Ten Essential BS 7671 Tables for the 18th Edition Exam

Most candidates walk into the 18th Edition exam expecting to be tested on regulations. In practice, a large share of the paper is testing something narrower: can you read a table correctly under time pressure?

 

That’s good news. Regulations require understanding; tables require technique. And technique can be drilled in an evening. The same handful of tables appear in almost every sitting of City & Guilds 2382-26, and once you recognise their layout, each one becomes a ten-second lookup rather than a two-minute puzzle.

 

Below are the ten tables (some of them groups, so the real count is higher) that are worth knowing cold — drawn from Parts 4, 5 and 6 of BS 7671, plus the two calculations that sit alongside them.

Before You Start: The Table of Tables

At the back of BS 7671, immediately before the index, there is a section called the Table of Tables. It lists every table in the book in order of appearance, with its number, its title, and — most usefully — its page number.

 

Exam tip: Tab the first page of the Table of Tables. Table numbers stay stable between amendments; page numbers do not. If you learned page references from an older Amendment 2 brown book, they will not match the Amendment 4 orange book. Learn the numbers, look up the pages.

 

1. Tables 41.1 to 41.6 — Earth Fault Loop Impedance

This is the single most productive group in the book, and it is guaranteed to appear. The sequence is logical: first establish the required disconnection time, then find the maximum Zs that achieves it.

 

Table	Covers	Disconnection Time
41.1	Maximum disconnection times by system and voltage band	Sets the target
41.2	Fuses — final circuits	0.4 s
41.3	Circuit breakers and RCBOs — both cases in one table	0.4 s and 5 s
41.4	Fuses — distribution circuits	5 s
41.5	Circuits relying on RCDs (typically TT)	—
41.6	Reduced low voltage — 55 V single-phase, 63.5 V three-phase	—

 

Table 41.1 asks three things: what is the nominal voltage U₀, is it AC or DC, and is it TN or TT? Note the difference between the “less than” and “less than or equal to” symbols in the voltage bands. A domestic supply that measures 240 V still has a nominal U₀ of 230 V — always use nominal voltages for these tables.

 

Worked example: What is the maximum disconnection time for a TN-S domestic final circuit at 230 V AC nominal? Find the band where U₀ is greater than 120 V and not more than 230 V, find AC, find TN — 0.4 s.

 

Tables 41.2 and 41.4 are the classic trap. They look nearly identical, both list BS 88 fuses, and both use the same layout. The only difference is the heading: 41.2 is 0.4 s, 41.4 is 5 s. A 20 A BS 88-3 system C fuse gives a maximum Zs of 1.93 Ω at 0.4 s, but 3.22 Ω at 5 s. Same device, very different answer.

 

Key point: In an exam, if a question says final circuit and gives no disconnection time, assume 0.4 s. If it says distribution circuit, assume 5 s.

 

Table 41.3 is friendlier — circuit breakers and RCBOs, with 0.4 s and 5 s columns side by side. A 32 A Type B to BS EN 60898 on a 5 s distribution circuit gives 1.37 Ω.

 

Table 41.5 covers circuits that rely on an RCD because the overcurrent device cannot achieve the required Zs — most often TT systems. The values come from a simple sum: 50 V touch voltage divided by the RCD rating.

 

RCD Rating	Calculation	Maximum Zs
30 mA	50 ÷ 0.03	1667 Ω
100 mA	50 ÷ 0.1	500 Ω
300 mA	50 ÷ 0.3	167 Ω
500 mA	50 ÷ 0.5	100 Ω

 

Important: You cannot simply declare every circuit has a 1667 Ω limit because RCBOs are fitted. Table 41.5 applies only where the correct Zs cannot be achieved by other means — shortening the circuit, reducing the device rating, or changing a Type C to a Type B. Exam questions test whether you know that.

 

Remember that measured values must also be compared correctly against the tabulated figures — see our guide to Maximum Zs and the 80% Rule for why a reading of 1.15 Ω against a 1.37 Ω limit can still be a fail.

 

2. Table 42.1 — Temperature Limits of Accessible Parts

A popular and very easy table. It answers: how hot can a part get before it risks burning someone?

