TT Systems: How to Check R1+R2 Using the Zs and Ze Tables

If you install a new circuit in a TT dwelling, how do you know your R1+R2 is any good? There are no maximum Zs tables for TT systems in BS 7671 — the tables in Chapter 41 and Appendix 14 assume a TN supply, and for TT installations the regulations point you at RCD protection instead of a loop impedance limit.

 

That leaves a genuine gap for the working electrician. You’ve measured your R1+R2, you’ve got a number, but there’s nothing obvious to compare it against. The answer is a simple bit of transposition combined with the TN-C-S tables you already own — and it’s a technique that gets tested indirectly in the 18th Edition exam, because it forces you to understand exactly what Zs, Ze and R1+R2 each represent.

A Quick Recap on TT Systems

In a TT installation, the supply is typically taken from an overhead distribution network. A transformer and fuses are mounted on the pole, and a line and neutral are run to the dwelling. Crucially, there is no metallic earth conductor between the supply and the property.

 

Instead, an earth electrode is installed at the property and another at the supply transformer. During an earth fault, the return path relies on the general mass of earth — current passes from the installation’s electrode, through the soil, and is picked up by the electrode at the transformer.

 

System	Earth Path	Typical Ze	Fault Protection
TN-S	Separate metallic CPC from supply	Up to 0.8 Ω	Overcurrent device or RCD
TN-C-S	Combined PEN separated at origin	Up to 0.35 Ω	Overcurrent device or RCD
TT	General mass of earth via electrodes	Highly variable — often tens of ohms	RCD essential

 

Key point: In a TT system the earth fault loop impedance is dominated by soil resistance, which is why Regulation 411.5.3 requires RA × IΔn ≤ 50 V rather than giving a maximum Zs. An overcurrent device alone cannot be relied upon to clear an earth fault on a TT installation.

 

If you want to revisit the underlying earthing arrangements in more detail, our guide on Earthing and Bonding: TN-C-S, TN-S and TT Systems covers each configuration and where BS 7671 treats them differently.

 

What Zs, Ze and R1+R2 Actually Mean

Before you can use the method, you need absolute clarity on the three quantities. Candidates lose marks in the exam not because the maths is hard, but because they mix up which part of the loop each symbol refers to.

 

Symbol	Name	What It Covers	Live or Dead?
Zs	Earth fault loop impedance	The whole loop — from the point of fault, out through the CPC, through the earth path to the transformer, and back along the line conductor	Live test
Ze	External earth fault loop impedance	The outside part only — from the origin (consumer unit) out to the transformer and back	Live test
R1+R2	Line plus CPC resistance	The inside part only — from the consumer unit to the furthest point of the circuit and back	Dead test

 

Remember: We call Zs and Ze impedances because they are live measurements on an AC system. R1+R2 is a resistance because it is measured dead. All three are in ohms, but the terminology matters — exam questions do test it.

 

Every circuit has its own R1+R2. The lighting circuit has one, the immersion heater has another, the socket circuit has a third. But they all share the same Ze, because the external part of the loop is common to the whole installation.

 

So the relationship is:

 

Ze + (R1+R2) = Zs

 

In plain terms: the outside part plus the inside part equals the whole thing.

 

Transposing the Formula

You measure Zs at the point of use and Ze at the origin. To get the inside part on its own, subtract Ze from both sides:

 

Step	Expression
1	Ze + (R1+R2) = Zs
2	Ze + (R1+R2) − Ze = Zs − Ze
3	R1+R2 = Zs − Ze

 

Exam tip: Transposition questions like this appear regularly. If you’re rusty on rearranging formulae, work through our guide to Transposition of Electrical Formulae — it’s the same maths that underpins the adiabatic equation and voltage drop calculations.

 

Why TT Systems Have No Maximum Zs Table

Table 41.3 gives maximum Zs values for TN systems because the external loop impedance is predictable and declared by the distributor. In a TT installation, it isn’t.

 

Season	Soil Condition	Effect on Ze and Zs
Winter / wet	Moist, conductive soil	Ze falls — Zs measures lower
Summer / dry	Dry, resistive soil	Ze rises — Zs measures higher
Frozen ground	Ice reduces conductivity	Ze can rise sharply

 

The critical observation is this: it is the external part that moves. The internal wiring — the copper in your cables — stays at a near-constant resistance regardless of the weather. So R1+R2 is the one figure in the equation you can genuinely stand behind as evidence that your work is sound.

 

The Method: Borrowing the TN-C-S Figures

If there’s no TT table, use the TN-C-S one and strip out the part that doesn’t apply.

 

Both the On-Site Guide (Table B6, page 145 in the Amendment 2 printing) and the Electrician’s Guide to the Building Regulations (Table 4.1.2A, page 69) give maximum measured earth fault loop impedance for BS EN 60898 circuit breakers, BS 3871 breakers and BS EN 61009 RCBOs, together with the assumed Ze for TN-C-S and TN-S supplies.

 

The Four Steps

 

Step	Action
1	Look up the maximum measured Zs for your device type and rating in the TN-C-S column
2	Subtract the assumed TN-C-S Ze of 0.35 Ω
3	The result is your benchmark maximum R1+R2 for that device
4	Compare your measured R1+R2 (= Zs − Ze) against it

 

Important: This is a sanity check on your own wiring, not a substitute for the TT fault protection requirements. Your TT circuit still needs RCD protection to Regulation 411.5.3, and the RA × IΔn ≤ 50 V condition still has to be satisfied. The method tells you the cable you installed is the right size and correctly terminated — it does not tell you the installation is compliant on its own.

