Electrical Principles and Wiring Materials

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Electrical Principles and Wiring Materials
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Direct Current DC

• a circuit containing a battery is a DC circuit
• in a DC circuit the current always flows in the
  same direction

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Alternating Current (AC)

• In an AC circuit the current reverses direction
  periodically
• AC is what you get from the power companies

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Electrical Safety

• Shock and Fire
• Never disconnect any safety device
• Dont touch electrical items with wet hands or
  feet
• Discontinue use of extension cord that feels warm
• Dont put extension cords under carpet

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Electrical Safety

• Install wiring according to regulations
• Determine cause of blown fuse or breaker
• Dont replace fuse with larger fuse
• Dont leave heat producing appliances unattended
• Heaters lamps away from combustibles
• Dont remove back of TV (30,000v when off)

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Electrical Safety

• Keep appliances dry
• Dont use damaged switches, outlets, fixtures,
  extension cords
• Follow manufacturers instructions for
  installation and use of electrical equipment

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Service Entrance

• Power from from power company
• Transformer drops volts from 25,000 volts to 240
  volts
• Service drop wires etc from transformer to house
• Entrance head weather-proof at house
• Meter
• Service Entrance Panel (SEP) box with fuses or
  breakers

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Electric Meter

• Kilowatthours how electricity is sold
• Kilo 1000
• Watthour use of 1 watt for one hour
• 100 watt light bulb for 1 hour - 100 watthours
• Kilowatthour 1000 watts for one hour

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Voltage Drop

• loss of voltage as it travels along a wire
• lights dim, motors overheat
• larger wires have less voltage drop for a given
  amount of current
• longer wire greater problem
• must increase wire size as distance increases

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Wire Identification

• Type of outer covering, individual wire covering,
  cable construction, number of wires
• Wire type stamped on outer surface

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Fuse and Circuit Breakers

• When a wire carries more current than it should,
  it is said to be overloaded. To prevent
  overloading, fuses or circuit breakers are
  installed in circuits. They act like switches to
  open the circuit when the current exceeds some
  particular value.

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Fuse

• An electrical protective device consisting of a
  fusible (easily melted) metal-alloy strip of wire
  encased in a cartridge
• If current exceeds a predetermined value,
  sufficient heat is generated to melt the fuse
  causing an open circuit
• Protects the circuit from possible damage due to
  excessive current

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Circuit Breaker

• An electrical device that opens when the current
  exceeds a predetermined value for the circuit
• Circuit breakers trip open, and may be reset when
  the operators are ready to close the circuit
  breaker
• Circuit breakers may be operated manually or
  electrically, and can be used as switches in some
  circuits

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Circuit overload

• if you have too many things plugged into the same
  circuit, the voltage may drop.
• you may notice that a lamp plugged into the same
  outlet as a hair dryer dims a bit when you turn
  on the hair dryer because a hair dryer draws a
  lot of current
• according to Ohm V I R, a big I can cause
  enough drop in the voltage to be noticeable!

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Fuse and Circuit Breakers
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Function of Fuse

• For example, a 20 A fuse or circuit breakers
  opens when the current passing through it exceed
  20 A. This maybe due to too many devices drawing
  current in that circuit or there might be a fault
  such as short circuit somewhere.

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Example

• Will the fuse blow? Determine the total current
  drawn by all the devices in the circuit.
• I P / V
• Light bulb 100W / 120V 0.8 A
• Heater 1800W / 120V 15.0 A
• Stereo 350 W / 120V 2.9 A
• Hair Dryer 1200 W / 120 V 10.0 A
• So the total current used are
• 0.8 A 15.0 A 2.9 A 10.0 A 28.7 A

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Electric power generation distribution

• It is more efficient to transmit electrical
• power (P IV) at high voltage and low current.
• The losses along the transmission lines are
• reduced compared to transmission at low V.