Electrical Safety | Physics for GCSE/IGCSE - Year 11 PDF Download

Electrical Hazards

• Mains electricity can be extremely dangerous - even a small voltage difference of 50 volts can present a significant risk to individuals.

The risk of electrocution is indicated by hazard signs but other risks which would not be signposted are listed below

 Common Hazards in Electrical Systems

• Damaged Insulation: When the protective covering of a wire is compromised, direct contact can result in a dangerous electric shock. For instance, if a person touches an exposed wire accidentally.
• Overheating of Cables: Passing excessive current through a wire that is too small or tightly coiled can lead to overheating. This overheating has the potential to cause fires or even melt the insulation, exposing live wires. An example could be running a high-power appliance on a thin extension cord.
• Damp Conditions: When moisture interacts with live wires, it can conduct electricity, leading to a variety of risks. For example, moisture causing a short circuit in a device or creating an electrocution hazard. This risk is evident in areas where water can come into contact with electrical outlets or appliances.
• Excess Current from Overloading: Overloading plugs, extension leads, or sockets with too many devices can generate excessive heat, potentially leading to fires. An illustration would be plugging multiple high-power devices into a single socket.

Mains Circuits

• Mains electricity refers to the power supplied by centralized power stations and distributed through the National Grid for various household and industrial uses.
  • When we plug in devices like phone chargers or kettles, we are connecting them to the mains electricity supply.
• The electricity provided through mains is in the form of alternating current (a.c.), which means the flow of current changes direction periodically.
  • In the United Kingdom, the standard domestic electricity has a frequency of 50 Hz and an approximate potential difference of 230 V. This frequency indicates how many times the current changes direction in a second.
• Unlike direct current (d.c.), which has positive and negative terminals, alternating current does not have these polarities. Instead, we refer to the ends of the circuit as live and neutral.

• Safety features are integrated into household appliances to safeguard both users and devices.
  • Double Insulation: Provides an extra layer of protection against electric shock.
  • Earthing: Safely directs excess electricity into the ground.
  • Fuses: Prevent excessive currents from damaging appliances.
  • Circuit Breakers: Automatically cut off power during electrical faults.

Insulation & Double Insulation

• The conducting part of a wire is typically made of copper or another metal. 
  • If this comes into contact with a person, there is a risk of electrocution. 
• To prevent this, wires are coated with an insulating material like rubber.

• Some appliances lack metal cases, eliminating the risk of becoming electrified. 
• These appliances are referred to as double insulated, featuring two layers of insulation:
  • Double Insulated Appliances: Appliances without metal cases are considered double insulated.
• Two Layers of Insulation: Double insulated devices have two layers of insulation: one surrounding the wires and a non-metallic case providing an additional insulation layer.

Electrical Appliance Safety

• Several electrical appliances feature metal casings which can pose a safety risk. If a live wire inside the appliance touches the case, the casing may become charged, endangering anyone who touches it.
• This scenario highlights the importance of the earth wire, an additional safety measure that mitigates the risk of electrocution.
  • If a live wire inside an appliance makes contact with the metal casing, the casing can become electrified, putting individuals at risk of electrocution.
• Why Earth Wire is Important for Safety
  • The earth wire offers a path of minimal resistance to the ground.
  • When a fault occurs, such as a short circuit, the earth wire diverts excess current safely.
  • This excess current triggers the fuse to melt and interrupt the electricity supply, preventing hazards.
  • Ultimately, this mechanism safeguards the appliance and its users.

Fuses & Trip Switches

• A fuse serves as a safety mechanism that interrupts the electricity supply to an appliance in case of excessive current flow, triggered by a fault or surge.

The symbol used in circuits to represent a fuse is distinct from that of a resistor.

• Typically, fuses are constructed with a glass cylinder enclosing a delicate metal wire.
• If the current passing through the wire surpasses safe levels, the wire heats up, eventually melting.
• This melt-down of the wire leads to its rupture, thereby breaking the circuit and halting the current flow.

A trip switch, located within the Consumer Box at the point where electricity enters a building, functions similarly to a fuse.

• When the current exceeds the permissible limit, the switch 'trips,' automatically transitioning to the off position.
• This action effectively halts the flow of current within that specific circuit.

Understanding Trip Switches and Fuses

• A trip switch, located in the Consumer Box, serves a similar purpose to a fuse. It automatically 'trips' when current levels are excessive, halting the flow of current in that specific circuit.
• When the current surpasses safe levels, the switch flips off, preventing further current flow.

Choosing the Right Fuse

• Fuses are available in various sizes, such as 3A, 5A, and 13A. To select the appropriate fuse, understanding the appliance's current requirements is crucial.
• By knowing the appliance's power and the mains voltage, you can calculate the current using a specific equation.

Fuse Selection Guidelines

• When selecting a fuse, ensure that its current rating exceeds the appliance's needs but isn't excessively high. Opt for the next size up for safety.
  
• Examples:
  • An appliance using 3.1 amps would need a fuse slightly higher than this value to function correctly.
  • A 3-amp fuse is too low and would blow upon activation.
  • A 13-amp fuse is excessively high, risking the appliance and safety.
  • A 5-amp fuse is the suitable choice, providing a margin above the appliance's requirement.