How do I test for an open circuit earth in an installation?

An open circuit earth can occur in the earthing conductor (between the MET and the earth electrode or PME terminal), in the main bonding conductors (between the MET and bonded services), or in a circuit protective conductor (between the consumer unit and an accessory). To test for open circuit earth in the earthing conductor: carry out a continuity test between the MET and a known-good external earth reference (such as an electrode driven into the ground at a different location from the installation electrode). A reading of infinity or very high resistance indicates an open circuit. For main bonding conductors: continuity test between the MET and the bonded service (gas meter, water pipe, structural steel). For circuit CPCs: continuity test between the MET and the earth terminal at each accessory on the circuit.

What is the maximum acceptable earth electrode resistance?

For a TT installation protected by an RCD, the maximum permissible earth electrode resistance is determined by the formula: Ra × IΔn ≤ 50V, where Ra is the electrode resistance and IΔn is the rated residual operating current of the RCD in amperes. For a 30mA (0.03A) RCD, the maximum Ra is 50 ÷ 0.03 = 1667Ω. For a 100mA RCD, the maximum Ra is 500Ω. In practice, a lower electrode resistance is better — target below 200Ω for a TT installation with a 30mA RCD to provide adequate margin. For earthing systems where the electrode provides the main fault current return path (such as some agricultural or caravan park installations), much lower electrode resistances (below 20Ω) are required. BS 7430 provides detailed guidance on electrode resistance requirements for different applications.

How does the three-electrode fall-of-potential method work?

The fall-of-potential method (also called the three-electrode method) is the standard technique for measuring earth electrode resistance specified in BS 7430. It uses three connections: the electrode under test (E), a current electrode (C) driven into the ground at a distance of approximately 10 times the electrode length from the test electrode (for a typical 1.2m earth rod, approximately 12m away), and a potential electrode (P) driven into the ground at approximately 62% of the distance between E and C (at approximately 7.4m from E). The earth resistance tester injects a test current between E and C, and measures the voltage between E and P. The ratio V/I gives the electrode resistance. The 62% rule gives a reliable measurement for most electrode configurations. The measurement is repeated with P at 52% and 72% of EC distance to verify consistency — if the three readings are within 10% of each other, the result is reliable.

What signs indicate a corroded or failing earth electrode?

Signs of a corroded or failing earth electrode include: measured Ze (external earth fault loop impedance) significantly higher than expected for the supply type; earth fault loop impedance tests that fail the Appendix 3 table limits on circuits that previously passed; RCDs that take longer to trip (or fail to trip) under test; and visual signs of corrosion at the electrode clamp or connection where it emerges from the ground. On TT systems, a rising electrode resistance is often the first sign of electrode deterioration. Check the electrode connection clamp — this is frequently the weakest point. Stainless steel or copper-clad steel electrodes resist corrosion better than plain steel rods. In corrosive soils (high salt content, acid soils), electrode life is shorter and more frequent testing is required.

What is a PME (TN-C-S) supply and why is the lost neutral dangerous?

PME (Protective Multiple Earthing) is the most common supply arrangement in the UK. The distributor's network uses a combined Protective Earth and Neutral (PEN) conductor rather than separate PE and N conductors. At the intake point, the PEN is split into a separate PE and N — the PE connects to the main earthing terminal and the N connects to the neutral bar of the consumer unit. The safety of the PME earthing depends entirely on the integrity of the PEN conductor between the premises and the transformer. If the PEN conductor breaks (due to cable damage, a loose neutral joint in a feeder pillar, or a network fault), the protective earth terminal in the premises is no longer connected to the transformer neutral. Under load, the voltage on the PE terminal rises — potentially approaching the phase voltage of 230V. All metalwork bonded to the PE (gas pipes, water pipes, appliance chassis, structural steel) rises to this dangerous voltage. Anyone touching bonded metalwork while standing on a different earth potential (the true earth potential outside) risks a severe or fatal electric shock.

