Max Zs calculator: maximum Zs for MCBs, RCBOs and fuses
Maximum Zs is the highest earth fault loop impedance a circuit may have and still have its protective device disconnect an earth fault within the time BS 7671 allows. For a circuit-breaker it is 230 V × 0.95 divided by the current that trips it instantly, which makes a B32 MCB 1.37 Ω, judged against 1.09 Ω when the reading is taken cold.
Pick the device and its rating and read the limit, worked out from Regulation 411.4.4 rather than thumbed from a table. Type in what you measured and it says pass or fail.
Regulation 411.4.4 of BS 7671:2018+A4:2026, with Cmin, and Appendix 14 for the 80% figure. Checked against BS 7671:2018+A4:2026, updated 3 September 2026. Free, and nothing to sign in to.
What the instrument read at the far end of the circuit, cold. Leave it blank for the limit alone.
Maximum Zs for a B32 MCB
1.37 Ω
For a conductor at its working temperature. Compare a reading taken cold with 1.09 Ω, the 80% figure.
- 80% figure for a site reading The limit × 0.8, for a conductor read cold.
- 1.09 Ω
- Current that trips it instantly 5 × 32 A
- 160 A
How is the maximum Zs worked out?
For a circuit-breaker to BS EN 60898 or an RCBO to BS EN 61009 the limit is a calculation, not a lookup. Regulation 411.4.4 requires that Zs × Ia ≤ U₀ × Cmin: the loop impedance times the current that operates the device within the required time must not exceed the nominal voltage to earth, 230 V, reduced by Cmin, 0.95, for a supply sitting at the bottom of its permitted range.
Ia for these devices is their instantaneous magnetic trip, a fixed multiple of the rating: five times for a type B, ten for a type C, twenty for a type D. So a B32 trips at 160 A and its limit is 230 × 0.95 ÷ 160 = 1.37 Ω, the same figure the published table prints, because the table is this calculation done for every device. It does not change between 0.4 s and 5 s: the trip current is the same whichever time the circuit is allowed.
A fuse has no fixed multiple. Its operating current at 0.4 s and at 5 s is read off its time–current curve, so fuses carry a tabulated figure for each time, the 5 s one larger because a fuse allowed longer to clear tolerates more impedance. The fuse figures here are the same ones the Pascal certificate uses to fill column 13 of the schedule.
The 80% figure
The limit is for a conductor at its working temperature, and a Zs measured on site is read off one at room temperature, whose resistance is lower. Appendix 14 allows the comparison to be made against 80% of the limit instead of correcting the reading, and that is the figure an inspector compares with at the board: a B32 is judged against 1.09 Ω, not 1.37 Ω.
Worked example
A B32 protecting a 2.5/1.5 mm² radial to a cooker, measured at the cooker outlet at 0.92 Ω.
- Ia = 5 × 32 A = 160 A.
- Maximum Zs = 230 V × 0.95 ÷ 160 A = 1.37 Ω.
- Site figure = 1.37 Ω × 0.8 = 1.09 Ω.
- 0.92 Ω is inside 1.09 Ω by about 16%, so the circuit passes with the reading uncorrected.
Had the reading been 1.15 Ω, over 1.09 Ω but under 1.37 Ω, the honest next step is to correct it for temperature and compare the corrected figure with the full 1.37 Ω before recording either result.
Questions the trade asks
1.37 Ω, for a conductor at its working temperature: 230 V × 0.95 ÷ (5 × 32 A). A reading taken cold on site is compared against 80% of that, 1.09 Ω.
A device to BS EN 60898 or BS EN 61009 clears an earth fault on its instantaneous magnetic trip, at a fixed multiple of its rating: five times for a type B, ten for a type C, twenty for a type D. That current is reached whether the circuit is allowed 0.4 s or 5 s, so the limit does not move. A fuse is different: its time–current curve gives a lower Zs for 0.4 s than for 5 s, which is why fuses carry a figure for each.
Cmin is 0.95, the factor Regulation 411.4.4 applies to the nominal 230 V to allow for the supply sitting at the bottom of its permitted range when the fault happens. It is why a B32 that once read 1.44 Ω in the tables now reads 1.37 Ω.
On a TN supply, 0.4 s for a final circuit up to 32 A supplying fixed equipment or up to 63 A with socket outlets (Table 41.1), and 5 s for a distribution circuit or a final circuit above those ratings (Regulation 411.3.2.3). A TT installation is normally disconnected by an RCD, whose own limit applies instead.
Not necessarily. The 80% figure is a shortcut for a reading taken on a cold conductor against a limit set for a hot one. Appendix 14 lets you correct the reading for temperature properly instead and compare that with the full limit. If the corrected figure is still over, the circuit fails.
Where the RCD is what provides the fault protection (every TT installation, and a TN circuit designed that way under Regulation 411.4.5) the limit is the RCD's: Zs × IΔn must not exceed 50 V, which is 1,667 Ω for a 30 mA device (Table 41.5). An RCD fitted as additional protection on a TN circuit does not relax the circuit-breaker's own figure.
Next door
- R1+R2 calculator: R1+R2 values and expected ZsR1+R2 for any copper cable and length, at 20 °C or 70 °C, and the earth fault loop impedance it gives with your Ze. Free, no sign-up.
- Prospective fault current calculator: PEFC, PSCC and IpfProspective earth fault and short-circuit current from Ze or the line–neutral loop, in kA, checked against the breaking capacity of your devices. Free, no sign-up.
- Ring final circuit test calculator: r1, rn, r2 and (r1+r2)/4Check r1, rn and r2 on a ring final circuit, get the (r1+r2)/4 figure every socket should read, and the R1+R2 to record. Free, no sign-up.
The figures go straight onto the certificate
In Pascal the maximum Zs is filled in for every circuit as you pick the device, the schedule of test results carries all 32 columns, and the observation library suggests the wording, code and regulation as you type. EICs, EICRs and minor works, from £25 a month.
See the certificate softwareA calculator applies a method to the numbers you give it. It does not know the installation in front of you, and it is no substitute for BS 7671, the guidance or the judgement of the person signing the certificate. Check anything you rely on.