 

A Worked Example: Immersion Heater on a TT Supply

Here’s the scenario in full:

 

Detail	Value
Load	Immersion heater, 2.8 kW at 230 V AC
Cable	2.5 mm² / 1.5 mm² twin and earth, clipped direct
Circuit length	40 m
Protective device	16 A Type B MCB
Measured Zs	72.08 Ω
Measured Ze	71.30 Ω

 

Step 1 — Find the Actual R1+R2

 

R1+R2 = Zs − Ze

R1+R2 = 72.08 − 71.30 = 0.78 Ω

 

Note how large the Zs and Ze figures are. That’s entirely normal for a TT system — the electrode resistance dominates. The circuit’s own contribution is less than one ohm.

 

Step 2 — Find the Benchmark

 

From the tables, for a 16 A Type B breaker:

 

Figure	Value
Maximum measured Zs (TN-C-S)	2.2 Ω
Assumed TN-C-S Ze	0.35 Ω
Benchmark maximum R1+R2	1.85 Ω

 

2.2 − 0.35 = 1.85 Ω

 

Step 3 — Compare

 

Measured R1+R2	Benchmark	Verdict
0.78 Ω	1.85 Ω	Acceptable

 

At 0.78 Ω the measured value is comfortably below the 1.85 Ω benchmark, so the internal wiring of this circuit is sound — correct cable size, correct length, properly terminated.

 

Key point: If your measured R1+R2 is equal to or less than the figure extracted from the tables, the circuit values are acceptable. If it’s higher, something is wrong — undersized CPC, an excessively long run, a loose termination, or a high-resistance joint. Investigate before you certify.

 

For the practical side of taking this measurement in the first place, see Continuity of Protective Conductors: The Essential R1+R2 Test Explained.

 

Checking the Voltage Drop

Getting the loop impedance right is only half the job. A long circuit can pass on R1+R2 and still fail on voltage drop, so always check both.

 

The Electrician’s Guide table also lists recommended maximum circuit lengths. For this cable size and breaker on an immersion heater circuit, the table gives 50 m. At 40 m we’re inside that, so the voltage drop should be well within limits — but let’s confirm.

 

Quantity	Value	Source
Design current, Ib	12.17 A	2800 W ÷ 230 V
Circuit length, L	40 m	Given
mV/A/m	18	On-Site Guide Table F6
Maximum permitted drop	11.5 V	5% of 230 V, Reg. 525

 

The calculation:

 

Vd = (mV/A/m × Ib × L) ÷ 1000

Vd = (18 × 12.17 × 40) ÷ 1000 = 8.76 V

 

At 8.76 V we’re comfortably below the 11.5 V maximum. Note the division by 1,000 — the tabulated figure is in millivolts per amp per metre, so you must convert to volts.

 

Exam tip: The 5% limit (11.5 V at 230 V) applies to power circuits. Lighting circuits are limited to 3% — 6.9 V at 230 V. Regulation 525 and Appendix 12 are worth tabbing.

 

Where This Comes Up in the Exam

You may not be asked to reproduce this exact procedure in the 2382-26 paper, but every component of it supports examinable testing principles:

 

Concept	Typical Question Style
Zs = Ze + (R1+R2)	“Which of the following expressions gives the earth fault loop impedance?”
Transposition	”Ze is 0.28 Ω and Zs is 1.06 Ω. What is R1+R2?”
TT fault protection	”Which device is essential for fault protection on a TT installation?”
Definition of Ze	”The external earth fault loop impedance is measured from…”
Dead vs live tests	”Which of the following is a dead test?”
Voltage drop limits	”The maximum permitted voltage drop for a 230 V lighting circuit is…”

 

Remember: The exam will always assume BS 7671 as the authority. The On-Site Guide and the Electrician’s Guide are non-statutory guidance publications — useful on site and in this method, but if a question asks what BS 7671 requires, the answer comes from the regulations themselves.

 

Common Mistakes With This Method

 

Mistake	Why It Matters
Using the TN-S Ze (0.8 Ω) with a TN-C-S Zs figure	Mixing columns gives you a benchmark that’s too tight — you’ll fail circuits that are actually fine
Forgetting the tables are “maximum measured”	Maximum measured values already include the temperature correction. Don’t apply the 0.8 factor twice
Treating the benchmark as a compliance limit	It’s a self-check on your own wiring, not proof of TT fault protection compliance
Measuring Ze without disconnecting the main earthing conductor	Parallel paths through bonding give an artificially low Ze — and therefore an artificially high apparent R1+R2
Ignoring voltage drop	A circuit can pass on impedance and still exceed 3% or 5% on a long run
Testing at the wrong point	Zs must be measured at the furthest point of the circuit, or your R1+R2 will be understated

 

Bottom line: R1+R2 = Zs − Ze is the most useful piece of transposition in inspection and testing. Learn it, understand which part of the loop each term describes, and you can verify a TT circuit with nothing more than a loop tester and the On-Site Guide.

 

For the equivalent method on TN systems — where the tables do exist and the 80% correction is applied directly — see Maximum Zs and the 80% Rule: Using the Tables Correctly.

 

Practice and Further Study

Inspection and testing sits in Part 6 of BS 7671, but questions on Zs, Ze and R1+R2 draw on Part 4 (protection and disconnection) and Part 5 (conductor selection) too. Test yourself across all three:

• 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, including earth fault loop impedance, testing-sequence, and earthing-system questions. You’ll also find topic-specific quizzes and full timed mock exams. Pair them with the current Amendment 4 revision guide.

 

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