High Ze on an EICR: what external impedance means

A high Ze reading means the measured impedance of the external earth fault path at the installation origin is higher than expected for the identified earthing arrangement or higher than the value on which the safety assessment relies. It can contribute to a failed—or, more precisely, unsatisfactory—EICR if the inspector cannot confirm effective fault protection.

The number alone does not set a universal classification. The inspector must check the declared earthing system, measurement method, supply conditions, bonding and circuit earth fault loop impedance. A high or unstable value may require the distribution network operator or another supply provider to investigate; a problem with the installation's own earthing conductor remains the owner's responsibility.

A C2 observation makes an EICR unsatisfactory. Under the current A4:2026 EICR format, an FI observation is advisory and does not by itself make the overall assessment unsatisfactory. A bare Ze figure in the test schedule is not a classification by itself. Use the classification hub for the broad meanings of C1, C2, C3 and FI. This guide stays focused on high external earth fault loop impedance in London properties.

Ze is the earth fault loop impedance external to the consumer's installation, measured at the origin with the installation's main earthing conductor disconnected under controlled conditions where the recognised test method is used. The loop includes the supply transformer winding and line conductor, plus the external protective conductor or the earth return path appropriate to the supply.

That is different from Zs, the total earth fault loop impedance at a point on a circuit. In simplified terms, Zs includes the external value and the resistance of the circuit line and protective conductors, although real measurements can be influenced by parallel paths and test conditions. Protective-device disconnection is normally assessed at circuit level.

The test is safety-critical. Disconnecting an earthing conductor and carrying out live loop testing can create danger if done incorrectly, so Ze is not something a landlord or homeowner should try to verify with a retail multimeter or plug-in tester.

TN-C-S, TN-S and TT systems do not share one acceptable Ze figure. In a TN system the network normally provides the metallic earth return. In a TT system the consumer's earth electrode and the mass of earth form part of the return path, so values and protective-device strategy are fundamentally different.

Common reference figures such as 0.35 ohms for TN-C-S, 0.8 ohms for TN-S and 21 ohms for a distributor-provided TT earth are often quoted as maximum external values from supply-industry guidance. They are useful prompts, not a complete pass-or-fail table for every installation. The distributor's declared value, network arrangement and applicable design requirements must be considered.

A London conversion can also be misleading at first glance. Several flats may have separate meters, a landlord's distribution board and shared intake equipment. The earthing terminal being tested must correspond to the correct supply, and parallel metallic paths can affect readings. The EICR should state the arrangement actually verified rather than assume it from conductor colours.

Locate the earthing arrangement, measured Ze and details of the supply protective device in the report's supply characteristics. Then read the numbered observation that refers to the value. A well-written entry explains whether the concern is an unexpectedly high measurement, no reliable means of earthing, an inaccessible origin, or uncertainty requiring further investigation.

Compare that with circuit schedules. Zs results, protective-device types, RCD tests and continuity results show how the external finding affects downstream fault protection. Do not perform your own arithmetic and override the report: measured and calculated values, temperature correction, device standards and parallel paths all require competent interpretation.

Check limitations too. The inspector may have been unable to isolate a whole building, access a sealed intake or disconnect parallel services. A recorded value obtained by an alternative method should be described accordingly. Missing context is a reason to ask the inspector, not a reason to assume the supply is safe or defective.

• A deteriorated, loose or absent network earth connection on a TN supply.
• An incorrect assumption about whether the supply is TN-S, TN-C-S or TT.
• Poor continuity in the consumer's earthing conductor or connection to the main earthing terminal.
• A high-resistance earth electrode or adverse ground conditions on a TT installation.
• Measurement through unexpected parallel paths, or a test made without the isolation needed for a true Ze value.
• Temporary network conditions, test-lead errors, instrument settings or a reading that is unstable and needs confirmation.
• Shared or inaccessible intake arrangements in converted buildings, leaving the origin or responsibility boundary unclear.

C2 may be justified where the evidence shows the fault-protection arrangement is potentially dangerous—for example, the earth path cannot support required disconnection and no suitable alternative protective measure compensates. The exact observation should connect the high reading to that safety consequence.

FI can be appropriate where an anomalous measurement raises a significant concern but the cause or effective protective arrangement cannot be established within the EICR. This often occurs when supply-provider attendance, access to a communal intake or additional isolation is needed. Under the current A4:2026 EICR format, that FI observation is advisory and does not by itself make the overall report unsatisfactory. Where the report requires further investigative work, landlord follow-up is a separate regulatory duty.

The reading does not automatically mean C1. C1 is reserved for danger present, not merely a value outside an expected range. Nor should high Ze be reduced automatically to C3 because it might belong to the network. Classification concerns occupant safety; responsibility for correction is a separate question.

