Protection & Fault Analysis Calculators

Verify earth fault loop impedance, prospective fault current, protective conductor sizing, device selection, and discrimination to BS 7671.

Ze — External earth fault loop impedance (Ω)

Cable length (m)

R1 — Line conductor cable size

R2 — CPC cable size

Apply temperature correction factor (1.20 for PVC at 70°C)

Protective device type (optional)

Device rating (A)

Disconnection time

Maximum Zs, BS 7671 Table 41.3 (Ω, U₀ 230 V, Cmin 0.95)

Rating	Type B (0.4 s and 5 s)	Type C (0.4 s and 5 s)	Type D 0.4 s	Type D 5 s
3A	14.57	—	—	—
6A	7.28	3.64	1.82	3.64
10A	4.37	2.19	1.09	2.19
16A	2.73	1.37	0.68	1.37
20A	2.19	1.09	0.55	1.09
25A	1.75	0.87	0.44	0.87
32A	1.37	0.68	0.34	0.68
40A	1.09	0.55	0.27	0.55
50A	0.87	0.44	0.22	0.44
63A	0.69	0.35	0.17	0.35
80A	0.55	0.27	0.14	0.27
100A	0.44	0.22	0.11	0.22
125A	0.35	0.17	0.09	0.17

Limits with conductors at their maximum operating temperature. Fuse limits are in Tables 41.2 (0.4 s) and 41.4 (5 s) — see the Zs Values quick reference.

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Practice 690+ exam questions covering protection, fault analysis, and all BS 7671 topics.

Protection & Fault Analysis Calculators

Earth Fault Loop Impedance (Zs) Calculator

Maximum Zs, BS 7671 Table 41.3 (Ω, U₀ 230 V, Cmin 0.95)

Prospective Fault Current (PFC) Calculator

Adiabatic Equation Calculator

MCB / RCBO Selector

Discrimination / Selectivity Calculator

Upstream (Main) Device

Downstream (Circuit) Device

Study BS 7671 on the Go

Protection & Fault Analysis Calculators

Earth Fault Loop Impedance (Zs) Calculator

Maximum Zs, BS 7671 Table 41.3 (Ω, U0 230 V, Cmin 0.95)

Prospective Fault Current (PFC) Calculator

Adiabatic Equation Calculator

MCB / RCBO Selector

Discrimination / Selectivity Calculator

Upstream (Main) Device

Downstream (Circuit) Device

Study BS 7671 on the Go

Maximum Zs, BS 7671 Table 41.3 (Ω, U₀ 230 V, Cmin 0.95)