23.8 Electrical Safety: Systems and Devices

23.8 Electrical Safety: Systems and Devices

  • You can learn how to make a bulb light using magnets.
  • There are two dangers to electricity.
    • Both dangers have been discussed.
    • We will focus on systems and devices that prevent electrical hazard.
  • The schematic for a simple AC circuit has no safety features.
    • This is not how power is distributed.
  • The circuit breaker is the first thing to be done to prevent overload.
    • There is a protective case around the appliance.
    • The case's safety feature helps prevent shocks by preventing a person from touching exposed wires and coming into electrical contact with the circuit.
  • A single appliance is represented by the resistance in the schematic of a simple AC circuit.
    • There aren't any safety features in this circuit.
  • The neutral wire is connected to the earth at the source of the current and the user location, so that it can return the current to the source.
    • The appliance is also grounded to zero volts.
  • It is important to check the wire insulation colors in your area to determine which color codes are used.
  • An earth/ground connection is a low-resistance path to the earth.
    • The neutral wire is named after it because of the two earth/ground connections on it.
    • Even if the insulation is missing, this wire is safe to touch.
    • The return path for the current is the neutral wire.
    • An alternative path through the earth, a good conductor, is provided by the two earth/ground connections.
    • The generating plant could be the location of the earth/ground connection.
  • The case of the appliance is forced to be at zero volts by the third earth/ground through the green earth/ground wire.
    • The live wire is used to supply the appliance's voltage and current.
  • To assure proper function of the three-wire system, the standard three-prong plug can only be inserted in one way.
  • Insulating plastic is color-coded to identify live/hot, neutral and ground wires but these codes vary around the world.
    • Live wires can be brown, red, black, blue or grey.
    • Blue, black or white neutral wire can be used.
    • It is important to determine the color code in your region since the same color may be used in different parts of the world.
    • The earth/ground wire is usually green but may be yellow or just bare.
    • Those who are colorblind can benefit from striped coating.
  • The older two-wire system did not have an earth/ground wire.
    • Under normal circumstances, insulation on the live/hot and neutral wires prevents the case from being directly in the circuit, so that the earth/ground wire may seem like double protection.
    • More than one problem can be solved by grounding the case.
    • A severe shock is possible if you don't have an earth/ground connection.
  • This is particularly dangerous in the kitchen, where a good connection to the ground can be found through water on the floor or a water faucet.
    • The circuit breaker will trip if the earth/ground connection is not intact.
    • Power tools with impact resistant plastic cases are doubly insulated and have nonconducting cases.
    • Modern two-prong plugs can be inserted into the asymmetric standard outlet in one way to ensure proper connection of live/hot and neutral wires.
  • The metal case of this appliance allows the live wire to come into contact with it.
    • In this situation, the appliance may operate normally.
  • A more subtle problem can be solved by grounding the case.
    • An emf can be caused by the AC current in appliances.
    • Current driven by the case emf is referred to as a leakage current.
  • An emf can be caused by AC currents.
    • It can be large enough to cause a shock.
    • The emf is kept near zero if the case is grounded.
  • A ground fault interrupter is a safety device found in updated kitchen and bathroom wiring.
    • The currents in the wires are compared.
    • When live/hot and neutral currents aren't equal, it's usually because the neutral current is less than the live wire.
    • Some of the current is coming back to the source by a path other than through the neutral wire.
    • The accepted maximum harmless shock is usually set to interrupt the circuit if the leakage current is greater than 5 mA.
    • If the leakage current goes through an intact earth/ground wire, it will trip, forcing the repair of the leak.
  • If the difference between the currents in the live and neutral wires exceeds a safe value, a ground fault interrupter will trip.
    • An intact earth/ground wire could have prevented the leak current here.
  • The figure shows how aGFI works.
    • If the currents in the live/hot and neutral wires are equal, the emfs in the coil will be equal.