A Zs value that exceeds the BS 7671 maximum means the protective device may not disconnect quickly enough during an earth fault — creating a serious electric shock risk. This guide explains what 'too high' means, every common cause, and the practical solutions available.
Free for 7 days · No charge until day 8 · Cancel anytime · Used by 1,000+ UK electricians
Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.
1,000+
UK electricians
“Replaced three separate apps with Elec-Mate. Certs, quotes, and scheduling all in one place.”
Daniel Palmer — DP Electrical
Earth fault loop impedance (Zs) is the total impedance of the path that fault current takes when a live conductor contacts earth — from the point of fault, through the protective conductor back to the distribution board, through the supply earth, and back to the transformer. BS 7671 sets maximum values for Zs because this impedance determines how much fault current flows during an earth fault, which in turn determines how quickly the protective device (MCB, fuse, or RCBO) disconnects the circuit.
If Zs is too high, the fault current during an earth fault is too low to trip the protective device within the required disconnection time. BS 7671 requires disconnection within 0.4 seconds for circuits supplying socket outlets and portable equipment, and within 5 seconds for distribution circuits. If the protective device does not disconnect quickly enough, the metalwork of the faulty equipment remains energised for a dangerous period, creating a severe electric shock risk.
BS 7671:2018+A4:2026 Regulation 411.3.4 requires that, within domestic (household) premises, AC final circuits supplying luminaires shall be provided with additional protection by an RCD with a rated residual operating current not exceeding 30 mA. For EICR work, a domestic lighting circuit without 30 mA RCD protection installed before this amendment will require a C2 or C3 observation on the Schedule of Inspections. Where high Zs is found on a lighting circuit during an EICR, fitting an RCBO satisfies both the Zs disconnection requirement and the new Reg 411.3.4 RCD obligation in a single device.
Enter the protective device type and rating and Elec-Mate instantly shows the maximum permitted Zs from BS 7671 Table 41.3 (MCBs) or Table 41.2 (fuses)…
Try it free for 7 days
Every key change in the 2026 amendment on one page. AFDDs, TN-C-S protection, new schedule columns, model forms. Pinned on your van dash.
This is the table to check your reading against. The values below are the maximum permitted earth fault loop impedance (Zs) for circuit breakers at 230 V, 0.4 s disconnection (final circuits), from BS 7671:2018+A4:2026 Table 41.3. If your measured Zs is higher than the figure for your device, the circuit is non-compliant.
MCB to BS EN 60898 / RCBO to BS EN 61009-1 · tabulated values (apply the GN3 0.8 factor for the cold-measured site limit)
Values per BS 7671:2018+A4:2026 Table 41.3, Reg 411.4.204, 230 V, 0.4 s. A higher trip type needs a lower Zs (Type D needs roughly a quarter of the Type B limit). For the on-site pass/fail figure, multiply by 0.8 — e.g. a 32 A Type B limit of 1.37 Ω gives a maximum cold-measured site reading of 1.10 Ω (GN3, Appendix 3).
If your reading is over the figure for your device, work through the common causes below — start by measuring Ze at the origin to split the problem between the supply (Ze) and your circuit (R1+R2).
Since Zs equals Ze (external impedance) plus R1 + R2 (internal circuit impedance), a high Zs reading means either the external earth is poor, the internal circuit conductors have high resistance, or both. Understanding which component is elevated is essential for choosing the correct solution.
The most common cause. A loose, corroded, or damaged main earthing conductor, earth clamp, or earth terminal increases the resistance of the earth path. On TN-S systems, the earth clamp on the cable sheath often corrodes over decades. On TN-C-S systems, a loose connection at the main earthing terminal can introduce significant resistance. Check and retighten all earthing connections, and replace corroded earth clamps with new BS 951 compliant clamps.
