Verification

BS 7671 earthing and fault-current verification

Fixed worked examples run through MEPCalc's earthing, fault-current and protective-device calculations, compared line by line with the values printed in BS 7671:2018+A2:2022 and the IET On-Site Guide.

Result: 46 of 46 checks pass
Cases: 16 worked examples
Version / published: v1.0 · 2026-09-03
Engine fingerprint: 73baceb34ff8

Why we publish this

No formal certification scheme covers BS 7671 earth-fault loop or fault-current calculations. So we publish our own evidence: a fixed set of worked examples, each with its expected value cited to the table, regulation and printed page it comes from, run through the same calculations our users run. Conductor resistances come from the On-Site Guide's Table I1 and are corrected to operating temperature with its Table I3; the Zs limits are the Chapter 41 tables on the Cmin = 0.95 basis; and a measured Zs is checked against the Appendix 3 rule, Zs(m) ≤ 0.8 × U0 × Cmin / Ia.

What these calculations do not model

TT systems and Table 41.5. On a TT supply the earthing calculation reports that an RCD is required and does not verify disconnection by Zs alone; the Table 41.5 RCD limits are not implemented, so no TT case appears above. A regression test holds that behaviour so the page changes when it does.
Separate protective conductors (Table 54.2). The adiabatic check covers a protective conductor incorporated in a cable or bunched with cables (Table 54.3) and cable armour or sheath (Table 54.4). A separate, unbunched protective conductor cannot be selected, so Table 54.2 is not exercised.
Fuses to BS 88-3 and BS 1362. Tables 41.2 and 41.4 also tabulate these; the calculations offer BS 88-2 and BS 3036 only.
On-site transformer sources. The fault-current calculation can model a transformer from its rating and impedance, but that relation is not printed in BS 7671, so every fault-current case above uses a distributor-declared Ze. Conductor resistances above 50 mm² are likewise outside the On-Site Guide's Table I1 and are not asserted here.
Ze as the whole source. The declared Ze is treated as the line-to-neutral loop for the single-phase fault and halved for the per-phase three-phase fault, which is how Appendix 14's line-to-neutral × 2 relation is applied; the neutral and protective conductor are taken as the same size as the line conductor.

How to read the tables

Expected is the value printed in the standard, or a hand calculation from printed values. The working is shown under each quantity.
Tolerance is exact for tabulated values and selected sizes, and ±2 % for calculated quantities such as R1 + R2, earth-fault loop impedance, prospective fault current, cable reactance and adiabatic conductor size.
Every check re-runs automatically whenever the calculation code changes, and this page is regenerated from those results. The engine fingerprint above identifies the calculation code the results came from.

Sources

BS 7671:2018+A2:2022
IET On-Site Guide, 18th Edition, Amendment 2:2022

Printed page numbers refer to the editions above. Amendment 3:2024 to BS 7671 adds two definitions and one regulation on bidirectional protective devices and does not change any page cited here.

Worked examples

EF-01

TN-C-S radial — 2.5/1.5 mm² flat twin-and-earth, 20 m, 32 A Type B

Inputs

TN-C-S supply, Ze = 0.35 Ω (typical distributor maximum)
2.5 mm² line with 1.5 mm² protective conductor, copper, 70 °C thermoplastic, 20 m
Type B circuit-breaker, 32 A, final circuit (0.4 s)

The 0.35 Ω Ze is the On-Site Guide's typical TN-C-S value (Introduction section 1.1 item (d), printed p13). A 1.5 mm² protective conductor is below the Table 54.7 minimum for a 2.5 mm² line conductor, which is why flat twin-and-earth relies on Regulation 543.1.3.

