Part 3 - Assessment of General Characteristics

Overview

Part 3 of BS 7671 addresses the essential assessments that must be carried out before an electrical installation is designed. With an exam weighting of 7, this is one of the more heavily tested areas of the syllabus and demands thorough study.

The core principle is straightforward: you cannot design a safe and effective installation without first understanding the characteristics of the supply, the demands that will be placed upon it, and the environment in which it will operate.

Part 3 requires the designer to gather and assess information across five key areas:

1. The purpose, supplies, and structure of the installation
2. The external influences that will affect it
3. The compatibility of equipment
4. The maintainability of the completed installation
5. The safety services required

Each of these areas directly influences the choices made in Parts 4, 5, and 6.

Key Sections

Chapter 31 - Purposes, Supplies and Structure (Regulations 311-313)

Regulation 311 — Purpose of the Installation

The designer must assess:

• The intended use of the building or premises
• The maximum demand that the installation will need to support
• Any specific load requirements (e.g., heavy industrial equipment, commercial kitchen loads)

Regulation 312 — Characteristics of the Supply

This is one of the most critical assessments. Before design begins, the following supply characteristics must be determined:

• Nature of current — AC or DC
• Nominal voltage — e.g., 230V single phase, 400V three phase
• Frequency — typically 50 Hz in the UK
• Prospective short-circuit current at the origin of the installation
• Earth fault loop impedance of the supply (Ze)
• Type of earthing arrangement — TN-S, TN-C-S, or TT
• Suitability of the supply for the intended maximum demand

Regulation 313 — Division into Circuits

The installation must be divided into circuits to:

• Avoid danger and minimise inconvenience in the event of a fault
• Allow safe inspection and testing of individual circuits
• Reduce the effects of electromagnetic interference
• Prevent the operation of one circuit from adversely affecting another

Certain areas require dedicated circuits, including:

• Smoke alarm systems
• Security systems
• Emergency lighting
• Any equipment whose failure would cause a significant hazard

Chapter 32 - Classification of External Influences (Regulations 320-322)

External influences are classified using an alphanumeric coding system with three categories:

Category	Code Letter	Meaning
Environment	A	Conditions surrounding the installation
Utilisation	B	How the installation is used and by whom
Construction	C	The construction of the building

Each category uses a second letter for the specific influence and a number for the severity class. Commonly tested codes include:

• AD — Presence of water (AD1 = negligible, AD4 = splashing, AD8 = submersion)
• AE — Presence of foreign bodies (dust, etc.)
• AA — Ambient temperature range
• BA — Capability of persons (BA1 = ordinary, BA4 = instructed, BA5 = skilled)

IP ratings are closely linked to external influences. The degree of protection required for equipment is determined by the environmental conditions present:

• IP20 — Indoor dry locations (standard domestic)
• IP44 — Bathrooms and sheltered outdoor areas
• IP55 — Exposed outdoor locations
• IP65 — Wash-down environments (e.g., commercial kitchens)

Chapter 33 - Compatibility (Regulation 331)

The designer must assess whether equipment will cause harmful effects to other equipment or to the supply. Key considerations include:

• Transient overvoltages — Voltage spikes that could damage sensitive equipment
• Harmonic currents — Generated by non-linear loads such as LED drivers, variable speed drives, and IT equipment
• High starting currents — From motors and compressors that may cause voltage dips
• Rapidly fluctuating loads — That could cause flicker affecting other users

With the growth of electronic equipment and LED lighting, harmonic distortion is an increasingly important consideration in modern installations.

Chapter 35 - Maintainability (Regulation 351)

The assessment of maintainability requires the designer to consider:

• The frequency and quality of maintenance that can reasonably be expected
• Whether qualified or instructed persons will carry out the maintenance
• The accessibility of equipment for inspection, testing, and repair
• The expected lifespan of the installation

An installation that cannot be properly maintained cannot remain safe throughout its intended life.

Important Regulation Numbers

• Regulation 311.1 - Requires assessment of the purpose of the installation and maximum demand
• Regulation 312.1 - Requires determination of the supply characteristics including nominal voltage and frequency
• Regulation 312.2 - Requires determination of the prospective short-circuit current at the origin
• Regulation 312.3 - Requires determination of the earth fault loop impedance of the supply (Ze)
• Regulation 313.1 - Requires the installation to be divided into circuits to avoid hazard and minimise inconvenience in the event of a fault
• Regulation 313.2 - Requires separate circuits for parts of the installation that need to be independently controlled
• Regulation 320.1 - Requires external influences to be assessed to determine the requirements for equipment selection
• Regulation 331.1 - Requires assessment of characteristics that may have harmful effects on other equipment or the supply
• Regulation 351.1 - Requires consideration of the frequency and quality of maintenance

Common Exam Topics

• Identifying the supply characteristics that must be determined before design (especially prospective short-circuit current, Ze, and earthing arrangement)
• External influences and the IP classification system — matching conditions to the correct IP rating or code
• Why certain areas require dedicated circuits (Regulation 313)
• Load assessment and maximum demand calculations
• Compatibility questions referencing harmonic distortion from modern electronic loads
• The difference between Part 3 (what you assess) and Parts 4/5 (what you design)

Study Tips

• Memorise the supply characteristics listed in Regulation 312. Being able to list what must be assessed before design begins is a common exam question format.
• Learn the external influences coding system. You do not need to memorise every code, but understand the structure (environment A, utilisation B, construction C) and know the most common ones such as AD (water), AE (foreign bodies), and BA (capability of persons).
• Know the common IP ratings: IP20 for indoor dry locations, IP44 for bathrooms and outdoor areas, IP65 for wash-down environments. These are frequently tested.
• Understand why an installation must be divided into circuits. The reasons include safety, convenience, and the need to minimise the effects of a fault on other circuits.
• Remember that Part 3 is about what you assess before you design. If a question asks about design decisions, it belongs to Parts 4 or 5. If it asks about what information you need first, it belongs to Part 3.
• Practise maximum demand calculations using diversity factors. These are a practical application of Regulation 311.

Related Topics

• Part 1 - Scope, Object and Fundamental Principles
• Part 2 - Definitions
• Part 4 - Protection for Safety
• Part 5 - Selection and Erection of Equipment