What is the max Zs for a 32A Type B MCB?

The maximum Zs for a 32A Type B MCB to BS EN 60898 is 1.37Ω at 0.4s disconnection per BS 7671:2018 Table 41.3, Cmin corrected.

Cmin is a minimum voltage factor of 0.95 applied to the nominal voltage (230V × 0.95 = 218.5V) when calculating maximum Zs values per BS 7671:2018. For example, a 32A Type B MCB: the uncorrected value is Uo/(5×In) = 230/160 = 1.44Ω, with Cmin applied it becomes 218.5/160 = 1.37Ω.

What is the difference between Ze and Zs?

Ze is the external earth fault loop impedance measured at the origin of the installation. Zs is the total earth fault loop impedance at any point in a circuit: Zs = Ze + (R1 + R2), where R1 is the resistance of the line conductor and R2 is the resistance of the circuit protective conductor from the distribution board to the furthest point of the circuit.

What are the max Zs values for BS EN 60898 MCBs?

Max Zs values for BS EN 60898 MCBs at 0.4s disconnection (Cmin corrected): Type B 32A = 1.37Ω, Type C 32A = 0.68Ω, Type D 32A = 0.34Ω. Full tables for all ratings from 3A to 125A are available at maxzs.co.uk.

What are the max Zs values for BS EN 60947-2 MCCBs?

Generic max Zs values for BS EN 60947-2 MCCBs follow the same Type B/C/D curves as BS EN 60898, extended to 250A. However, MCCBs with adjustable trip units have manufacturer-specific values. Schneider NSX, Eaton NZM, and Dorman Smith Loadline tables are available at maxzs.co.uk.

What are the max Zs values for the 18th Edition?

The 18th Edition (BS 7671:2018) max Zs values include Cmin voltage correction (×0.95). For example, a 32A Type B MCB: the uncorrected value is Uo/(5×In) = 230/160 = 1.44Ω, with Cmin applied it becomes 218.5/160 = 1.37Ω. The 1.37Ω value is what appears in BS 7671:2018 Table 41.3.

Where can I find Schneider MCCB max Zs values?

Schneider Electric MCCB max Zs values for NSX TMD, NSX Micrologic, NSC100, and Minipact ranges are published at maxzs.co.uk, sourced from Schneider Electric technical documentation.

Max Zs Values — BS 7671:2018 Earth Fault Loop Impedance Tables

Amendment 4 publishes 15 April 2026 — see what's changing →

Maximum earth fault loop impedance (Z_s)

Maximum permitted Z_s values from BS 7671:2018 Tables 41.2–41.4 with C_min voltage correction (×0.95). All values in ohms (Ω).

▶ What is Z_s & how to use these tables

What is Z_s?

Earth fault loop impedance (Z_s) is the total impedance of the fault current path from the point of fault, through the earth, back to the source transformer. It determines whether a protective device will disconnect fast enough to prevent electric shock. Z_s = Z_e + (R₁ + R₂), where Z_e is the external earth fault loop impedance and R₁ + R₂ is the resistance of the line and circuit protective conductors.

The measured Z_s at any point in a circuit must not exceed the maximum values in BS 7671 Tables 41.2–41.4 for the protective device type and rating. If Z_s is too high, the fault current will be too low to trip the device within the required disconnection time (0.4s for final circuits, 5s for distribution circuits in a TN system).

How to use these tables

Find your protective device type (MCB Type B/C/D, fuse, or MCCB), then look up the rating in amps. The value shown is the maximum permitted Z_s in ohms. Your measured reading must not exceed this value. For on-site testing at ambient temperature, readings at or below 80% of the tabulated value are considered compliant without further calculation (BS 7671 Appendix 14).

Table 1 — Maximum Z_s for MCBs / RCBOs (BS EN 60898)

Circuit-breakers to BS EN 60898 or RCBOs to BS EN 61009
A	Type B	Type C	Type D
0.4s disconnection (final circuits)
3	14.57	N/A	N/A
6	7.28	3.64	1.82
10	4.37	2.19	1.09
16	2.73	1.37	0.68
20	2.19	1.09	0.55
25	1.75	0.87	0.44
32	1.37	0.68	0.34
40	1.09	0.55	0.27
50	0.87	0.44	0.22
63	0.69	0.35	0.17
80	0.55	0.27	0.14
100	0.44	0.22	0.11
125	0.35	0.17	0.09

