Chapter 16: Practical Electricity

Question 1

What are the risks of current?

Answer 1

• The human body can only withstand a small current of about 50 mA.
• Larger amount of current can cause burns or damage to tissues or
  organs.

Question 2

What are the different electrical hazards?

Answer 2

• Damaged Insulation
• Overheating of Cables
• Damp Environment

Question 3

Why is over-heating of cables a danger of electricity

Answer 3

• Overheating of cables occurs when an unusually large current flows through the conducting wires. (P = I²R)
• Electrical appliances are connected in parallel to the mains at the socket. Over-loading of a socket draws a huge current from the mains which leads to a risk of over-heating and may result in a fire
• Poor contact between the plug and socket, may also cause overheating of cables

Question 4

Which wire is connected to the metal casing?

Answer 4

The Earth (E) wire (green and yellow)

Question 5

What is the purpose of a circuit breaker?

Answer 5

• A circuit breaker is a safety device to prevent excessive current flow.
• Unlike fuses, a circuit breaker is not added to the plug of an electric appliance.
• Circuit breakers are part of the main electrical wiring to an apartment or room, as shown in Figure 18.12
• Circuit breakers also have current ratings.
• If the current exceeds its rating, then the
  circuit breaker will trip, causing an open circuit.
• When a circuit breaker trips, the faulty
  appliance causing the trip has to be identified
  and removed from the circuit. Then, the circuit
  breaker may be turned on again.
• Figure 18.13 shows a circuit breaker in a
  distribution board box.

Question 6

Which wire is connected to the fuse?

Answer 6

Live (L) wire (brown)

Question 7

What is the purpose of the earth metal case?

Answer 7

• An earth metal casing is a metal housing that encloses the
  electrical parts of an appliance, keeping the user safe in the
  event of an electrical fault.
• The live current passing through the earth metal casing flows to
  the ground.
• Since the earth wire has almost no resistance, a large current
  flows and causes the circuit breaker to trip or the fuse to blow. This prevents further current flow.

Question 8

What is the use of double insulation?

Answer 8

• Not all appliances use earth metal casings as this can
  increase the cost of the appliances.
• Some appliances use double insulation as a safety
  feature to keep users safe in the event of a fault.
• These appliances are enclosed in a plastic casing which is
  not earthed.
• Double insulation has two layers of insulation to enclose
  all exposed metalwork and “live” parts.

Question 9

What is the colour of the Earth wire, and what is the purpose of it?

Answer 9

The earth wire is yellow and green. It is a safety wire that is needed to earth appliances with a metal case

Question 10

What is the Neutral wire?

Answer 10

The neutral wire is blue and is kept at zero voltage. It completes the path for current to flow through appliances in the electrical circuit

Question 11

What is the live wire?

Answer 11

The live wire is usually brown. It carries both the high voltage and the current

Question 12

Why is damaged insulation a danger of electricity?

Answer 12

• Bending and twisting of electricial cables may cuase the elctricial insulation to crack and break, thus exposing the conducting wires inside.
• Exposted live wire (which carries current from the mains that is 240V ) can cause severe electric shcok to the user when touched, leading to serious injuriejs or even death

Question 12

Why is damp conditions a danger of electricity?

Answer 12

• Pure water is an electrical insulator but tap water cotnains mineral ions which help to conduct electricity. Hence, water can provide a conducting patht for a large current to flow. (A current between 0.100A and 0.200A through the body is lethal).
• Wet skin (a few hundred ohms) has less resistance than dry skin (100 000Ω). For example, if a person uses a hair dryer with the mains power switched on when in the bathroom, the person may get electrocuted when holding the hairdryer, plug or socket with wet hands should the water touches the live wire

Question 13

What is a short-circuit?

Answer 13

• A short circuit is a condition in an electric current when excessively large current flows through an unintended shorter or low-resistance pathway instea dof through the circuit.
• Short circuits are dangerous because they can cause electric fires and electrocution
• One example of a short circuit is the contact between live wire and neutral wire due to damaged insulation. This leads to a large current causing heat (P = I²R) to start a fire.

Question 13

What is a fuse and how does it work?

Answer 13

• A fuse is a safety device included in an electrical circuit to prevent excessive current flow. it uses the heating effect of current to interrupt the current that exceeds its current to interrupt the current that exceeds its current rating.
• It is a short thin piece of wire which becomes hot (P = I²R) and melts when the current flowing through it is greater than its rated value.
• A fuse should have a current rating just slightly higher than the normal operating current an electrical appliance will use. For example, an 13A fuse is commonly used in a kettle than drawas about 11A.
• Fuse rating is the amount of current needed for the fuse element to blow or melt. Examples of fuse ratings: 2.5A, 5A, 10A, 15A
• A fuse should be connected to the live wire so that the appliance will not become ‘live’ after the fuse had melted due to an overflow of current.
• Always switch off the mains power supply before changing the fuse

Question 14

What is a circuit breaker and how does it work?

Answer 14

• A circuit breaker is a safety deveice that can switch off the electrical supply (or ‘trip’) when there is excessive current in the circuit. It may mak use of the magnetic effect of current to interrupt the current that exceeds its current rating
• The main difference between a circuit breaker and a fuse is that the cirucit breaker can be reused over and over again by pushing the reset button while the blown fuse needs to be replaced.

Question 15

Where should switches be placed, and which wire should it be placed?

Answer 15

• Switches are used to open to close the electrical circuit.
• The switch should be connected to the live wire.
• If the switch is fitted onto the neutral wire, the appliance will be ‘live’ even if the switch is ‘off’