 

Two questions get you the answer: how would the person touch it, and is it metallic or non-metallic?

 

Accessible Part	Metallic	Non-Metallic
Hand-held in normal use	55 °C	65 °C
Intended to be touched, not hand-held	70 °C	80 °C
Not intended to be touched in normal use	80 °C	90 °C

 

Watch for this: Some answer options are given in Fahrenheit. BS 7671 always states temperatures in Celsius. That option is there purely to catch candidates who aren’t reading carefully.

 

3. Table 43.1 — k Values and the Adiabatic Equation for Time

The k value is a material factor describing how a conductor responds to fault current. The one you’ll use most is 70 °C thermoplastic (PVC) insulation with copper conductors up to 300 mm² — twin and earth — which has a k of 115.

 

Insulation	Conductor	k Value
70 °C thermoplastic	Copper	115
70 °C thermoplastic	Aluminium	76
90 °C thermosetting	Copper	143
90 °C thermosetting	Aluminium	94

 

That table feeds straight into the adiabatic equation for time, from Regulation 434.5.2:

 

t = k² × S² ÷ I²

 

This tells you how long a conductor of cross-sectional area S can carry fault current I before reaching its limiting temperature.

 

Worked example: A 4 mm² copper conductor in 70 °C thermoplastic sheathing, k = 115, fault current 800 A. t = (115² × 4²) ÷ 800² = (13,225 × 16) ÷ 640,000 = 0.33 s

 

Now look at what that means. If this is a final circuit needing 0.4 s disconnection, the cable reaches its limiting temperature before the device is guaranteed to operate — damage is possible. Increase the conductor to 6 mm² and the same sum gives 0.74 s, comfortably beyond the disconnection time. Try it yourself; the arithmetic is worth doing by hand once.

 

Our full walkthrough of both forms of the equation is in The Adiabatic Equation Explained.

 

4. Table 443.2 — Rated Impulse Voltage

U_W is the impulse withstand voltage — how much transient overvoltage a piece of equipment must survive without becoming dangerous. The table cross-references nominal system voltage against equipment category.

 

Category	Typical Equipment	U_W at 230/400 V
IV	Origin of installation, meters, main switchgear	6 kV
III	Distribution boards, fixed installation wiring	4 kV
II	Appliances, portable tools	2.5 kV
I	Sensitive electronic equipment	1.5 kV

 

Common trap: A domestic washing machine is Category II, and the table cell reads 2.5. Candidates then pick “2.5 V”. Read the column heading — the values are in kilovolts. The correct answer is 2.5 kV.

 

5. Table 444.2 — Separation of Power and Signal Cables

Treat this as two tables in one: one part indexed by voltage, another by current. Look up both, then take the worst case — the larger separation distance.

 

Worked example: A power cable carrying 100 A at 240 V. The voltage lookup gives 0.45 m; the current lookup gives 0.6 m. The required minimum separation is the greater of the two: 0.6 m.

 

Candidates lose this mark by finding one value and stopping. Always check both halves.

 

6. Table 52.3 — Minimum Cross-Sectional Area of Conductors

This table has nothing to do with load current or voltage drop. It answers a purely mechanical question: how thin is too thin?

 

Four steps: type of wiring system → what the circuit is used for → conductor material (copper or aluminium) → read off the minimum size.

 

Worked example: A domestic power circuit in sheathed twin and earth cable with copper conductors must have a conductor CSA of not less than 1.0 mm².

 

Exam tip: When a question points you at Table 52.3, ignore current-carrying capacity, grouping factors and voltage drop entirely. Answer from the table and move on.

 

7. Tables A53.1 and A53.2 — Device Function and Coordination

Table A53.1 answers “what can this device actually do?” Rows are device types with their BS numbers; the final columns show the functions each performs — isolation, switching, overcurrent protection, residual current protection.

 

Device	Primary Function
MCB	Overcurrent protection
RCCB	Residual current protection only
RCBO	Overcurrent and residual current protection

 

Exam tip: “All of the above” is not always the right answer. An RCCB provides residual current protection — it does not provide overcurrent protection. That distinction is exactly what the question is testing.

