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Zs = 230 × 0.95 / (multiplier × In). MCBs and RCBOs to BS EN 60898/61009 only — use BS 7671 tables for fuses. Guidance only — always verify against BS 7671 and the On-Site Guide for the actual installation method.
Every value below is calculated at Cmin × U0 = 0.95 × 230V = 218.5V, consistent with BS 7671:2018+A2. The design column is the tabulated maximum; the measured column applies the 80% rule for comparing a cold test reading. These tables apply to BS EN 60898 MCBs and BS EN 61009 RCBOs only — take fuse values from the BS 7671 tables directly.
| Rating (In) | Trip current (Ia) | Max Zs — design (Ω) | Max Zs — measured, 80% (Ω) |
|---|---|---|---|
| 6A | 30A | 7.28 | 5.83 |
| 10A | 50A | 4.37 | 3.50 |
| 16A | 80A | 2.73 | 2.19 |
| 20A | 100A | 2.19 | 1.75 |
| 25A | 125A | 1.75 | 1.40 |
| 32A | 160A | 1.37 | 1.09 |
| 40A | 200A | 1.09 | 0.87 |
| 45A | 225A | 0.97 | 0.78 |
| 50A | 250A | 0.87 | 0.70 |
| 63A | 315A | 0.69 | 0.55 |
| Rating (In) | Trip current (Ia) | Max Zs — design (Ω) | Max Zs — measured, 80% (Ω) |
|---|---|---|---|
| 6A | 60A | 3.64 | 2.91 |
| 10A | 100A | 2.19 | 1.75 |
| 16A | 160A | 1.37 | 1.09 |
| 20A | 200A | 1.09 | 0.87 |
| 25A | 250A | 0.87 | 0.70 |
| 32A | 320A | 0.68 | 0.55 |
| 40A | 400A | 0.55 | 0.44 |
| 45A | 450A | 0.49 | 0.39 |
| 50A | 500A | 0.44 | 0.35 |
| 63A | 630A | 0.35 | 0.28 |
| Rating (In) | Trip current (Ia) | Max Zs — design (Ω) | Max Zs — measured, 80% (Ω) |
|---|---|---|---|
| 6A | 120A | 1.82 | 1.46 |
| 10A | 200A | 1.09 | 0.87 |
| 16A | 320A | 0.68 | 0.55 |
| 20A | 400A | 0.55 | 0.44 |
| 25A | 500A | 0.44 | 0.35 |
| 32A | 640A | 0.34 | 0.27 |
| 40A | 800A | 0.27 | 0.22 |
| 45A | 900A | 0.24 | 0.19 |
| 50A | 1000A | 0.22 | 0.17 |
| 63A | 1260A | 0.17 | 0.14 |
Because an MCB relies on its instantaneous magnetic trip to meet earth-fault disconnection times, the same maximum Zs applies whether the required disconnection time is 0.4s or 5s. Where a 30mA RCD provides the fault protection, BS 7671 permits a maximum Zs of 1667Ω — but good practice is still to meet the overcurrent device's value where practicable.
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This calculator gives the maximum permitted earth fault loop impedance (Zs) for circuit breakers and RCBOs to BS EN 60898 and BS EN 61009. It is a quick reference for UK electricians testing and certifying circuits: pick the device curve and rating and read off the limit your measured Zs must not exceed.
Rather than looking up a table, the tool calculates the value directly from the device trip characteristics. Type B devices must trip instantaneously at 5 times their rating, Type C at 10 times and Type D at 20 times. The maximum Zs is the loop impedance that still lets that trip current flow at the minimum supply voltage, using the Cmin factor of 0.95 applied to 230V.
The calculator can also apply the 80% rule used to compare hot design limits with measurements taken on cold conductors, giving you the figure to check test results against directly. The full 18th Edition tables for every common rating are printed below the tool.
