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Domestic electrical systems
 

Domestic electrical systems

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    Domestic electrical systems Domestic electrical systems Presentation Transcript

    • Domestic Electrical Systems Core source of energy in modern developed societies
    • Overview of unit
      • What is electricity?
      • What is an electrical circuit?
      • How is electricity generated?
      • How is electricity distributed?
      • Domestic electrical systems
      • Safety
    • What is electricity?
      • Nobody really knows what it is, but they can describe its actions
      • All matter consists of particles which can have one of three charges
        • Neutral (effectively no charge)
        • Positive (+ve)
        • Negative (-ve)
      • Like charges repel each other, opposite charges attract each other
    • Charges in an atom -ve electrons +ve nucleus In a neutral atom, the number of positive charges in the nucleus equals the number of negative electrons orbiting it
    • Electrical potential
      • If two bodies have different electrical charges, an electrical potential exists between them
      • If those two bodies are linked by a conductor, the –ve electrons will flow from the –ve source to the +ve sink
      • This flow of electrons is an electrical current and it is a form of energy
    • Lightning
      • Thunder clouds generate enormous –ve charges
      • The Earth, neutrally charged, is effectively +ve in relation to the clouds
      • When the potential becomes big enough, the air becomes conducting and the current flows
      -ve -ve -ve -ve -ve -ve -ve -ve -ve -ve +ve +ve +ve +ve +ve +ve +ve +ve +ve +ve +ve -ve -ve -ve -ve -ve -ve -ve -ve -ve -ve -ve -ve -ve -ve -ve -ve -ve -ve -ve -ve
    • What is an electrical circuit?
      • To harness electrical energy safely, we need to use closed electrical circuits, which contain the current in a solid conductor
      -ve +ve supply Energy converter (light bulb, CD player, tumble drier etc.) conductor conductor The potential difference between positive and negative is measured in VOLTS Flow of –ve electrons, the current
    • Controlling a circuit by breaking it -ve +ve Conductor: The most suitable, domestically, is copper. Silver is best, but too expensive. Aluminium is used in overhead cables because it is light. -ve +ve Circuit closed, current flows Circuit broken, no current switch
    • Units of electricity
      • The potential difference (think of it like pressure in water) is measured in volts – V
      • The current (think of it like the quantity of water flowing through a pipe) is measured in amps – A
      • The power (ability to do work) is measured in watts – W
      • W = VA ; A = W/V ; V = W/A
    • Domestic electricity
      • In UK: domestic current is 13A voltage is 240V
      • Max power from a domestic socket is W=VA 13x240=3120 or about 3kW
      • If you plug in a power tool requiring say 5kW, what happens?
    • How is electricity generated?
      • Overwhelmingly, electricity is created by converting mechanical energy into electrical energy in an electrical generator
      A rotor, bound with copper wire, is spun between strong magnets. This induces an electrical current in the wires. This spinning rotor is at the core of all major electrical power generation
    • All use rotating generators Hydro power Nuclear power Tidal power Wind power
    • How is electricity distributed?
      • Overhead power lines
      • Potentially inefficient as electricity travelling through a conductor generates heat
      • Usable energy is lost to the environment
      • But, heat generated is proportional to the current, which leads to a solution…
    • High voltage power distribution
      • 240V at 13A gives us about 3kW
      • 765,000V at 0.004A gives about 3kW
      • Electricity is distributed at very high voltage to minimise energy loss
    • AC~DC
      • The steady current from a battery is a direct current, DC. Efficient, but difficult to use in large installations
      • Voltage level is changed most efficiently in a solid state transformer if the current is alternating current, AC
      • All distribution and supply of electrical power is AC
    • AC power supply in principle Live wire: voltage change from -240V to +240 volts Neutral wire: voltage constant at 0 volts Current alternates direction 50 times per second, 50Herz When an electrical unit hums, it does so at a tone of 50Herz
    • Domestic electrical systems
      • UK domestic electrical power supply: 13A, 240V, 50Hz
      • Some special sockets (e.g. electric cooker outlets, electric showers) are rated at 15A and use different, round pin plugs and separate protected circuits
      • Power outlets rated at 13A
      • Lighting outlets rated at 5A
    • Domestic control systems
      • All circuits are switched in the live wire
      • All live wires have a fuse , designed to fail and break the circuit if too much current flows
      • Modern systems use an electro-magnetic circuit breaker in place of the fuse
      Live wire Neutral wire fuse switch
    • Ring mains
      • Typically, in a house, each floor is served by two ring mains , one for the power sockets and one for the lights
      Switch here Live ring Neutral ring Earth ring -240V>+240V 0V
    • Safety
      • Insulation
      • Fuses and circuit breakers
      • Earthing
      • Double insulation
    • Insulation
      • Cover the conductor with a non-conductor
      • Most widely used in PVC
      • Can be coloured to show which wire is which
      Ring main cable Appliance cable
    • Fuses and circuit breakers
      • Fuses are weak links, designed to burn out and break the circuit if the current gets too high. Mainly used in plugs and cars now.
      • “ Fuse boards” have been replaced in houses by “circuit breaker panels”
    • Circuit breakers
      • Electro-magnetic switch, which pops open when the current becomes too large
      • A CB is used for each ring main and each isolated power unit
      • A double switch isolates both live and neutral supplies
    • Earthing
      • Fundamental safety feature
      • Any conducting external part of an appliance must be connected to an Earth wire, e.g. the metal case of a CD player
      • This Earth wire is connected to a copper conductor buried in the ground
    • Earth leakage detection
      • If live wire connects to the casing accidentally a current flows through the earth wire to the ground
      • The earth leakage detector detects this and breaks the whole circuit.
      Metal casing Neutral wire Live wire Electrical device
    • Double insulation
      • If the appliance enclosure is completely non-conducting it does not need to be earthed, it is “double insulated”
      • A double insulated appliance is identified by a double square symbol and will not have an Earth wire
    • Regulation of electrical systems
      • BS 7671 (The IEE Wiring Regulations) currently in the 17 th edition, published by the Institution of Engineering and Technology
      • Building Regulations Part P “Electrical safety
      • Always get electrical circuits checked by a qualified electrical engineer. Don’t rely on visual inspection.