Part 4 - Protection for Safety

Overview

Part 4 of BS 7671 addresses the fundamental requirements for protecting people, livestock, and property against the hazards that can arise from electrical installations. This section carries a significant exam weighting of 12%, making it one of the most heavily tested areas in the IET Wiring Regulations examination.

The core principle running through Part 4 is that every installation must provide both basic protection (protection against contact with live parts under normal conditions) and fault protection (protection in the event of a fault). Understanding how these two layers of protection work together through specific protective measures is essential for both exam success and safe installation practice.

Key Sections

Chapter 41 - Protection Against Electric Shock (Regulations 410-415)

Chapter 41 is the cornerstone of Part 4. It establishes that protection against electric shock must be provided by an appropriate protective measure, each consisting of a combination of basic protection and fault protection.

Automatic Disconnection of Supply (ADS) — Regulation 411

The primary protective measure for most installations is automatic disconnection of supply (ADS). It works through two layers:

• Basic protection is provided by:

  • Insulation of live parts
  • Barriers and enclosures

• Fault protection is achieved by:

  • Protective earthing
  • Protective bonding
  • Automatic disconnection by a protective device in the event of a fault

Key Disconnection Times (Regulation 411.3.2)

The maximum disconnection times for final circuits not exceeding 32A are:

System Type	Maximum Time	Circuit Type
TN systems	0.4 seconds	Final circuits ≤ 32A
TT systems	0.2 seconds	Final circuits ≤ 32A
TN systems	5 seconds	Distribution circuits (Reg 411.3.2.3)

Every circuit must have a circuit protective conductor (cpc) connected to the main earthing terminal (Regulation 411.3.3).

Additional Protection — 30mA RCD (Regulation 415.1)

RCDs with a rated residual operating current not exceeding 30mA must be provided for:

• Socket outlets with a rated current not exceeding 32A
• Mobile equipment with a rated current not exceeding 32A used outdoors
• Circuits in locations of increased shock risk, such as bathrooms (Section 701)

Other Protective Measures

Part 4 also recognises alternative protective measures for specific situations:

• Double or reinforced insulation (Class II equipment) — Regulation 412
• Electrical separation — Regulation 413
• SELV (Separated Extra-Low Voltage) — Regulation 414, provides the highest level of shock protection with no earthing required
• PELV (Protective Extra-Low Voltage) — Regulation 414, permits an earth connection at the secondary side

Chapter 42 - Protection Against Thermal Effects (Regulations 421-422)

This chapter requires that electrical equipment must not present a fire risk to adjacent materials.

Key requirements include:

• Fixed equipment must be selected and erected so that its highest operating temperature does not cause a fire
• Regulation 421.1.201 — Requirements for installations in locations with combustible materials
• Regulation 422 — Additional precautions where particular risks of fire exist, including locations classified as:
  • BD2 — Propagation of fire (e.g., timber-framed buildings)
  • BD3 — Low fire load
  • BD4 — High fire load (e.g., barns, woodworking shops)

Chapter 43 - Protection Against Overcurrent (Regulations 431-434)

Overcurrent protection covers two distinct scenarios:

Overload Protection (Regulation 433.1)

The fundamental overload coordination rule requires all of the following conditions to be met:

• Ib ≤ In ≤ Iz — The design current (Ib) must not exceed the nominal rating of the protective device (In), which must not exceed the current-carrying capacity of the cable (Iz)
• I2 ≤ 1.45 × Iz — The conventional operating current of the device (I2) must not exceed 1.45 times the cable’s current-carrying capacity

A single device may provide protection against both overload and short-circuit current, provided it has adequate breaking capacity (Regulation 432.1).

Short-Circuit Protection (Regulation 434.5.1)

For short-circuit protection, the energy let-through of the protective device must not exceed the energy withstand of the conductor:

• I²t ≤ k²S² — The energy let-through (I²t) of the device must not exceed the thermal withstand (k²S²) of the conductor
• This is known as the adiabatic equation and links to Appendix 3

Important Regulation Numbers

• 411.3.2 - Maximum disconnection times for final circuits (0.4s TN, 0.2s TT)
• 411.3.3 - Every circuit must have a circuit protective conductor
• 411.4.5 - Additional protection by RCDs in TT systems
• 415.1 - Additional protection requirements (30mA RCD for socket outlets up to 32A)
• 421.1 - Protection against fire caused by electrical equipment
• 422.1 - Precautions in locations with combustible materials
• 431-434 - Overcurrent protection requirements and coordination rules
• 433.1 - Overload protection coordination (Ib ≤ In ≤ Iz)
• 434.5.1 - Short-circuit energy let-through (adiabatic equation)

Common Exam Topics

• Disconnection times for TN and TT systems and the difference between final and distribution circuits
• Where 30mA RCD protection is mandatory (socket outlets up to 32A, bathrooms, outdoor mobile equipment)
• The distinction between SELV and PELV, including earthing requirements
• The overload coordination rule: Ib ≤ In ≤ Iz and I2 ≤ 1.45 × Iz
• Breaking capacity requirements for short-circuit protective devices
• Conditions under which basic protection or fault protection can be omitted
• Thermal constraints on cables and the adiabatic equation

Study Tips

• Memorise the disconnection times table: 0.4s (TN final), 0.2s (TT final), 5s (TN distribution). These appear frequently in exam questions.
• Understand the overload coordination formula thoroughly. Questions often present scenarios where you must determine whether a device and cable combination is compliant.
• Pay special attention to the 30mA RCD requirements in Regulation 415.1. Know the specific circumstances where additional protection is required versus where it is recommended.
• Practice questions that ask you to distinguish between basic protection and fault protection within each protective measure.
• Link this section back to Part 2 (definitions) for terms such as “extra-low voltage”, “basic protection”, and “fault protection”.

Related Topics

• Part 1 - Scope, Object and Fundamental Principles
• Part 2 - Definitions
• Part 5 - Selection and Erection of Equipment
• Part 6 - Inspection and Testing