How is the main earthing terminal inspected and tested?

The main earthing terminal (MET) is the central connection point for the earthing system. It should be accessible for inspection and testing. On inspection: verify the MET label is present and correct; check all connections are tight and show no signs of corrosion or overheating; verify the earthing conductor is correctly sized for the supply (typically 16mm² or 25mm² for domestic PME supplies, depending on the supply conductor size per Table 54.7 of BS 7671); verify that all main protective bonding conductors are connected (gas, water, oil, other metallic services as applicable). On testing: measure continuity between the MET and each bonded service — resistance should typically be below 0.05Ω. Measure Ze at the MET to verify the external earth fault loop impedance and confirm the PME or TN-S earthing arrangement is intact.

When should I recommend upgrading from TT to PME earthing?

Recommending a change from TT to PME earthing is not straightforward because it requires the agreement and involvement of the distributor. TT installations are common in rural areas and properties remote from the distributor's PME network. TT earthing has inherent limitations — the electrode resistance can be high (particularly in dry or rocky soils), requiring RCD protection throughout the installation, and the electrode can deteriorate over time. However, PME earthing is not always superior — in some locations (particularly near rivers, in areas with significant metallic pipework, or where there is a history of neutral faults on the network), PME carries its own risks. The decision to upgrade from TT should be made in consultation with the distributor, and the new PME earthing arrangement must comply with BS 7671 and the distributor's network requirements.

FAULT FINDING GUIDE

Earthing System Fault Finding: Open Circuit Earth, Electrode Resistance, and PME Dangers

A complete guide to diagnosing earthing system faults for UK electricians. Covers open circuit earth, poor main earth connections, corroded earth electrodes, measuring electrode resistance using BS 7430 methods, and the dangers of a lost PME neutral.

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13 min readUpdated 2026-05-18Andrew Moore, Founder of Elec-Mate

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.

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Key Takeaways

1The earthing system comprises the main earthing terminal (MET), the earthing conductor connecting the MET to the earth electrode or PME terminal, the earth electrode (on TT installations), and the main protective bonding conductors to metallic services.
2An open circuit earth conductor leaves the installation without effective protection — any fault to exposed metalwork creates a shock hazard and the protective device may not operate within the required disconnection time.
3Earth electrode resistance can be measured using the three-electrode fall-of-potential method (BS 7430) or using a dedicated earth loop tester. The maximum electrode resistance for a TT installation depends on the RCD trip current: R × I_Δn ≤ 50V (typically 1667Ω for a 30mA RCD).
4On PME (TN-C-S) supplies, the protective earth and neutral are combined in the distributor's network. If the combined PEN conductor breaks upstream, the voltage on the PME earth terminal and all bonded metalwork in the premises can rise to a dangerous level — potentially approaching phase voltage (230V). This is known as a lost neutral event.
5BS 7430 (Code of practice for protective earthing of electrical installations) is the UK standard governing the design and testing of earthing systems. It specifies measurement methods for earth electrode resistance and requirements for buried electrode installations.

01 · Fault Finding Guide

Earthing System Basics

The earthing system of an electrical installation provides the return path for fault current in the event of a live-to-earth fault, ensuring that the fault current is sufficient to operate the protective device within the required time. Without an effective earthing system, fault current may be too low to trip the MCB or fuse, exposed metalwork remains at a dangerous voltage, and electric shock risk is not adequately controlled.

The earthing system comprises several interconnected components:

Earth electrode — a buried conductor (typically a copper-clad steel rod) that makes electrical contact with the general mass of earth. Required on TT installations; not used as the primary earth on PME (TN-C-S) supplies.
Earthing conductor — connects the main earthing terminal to the earth electrode (TT) or to the PME terminal provided by the distributor (TN-C-S).
Main earthing terminal (MET) — the central connection point in the installation to which all protective conductors, bonding conductors, and the earthing conductor connect.
Main protective bonding conductors — connect metallic services entering the building (gas, water, oil pipes) to the MET.
Circuit protective conductors (CPCs) — run with each circuit and connect exposed metalwork of equipment and accessories back to the MET.

