3.2 Earthing and Excess Current Protection

• Electrical hazards in domestic installations and how they arise
• The purpose and principle of earthing of appliances and installations
• Components of an earthing system: earth electrode, earth clamp and earth continuity conductor
• Types of earthing system: TT, TN-S and TN-C-S
• Lightning arresters and protection against voltage surges
• Fuses: rewirable, cartridge and high rupturing capacity types
• Miniature circuit breakers (MCBs) and their operation
• Residual current devices (RCDs) and earth leakage protection
• Selecting and coordinating protective devices in a domestic installation
• Installing and connecting protective devices and earthing conductors

• explain how earthing and excess current protection ensures safety in electrical installations
• describe the operation of different types of protective devices used in domestic installations
• identify types of earthing system used in domestic installations
• install protective devices in domestic installations
• appreciate the importance of protective devices in domestic installations

“Why is the earthing of electrical appliances important?”

“How are protective devices connected in electrical installations?”

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1. Electrical hazards and the need for protection
   40 min · 1 practice tasks · quiz
   Identify electrical hazards and explain how earthing and excess current protection keep an installation safe.
2. The principle of earthing
   40 min · 1 practice tasks · quiz
   Explain how an earth connection carries fault current and why a low earth-loop resistance matters.
3. Types of earthing systems
   40 min · 1 practice tasks · quiz
   Describe the TT, TN-S and TN-C-S earthing systems and choose one for a given supply.
4. Earth electrodes, conductors and lightning arresters
   40 min · 1 practice tasks · quiz
   Describe the components of an earthing system and how a good earth electrode is installed.
5. Fuses and their operation
   40 min · 1 practice tasks · quiz
   Describe how a fuse works, name the types of fuse and select a fuse rating.
6. Miniature circuit breakers (MCBs)
   40 min · 1 practice tasks · quiz
   Explain how an MCB trips on overload and fault, and select a suitable type.
7. Residual current devices (RCDs)
   40 min · 1 practice tasks · quiz
   Explain how an RCD senses earth leakage and protects people from electric shock.
8. Selecting and coordinating protective devices
   40 min · 1 practice tasks · quiz
   Match protective devices to their jobs and arrange them so the nearest device trips first.
9. Installing and connecting protective devices
   40 min · 1 practice tasks · quiz
   Describe the parts of a consumer unit and the order in which devices are connected safely.