Checks for case EF-01
Quantity	Expected	Source	MEPCalc	Tolerance	Result
R1 + R2 at operating temperature 19.51 mΩ/m (2.5/1.5 row) × 1.20 × 20 m ÷ 1000 = 0.4682 Ω	0.4682Ω	IET On-Site Guide, 18th Edition, Amendment 2:2022 Appendix I Table I1, printed p218; Appendix I Table I3 (incorporated in a cable, 70 °C thermoplastic), printed p220	0.4682Ω	±2 %	Pass
Earth-fault loop impedance, Zs 0.35 + 0.4682 = 0.8182 Ω	0.8182Ω	IET On-Site Guide, 18th Edition, Amendment 2:2022 Appendix I, Zs = Ze + (R1 + R2), printed p217	0.8182Ω	±2 %	Pass
Maximum Zs for the device	1.37Ω	BS 7671:2018+A2:2022 Table 41.3, printed p68 (Type B, 32 A)	1.37Ω	exact	Pass
Minimum protective conductor, Table 54.7	2.5mm²	BS 7671:2018+A2:2022 Table 54.7, printed p201 (S ≤ 16 → S)	2.5mm²	exact	Pass

EF-02

TN-S radial — 6/2.5 mm² flat twin-and-earth, 25 m, 32 A Type B

Inputs

TN-S supply, Ze = 0.8 Ω (typical distributor maximum)
6 mm² line with 2.5 mm² protective conductor, copper, 70 °C thermoplastic, 25 m
Type B circuit-breaker, 32 A, final circuit (0.4 s)

The 0.8 Ω Ze is the On-Site Guide's typical TN-S value (Introduction section 1.1 item (d), printed p13).

Checks for case EF-02
Quantity	Expected	Source	MEPCalc	Tolerance	Result
R1 + R2 at operating temperature 10.49 mΩ/m (6/2.5 row) × 1.20 × 25 m ÷ 1000 = 0.3147 Ω	0.3147Ω	IET On-Site Guide, 18th Edition, Amendment 2:2022 Appendix I Table I1, printed p218; Appendix I Table I3 (incorporated in a cable, 70 °C thermoplastic), printed p220	0.3147Ω	±2 %	Pass
Earth-fault loop impedance, Zs 0.8 + 0.3147 = 1.1147 Ω	1.1147Ω	IET On-Site Guide, 18th Edition, Amendment 2:2022 Appendix I, Zs = Ze + (R1 + R2), printed p217	1.1147Ω	±2 %	Pass
Maximum Zs for the device	1.37Ω	BS 7671:2018+A2:2022 Table 41.3, printed p68 (Type B, 32 A)	1.37Ω	exact	Pass

EF-03

TN-S lighting circuit — 1.5/1.0 mm² flat twin-and-earth, 40 m, 6 A Type B

Inputs

TN-S supply, Ze = 0.8 Ω
1.5 mm² line with 1.0 mm² protective conductor, copper, 70 °C thermoplastic, 40 m
Type B circuit-breaker, 6 A, final circuit (0.4 s)

Checks for case EF-03
Quantity	Expected	Source	MEPCalc	Tolerance	Result
R1 + R2 at operating temperature 30.20 mΩ/m (1.5/1 row) × 1.20 × 40 m ÷ 1000 = 1.4496 Ω	1.4496Ω	IET On-Site Guide, 18th Edition, Amendment 2:2022 Appendix I Table I1, printed p218; Appendix I Table I3 (incorporated in a cable, 70 °C thermoplastic), printed p220	1.4496Ω	±2 %	Pass
Earth-fault loop impedance, Zs 0.8 + 1.4496 = 2.2496 Ω	2.2496Ω	IET On-Site Guide, 18th Edition, Amendment 2:2022 Appendix I, Zs = Ze + (R1 + R2), printed p217	2.2496Ω	±2 %	Pass
Maximum Zs for the device	7.28Ω	BS 7671:2018+A2:2022 Table 41.3, printed p68 (Type B, 6 A)	7.28Ω	exact	Pass

EF-04

TN-C-S — 4 mm² line and 4 mm² protective conductor, 30 m, 20 A Type C

Inputs

TN-C-S supply, Ze = 0.35 Ω
4 mm² line and 4 mm² protective conductor, copper, 70 °C thermoplastic, 30 m
Type C circuit-breaker, 20 A, final circuit (0.4 s)

