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:

System	Earth Source	Key Characteristics
TN-S	Supply cable sheath	Separate protective conductor provided by the distributor; neutral and earth separate throughout the entire system
TN-C-S (PME)	Combined PEN conductor	Supply neutral and earth combined in a single conductor for part of the system, then separated within the consumer’s installation
TT	Consumer’s own earth electrode	No direct metallic connection to the supply earth; common in rural areas where a TN supply is unavailable
IT	No direct connection to earth	Live parts have no direct connection to earth, or connected through a high impedance; used in specialist applications such as medical locations

Each system type has different implications for:

Implication	Affected By System Type
Fault loop impedance values	Different Ze values for TN-S, TN-C-S, and TT systems
Disconnection times	Maximum disconnection times vary by system type
Protective device selection	Choice of MCB, RCBO, or fuse depends on system impedance
RCD requirements	TT systems generally require RCD protection for all circuits

Extra-Low Voltage Systems

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

Type	Full Name	Key Features	Earth Connection
SELV	Separated Extra-Low Voltage	Highest level of shock protection; electrical separation from all other circuits; max 50V AC or 120V DC	No connection to earth
PELV	Protective Extra-Low Voltage	Similar to SELV; still requires separation from other circuits; used where earthing is needed for functional reasons	Permits a connection to earth
FELV	Functional Extra-Low Voltage	Used where extra-low voltage is needed for functional reasons but full SELV/PELV requirements are not met; additional protective measures required (basic protection by barriers or insulation, fault protection by automatic disconnection)	Depends on installation

Circuit and Equipment Terms

Key terms that are frequently tested:

Term	Definition
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)

Exam tip: The distinction between exposed-conductive-parts and extraneous-conductive-parts is one of the most commonly tested definitions in the IET exam.

Important Regulation Numbers

Term	Definition
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