10.6 Household Wiring and Electrical Safety

Household electrical systems power our homes, but they can be dangerous if not properly understood and maintained. From the service entrance to branch circuits, each component plays a crucial role in safely distributing electricity throughout our living spaces.

Electrical safety is paramount in home wiring. Thermal hazards can cause fires, while shock hazards pose risks to human life. Understanding the physiological effects of electrical shock and the importance of protection devices like fuses and circuit breakers is essential for every homeowner.

Household Electrical Systems

Components of household electrical systems

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• Electrical service entrance connects the house to the utility company's power lines typically consists of a meter and a main service panel
• Main service panel contains the main circuit breaker or fuse distributes power to branch circuits throughout the house
• Branch circuits supply power to specific areas (kitchen) or appliances (refrigerator) protected by circuit breakers or fuses in the main service panel
• Grounding provides a safe path for electrical current to flow in case of a fault achieved through a grounding rod driven into the earth establishes a reference point for voltage measurements
• Wiring consists of hot (live) wires carry the electrical current to the load neutral wires return the current to the source ground wires provide a safe path for fault currents
  • Wiring materials include copper conductors for good electrical conductivity and low resistance (measured in ohms) plastic insulation (PVC) for protection against short circuits and electrical shock
  • Wiring gauges (thickness) are selected based on the expected current load thicker wires (lower gauge numbers) are used for high-current applications (electric stove)

Electrical Load and Safety Devices

• Electrical load refers to the amount of power consumed by devices connected to a circuit
• Junction boxes are used to safely connect and house wire connections
• Arc fault circuit interrupters (AFCIs) detect and interrupt electrical arcs that can cause fires
• Electrical codes provide safety standards for wiring and installation practices

Electrical Safety

Thermal vs shock hazards

• Thermal hazards occur when electrical equipment or wiring generates excessive heat can cause fires (electrical panel) or burns (frayed appliance cord)
  • Overloaded circuits when too many devices are connected drawing more current than the circuit is designed to handle
  • Short circuits when a live wire comes into direct contact with a neutral or ground wire causing a rapid increase in current
  • Faulty appliances with damaged wiring or components leading to increased resistance and heat generation
• Shock hazards occur when a person comes into contact with live electrical components can cause injury or death through electrical shock
  • Exposed wires due to damaged insulation or improper installation posing a risk of direct contact
  • Faulty insulation allowing current to leak from live components to accessible surfaces (metal appliance housing)
  • Improper grounding failing to provide a safe path for fault currents increasing the risk of shock

Physiological effects of electrical shock

• Current intensity measured in milliamperes (mA) determines the severity of electrical shock on the human body
  1. 1 mA perception threshold causes a tingling sensation
  2. 5-10 mA slight shock leads to involuntary muscle reactions
  3. 10-20 mA painful shock but muscle control is maintained
  4. 20-50 mA severe shock causes respiratory arrest and muscle paralysis
  5. 50-100 mA ventricular fibrillation potentially fatal without immediate medical attention
  6. 100-2000 mA ventricular fibrillation cardiac arrest and severe burns
  7. Over 2000 mA cardiac arrest severe tissue damage and high risk of fatality
• Duration of exposure longer exposure to electrical current increases the risk and severity of injury
• Pathway of current flow through the body current passing through vital organs (heart) is more dangerous than current flowing through extremities (hand to foot)

Protection by fuses and circuit breakers

• Fuses and circuit breakers are overcurrent protection devices that interrupt the flow of electrical current when it exceeds a safe level
• Fuses contain a metal wire or strip (element) that melts when exposed to excessive current permanently breaking the circuit must be replaced after operating
  • Fuse ratings (amperage) are selected based on the expected current load of the circuit
  • Time-delay fuses allow brief current surges (motor startup) without melting reducing nuisance trips
• Circuit breakers contain an electromagnet and a switch the electromagnet trips the switch when exposed to excessive current can be reset after operating
  • Circuit breaker ratings (amperage) are selected based on the expected current load of the circuit
  • Thermal-magnetic circuit breakers combine a bimetallic strip (thermal protection) and an electromagnet (magnetic protection) for improved response to both overloads and short circuits
• Protection against overloads fuses and circuit breakers prevent overheating and potential fire hazards by interrupting the current when too many devices are connected to a circuit
• Protection against short circuits fuses and circuit breakers quickly interrupt the high current flow caused by a live wire contacting a neutral or ground wire preventing damage to the electrical system and potential fire hazards