Zs Tables Explained: Which Table to Use and How to Get the Answer Right

Ask any exam invigilator which topic generates the most candidate confusion and Zs will be near the top of the list. It isn’t that the maths is hard — the formula fits on one line. The problem is that BS 7671 contains several Zs tables that look almost identical, and picking the wrong one gives you an answer that appears perfectly reasonable and is completely wrong.

 

This guide walks through where the numbers come from, which table applies to which circuit, and the specific traps that catch candidates out every sitting.

The Book You Can Take In

Let’s clear up the most common question first, because it costs people marks before they’ve even opened a paper.

 

The only book permitted in the 18th Edition exam is your copy of BS 7671. Not the On-Site Guide. Not the Guidance Notes. Every answer to every question is in the regulations book.

 

Important: The On-Site Guide publishes Zs measured (Zsm) values. BS 7671 publishes Zs tabulated values. They are different numbers for the same device — the OSG figures are 80% of the regulations book figures. Study from the wrong book and your memorised values won’t match any of the four options in front of you.

 

What Zs Actually Represents

Zs is the earth fault loop impedance: the total impedance of the fault path from the furthest point of the circuit, along the CPC, back through the earth return to the transformer, and out again along the line conductor.

 

The question every Zs table answers is this: what impedance is low enough to let sufficient fault current flow to operate the device in the required time?

 

The standard formula is:

 

Zs = (U₀ × Cmin) ÷ Ia

 

Symbol	Meaning	Typical Value
Zs	Maximum earth fault loop impedance	The answer you’re calculating
U₀	Nominal line-to-earth voltage	230 V for a domestic supply
Cmin	Voltage factor allowing for supply fluctuations	0.95
Ia	Fault current needed to operate the device in time	From Appendix 3

 

Key point: Note that U₀ is the nominal voltage, not what your meter reads. If you measure 240 V on site, you still use 230 V in the calculation. Cmin then knocks that back further to allow for low-voltage conditions.

 

Step 1 — Find the Disconnection Time

Before you can find Ia, you need to know how fast the device must operate. Regulation 411.3.2.2 sends you to Table 41.1 for final circuits.

 

Regulation 411.3.2.2 applies to final circuits up to 63 A where socket outlets are involved, and up to 32 A for fixed current-using equipment.

 

System	Circuit Type	Maximum Disconnection Time	Regulation
TN (TN-S and TN-C-S)	Final circuits	0.4 s	411.3.2.2
TT	Final circuits	0.2 s	411.3.2.2
TN	Distribution circuits	5 s	411.3.2.3
TT	Distribution circuits	1 s	411.3.2.4

 

Exam tip: Table 41.1 is banded by voltage. A 230 V supply sits in the 50 V < U₀ ≤ 230 V band — not the band above it. Candidates routinely read across from the wrong row because 230 sits at the boundary. Check which side of the boundary you’re on before reading the time.

 

The two regulations sitting immediately below Table 41.1 — 411.3.2.3 (5 s for TN distribution circuits) and 411.3.2.4 (1 s for TT distribution circuits) — appear in exams regularly. Tab that page.

 

Step 2 — Find Ia in Appendix 3

Appendix 3 contains the time/current characteristics for every common protective device. Each one gets a full-page graph and a small table in the corner.

 

Ignore the graph. For exam purposes you want the table, which lists the device rating on the left and the current required for operation between 0.1 s and 5 s on the right.

 

Device	Rating	Ia for Required Disconnection
Type B MCB	16 A	80 A
Type B MCB	20 A	100 A
Type B MCB	32 A	160 A
Type C MCB	20 A	200 A
Type C MCB	32 A	320 A

 

Remember: There’s a shortcut worth memorising. For Type B, Ia = 5 × In. For Type C, Ia = 10 × In. For Type D, Ia = 20 × In. Check any of the values above against the rule and you’ll see it holds.

 

This is why a Type C breaker needs a much lower Zs than a Type B of the same rating — it responds more slowly to fault current, so it needs more of it, which means less impedance in the loop.

 

Step 3 — Calculate or Look Up Zs

Take a worked example. A domestic property has a nominal 230 V TN-C-S supply. What is the maximum permitted Zs for a final circuit protected by a 32 A BS EN 60898 Type B breaker?