Table 2 — Maximum Z_s for fuses

Fuses
	BS 88-2 (gG)		BS 1361 / BS 88-3		BS 3036
A	0.4s	5s	0.4s	5s	0.4s	5s
2	33.10	44.00	N/A	N/A	N/A	N/A
4	15.60	21.00	N/A	N/A	N/A	N/A
5	N/A	N/A	9.93	14.60	9.10	16.80
6	7.80	12.00	N/A	N/A	N/A	N/A
10	4.65	6.80	N/A	N/A	N/A	N/A
15	N/A	N/A	2.30	3.90	2.43	5.08
16	2.43	4.00	2.30	3.90	N/A	N/A
20	1.68	2.80	1.93	3.20	1.68	3.64
25	1.29	2.20	N/A	N/A	N/A	N/A
30	N/A	N/A	0.91	1.60	1.04	2.51
32	0.99	1.70	0.91	1.60	N/A	N/A
40	0.75	1.30	N/A	N/A	N/A	N/A
45	N/A	N/A	0.57	1.00	0.56	1.51
50	0.57	0.99	N/A	N/A	N/A	N/A
60	N/A	N/A	N/A	N/A	0.40	1.07
63	0.44	0.78	0.36	0.68	N/A	N/A
80	N/A	0.55	N/A	0.51	N/A	N/A
100	N/A	0.42	N/A	0.38	N/A	0.51
125	N/A	0.32	N/A	N/A	N/A	N/A
160	N/A	0.27	N/A	N/A	N/A	N/A
200	N/A	0.18	N/A	N/A	N/A	N/A

BS 1362 plug fuses: 3A = 15.60 (0.4s) / 22.00 (5s) · 13A = 2.30 (0.4s) / 3.64 (5s)

Table 3 — 5s disconnection (distribution circuits)

A	Type B	Type C	Type D
6	7.28	3.64	0.36
10	4.37	2.19	2.19
16	2.73	1.37	1.37
20	2.19	1.09	1.09
25	1.75	0.87	0.87
32	1.37	0.68	0.68
40	1.09	0.55	0.55
50	0.87	0.44	0.44
63	0.69	0.35	0.35
80	0.55	0.27	0.27
100	0.44	0.22	0.22
125	0.35	0.17	0.17

Type B & C: same values at 0.4s and 5s. Type D differs at 5s. 5s fuse values in Table 2.

BS EN 60947-2 — MCCB Max Z_s by Manufacturer

MCCBs have manufacturer-specific Z_s values depending on trip unit type and settings. Select a manufacturer below.

Schneider MCCB Max Z_s

NSX100/160/250 TMD

Thermal-magnetic distribution · Fixed Ir, fixed Im

A	0.4s	5s
16	0.96	1.20
25	0.61	0.92
32	0.46	0.73
40	0.36	0.58
50	0.36	0.55
63	0.36	0.44
80	0.29	0.29
100	0.23	0.23
125	0.14	0.14
160	0.14	0.14

NSX Micrologic 2.2

Electronic adjustable · Isd 1.5–10 × In

A	Z_s range (Ω)
40	0.49 – 8.17
100	0.20 – 3.68
160	0.12 – 2.34
250	0.08 – 1.47

NSX400/630 Micrologic 2.3

Electronic adjustable

A	Z_s range (Ω)
250	0.08 – 2.10
400	0.05 – 0.92
630	0.03 – 0.59

NSC100

Compact thermal-magnetic

A	0.4s	5s
16	0.30	1.84
25	0.30	1.18
32	0.30	0.92
40	0.18	0.74
50	0.18	0.78
63	0.18	0.62
80	0.18	0.48
100	0.14	0.31

Minipact

Compact MCCB · 16–60A

A	0.4s	5s
16	0.30	1.84
25	0.30	1.18
32	0.30	0.92
40	0.18	0.74
60	0.18	0.50

Sources: Schneider Electric UK FAQ FA132586, FA132588, FA281742 (se.com/uk); Schneider Electric NSX and NS MCCB Zs Values reference table. Values at 230V, C_min (×0.95) applied.

Eaton NZM MCCB Max Z_s

NZM1 (16–160A)

Thermal-magnetic adjustable · Ir adjustable 0.8–1 × In

A	0.4s range	5s range
20	0.68 – 1.09	0.74 – 1.19
32	0.68 – 1.14	0.74 – 0.94
40	0.55 – 0.91	0.59 – 0.75
50	0.44 – 0.73	0.48 – 0.60
63	0.35 – 0.58	0.38 – 0.48
80	0.27 – 0.46	0.30 – 0.37
100	0.22 – 0.36	0.24 – 0.30
125	0.18 – 0.29	0.19 – 0.24
160	0.14 – 0.23	0.15 – 0.19

NZM2 (125–250A)

Thermal-magnetic adjustable

A	0.4s range	5s range
125	0.18 – 0.29	0.19 – 0.24
160	0.14 – 0.23	0.15 – 0.19
200	0.11 – 0.18	0.12 – 0.15
250	0.09 – 0.15	0.10 – 0.12

NZM2 Electronic (100–250A)

Electronic adjustable trip

A	Z_s range (Ω)
100	0.20 – 1.90
160	0.12 – 1.19
250	0.08 – 0.76

NZM3 (250–400A)

Thermal-magnetic adjustable

A	0.4s range	5s range
250	0.09 – 0.15	0.10 – 0.12
320	0.07 – 0.11	0.07 – 0.09
400	0.06 – 0.09	0.06 – 0.08

PDE Electronic (800A)

Electronic trip unit

A	0.4s	5s
800	0.031	0.034

Sources: Eaton Power Distribution Components Catalogue CA014013EN (2024), NZM MCCB section 9.2; Eaton UK Technical Support. Values at 230V, C_min (×0.95) applied.