Faults in any part of this system can compromise the protection of the entire installation. Earthing system fault finding requires a systematic check of each component, from the electrode through the earthing conductor to the MET and then throughout the circuit CPCs.

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02 · Fault Finding Guide

Open Circuit Earth — Detecting a Broken Earthing Conductor

An open circuit in the earthing conductor (between the MET and the earth electrode or PME terminal) is one of the most serious faults an electrician can find on a periodic inspection. If the earthing conductor is broken, the entire installation may have no effective earth connection — depending on the supply arrangement.

On a TN-C-S (PME) supply — the PME earthing depends on the distributor's combined PEN conductor. If the internal earthing conductor is broken but the external PEN is intact, the installation may still have some earth connection via residual paths (bonded metalwork in contact with external metalwork). However, this is not reliable and the fault must be repaired immediately. Coded C1 on an EICR.
On a TT supply — if the earthing conductor to the electrode is broken, the installation has no earth connection at all. All fault protection relies on the integrity of a conductor that is broken. This is an immediate danger (C1) requiring the installation to be taken out of service or the fault repaired before re-energising.

To detect an open circuit earthing conductor: measure Ze at the origin — if Ze is very high (or the tester shows an error indicating no earth path), the earthing conductor or external PME/TT earth is compromised. Follow up with continuity testing between the MET and the electrode rod connection.

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03 · Fault Finding Guide

Poor Main Earth Connection

Even without an open circuit, a high-resistance connection at any point in the earthing system raises the measured Ze and can cause circuits to exceed their Appendix 3 maximum Zs. Common locations for poor connections are:

MET connections — corroded or loose connections at the main earthing terminal. Inspect and retighten all connections. Clean any corrosion with emery cloth before reassembling. Check that the earthing conductor is correctly terminated (lugged or effectively crimped — not just wrapped around a terminal bolt).
Electrode rod connection clamp — the brass or stainless steel clamp connecting the earthing conductor to the electrode rod can corrode, particularly in damp soil. Inspect the clamp and re-make the connection if necessary. Use corrosion-resistant compound (such as Denso paste) around the connection.
PME terminal connection — the connection at the distributor's cut-out (the sealed fuse unit) includes a PME earthing terminal. The earthing conductor from the MET connects here. This connection can loosen over time. Note that the cut-out itself is the distributor's property — if the PME terminal appears damaged or loose, contact the distributor, do not work inside the sealed cut-out.

04 · Fault Finding Guide

Corroded Earth Electrode

On TT installations, the earth electrode is the sole earth connection for the installation. Electrode corrosion progressively increases its resistance, eventually exceeding the maximum permissible value for the RCD protection to operate within the required time.

Signs of Corrosion

Visual signs at the electrode connection point (where it emerges from the ground): green or white oxide deposits on copper conductors; rust and pitting on steel rods; loose or detached connection clamp; conductor insulation that has degraded at the soil interface. Measured sign: electrode resistance significantly higher than previous test results (tracked on EICR records).

Remediation

A corroded electrode that has high resistance should be supplemented or replaced. Drive a new electrode adjacent to the existing one and connect them in parallel — parallel electrodes reduce total resistance. Alternatively, if the existing electrode has only surface corrosion, excavate, clean, apply corrosion protection, and check the connection. Deep-driven copper-clad steel rods resist corrosion better than surface rods.

The electrode resistance must be re-measured after any remediation to confirm it now meets the BS 7430 and BS 7671 requirements for the installation. Record the before and after values on the EICR.