 

Step	Working
Disconnection time (Table 41.1)	0.4 s
Ia (Appendix 3, or 5 × 32)	160 A
Zs = (230 × 0.95) ÷ 160	218.5 ÷ 160 = 1.37 Ω

 

Now turn to Table 41.3. Find Type B along the top, move to the 32 A column, and read off 1.37 Ω. Identical.

 

Key point: The tables in Chapter 41 have done the Appendix 3 work for you. Unless a question explicitly asks you to derive Ia or use the formula, go straight to Chapter 41 and read the answer. That’s minutes saved on a paper where you have exactly two minutes per question.

 

If you want the full derivation of this relationship with more worked examples, our guide to the Zs test procedure, formula and maximum values covers it from the testing side.

 

Which Table? The Three-Page Layout

This is where most marks are lost. Chapter 41 lays the Zs tables out across three consecutive pages, and they alternate between fuses and breakers:

 

Table	Devices	Circuit Type	Disconnection Time
41.2	Fuses	Final circuits	0.4 s
41.3	Circuit breakers and RCBOs	Final and distribution	0.4 s and 5 s
41.4	Fuses	Distribution circuits	5 s

 

Why do breakers get one table and fuses get two? Because circuit breakers are engineered with a magnetic trip that responds within a narrow band — the same Ia satisfies both 0.4 s and 5 s. Fuses work on a completely different principle, and their operating characteristic varies enough between the two time bands that they need separate tabulations.

 

Exam tip: Tables 41.2 and 41.4 sit either side of 41.3 and look near enough identical at a glance. If the question mentions a fuse, decide first whether it’s a final or distribution circuit, then turn to the table. Getting 41.2 and 41.4 mixed up is one of the easiest marks to throw away in the whole paper.

 

Final Circuits vs Distribution Circuits

Since the whole table choice hangs on this distinction, it’s worth being precise about it.

 

Circuit Type	Definition	What’s Downstream	Disconnection Time
Final circuit	Connected directly to current-using equipment or socket outlets	Nothing — this is the last leg	0.4 s
Distribution circuit	Feeds another distribution board or consumer unit	More protective devices	5 s

 

A cable running from a distribution board to a downstream consumer unit is a distribution circuit — there are more fuses after it. The circuits leaving that consumer unit to sockets and lights are final circuits.

 

The catch: the same distribution board can also feed a final circuit directly. A lighting circuit taken straight off the DB is a final circuit even though everything around it is distribution.

 

Remember: A fused connection unit does not count as a downstream protective device for this purpose. The circuit feeding the boiler FCU is still a final circuit at 0.4 s — don’t be talked into 5 s because there’s a 3 A fuse somewhere along it.

 

In a typical domestic installation there is a cut-out fuse, a meter, tails to the consumer unit, and then everything leaving the consumer unit is a final circuit at 0.4 s. Simple — provided you don’t overthink it.

 

Table 41.5 — When an RCD Provides Fault Protection

Sometimes the required maximum Zs simply cannot be achieved. This happens most often on TT systems, where the earth electrode gives you a high Ze before you’ve even started.

 

Regulation 411.4.204 covers this: where the required disconnection time can’t be met by an overcurrent device alone, an RCD may be used for fault protection, and the Zs must then not exceed the values in Table 41.5 on the following page.

 

Those values look strange until you see where they come from. Regulation 411.5.3 gives the reasoning, based on limiting touch voltage to 50 V:

 

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

 

Important: RCDs are primarily an additional protection device. Using one to solve a high-Zs problem is a last resort, permitted only after you’ve exhausted the practical ways of reducing the circuit impedance. An exam question phrased as “what is the best way to achieve compliance” rarely wants “fit an RCD” as the first answer.

 

If TT systems are your weak spot, our walkthrough on checking R1+R2 using the Zs and Ze tables works through the arithmetic.

 

Table 41.6 — Reduced Low Voltage Systems

Table 41.6 covers 55 V and 63.5 V systems — the reduced low voltage supplies you get from 110 V centre-tapped-to-earth transformers on construction sites. Plenty of candidates never open this table in working life, then meet it under exam conditions.

