Michal Kabrhel. Electric current

Transcription

1 Michal Kabrhel Electric current An electric current is a flow of electric charge. Electric charge flows when there is voltagepresent across a conductor. In electric circuits charge is often carried by moving electronsin awire. TheSIunit for measuring an electric current is theampere(a) The conventional symbol for current is I Ohm's law V is the potential difference measured across the conductor in units ofvolts, and R is the resistanceof the conductor in units ofohms = 1

2 Power grid Power station Apowerstation isindustrial facility for the generationofelectric 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 burnfossil fuelssuch as coal, oil, and natural gas to generate electricity, and some usenuclear power increasing use of cleaner renewable sources such as solar,windand water The system ofthree-phasealternating current electrical generation, transmission, and distribution was developed in the 19th century by Nikola Tesla,George Westinghouse and others 2

3 Power grid Transmissionlines becometransmissionnetworks(power grid, grid) Most transmission lines use high-voltagethree-phasealternating current(ac) Electricity is transmitted athigh voltages(110kv 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 Example32/04 32kV istransferedto 0,4kV=400V 3

4 Current Alternatingcurrent (AC,ac) the flow ofelectric charge periodically reverses direction AC is the form in which electric poweris delivered to businesses and residences. usual waveformof an AC powercircuit is asine wave In direct current (DC, dc), the flow of electric charge is only in one direction. Direct current is produced by sources such asbatteries,solar cells, and commutator-type electric machines of the dynamo type Direct current may be made into alternating current with an inverteror a motor-generator set. Current 4

5 Electrical wiring Electrical code is a set of regulations forelectrical wiring provide standards to ensure electrical wiring systems that are safe and unlikely to produce eitherelectric shockorfires devised by national or international technical organizations IEC is used as a basis for electrical codes in many European countries DIN VDE (German Institute for Standardization) published bydin-norms is used in Germany British Standard BS 7671 is the set of regulations for electrical wiring in the United Kingdom. NF C 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), carriesalternating currentbetween thepower gridand the household. Live (L). The neutral wire completes the electrical circuit by also carryingalternating currentbetween thepower gridand 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/400VThree-phase electric power(common in northern and central Europe) or 230Vsingle-phasebased household wiring 5

6 Networks International standard IEC 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

7 Socket Color code Flexible cable u Fixedcable 7

8 Conductors Copperconductors wireare 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 toelectrical overloads, compatibility withelectrical insulators, and ease of installation. Aluminium wirewas common in North American. Because of its greaterresistivity, 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 breakersandfuses 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 Protection A circuit breaker is an automatically operatedelectricalswitchdesigned to protect an electrical circuit from damage caused by overloadorshort circuit. Its basic function is to detect a fault condition and interrupt current flow. Acircuit breaker can be reset Fuseoperates once and then must be replaced Distribution board A distribution board (or panelboard) is a component of anelectricity supply system which divides an electrical power feed into subsidiarycircuits, while providing a protective fuseor circuit breakerfor each circuit, in a common enclosure. Distribution boards may be surface-mounted on a wall or may be sunk into the wall. 9

10 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

11 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 fromelectric 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 themainsearth to form anequipotentialzone. Electromagnetism Electric current produces amagnetic 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

12 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