Dorman Smith MCCB Max Z_s

YB1 1P Fixed (125A frame)

Fixed thermal/magnetic trip

A	0.4s	5s
16	0.30	1.14
20	0.30	0.91
25	0.30	0.73
32	0.30	0.57
40	0.30	0.46
50	0.30	0.36
63	0.18	0.29
80	0.18	0.23
100	0.12	0.18
125	0.12	0.15

YA3 1P Fixed (250A frame)

Fixed thermal/magnetic trip · 16–160A

A	0.4s	5s
16	1.14	1.22
20	0.91	1.00
25	0.73	0.78
32	0.57	0.59
40	0.46	0.48
50	0.36	0.39
63	0.29	0.31
80	0.23	0.25
100	0.18	0.20
125	0.14	0.16
160	0.14	0.12

YA3 3P/4P Adjustable (250A frame)

Adjustable thermal/magnetic · 20–250A

A	0.4s range	5s range
20	0.76 – 1.52	1.11 – 1.81
32	0.48 – 0.95	0.63 – 0.95
50	0.30 – 0.61	0.30 – 0.62
63	0.24 – 0.48	0.24 – 0.48
100	0.15 – 0.30	0.15 – 0.30
125	0.12 – 0.24	0.12 – 0.24
160	0.09 – 0.19	0.11 – 0.19
200	0.07 – 0.15	0.07 – 0.15
250	0.07 – 0.12	0.07 – 0.12

YA5 Thermal-Magnetic (400A frame)

Adjustable · 250–400A

A	Z_s range (Ω)
250	0.061 – 0.121
400	0.038 – 0.076

YA6 Thermal-Magnetic (630A frame)

Adjustable · 400–800A

A	Z_s range (Ω)
400	0.029 – 0.058
630	0.029 – 0.058
800	0.023 – 0.046

YA7 & YA8 Electronic

Electronic trip · Large frame

A	Z_s range (Ω)
1250	0.015 – 0.061
1600	0.012 – 0.048

Sources: Dorman Smith Loadline datasheets V3 (2024) — YB1, YA2, YA3, YB3, YA5, YA6, YA7, YA8 (dormansmithswitchgear.com). Values at 230V, C_min (×0.95) applied.

ABB Tmax MCCB Max Z_s

ABB Tmax T1–T6 MCCB Z_s data — coming soon If you have ABB Tmax technical catalogues with earth fault loop impedance data, please get in touch via pug.app

Table 41.5 — RCDs (BS EN 61008 / BS EN 61009)

Maximum Z_s for RCD-protected circuits at 230V nominal. Where an RCD provides fault protection, the Z_s must ensure the residual current operates the RCD.

IΔn	Max Z_s (Ω)
30 mA	1667
100 mA	500
300 mA	167
500 mA	100

BS 3871 MCBs (Withdrawn 1994)

Values from BS 7671 Table B6 for 0.1s to 5s disconnection. Required for EICR inspections on older installations. Type 1 ≈ Type B, Type 3 ≈ Type C.

Circuit-breakers to BS 3871
A	Type 1	Type 2	Type 3
0.1s to 5s disconnection
5	8.74	5.00	3.49
6	7.28	4.20	2.91
10	4.40	2.50	1.75
15	2.93	1.67	1.16
16	2.76	1.58	1.09
20	2.20	1.25	0.87
25	1.76	1.00	0.70
30	1.47	0.83	0.58
32	1.38	0.79	0.55
40	1.10	0.63	0.44
45	0.98	0.56	0.38
50	0.88	0.50	0.35
63	0.70	0.40	0.27
100	0.44	0.25	0.17

C_min: All values include C_min = 0.95 (U_o × 0.95 = 218.5V).

0.4s: Final circuits, TN system (Reg 411.3.2.2). 5s: Distribution circuits (Reg 411.3.2.3).

80% rule: If measured Z_s at ambient ≤ 80% of tabulated max, circuit is compliant without further calculation (Appendix 14).

N/A: Device not available in that rating. Some manufacturers' devices operate at higher Z_s than shown.

Amendment 4: Max Z_s tables (41.2–41.4) are unchanged in Amendment 4. See what is changing at bs7671amendment4.co.uk

These tables are built into PUG — certification software that validates Z_s automatically against every circuit.

Try PUG free →

Max Zs Values

Maximum earth fault loop impedance (Zs)

What is Zs?