Checks for case EF-04
Quantity	Expected	Source	MEPCalc	Tolerance	Result
R1 + R2 at operating temperature 9.22 mΩ/m (4/4 row) × 1.20 × 30 m ÷ 1000 = 0.3319 Ω	0.3319Ω	IET On-Site Guide, 18th Edition, Amendment 2:2022 Appendix I Table I1, printed p218; Appendix I Table I3 (incorporated in a cable, 70 °C thermoplastic), printed p220	0.3319Ω	±2 %	Pass
Earth-fault loop impedance, Zs 0.35 + 0.3319 = 0.6819 Ω	0.6819Ω	IET On-Site Guide, 18th Edition, Amendment 2:2022 Appendix I, Zs = Ze + (R1 + R2), printed p217	0.6819Ω	±2 %	Pass
Maximum Zs for the device	1.09Ω	BS 7671:2018+A2:2022 Table 41.3, printed p68 (Type C, 20 A)	1.09Ω	exact	Pass
Minimum protective conductor, Table 54.7	4mm²	BS 7671:2018+A2:2022 Table 54.7, printed p201 (S ≤ 16 → S)	4mm²	exact	Pass

EF-05

TN-C-S distribution circuit — 16/16 mm², 40 m, 32 A Type D (5 s)

Inputs

TN-C-S supply, Ze = 0.35 Ω
16 mm² line and 16 mm² protective conductor, copper, 70 °C thermoplastic, 40 m
Type D circuit-breaker, 32 A, distribution circuit (5 s, Regulation 411.3.2.3)

The 0.4 s row of Table 41.3 gives 0.34 Ω for the same device; the 5 s row applies to a distribution circuit.

Checks for case EF-05
Quantity	Expected	Source	MEPCalc	Tolerance	Result
R1 + R2 at operating temperature 2.30 mΩ/m (16/16 row) × 1.20 × 40 m ÷ 1000 = 0.1104 Ω	0.1104Ω	IET On-Site Guide, 18th Edition, Amendment 2:2022 Appendix I Table I1, printed p218; Appendix I Table I3 (incorporated in a cable, 70 °C thermoplastic), printed p220	0.1104Ω	±2 %	Pass
Earth-fault loop impedance, Zs 0.35 + 0.1104 = 0.4604 Ω	0.4604Ω	IET On-Site Guide, 18th Edition, Amendment 2:2022 Appendix I, Zs = Ze + (R1 + R2), printed p217	0.4604Ω	±2 %	Pass
Maximum Zs for the device	0.68Ω	BS 7671:2018+A2:2022 Table 41.3, printed p68 (Type D, 32 A, 5 s row)	0.68Ω	exact	Pass
Minimum protective conductor, Table 54.7	16mm²	BS 7671:2018+A2:2022 Table 54.7, printed p201 (S ≤ 16 → S)	16mm²	exact	Pass

EF-06

TN-C-S distribution circuit — 25/16 mm², 50 m, 63 A BS 88-2 fuse (5 s)

Inputs

TN-C-S supply, Ze = 0.35 Ω
25 mm² line with 16 mm² protective conductor, copper, 70 °C thermoplastic, 50 m
BS 88-2 gG fuse, 63 A, distribution circuit (5 s)

Checks for case EF-06
Quantity	Expected	Source	MEPCalc	Tolerance	Result
R1 + R2 at operating temperature 1.877 mΩ/m (25/16 row) × 1.20 × 50 m ÷ 1000 = 0.1126 Ω	0.1126Ω	IET On-Site Guide, 18th Edition, Amendment 2:2022 Appendix I Table I1, printed p218; Appendix I Table I3 (incorporated in a cable, 70 °C thermoplastic), printed p220	0.1126Ω	±2 %	Pass
Earth-fault loop impedance, Zs 0.35 + 0.1126 = 0.4626 Ω	0.4626Ω	IET On-Site Guide, 18th Edition, Amendment 2:2022 Appendix I, Zs = Ze + (R1 + R2), printed p217	0.4626Ω	±2 %	Pass
Maximum Zs for the device	0.78Ω	BS 7671:2018+A2:2022 Table 41.4, printed p69 (BS 88-2, 63 A)	0.78Ω	exact	Pass
Minimum protective conductor, Table 54.7	16mm²	BS 7671:2018+A2:2022 Table 54.7, printed p201 (16 < S ≤ 35 → 16)	16mm²	exact	Pass