Every metre of cable adds resistance to the earth fault loop. Long runs — common on outbuilding circuits, detached garage circuits, and garden lighting — accumulate enough R1 + R2 to push Zs above the permitted maximum. The problem is compounded when the CPC is undersized relative to the cable length. For example, a 30-metre run of 2.5 mm twin and earth cable has an R1 + R2 of approximately 0.70 ohms, which when added to a typical Ze of 0.35 ohms gives a Zs of 1.05 ohms — very close to the limit for a 32A Type B MCB.
Ze is the external earth fault loop impedance — the portion of the earth path that belongs to the DNO's network. Typical values are 0.8 ohms or less for TN-S and 0.35 ohms or less for TN-C-S. If Ze is significantly higher than these values, the supply earth may be deteriorating. On TN-S systems, a corroded lead cable sheath increases Ze. On TN-C-S (PME) systems, a high Ze can indicate a poor neutral-earth connection in the supply network. High Ze is the DNO's responsibility — report it.
Every connection in the earth path — at the consumer unit, junction boxes, accessory earth terminals, and the main earthing terminal — adds resistance. A single loose connection can add several tenths of an ohm, potentially pushing Zs above the maximum. Over time, connections loosen due to thermal cycling (heating and cooling as load changes) and mechanical movement. Tightening all connections in the earth path often reduces Zs by a measurable amount.
Corrosion at any point in the earth path increases resistance. This is particularly common at earth clamps on metallic water or gas pipes (for bonding), at the main earth terminal, and at the earth clamp on a TN-S cable sheath. Copper conductors corrode less than the steel clamps used to attach them. Green verdigris on copper or rust on clamps are visual indicators. Replace corroded clamps and clean connection surfaces.
The correct solution depends on whether the high Zs is caused by external impedance (Ze — the DNO's responsibility) or internal impedance (R1 + R2 — your responsibility). Start by measuring Ze at the origin of the installation with all circuits isolated.
Inspect the main earthing conductor, the earth clamp (on TN-S), and the main earthing terminal. Check for corrosion, loose connections, and correct tightening torques. Replace corroded clamps with new BS 951 compliant types. Ensure the main earthing conductor is of adequate cross-sectional area per BS 7671 Table 54.7. This single step often reduces Zs by 0.1 to 0.3 ohms.
If R2 (protective conductor resistance) is the main contributor to high Zs, running a supplementary CPC of larger cross-sectional area alongside the existing cable can reduce R2 significantly. For example, replacing a 1.0 mm CPC with a separate 4.0 mm CPC run in parallel reduces R2 by approximately 75% for that circuit. Use Elec-Mate's R1+R2 calculator to check that the proposed CPC size gives a compliant Zs before starting the work.
In specific locations (bathrooms per Regulation 701.415.2, if the disconnection time cannot be met), supplementary bonding can provide an alternative means of shock protection by equalising potential between simultaneously accessible exposed-conductive-parts and extraneous-conductive-parts. This does not reduce Zs itself but provides additional protection where high Zs cannot be economically reduced.
If Zs exceeds the limit for one device type but is within limits for another, changing the protective device may be appropriate. For example, a Type C MCB requires a higher fault current to magnetically trip than a Type B MCB, so it has a lower maximum Zs. However, changing from Type B to Type C is only appropriate if the load characteristics suit a Type C characteristic. Fitting an RCBO ensures earth fault disconnection via the 30 mA function regardless of Zs, though the adiabatic equation must still be checked to verify the CPC can withstand the fault energy.
For TT earthing systems (common in rural areas), the earth path goes through the general mass of earth via an earth rod, which inherently has much higher impedance than a metallic return path. On TT systems, Zs will often exceed the MCB maximum values, which is why BS 7671 requires RCD protection (not overcurrent protection) as the primary means of fault disconnection in TT systems.
