Download Electric current

9.5.2013
Michal Kabrhel
Electric current
• An electric current is a flow of electric charge. Electric charge
flows when there is voltage present across a conductor.
• In electric circuits charge is often carried by moving electrons in
a wire.
• The SI unit for measuring an electric current is the ampere (A)
• The conventional symbol for current is I
• Ohm's law
• V is the potential difference measured across the conductor in units
of volts, and R is the resistance of the conductor in units of ohms
=
1
9.5.2013
Power grid
Power station
• A power station is industrial facility for the generation of electric power
• At the center of nearly all power stations is a generator, a rotating machine
that converts mechanical power into electrical power
• Most power stations in the world burn fossil fuels such as coal, oil, and natural
gas to generate electricity, and some use nuclear power
• increasing use of cleaner renewable sources such as solar, wind and water
• The system of three-phase alternating current electrical generation,
transmission, and distribution was developed in the 19th century by Nikola
Tesla, George Westinghouse and others
2
9.5.2013
Power grid
• Transmission lines become transmission networks (power grid, grid)
• Most transmission lines use high-voltage three-phase alternating current (AC)
• Electricity is transmitted at high voltages (110 kV or above) to reduce the energy
lost in long-distance transmission.
• Electric power - distribution local wiring between high-voltage substations and
customers
Transformer (substation)
• A transformer is a static electrical device that transfers energy
by inductive coupling between its winding circuits
• Example 32/04
• 32kV is transfered to 0,4kV=400V
3
9.5.2013
Current
• Alternating current (AC,ac) the flow of electric charge periodically
reverses direction
• AC is the form in which electric power is delivered to businesses and
residences.
• usual waveform of an AC power circuit is a sine wave
• In direct current (DC, dc), the flow of electric charge is only in one
direction.
• Direct current is produced by sources such as batteries, solar cells,
and commutator-type electric machines of the dynamo type
• Direct current may be made into alternating current with
an inverter or a motor-generator set.
Current
4
9.5.2013
Electrical wiring
• Electrical code
• is a set of regulations for electrical wiring
• provide standards to ensure electrical wiring systems that are safe and unlikely
to produce either electric shock or fires
• devised by national or international technical organizations
• IEC 60364 is used as a basis for electrical codes in many European countries
• DIN VDE (German Institute for Standardization) published by DIN-Norms is used in
Germany
• British Standard BS 7671 is the set of regulations for electrical wiring in the United
Kingdom.
• NF C 15-100 is used for low voltage installations in France
• National Electrical Code has been adopted for electrical wiring in the United States and
for Mexico, Costa Rica, Venezuela and Colombia
Building wiring
• The live wire (also known as phase, hot or active contact),
carries alternating current between the power grid and the household. Live
(L).
• The neutral wire completes the electrical circuit by also carrying alternating
current between the power grid and the household. The neutral is
connected to the ground, and therefore has nearly the same electrical
potential as the earth. This prevents the power circuits from rising beyond
earth, such as when they are struck by lightning or become otherwise
charged. Neutral (N).
• The earth wire or ground connects cases of equipment to earth ground as a
protection against faults (Electric Shock). Protective earth (PE)-conductor
connects the exposed metallic parts of the consumer's electrical installation.
• Mixed 230V/400V Three-phase electric power (common in northern and
central Europe) or 230V single-phase based household wiring
5
9.5.2013
Networks
• International standard IEC 60364 distinguishes three families of earthing
arrangements (TN, TT, IT)
• TN−S PE and N are separate conductors that are connected together only
near the power source. This arrangement is the current standard for most
residential and industrial electric systems partially in Europe.
Networks
• TN−C-S Part of the system uses a combined PEN conductor, which is
at some point split up into separate PE and N lines. The combined
PEN conductor typically occurs between the substation and the entry
point into the building, and separated in the service head.
6
9.5.2013
Socket
Color code
• Flexible cable
u
Fixed cable
7
9.5.2013
Conductors
• Copper conductors wire are common because of their multiple
beneficial properties, including their high electrical
conductivity, tensile strength, ductility, creep resistance, corrosion
resistance, thermal conductivity, coefficient of thermal
expansion, solderability, resistance to electrical overloads,
compatibility with electrical insulators, and ease of installation.
• Aluminium wire was common in North American. Because of its
greater resistivity, aluminium wiring requires larger conductors
than copper.
Electrical wiring
• Small portable electrical equipment is connected to the power
supply through flexible cables terminated in a plug, which is
then inserted into a fixed receptacle (socket).
• Larger household electrical equipment and industrial
equipment may be permanently wired to the fixed wiring of
the building.
• Circuit breakers and fuses are used to detect short
circuits between the live and neutral wires, or the drawing of
more current than the wires are rated to handle to prevent
overheating and fire.
• Protective devices are usually mounted in a central panel in a
building
8
9.5.2013
Protection
• A circuit breaker is an automatically
operated electrical switch designed to protect an electrical
circuit from damage caused by overload or short circuit.
• Its basic function is to detect a fault condition and
interrupt current flow.
• A circuit breaker can be reset
• Fuse operates once and then must be replaced
Distribution board
• A distribution board (or panelboard) is a component of
an electricity supply system which divides an electrical power
feed into subsidiary circuits, while providing a
protective fuse or circuit breaker for each circuit, in a
common enclosure.
• Distribution boards may be surface-mounted on a wall or may
be sunk into the wall.
9
9.5.2013
Residential electrical installation - Circuits
• National standards commonly recommend the subdivision of circuits
according to the number of utilization categories in the installation
concerned
• At least 1 circuit for lighting. Each circuit supplying a maximum of 8 lighting
points
• At least 1 circuit for socket-outlets rated 10/16 A, each circuit supplying a
maximum of 8 sockets. These sockets may be single or double
• 1 circuit for each appliance such as water heater, washing machine, dishwashing machine, cooker, refrigerator, etc. Recommended numbers of
10/16 A (or similar) socket-outlets and fixed lighting points
Bathroom electrical installation
• Bathrooms and showers rooms are areas of high risk, because of the very
low resistance of the human body when wet or immersed in water.
• The definition of zones, numbered 0,1, 2, 3 in which the placement (or
exclusion) of any electrical device is strictly limited or forbidden and,
where permitted, the electrical and mechanical protection is prescribed
10
9.5.2013
Electrical bonding
• Electrical bonding is the practice of intentionally
electrically connecting all exposed metallic items
not designed to carry electricity in a room or
building as protection from electric shock.
• If a failure of electrical insulation occurs, all bonded
metal objects in the room will have substantially
the same electrical potential, so that an occupant
of the room cannot touch two objects with
significantly different potentials.
• In a building with electricity it is normal for safety
reasons to connect all metal objects such as pipes
together to the mains earth to form
an equipotential zone.
Electromagnetism
• Electric current produces a magnetic field. The magnetic
field can be visualized as a pattern of circular field lines
surrounding the wire that persists as long as the current
flows.
11
9.5.2013
Stray voltage
• Stray voltage is the occurrence of electrical potential between two
objects that ideally should not have any voltage difference between
them.
• Stray voltage is not related to power system faults, and is generally
not considered hazardous.
12