EF-07

TN-C-S radial on a semi-enclosed fuse — 2.5/1.5 mm², 20 m, 30 A BS 3036

Inputs

TN-C-S supply, Ze = 0.35 Ω
2.5 mm² line with 1.5 mm² protective conductor, copper, 70 °C thermoplastic, 20 m
BS 3036 semi-enclosed fuse, 30 A, final circuit (0.4 s)

Checks for case EF-07
Quantity	Expected	Source	MEPCalc	Δ	Tolerance	Result
Earth-fault loop impedance, Zs 0.35 + 19.51 mΩ/m × 1.20 × 20 m ÷ 1000 = 0.8182 Ω	0.8182Ω	IET On-Site Guide, 18th Edition, Amendment 2:2022 Appendix I, Zs = Ze + (R1 + R2), printed p217; Appendix I Table I1, printed p218; Appendix I Table I3 (incorporated in a cable, 70 °C thermoplastic), printed p220	0.8182Ω	0.00 %	±2 %	Pass
Maximum Zs for the device	1.04Ω	BS 7671:2018+A2:2022 Table 41.2, printed p67 (c) (BS 3036, 30 A)	1.04Ω	0.00 %	exact	Pass

EF-08

TN-C-S aluminium distribution circuit — 35/35 mm², 60 m, 80 A BS 88-2 fuse (5 s)

Inputs

TN-C-S supply, Ze = 0.35 Ω
35 mm² line and 35 mm² protective conductor, aluminium, 70 °C thermoplastic, 60 m
BS 88-2 gG fuse, 80 A, distribution circuit (5 s)

Checks for case EF-08
Quantity	Expected	Source	MEPCalc	Tolerance	Result
R1 + R2 at operating temperature 1.74 mΩ/m (35/35 row, aluminium) × 1.20 × 60 m ÷ 1000 = 0.1253 Ω	0.1253Ω	IET On-Site Guide, 18th Edition, Amendment 2:2022 Appendix I Table I1, printed p218 (aluminium column); Appendix I Table I3 (incorporated in a cable, 70 °C thermoplastic), printed p220	0.1253Ω	±2 %	Pass
Earth-fault loop impedance, Zs 0.35 + 0.1253 = 0.4753 Ω	0.4753Ω	IET On-Site Guide, 18th Edition, Amendment 2:2022 Appendix I, Zs = Ze + (R1 + R2), printed p217	0.4753Ω	±2 %	Pass
Maximum Zs for the device	0.55Ω	BS 7671:2018+A2:2022 Table 41.4, printed p69 (BS 88-2, 80 A)	0.55Ω	exact	Pass
Minimum protective conductor, Table 54.7	16mm²	BS 7671:2018+A2:2022 Table 54.7, printed p201 (16 < S ≤ 35 → 16)	16mm²	exact	Pass

EF-09

Protective conductor for a 95 mm² line conductor — Table 54.7 with Regulation 543.1.4

Inputs

TN-C-S supply, Ze = 0.35 Ω
95 mm² line with 50 mm² protective conductor, copper, 30 m
Type B circuit-breaker, 100 A, distribution circuit (5 s)

No R1 + R2 check on this case: the On-Site Guide tabulates conductor resistance to 50 mm² only.

