Zs Values
Maximum earth fault loop impedance for circuit-breakers, RCBOs and fuses from BS 7671 Tables 41.2, 41.3 and 41.4 (Cmin = 0.95).
Type B Circuit-Breakers / RCBOs (Table 41.3)
Type B — Max Zs (Table 41.3(a))
| Rating (A) | Max Zs, 0.4 s and 5 s (Ω) |
|---|---|
| 3 | 14.57 |
| 6 | 7.28 |
| 10 | 4.37 |
| 16 | 2.73 |
| 20 | 2.19 |
| 25 | 1.75 |
| 32 | 1.37 |
| 40 | 1.09 |
| 50 | 0.87 |
| 63 | 0.69 |
| 80 | 0.55 |
| 100 | 0.44 |
| 125 | 0.35 |
Circuit-breakers to BS EN 60898 and the overcurrent characteristics of RCBOs to BS EN 61009-1. One figure serves both the 0.4 s (Reg 411.3.2.2) and the 5 s (Reg 411.3.2.3) disconnection time: Zs = 230 × 0.95 / 5Iₙ.
Values from BS 7671:2018+A4:2026 for U₀ = 230 V, determined using Cmin = 0.95. They are the limits with conductors at their maximum operating temperature — adjust measured values for temperature (NOTE 2 to the table; Appendix 3).
Type C Circuit-Breakers / RCBOs (Table 41.3)
Type C — Max Zs (Table 41.3(b))
| Rating (A) | Max Zs, 0.4 s and 5 s (Ω) |
|---|---|
| 6 | 3.64 |
| 10 | 2.19 |
| 16 | 1.37 |
| 20 | 1.09 |
| 25 | 0.87 |
| 32 | 0.68 |
| 40 | 0.55 |
| 50 | 0.44 |
| 63 | 0.35 |
| 80 | 0.27 |
| 100 | 0.22 |
| 125 | 0.17 |
One figure serves both the 0.4 s and the 5 s disconnection time: Zs = 230 × 0.95 / 10Iₙ.
Values from BS 7671:2018+A4:2026 for U₀ = 230 V, determined using Cmin = 0.95. They are the limits with conductors at their maximum operating temperature — adjust measured values for temperature (NOTE 2 to the table; Appendix 3).
Type D Circuit-Breakers / RCBOs (Table 41.3)
Type D — Max Zs (Table 41.3(c))
| Rating (A) | Max Zs 0.4 s (Ω) | Max Zs 5 s (Ω) |
|---|---|---|
| 6 | 1.82 | 3.64 |
| 10 | 1.09 | 2.19 |
| 16 | 0.68 | 1.37 |
| 20 | 0.55 | 1.09 |
| 25 | 0.44 | 0.87 |
| 32 | 0.34 | 0.68 |
| 40 | 0.27 | 0.55 |
| 50 | 0.22 | 0.44 |
| 63 | 0.17 | 0.35 |
| 80 | 0.14 | 0.27 |
| 100 | 0.11 | 0.22 |
| 125 | 0.09 | 0.17 |
0.4 s: Zs = 230 × 0.95 / 20Iₙ. 5 s: Zs = 230 × 0.95 / 10Iₙ.
Values from BS 7671:2018+A4:2026 for U₀ = 230 V, determined using Cmin = 0.95. They are the limits with conductors at their maximum operating temperature — adjust measured values for temperature (NOTE 2 to the table; Appendix 3).
BS 88-2 gG/gM Fuses (Tables 41.2 / 41.4)
BS 88-2 Fuse Systems E and G — Max Zs
| Rating (A) | Max Zs 0.4 s, Table 41.2 (Ω) | Max Zs 5 s, Table 41.4 (Ω) |
|---|---|---|
| 2 | 33.1 | 44 |
| 4 | 15.6 | 21 |
| 6 | 7.80 | 12 |
| 10 | 4.65 | 6.8 |
| 16 | 2.43 | 4.0 |
| 20 | 1.68 | 2.8 |
| 25 | 1.29 | 2.2 |
| 32 | 0.99 | 1.7 |
| 40 | 0.75 | 1.3 |
| 50 | 0.57 | 0.99 |
| 63 | 0.44 | 0.78 |
| 80 | — | 0.55 |
| 100 | — | 0.42 |
| 125 | — | 0.32 |
| 160 | — | 0.27 |
| 200 | — | 0.18 |
General purpose (gG) and motor circuit application (gM) fuses to BS 88-2, fuse systems E (bolted) and G (clip-in).
— : not tabulated for that disconnection time.
Values from BS 7671:2018+A4:2026 for U₀ = 230 V, determined using Cmin = 0.95. They are the limits with conductors at their maximum operating temperature — adjust measured values for temperature (NOTE 2 to the table; Appendix 3).
BS 88-3 Fuse System C (Tables 41.2 / 41.4)
BS 88-3 Fuse System C — Max Zs
| Rating (A) | Max Zs 0.4 s, Table 41.2 (Ω) | Max Zs 5 s, Table 41.4 (Ω) |
|---|---|---|
| 5 | 9.93 | 14.6 |
| 16 | 2.30 | 3.9 |
| 20 | 1.93 | 3.2 |
| 32 | 0.91 | 1.6 |
| 45 | 0.57 | 1.0 |
| 63 | 0.36 | 0.68 |
| 80 | — | 0.51 |
| 100 | — | 0.38 |
— : not tabulated for that disconnection time.
Values from BS 7671:2018+A4:2026 for U₀ = 230 V, determined using Cmin = 0.95. They are the limits with conductors at their maximum operating temperature — adjust measured values for temperature (NOTE 2 to the table; Appendix 3).
BS 3036 Fuses (Tables 41.2 / 41.4)
BS 3036 Fuses — Max Zs
| Rating (A) | Max Zs 0.4 s, Table 41.2 (Ω) | Max Zs 5 s, Table 41.4 (Ω) |
|---|---|---|
| 5 | 9.10 | 16.8 |
| 15 | 2.43 | 5.08 |
| 20 | 1.68 | 3.64 |
| 30 | 1.04 | 2.51 |
| 45 | 0.56 | 1.51 |
| 60 | 0.40 | 1.07 |
| 100 | — | 0.51 |
— : not tabulated for that disconnection time.
Values from BS 7671:2018+A4:2026 for U₀ = 230 V, determined using Cmin = 0.95. They are the limits with conductors at their maximum operating temperature — adjust measured values for temperature (NOTE 2 to the table; Appendix 3).
BS 1362 Fuses (Tables 41.2 / 41.4)
BS 1362 Fuses — Max Zs
| Rating (A) | Max Zs 0.4 s, Table 41.2 (Ω) | Max Zs 5 s, Table 41.4 (Ω) |
|---|---|---|
| 3 | 15.6 | 22.0 |
| 13 | 2.30 | 3.64 |
Values from BS 7671:2018+A4:2026 for U₀ = 230 V, determined using Cmin = 0.95. They are the limits with conductors at their maximum operating temperature — adjust measured values for temperature (NOTE 2 to the table; Appendix 3).
Get the Full Reference in the App
Access the complete Quick Reference with search, bookmarks, and 690+ exam questions — all offline on your phone.