 

Take a 55 V single-phase circuit protected by a 20 A BS EN 60898 Type B breaker. Follow the path: Type B section, 55 V column, 20 A row. Maximum Zs = 0.52 Ω.

 

Compare that with Table 41.3, where a 20 A Type B gives 2.19 Ω. The two look irreconcilable until you scale for voltage:

 

Check	Working	Result
Scale 55 V value to 230 V	(230 ÷ 55) × 0.52	2.18 Ω — matches Table 41.3
Verify fault current at 55 V	55 ÷ 0.52	106 A — comfortably above the 100 A Ia needed

 

A lower supply voltage pushes less current through the same impedance, so the permitted impedance has to come down to compensate. The physics is consistent; only the numbers change.

 

Watch for this: In Table 41.6, the layout makes it look as though Type C applies only to 55 V and Type D only to 63.5 V. It doesn’t. Both breaker types are tabulated for both voltages. This is one of the most reliable trap questions in the whole Zs topic — read the column headings carefully rather than assuming the pattern.

 

Zs Tabulated vs Zs Measured

Everything so far gives Zs tabulated — values calculated at conductor operating temperature (70 °C for standard thermoplastic insulation). When you put a loop tester on a circuit, the cable is cold. Copper resistance rises with temperature, so a cold reading understates the impedance the circuit will actually present under load.

 

The correction is the 80% rule, and the formula sits in Appendix 3:

 

Zs measured ≤ 0.8 × Zs tabulated

 

Worked through for our 32 A Type B breaker:

 

	Value
Zs tabulated (Table 41.3)	1.37 Ω
× 0.8	1.09 Ω
Meaning	Maximum acceptable reading on your test meter

 

Key point: Those 0.8-corrected figures are exactly the values printed in the On-Site Guide. Same physics, different presentation — which is precisely why bringing OSG numbers into the exam causes chaos.

 

The exam rule is simple: answer with the tabulated value from Tables 41.2, 41.3 or 41.4 unless the question specifically asks for a measured value. If it does, multiply by 0.8.

 

For a fuller treatment including the alternative 1.2 correction factor applied to calculated R1+R2 values, see Maximum Zs and the 80% Rule.

 

How to Reduce a Zs That’s Too High

Design questions often ask what you’d do when a circuit fails its Zs check. The permitted maximum and the actual value are two separate levers:

 

Approach	Method	Effect
Raise the permitted Zs	Reduce the device rating (32 A → 20 A)	Lower Ia, so higher permitted Zs
Raise the permitted Zs	Change device type (Type C → Type B)	Type B needs less fault current
Lower the actual Zs	Increase conductor CSA	Less resistance in R1+R2
Lower the actual Zs	Shorten the cable run	Less resistance in R1+R2
Last resort	Fit an RCD for fault protection	Zs limited by Table 41.5 instead

 

Note the direction of each lever. Reducing the breaker size doesn’t change the circuit’s impedance at all — it changes the number the impedance has to beat.

 

The Traps That Cost Marks

 

Trap	What Goes Wrong
Using On-Site Guide values	Your answer is 80% of every option on the paper
Mixing up Tables 41.2 and 41.4	Right device, wrong disconnection time, wrong answer
Reading the wrong voltage band in Table 41.1	230 V sits at a band boundary — easy to slip a row
Counting an FCU as a downstream device	Turns a 0.4 s final circuit into a 5 s distribution circuit
Assuming Table 41.6 splits Type C and D by voltage	Both types are tabulated at both 55 V and 63.5 V
Giving Zs measured when Zs tabulated was asked	Read the question wording before you reach for 0.8
Using measured voltage instead of 230 V	U₀ is always the nominal figure, never your meter reading

 

Exam tip: When a Zs question appears, work through the same four checks every time — device type, circuit type, voltage, tabulated or measured. Answer those four and the correct table picks itself.

 

Practice and Further Study

Zs sits right at the heart of Part 4, which carries roughly a quarter of the exam on its own. Table navigation is a skill that only comes from repetition — you want to be finding Table 41.3 and reading off a value in under fifteen seconds.

 

Test yourself on the topics Zs connects to:

• 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 a detailed explanation citing the specific regulation and table, plus full timed mock tests weighted the same way as the real paper — so you practise picking the right table under the same clock pressure you’ll face on the day.

 

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