Checks for case EF-09
Quantity	Expected	Source	MEPCalc	Δ	Tolerance	Result
Minimum protective conductor, Table 54.7 95 ÷ 2 = 47.5 mm², not a standard size → next larger standard size 50 mm²	50mm²	BS 7671:2018+A2:2022 Table 54.7, printed p201 (S > 35 → S/2); Regulation 543.1.4, printed p201	50mm²	0.00 %	exact	Pass
Maximum Zs for the device	0.44Ω	BS 7671:2018+A2:2022 Table 41.3, printed p68 (Type B, 100 A)	0.44Ω	0.00 %	exact	Pass

EF-10

Measured Zs at ambient — the Appendix 3 0.8 rule for a 32 A Type B

Inputs

TN-C-S supply, Ze = 0.35 Ω; 2.5/1.5 mm² flat twin-and-earth, 20 m
Type B circuit-breaker, 32 A, final circuit (0.4 s)
Measured Zs at ambient temperature = 1.05 Ω

The On-Site Guide tabulates the same limit as 1.1 Ω (Appendix B Table B6, Type B 32 A, printed p145).

Checks for case EF-10
Quantity	Expected	Source	MEPCalc	Δ	Tolerance	Result
Maximum Zs for the device	1.37Ω	BS 7671:2018+A2:2022 Table 41.3, printed p68 (Type B, 32 A)	1.37Ω	0.00 %	exact	Pass
Maximum measured Zs at ambient (0.8 rule) 0.8 × 1.37 = 1.096 Ω	1.096Ω	BS 7671:2018+A2:2022 Appendix 3, Zs(m) ≤ 0.8 × U0 × Cmin / Ia, printed p410; Table 41.3, printed p68	1.096Ω	0.00 %	±2 %	Pass

EF-11

Prospective fault current at the origin — TN-C-S, Ze = 0.35 Ω, single-phase

Inputs

Distributor's supply, declared Ze = 0.35 Ω
Single-phase 230 V, no cable (figures at the origin)

Checks for case EF-11
Quantity	Expected	Source	MEPCalc	Δ	Tolerance	Result
Prospective fault current, line to neutral 230 V ÷ 0.35 Ω = 657.1 A	0.6571kA	IET On-Site Guide, 18th Edition, Amendment 2:2022 Appendix I, I = U0 / Zs, printed p217	0.657kA	0.01 %	±2 %	Pass

EF-12

Prospective fault current at the end of 30 m of 6 mm² two-core — TN-C-S

Inputs

Distributor's supply, declared Ze = 0.35 Ω, single-phase 230 V
6 mm² two-core copper cable, 30 m, neutral and protective conductor the same size as the line

Checks for case EF-12
Quantity	Expected	Source	MEPCalc	Δ	Tolerance	Result
R1 + R2 at 20 °C 6.16 mΩ/m × 30 m ÷ 1000 = 0.1848 Ω at 20 °C	0.1848Ω	IET On-Site Guide, 18th Edition, Amendment 2:2022 Appendix I Table I1, printed p218 (6/6 row)	0.1848Ω	0.00 %	±2 %	Pass
Line-to-neutral loop impedance 0.35 + 0.1848 = 0.5348 Ω (reactance neglected at 6 mm²)	0.5348Ω	IET On-Site Guide, 18th Edition, Amendment 2:2022 Appendix I, Zs = Ze + (R1 + R2), printed p217	0.5348Ω	0.00 %	±2 %	Pass
Prospective fault current, line to neutral 230 V ÷ 0.5348 Ω = 430.1 A	0.4301kA	IET On-Site Guide, 18th Edition, Amendment 2:2022 Appendix I, I = U0 / Zs, printed p217	0.43kA	0.02 %	±2 %	Pass
Zs at operating temperature 0.35 + 1.20 × 0.1848 = 0.5718 Ω	0.5718Ω	IET On-Site Guide, 18th Edition, Amendment 2:2022 Appendix I, Zs = Ze + (R1 + R2), printed p217; Appendix I Table I3 (incorporated in a cable, 70 °C thermoplastic), printed p220	0.5718Ω	0.00 %	±2 %	Pass

EF-13

Three-phase prospective fault current at the origin — TN-C-S, Ze = 0.35 Ω, 400 V

Inputs

Distributor's supply, declared Ze = 0.35 Ω
Three-phase 400/230 V, no cable (figures at the origin)