Max Zs = (230 × 0.95) / (multiplier × In)
230 is the nominal supply voltage and 0.95 is the Cmin factor, which allows for the supply being below nominal. The multiplier is the instantaneous trip factor for the device curve: 5 for Type B, 10 for Type C and 20 for Type D. In is the device rating in amps. The product of multiplier and In is the current that guarantees instantaneous disconnection, and dividing the minimum voltage by that current gives the highest loop impedance at which the device still trips in time. When the 80% measured-value option is on, the result is multiplied by 0.8 to account for conductors being tested cold rather than at operating temperature.
Select Type B, C or D. This sets the instantaneous trip multiplier: 5×In for B, 10×In for C, 20×In for D.
Pick the rating In from 6A to 63A. The tool shows the resulting trip current (multiplier × In) so you can see what the device needs to disconnect instantaneously.
Leave it on to get the value for comparing against a measured Zs taken with conductors cold. Turn it off if you want the full design limit for calculated values.
Your measured Zs at the furthest point of the circuit must not exceed the 80% figure. If it does, investigate the loop path before energising — check terminations, conductor sizes and Ze.
The values match the approach in BS 7671:2018+A2 for devices to BS EN 60898 and BS EN 61009, using Cmin of 0.95. They ensure the device disconnects within the required time on an earth fault by forcing enough fault current to reach the instantaneous magnetic trip region of the breaker.
This calculator is for MCBs and RCBOs only. Fuses to BS 88, BS 3036 and BS 1361 have entirely different time-current characteristics, and their maximum Zs values must be taken from the relevant BS 7671 tables — do not use this tool for fused circuits.
The 80% rule is the common site practice for comparing a measured (ambient temperature) Zs with the tabulated (operating temperature) limit. Some engineers instead correct the measured value upwards; either way, a measured Zs close to the design limit deserves scrutiny rather than a bare pass.
Guidance only — always verify against BS 7671 and the On-Site Guide, and remember that RCD-protected circuits have separate disconnection considerations that this lookup does not cover.
Zs = 230 × 0.95 / (multiplier × In). MCBs and RCBOs to BS EN 60898/61009 only — use BS 7671 tables for fuses. Guidance only — always verify against BS 7671 and the On-Site Guide for the actual installation method.
Using Zs = (230 × 0.95) / (5 × 32), the design maximum is about 1.37Ω. Applying the 80% rule for measured values gives roughly 1.09Ω, which is the figure to compare a cold test result against. Always confirm against the current edition of BS 7671.
Tabulated maximum Zs values assume conductors at operating temperature, but you test them cold. Conductor resistance rises with temperature, so a measured value is reduced-limit checked: if the cold measurement is within 80% of the design maximum, the hot value should still comply.
Cmin is a factor from BS 7671 that allows for the supply voltage being below the 230V nominal. Multiplying 230V by 0.95 means the disconnection check still works when the supply is at the lower end of its permitted range.
No. The multiplier method only applies to MCBs and RCBOs to BS EN 60898 and BS EN 61009, which have a defined instantaneous trip band. Maximum Zs values for BS 88, BS 3036 and BS 1361 fuses come from their time-current curves and must be taken from the BS 7671 tables.
First re-test and check the meter and connections. Then look for the cause: a high Ze from the supply, undersized or overly long conductors, or poor terminations along the circuit. Increasing the CPC size, shortening the run or changing the protective device type or rating are the usual design remedies.
The full guide behind these tables — the Cmin 0.95 formula, the 80% rule and how to measure Zs on site.
The three-step continuity test that produces the R1+R2 behind your measured Zs.
What Ze, Zs and Zdb mean, how the fault loop works, and how to verify it against BS 7671 limits.
Calculated Zs for a circuit from external loop and conductor resistances.
Expected conductor resistance for a run from OSG Table I1 values.
Fault current from a measured loop impedance.
Volt drop for twin & earth over a run, checked against BS 7671 limits.
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