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:

Layer	Method	Provided By
Basic protection	Prevention of contact with live parts	Insulation of live parts; barriers and enclosures
Fault protection	Clearance of fault conditions	Protective earthing; protective bonding; automatic disconnection by a protective device

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:

Scenario	Requirement
Socket outlets	Rated current not exceeding 32A
Mobile equipment	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:

Measure	Regulation	Description
Double or reinforced insulation	412	Class II equipment
Electrical separation	413	Isolation from the main supply
SELV (Separated Extra-Low Voltage)	414	Highest level of shock protection with no earthing required
PELV (Protective Extra-Low Voltage)	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

Key formula: Ib ≤ In ≤ Iz — The design current must not exceed the nominal rating of the protective device, which must not exceed the current-carrying capacity of the cable.

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

Regulation	Requirement
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