 

Table A53.2 covers device coordination — selectivity, so the device nearest the fault operates first and the whole house doesn’t go dark. The table doesn’t give you rules; it gives you the regulation number to consult for each coordination type.

 

Remember: With roughly two minutes per question, do not turn to the regulation the table points at. The answer options are regulation numbers. Read them off the table and move on.

 

8. Table 54.1 and Table 54.7 — Earthing and Protective Conductors

Table 54.1 gives the minimum CSA of a buried earthing conductor. Three steps: level of protection against corrosion, level of mechanical protection, then material.

 

Condition	Copper	Steel
Protected against corrosion but not mechanical damage	16 mm²	16 mm²
Not protected against corrosion	25 mm²	50 mm²

 

That leads directly to the adiabatic equation for size, Regulation 543.1.3 — which almost always appears in some form:

 

S = √(I² × t) ÷ k

 

Worked example: If the calculation returns 3.57 mm², you select the nearest standard size not less than the result — 4 mm². Never round down; a smaller conductor exceeds its limiting temperature before the device clears the fault.

 

Table 54.7 is the alternative tabulated method for sizing a protective conductor — no calculation needed:

 

Line Conductor S	Protective Conductor	Example
S ≤ 16 mm²	Same as line conductor	6 mm² line → 6 mm² CPC
16 mm² < S ≤ 35 mm²	16 mm²	25 mm² line → 16 mm² CPC
S > 35 mm²	Half the line conductor	50 mm² line → 25 mm² CPC

 

9. Table 55.3 — Luminaire Symbols

The easiest table in the book — it’s all pictures. The question gives you a description in words and four symbols; you match one to the other.

 

Exam tip: Don’t analyse the symbols or try to reason about what they ought to mean. Read the wording beside each one, find the match, answer, move on. This is a free mark and should take fifteen seconds.

 

10. Table 64 — Insulation Resistance

Near-guaranteed to appear. The question gives you the nominal circuit voltage; the table gives you the test voltage and the minimum acceptable reading.

 

Circuit Nominal Voltage	Test Voltage (DC)	Minimum Insulation Resistance
SELV and PELV	250 V	0.5 MΩ
Up to and including 500 V	500 V	1.0 MΩ
Above 500 V	1000 V	1.0 MΩ

 

Common trap: The test voltage is DC, and answer options will often include an AC value alongside the correct DC one. Others mix ohms with amps. If the question just says “domestic dwelling”, assume 230 V AC nominal.

 

Full procedure and the practical pitfalls are covered in our Insulation Resistance Test guide.

 

How to Practise Table Lookups

Reading about these tables is not the same as being fast with them. Two hours for 60 questions is exactly two minutes each, and table questions should take you well under that so you have headroom for the ones that need real thought.

 

1. Sit with the book and a list of table numbers. Time yourself finding each one from the Table of Tables. Repeat until it’s automatic.
2. Read every heading out loud. Most table mistakes are heading mistakes — 0.4 s versus 5 s, volts versus kilovolts, AC versus DC.
3. Work the two adiabatic examples by hand. Both forms, with the calculator you’ll take into the exam.
4. Tab the six Zs tables as a single block. They’re consecutive, so one tab plus the knowledge of the running order is enough.
5. Answer from the table only. Resist the urge to verify against the regulation text. There isn’t time.

 

Bottom line: Roughly a third of exam questions can be answered by finding the right cell in the right table. That’s a third of the paper where speed, not knowledge, decides your score.

 

Practice and Further Study

The tables in this guide sit across Parts 4, 5 and 6 — the three highest-weighted areas of the exam. Test yourself on each:

• Part 4 — Protection for Safety quiz
• Part 5 — Selection and Erection of Equipment quiz
• Part 6 — Inspection and Testing quiz

 

Our app includes 690+ practice questions across all 8 parts, each with an explanation referencing the specific regulation or table number so you learn where the answer lives, not just what it is. Topic-specific quizzes let you drill the Zs tables or the adiabatic equation on their own, and full timed mock exams use the same weighted question distribution as the real paper — the best way to build the two-minutes-per-question rhythm before exam day.

 

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