Part 2 - Definitions

Overview

Part 2 of BS 7671 provides the definitions for all the specialised terms used throughout the Wiring Regulations. While it may seem like a dry reference section, a solid grasp of these definitions is critical for interpreting every other part of the standard correctly. Misunderstanding a single term can lead to selecting the wrong protective measure, specifying an incorrect cable, or failing an exam question that hinges on precise terminology.

The definitions in Part 2 are not arbitrary. Each term has been carefully worded to convey a specific technical meaning, and the exam tests whether candidates understand the distinction between closely related terms. Investing time in learning these definitions pays dividends across every other topic in the syllabus.

Key Sections

Protection Terminology

BS 7671 uses a layered approach to shock protection. The key terms are:

• Basic protection — Protection against electric shock under fault-free (normal) conditions. Typically provided by:

  • Insulation of live parts
  • Barriers and enclosures

• Fault protection — Protection against electric shock under single fault conditions. Typically provided by:

  • Automatic disconnection of supply
  • Protective earthing and protective bonding

• Double protection (or independent protection) — The combination of both basic protection and fault protection working together

Understanding these three terms is fundamental. Every protective measure described in Part 4 consists of some combination of basic and fault protection.

Earthing Arrangements

The standard defines several earthing system types that candidates must know thoroughly:

• TN-S system — The supply distributor provides the means of earthing via a separate protective conductor (typically the metal sheath of the supply cable). The neutral and earth conductors are separate throughout the entire system.

• TN-C-S system (also known as PME — Protective Multiple Earthing) — The supply neutral and earth are combined in a single conductor (the PEN conductor) for part of the system, then separated within the consumer’s installation.

• TT system — The installation has its own earth electrode, with no direct metallic connection to the supply earth. Common in rural areas where a TN supply is unavailable.

• IT system — No direct connection between live parts and earth, or connected through a high impedance. Used in specialist applications such as medical locations.

Each system type has different implications for:

• Fault loop impedance values
• Disconnection times
• Protective device selection
• RCD requirements

Extra-Low Voltage Systems

Candidates must understand the distinctions between three types of extra-low voltage:

• SELV (Separated Extra-Low Voltage) — Provides the highest level of shock protection. Requirements include:

  • Electrical separation from all other circuits
  • No connection to earth
  • Maximum voltage: 50V AC or 120V DC

• PELV (Protective Extra-Low Voltage) — Similar to SELV but:

  • Permits a connection to earth
  • Still requires separation from other circuits
  • Used where earthing is needed for functional reasons

• FELV (Functional Extra-Low Voltage) — Used where extra-low voltage is needed for functional reasons but:

  • The full requirements of SELV or PELV are not met
  • Additional protective measures are required (e.g., basic protection by barriers or insulation, and fault protection by automatic disconnection)

Circuit and Equipment Terms

Key terms that are frequently tested:

• Circuit protective conductor (cpc) — The conductor connecting exposed-conductive-parts to the main earthing terminal
• Protective bonding conductor — The conductor connecting extraneous-conductive-parts to the main earthing terminal
• RCD (Residual Current Device) — A mechanical switching device that disconnects a circuit when the residual current exceeds a predetermined value
• Exposed-conductive-part — A conductive part of equipment that can be touched and is not normally live, but may become live under fault conditions
• Extraneous-conductive-part — A conductive part not forming part of the electrical installation that is liable to introduce an earth potential (e.g., metal water pipes, structural steelwork)

The distinction between exposed-conductive-parts and extraneous-conductive-parts is one of the most commonly tested definitions.

Important Regulation Numbers

• Basic protection - Protection against electric shock under fault-free conditions
• Fault protection - Protection against electric shock under single fault conditions
• TN-S - Earth provided by supply cable sheath, separate neutral and earth throughout
• TN-C-S (PME) - Combined neutral and earth from the supply, separated within the installation
• TT - Installation earth provided by the consumer’s own earth electrode
• IT - No direct connection between live parts and earth at the supply
• SELV - Separated extra-low voltage with no earth connection and full separation
• PELV - Protective extra-low voltage, may be connected to earth
• RCD - Device that operates on residual current to disconnect the supply

Common Exam Topics

• The distinction between basic protection and fault protection
• Identifying earthing system types from a description or diagram (especially TN-C-S and TT)
• Differences between SELV, PELV, and FELV
• Distinguishing between exposed-conductive-parts and extraneous-conductive-parts
• The difference between a circuit protective conductor and a protective bonding conductor
• Scenario-based questions asking which definition applies

Study Tips

• Create flashcards for every definition in Part 2. The exam relies on precise wording, and close-but-wrong answers are common distractors.
• Draw diagrams of each earthing arrangement (TN-S, TN-C-S, TT, IT) and label every conductor. Visual memory is powerful for these system types.
• Learn the prefixes: TN means the supply is directly earthed and the installation uses the supply earth; TT means the supply is directly earthed but the installation has its own earth electrode.
• Do not confuse SELV with PELV. The key difference is that PELV may be connected to earth while SELV must not be.
• When revising, cross-reference definitions with the regulations in Parts 4 and 5 where they are applied. Seeing terms in context strengthens understanding.

Related Topics

• Part 1 - Scope, Object and Fundamental Principles
• Part 3 - Assessment of General Characteristics
• Part 4 - Protection for Safety