Checks for case EF-13
Quantity	Expected	Source	MEPCalc	Δ	Tolerance	Result
Prospective fault current, line to neutral 230 V ÷ 0.35 Ω = 657.1 A	0.6571kA	IET On-Site Guide, 18th Edition, Amendment 2:2022 Appendix I, I = U0 / Zs, printed p217	0.657kA	0.01 %	±2 %	Pass
Prospective fault current, three-phase 2 × 657.1 A = 1314.3 A	1.3143kA	BS 7671:2018+A2:2022 Appendix 14, printed p554 (fault between all line conductors ≈ line-to-neutral value × 2)	1.32kA	0.43 %	±2 %	Pass

EF-14

Single-phase fault current with cable reactance — 100 m of 50 mm² two-core

Inputs

Distributor's supply, declared Ze = 0.35 Ω, single-phase 230 V
50 mm² two-core copper cable, 100 m, neutral the same size as the line

Checks for case EF-14
Quantity	Expected	Source	MEPCalc	Δ	Tolerance	Result
Cable reactance, one-way 0.165 ÷ 2 = 0.0825 mΩ/m one-way × 100 m ÷ 1000 = 0.00825 Ω	0.00825Ω	BS 7671:2018+A2:2022 Table 4D2B, column 3 (x), printed p451 (50 mm², single-phase loop 0.165 mV/A/m)	0.0083Ω	0.61 %	±2 %	Pass
Cable loop impedance (line + return) √((0.774 × 100 ÷ 1000)² + (0.165 × 100 ÷ 1000)²) = √(0.0774² + 0.0165²) = 0.0791 Ω	0.0791Ω	IET On-Site Guide, 18th Edition, Amendment 2:2022 Appendix I Table I1, printed p218 (50/50 row); Table 4D2B, column 3 (x), printed p451	0.0791Ω	0.00 %	±2 %	Pass
Line-to-neutral loop impedance 0.35 + 0.0791 = 0.4291 Ω	0.4291Ω	IET On-Site Guide, 18th Edition, Amendment 2:2022 Appendix I, Zs = Ze + (R1 + R2), printed p217	0.4291Ω	0.00 %	±2 %	Pass
Prospective fault current, line to neutral 230 V ÷ 0.4291 Ω = 536.0 A	0.536kA	IET On-Site Guide, 18th Edition, Amendment 2:2022 Appendix I, I = U0 / Zs, printed p217	0.536kA	0.00 %	±2 %	Pass

EF-15

Protective device check — 20 A Type C on a 27 A cable, Table 41.3 limit

Inputs

Design current Ib = 18 A, device In = 20 A Type C, cable Iz = 27 A
Prospective fault current at the device 3 kA

Checks for case EF-15
Quantity	Expected	Source	MEPCalc	Δ	Tolerance	Result
Maximum Zs for the device, Table 41.3	1.09Ω	BS 7671:2018+A2:2022 Table 41.3, printed p68 (Type C, 20 A)	1.09Ω	0.00 %	exact	Pass

EF-16

Adiabatic protective conductor — aluminium, 70 °C thermoplastic, 6 kA for 0.4 s

Inputs

Fault current 6 kA, disconnection time 0.4 s
Aluminium protective conductor incorporated in a 70 °C thermoplastic cable, installed 50 mm²

Checks for case EF-16
Quantity	Expected	Source	MEPCalc	Δ	Tolerance	Result
Adiabatic constant, k	76	BS 7671:2018+A2:2022 Table 54.3, aluminium, 70 °C thermoplastic, printed p200	76	0.00 %	exact	Pass
Minimum protective conductor size, S S = √(6000² × 0.4) ÷ 76 = 3794.7 ÷ 76 = 49.93 mm²	49.93mm²	BS 7671:2018+A2:2022 Regulation 543.1.3, printed p199	49.93mm²	0.00 %	±2 %	Pass