BS 7671 Regulation 411.5.3 provides the quantitative RCD selection rule: Ra × Ign ≤ 50 V, where Ra is the sum of the resistances of the earth electrode and the protective conductor (in ohms), and Ign is the rated residual operating current of the RCD. For a 100 mA RCD: Ra must not exceed 500 Ω (50 V ÷ 0.1 A). For a 30 mA RCD: Ra must not exceed 1 667 Ω (50 V ÷ 0.03 A). In practice, most TT installations use a 100 mA time-delayed RCD at origin and 30 mA RCDs on final circuits. If the earth electrode resistance is too high even for RCD operation, driving the earth rod deeper, using multiple rods in parallel, or treating the soil with bentonite can reduce Ra.
BS 7671:2018+A4:2026 Regulation 421.1.7 recommends the installation of arc fault detection devices (AFDDs) on AC final circuits to mitigate the risk of fire caused by arc faults. When resolving high Zs by replacing wiring or fitting a new consumer unit, this is an appropriate point to consider AFDDs. They must conform to BS EN 62606. Note: Reg 421.1.7 is a recommendation, not a mandatory requirement for all circuits — but it should be considered and discussed with the client during any consumer unit replacement or rewire undertaken to address Zs deficiencies.
Enter Ze and R1+R2, select the protective device, and Elec-Mate instantly tells you whether Zs is within the BS 7671 maximum.
Try it free for 7 days16 certificate types, 70+ calculators, RAMS, quoting, invoicing, AI agents, and 46+ training courses — from £6.99/mo.
Start free trialThe external earth fault loop impedance (Ze) is the responsibility of the Distribution Network Operator (DNO). If Ze is abnormally high, it indicates a problem with the supply earth that the DNO must investigate and rectify. You should report to the DNO if:
Record the Ze value on the EICR and note it as a Code C2 observation with a recommendation to contact the DNO. The DNO contact details for the region can be found on the Energy Networks Association website. Do not attempt to modify the DNO's equipment — this is illegal and dangerous.
Elec-Mate provides several tools that help electricians verify earth fault loop impedance compliance quickly and accurately on site.
Select any protective device type (BS 88 fuse, BS 3036 fuse, BS 1361 fuse, Type B/C/D MCB) and rating…
Try it free for 7 daysEnter the cable type, conductor size, and cable length, and Elec-Mate calculates the expected R1+R2 value.
Try it free for 7 daysThe schedule of tests auto-validates every Zs measurement against the maximum for the protective device recorded on the circuit schedule. Non-compliant readings are flagged immediately and the app suggests the appropriate EICR observation code.
Verified reviews from the UK App Store.
Elec-Mate is my go to app for business and electrical work. It's feature rich without feeling cluttered. A true all in one app for quotes, certs, calculations, RAMS, EICRs, and more. I use it every day without fail, and it makes my workflow much smoother since I'm not jumping between apps anymore. The price-to-feature ratio is excellent. Any issues I've had, the developer responds within the hour and usually fixes them the same day. 100% recommend.
I've used the app and the web based version for a while now and it's well worth the investment. If you're an apprentice or experienced Spark give it a go, you won't be disappointed.
I've been using Elec-Mate for a while now, and honestly, it's one of the best apps I've ever downloaded. Every aspect of it feels thoughtfully designed, from the clean and intuitive interface to the powerful features that make everything so easy to manage. It's clear that a lot of care and attention went into building this app, and it shows in every detail.
Real feedback from real sparks
“Replaced three separate apps with Elec-Mate. Certs, quotes, and scheduling all in one place.”
Daniel Palmer
Sole Trader · DP Electrical
“I've won two contracts this month because I could turn quotes around same-day with the AI cost engineer.”
Nathan Perry
Electrician · NP Electrical Services
“The study centre got me through my AM2. Mock exams and flashcards are brilliant.”
Jake Pizey
3rd Year Apprentice · Apprentice
Zs lookup calculator, R1+R2 calculator, auto-validated schedule of tests, and digital EICR forms. Join 1,000+ UK electricians. 7-day free trial, cancel anytime.
“Replaced three separate apps with Elec-Mate. Certs, quotes, and scheduling all in one place.”
Daniel Palmer, DP Electrical
From £6.99/mo after trial — less than a coffee a week
We use cookies to improve the app and measure what works. Cookie Policy