In a Victorian conversion with a TN-S supply, a reading materially above the expected external value may be accompanied by sound circuit conductors but poor continuity at ageing intake equipment. The inspector can report the safety concern and refer the supply-side issue without instructing the landlord to alter sealed network equipment.

In a house on TT, a comparatively high number may be normal relative to TN figures yet still require assessment against electrode resistance, RCD operation and the disconnection strategy. Comparing it with 0.35 ohms would be meaningless. If the electrode connection is defective, work on the consumer's side may be needed.

In a modern block, parallel bonding and communal metalwork can make a loop reading appear low even if the intended protective conductor is compromised. Continuity and visual verification remain important. One reassuring number cannot replace inspection of the actual earthing connection.

The distribution network operator generally controls its service cable, cut-out and any earth terminal it provides. Meter operators and electricity suppliers have separate roles around metering and supply contracts. Electricians working for the landlord must not break seals, alter the cut-out or improvise connections to network equipment.

The property owner is normally responsible for the installation after the defined origin, including the main earthing conductor where it forms part of the consumer's installation, main protective bonding, distribution equipment and final circuits. In blocks, leases and building management arrangements can divide responsibility for communal distribution.

Ask the inspector to state why referral is needed and which party should be contacted. A DNO reference number, attendance note or declared earth value is useful evidence. Network attendance does not automatically resolve every EICR observation; the electrician may need to return and verify the installation after the supply issue is corrected.

“Any Ze above 0.35 ohms fails” confuses a commonly quoted TN-C-S external value with a universal limit. It ignores TN-S, TT, declared values and the complete protective arrangement. The report must identify the system before interpreting the result.

“RCDs make Ze irrelevant” is also incorrect. RCDs can provide fault protection in appropriate designs and are central to TT systems, but earthing still supports a reference to earth, protective equipotential bonding and operation under fault conditions. Device type, rated residual current and measured performance matter.

“It is outside the flat, so the landlord can ignore it” confuses ownership with risk. A network defect may require the DNO to repair it, yet the landlord still needs to pursue the issue and retain evidence where an EICR is unsatisfactory. Conversely, a contractor should not charge for speculative consumer-unit replacement when the unresolved defect is demonstrably supply-side.

The first stage is confirmation: review the instrument result, test method, earthing arrangement and accessible connections. The electrician may compare measured and calculated circuit values, check continuity and contact the DNO for a declared value or attendance. This is investigation, not an invitation to repeatedly live-test without a plan.

If the defect is property-side, suitable work might involve renewing an earthing conductor or termination, correcting a main earthing terminal, improving a TT electrode system, or redesigning protection. If it is network-side, the DNO decides its corrective action. Bonding upgrades or consumer-unit work should be specified only where the evidence supports them.

After correction, relevant tests must be repeated and results documented. Depending on scope, electrical certification and written confirmation against the EICR observations may be appropriate. Keep the original unsatisfactory report: remedial paperwork supplements it rather than erasing the historic finding.

Once the report findings are clear, use our checklist to estimate the repair cost. It is an indication of scope; the electrician confirms the work and quotation.

Send the earthing details and test schedules when you request an investigation or remedial quotation; the findings determine the scope.

Where the report requires further investigative or remedial work, regulation 3(4) can require a qualified person to do that work within 28 days, or any shorter period the report states, starting with the inspection and test. Regulation 3(2A) and 3(6A) also record a reasonable-steps position, including where a tenant prevents entry. A Ze figure does not, by itself, decide that duty.

Supply-provider involvement can complicate scheduling but does not make the observation disappear. Record when the issue was reported, appointment references, access attempts and the electrician's advice. If a statutory deadline is at risk, obtain competent advice rather than treating an open DNO job as automatic compliance.

The broader unsatisfactory EICR and remedial-work pages explain that process. For this specific issue, the useful question is: what evidence will confirm that effective earthing and fault protection are restored? Make that deliverable explicit when arranging return attendance.

Send the full EICR to the inspecting electrician or a competent remedial contractor and ask whether the value was directly measured, what earthing arrangement was verified, how circuit protection was affected and whether a DNO referral has been raised. Include all companion observations, especially bonding, earthing-conductor and Zs entries.

Do not touch the service head, remove seals, disconnect earthing or bonding conductors, or attempt live loop tests. If there are shocks, sparking, heat, burning smells, damaged intake equipment or loss of supply, keep clear and use the relevant emergency contact route. Routine email clarification is not suitable for immediate danger.

Sources checked on 3 October 2026. Read this guidance alongside your own EICR: its edition, observations, extent and limitations matter. Ask a qualified electrician to assess the findings at your property.

This article was editorially reviewed on 2026-10-03. It provides general operational guidance for London property decision-makers and is not personalised legal advice.

Electrical classification on an EICR depends on the installation, the observations recorded and competent professional judgement. Always read your own report and seek competent advice for your property.