domestic uses & safety

• DIRECT CURRENT (DC):

  Electrical current flowing in ONE direction.

  • produces by a direct potential difference which is either positive or negative the entire time
  • the charge is always flowing in the same direction
  • Usually found in BATTERY POWERED devices

  
• ALTERNATING CURRENT (AC)

  current that changes direction from positive to negative periodically.

  • caused by an alternating potential difference
  • An example of AC is the UK MAINS SUPPLY (Current from sockets in the home)

  
• Because the potential difference is always positive & it doesn't change direction.

  
• Electrical appliances are usually connected with a THREE-CORE cable:

  • LIVE WIRE – BROWN
  • NEUTRAL WIRE – BLUE
  • EARTH WIRE – GREEN & YELLOW
• LIVE WIRE:

  • BROWN
  • Carries the ALTERNATING POTENTIAL DIFFERENCE from the supply
  • always connected to the FUSE
  • This wire has an electric potential of 230V
• NEUTRAL WIRE

  • BLUE
  • Completes the circuit by carrying away current
  • electricity flows into the device through the live wire & flows out through the neutral wire
  • approximately at the EARTH POTENTIAL (0V)
• EARTH POTENTIAL

  an electric potential of 0V
• EARTH WIRE:

  • GREEN & YELLOW
  • used for SAFETY, to stop the appliance from becoming live, if the live wire came loose & touched the casing.
  • it provides an alternative pathway for the current to flow away
  • so any dangerous electricity will pass through the earth wire, rather than passing through us.
  • has an electric potential of 0V
  • only carries a current if there's a FAULT
• Main electricity

  A) earth

  B) neutral

  C) live

  D) fuse

  
• The potential difference between the wire can be calculated by finding the difference between electric potentials.
• The potential difference between the LIVE WIRE (230V) & EARTH WIRE (0V) is:

  • 230 V – 0V = 230V

  Connecting the LIVE WIRE to the EARTH WIRE can pose serious dangers due to the LARGE potential difference of 230V.

  • This can result in a LARGE CURRENT which can create a FIRE.

  
• The potential difference between the LIVE WIRE & A HUMAN, The human body has an electric potential of 0V so:

  • 230 V – 0V = 230V

  The live wire may be dangerous even when a switch in the mains circuit is open as it has an potential of 230V

  • so there will be a POTENTIAL DIFFERENCE of 230V across your body, which can cause CURRENT to flow through you, leading to DEATH.

  
• EARTH WIRE in an electrical system is crucial for SAFETY.

  • normally carries no electricity during regular appliance operation
  • The ELECTRIC POTENTIAL of the EARTH WIRE is 0V because it's connected to the ground
  • making it the same electric potential as the Earth
• METAL CASES on electrical appliances can be a safety hazard.

  • If the LIVE WIRE inside the appliance touches the metal case, the case could become LIVE.
  • Touching a LIVE metal case can lead to electrocution.
  • The EARTH WIRE helps to prevent this by providing a safe path for the electricity.
• The EARTH WIRE offers a LOW RESISTANCE PATH to the Earth, which is important if a fault occurs.

  • When a fault happens, the EARTH WIRE allows current to flow to the ground
  • creating a surge of current in the EARTH WIRE & potentially also in the LIVE WIRE.
  • This surge can trigger a FUSE to melt & break the circuit to stop the flow of electricity & makes the appliance safe to touch.
• The person could get an electric shock, because there's a current in the person.

  
• The charge flows through the earth wire instead of the person, because the resistance of the earth wire is much less than that of a person.

  
• Casing is not made of metal, so little to no chance of electric shock

  
• In the UK, the mains supply has a frequency of 50Hz & a voltage of 230V.
• One wire in the switch is live & the electrician is at earth potential, so there will be a large potential difference between the liver wire & the electrician, if they touched the wire.

  
• 50Hz has the lowest maximum let-go current, a different frequency would allow people to let go at a greater current.

  
• The potential of the live wire is 230V & the potential of the electrician is 0V, so there's a large potential difference between live wire & electrician & current passes through his body.

  
• oscilloscope

  Displays how voltage changes with time on graphs on a monitor

  
• Wires are made of copper to conduct electricity & coated in a layer of insulating plastic for safety

  
• In the uk, the large majority of the electricity comes from the national grid

  • which distributes power across the country to our homes, the electricity that this supply’s is the mains supply
  • The mains supply is an alternating current of 230 volts & it’s at 50Hz

  
• When we plug electrical appliance from our home into a plug socket, it’s connecting it to the main supply, this usually involves a three core cable (a cable with 3 wires inside)

  
• The difference in voltage causes electricity to pass from one to the other
• Humans have a potential difference of 0V

  • so if we touch the live wire, it will cause a large current to flow through us in the form of an electric shock
  • dont touch plug sockets, even if they’re switched off as the live wire still has a potential difference
  • any contact would result in a shock

  
• Electricity passes from the live wire to the neutral / earth wire, as they both have potential differences of 0V, while the live wire is at 230V
• Surges

  A sudden increase in current, caused when something changes in a circuit like

  • turning an appliance on or off
  • a fault in the circuit or appliance

  
• One risk in electric circuits is a surge in current, they can:

  • damage the appliance
  • cause a fire
  • give an electric shock

  
• Safety precautions for surges:

  • fuses
  • earth wires
  • double insulation
  • circuit breakers

  
• fuse

  a very thin piece of wire that’s connected to the live wire

  • if there’s a surge, a huge amount of current will flow through the live wire & flow through the fuse
  • this causes the wire of the fuse to heat up quickly & melt to break the circuit so no more current can flow through

  
• The rating of a fuse is the current that will cause the fuse to break the circuit

  • If an appliance that normally has a 3A current flowing through it, then you could use a 5A fuse
  • so that it only breaks when there’s a surge that makes the current rise above 5A

  
• Fuses come in little cases & comes in a range of different ratings depending on what current you want to break the circuit

  • The fuse should be a couple of amps above the rating of the appliance

  
• Using a fuse with a rating a few amps above the recommended current means that it won't break under normal use.

  • if there's a surge & the current rises significantly then it will surpass the rating & break the fuse.

  
• Fuses

  • simple & cheap
  • found in lots of individual appliances
  • but they’re permanently broken after a single surge
  • so they have to be replaced each time
• Circuit breakers

  breaks the circuit whenever there’s a surge

  • they get tripped & turn off the circuit
  • so they can reset than be replaced
  • but they’re a bit more expensive than fuses
• Double insulation

  where the entire appliance is covered in a plastic casing

  • so there’s no exposed metal parts that we can touch
  • as plastic doesn’t conduct electricity, we can’t get an electric shock from touching it
  • If an appliance has double insulation, then it doesn’t need an earth wire