to incandescent lamps.  Their light output is about 50% higher for the same amount of input current.
The inner capsule of a quartz lamp contains mercury vapor under high pressure rather than a vacuum or low-pressure inert gas.  Halogen lamps produce a cool white sunlight appearance rather than the warm yellow light of an incandescent lamp.
Quartz bulbs burn very hot and can explode, especially if oil or water is present on their surfaces.  For this reason, they are enclosed in an additional glass envelope for safety.

Pipe and Conduit Fittings
For threaded conduit, nipples are male-to-male fittings.  A close nipple is a short nipple that has threads over its entire length.  L’s make right angle turns.  Sleeves are female-to-female connectors.  Bushings and locknuts are used for connecting to boxes and other devices.  Various connectors and adapters are used for unthreaded conduit.  The threadless conduit pieces are slid into them and secured either with set screws or compression fittings.  The fittings include couplings to connect two lengths of conduit together and threaded adapters with locknuts for installation into boxes.

 

Ampacity
Ampacity is the current in amps that a conductor can carry continuously without exceeding its temperature rating.  Ambient temperature, which is the temperature of the area surrounding the conductor, directly affects this current capacity.  If the temperature is higher, the ampacity must be derated according to prescribed formulas or tables.
Installing more than three current carrying conductors in a single conduit also requires derating of ampacity.  If a conductor supplies two or more motors, the ampacity used must be the sum of the load currents + 25% of the highest rated load.

Raceway
A raceway is any type of enclosure for housing, protecting and organizing insulated electrical wires and cables.  This includes conduit pipe, tubing, and cable trays (also called wireways).  Any of these devices may be made of steel or aluminum, PVC, or other types of plastic.  For protection from the spread of flames, fire-retarding material may be used.  Raceways must always be rigidly supported.  Longer runs of raceways may need expansion joints to compensate for thermal expansion and contraction.
Wireways are troughs with hinged or removable covers that are generally placed in ceilings or floors of commercial and industrial buildings.  Power cables may use fittings in the tray to maintain clearance between the conductors, but most cables are installed without any special spacing.
Conduit is circular pipe that may be run underground or in concrete or may be fastened to exposed or concealed building structures.  Wires in conduits don’t dissipate heat as easily as in open air and wires can induce currents in adjacent conductors, so special rules apply for the number of wires that can be installed in a conduit and derating is required for power carrying conductors.