How to Pass the City & Guilds 2391-52 Exam: The Ultimate 2026 Guide

The City & Guilds 2391-52 (Level 3 Award in Inspection, Testing and Certification of Electrical Installations) is widely regarded as one of the most demanding qualifications in the UK electrical trade. Covering both initial verification of new installations and periodic inspection (EICR) of existing systems, passing requires a deep understanding of BS 7671:2018+A4:2026 and IET Guidance Note 3.

In this comprehensive guide, we break down the exam structure, key calculation formulas, open-book vs closed-book preparation, and practical testing sequences.

Understanding the 2391-52 Exam Structure

The C&G 2391-52 assessment consists of three core components:

1. Online Multiple-Choice Exam (2391-502 / 60 Questions, 2 Hours): An open-book examination covering statutory regulations, BS 7671 Part 6, testing methods, and certification rules.
2. Written Examination / Assignment (2391-503): Scenario-based inspection and condition reporting scenarios requiring calculation of earth fault loop impedance (Zs), voltage drop, and fault code classification.
3. Practical Inspection & Testing Assessment: Hands-on verification of a simulated installation board, safe isolation demonstration, and EIC / EICR documentation completion.

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5 Essential Rules for Exam Success

1. Memorise the Dead and Live Test Sequences

Under BS 7671 Regulation 643.1, tests must be performed in a strict logical order before energisation:

Order	Test Stage	Key Regulation / Standard
1	Protective Conductor Continuity ($R_1+R_2$)	Regulation 643.2.1 (200mA min test current)
2	Ring Final Circuit Continuity ($r_1, r_n, r_2$)	Regulation 643.2.2 / GN3 Section 2.7.5
3	Insulation Resistance	Table 64 (500V DC $\ge$ 1.0 MΩ)
4	Polarity (Dead Check)	Regulation 643.6
5	Earth Electrode Resistance	Regulation 643.7.2
6	Live Polarity & Loop Impedance ($Z_e$ / $Z_s$)	Regulation 643.7.3 / Tables 41.2–41.4
7	RCD Trip Times ($1/2\times, 1\times, 5\times$)	Regulation 643.8

2. Apply the 80% Rule for Maximum $Z_s$

Table values for maximum earth fault loop impedance ($Z_s$) in BS 7671 are stated at a conductor temperature of 10°C or 20°C. During field testing, conductors are at ambient temperature, so measured $Z_s$ must not exceed 80% of the tabulated maximum value:

$$Z_{s(\text{measured})} \le 0.8 \times Z_{s(\text{tabulated})}$$

Our interactive Max Zs Calculator automatically applies this 0.8 multiplier for Type B, C, and D MCBs.

3. Master Safe Isolation (GS38)

Failing safe isolation in your practical assessment is an immediate safety fail. Always use an HSE GS38 compliant voltage indicator with max 4mm exposed tips (or 2mm fused probes) and verify before and after against a known proving unit.

4. Practice Fault Coding (C1, C2, C3, FI)

Condition reporting questions require precise classification. Remember:

• Code C1: Danger present (e.g. exposed live conductors).
• Code C2: Potentially dangerous (e.g. missing main protective bonding).
• Code C3: Improvement recommended (e.g. outdated circuit labels).
• Code FI: Further investigation required (e.g. unverified hidden connections).

5. Drill Exam-Style Practice Questions

Confidence comes from familiarity with question formats and time management. Practice with our Interactive 2391 Quiz Engine and explore our recommended books and tools to ensure you are fully prepared on exam day.