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05 · Fault Finding Guide

Measuring Earth Electrode Resistance — BS 7430 Methods

BS 7430 (Code of practice for protective earthing of electrical installations) specifies the test methods for measuring earth electrode resistance. The primary method is the three-electrode fall-of-potential method:

1Disconnect the electrode from the installation (disconnect the earthing conductor at the MET). This ensures you are measuring only the electrode resistance, not a parallel path via the installation earthing system.
2Drive a current electrode (C) approximately 30m from the test electrode in the direction of the supply cable (away from the building). For a 1.2m rod, 30m is a typical distance to ensure the resistance areas do not overlap.
3Drive a potential electrode (P) at 62% of the C distance from the test electrode — approximately 18 to 19m from the test electrode.
4Connect the earth resistance tester and take the reading. Repeat with P at 52% and 72% of EC distance. If the three readings are within 10% of each other, the middle reading (62%) is the electrode resistance.
5Reconnect the earthing conductor and verify Ze at the installation origin. A high Ze relative to the measured electrode resistance indicates a poor connection in the earthing conductor.

An alternative method available on many modern loop testers is the two-electrode method — measuring the loop impedance at the origin with the incoming supply disconnected and the electrode connected. This gives an approximation of electrode resistance but is less accurate than the three-electrode method.

06 · Fault Finding Guide

PME Supply Issues — The Lost Neutral Danger

The PME (Protective Multiple Earthing) arrangement is inherently safe when the distributor's PEN conductor is intact. However, if the PEN conductor breaks upstream of the premises — in the street cable, at a joint, or in a feeder pillar — the consequences are severe and occur without warning.

What happens when PME neutral is lost — the protective earth terminal in the premises rises to a voltage determined by the load sharing between the remaining phases. In a severely unbalanced case, the PE terminal can rise to near phase voltage (230V). All bonded metalwork — gas pipes, water pipes, appliance bodies, structural steel — rises to this voltage.
Why PME is prohibited in certain locations — BS 7671 prohibits the use of PME earthing for caravan parks, marina berths, and similar locations where users may be standing on true earth potential (outdoor ground) while touching metalwork connected to the PME earth. In these locations, a TT system or a separate TN-S earth is required.
What to do if you suspect a lost neutral — if appliances across the premises are failing or behaving erratically, and if touching bonded metalwork gives a tingle or shock, suspect a lost neutral. Do not enter the premises — evacuate immediately and call the distributor's emergency number. Do not touch any metalwork. This is a network emergency.

Lost neutral protection devices (LNPDs) are available for PME premises at high risk — they monitor the PME voltage and disconnect the installation if the earth voltage rises above a safe threshold. These are used in marinas and some industrial premises as an additional safeguard.

07 · Fault Finding Guide

For Electricians: Earthing System Checks on Every EICR

The earthing system should be checked systematically on every EICR, not only when a fault is suspected. Here is a practical checklist:

Measure Ze at the Origin Every Time

Ze is the single most informative test for the earthing system as a whole. A Ze reading consistent with the supply type (below 0.35Ω for PME, below 0.8Ω for TN-S) confirms the external earth path is intact. An unexpectedly high Ze triggers further investigation of the earthing conductor and connections.

Test Main Bonding on Every EICR

Main protective bonding conductors to gas and water services are frequently missing, undersized, or disconnected — often as a result of plumbing work where a section of metal pipe was replaced with plastic. Test continuity between the MET and the gas/water service entry points. A reading above 0.05Ω warrants investigation. Missing bonding is a C2 observation.

Frequently Asked Questions About Earthing System Fault Finding

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Earthing System Fault Finding: Open Circuit Earth, Electrode Resistance, and PME Dangers

Key Takeaways

Earthing System Basics

Get the BS 7671 A4:2026 Cheat Sheet — free

Open Circuit Earth — Detecting a Broken Earthing Conductor

Record earthing system findings on your phone

Poor Main Earth Connection

Corroded Earth Electrode

Signs of Corrosion

Remediation

Try Elec-Mate free for 7 days

Measuring Earth Electrode Resistance — BS 7430 Methods

PME Supply Issues — The Lost Neutral Danger

For Electricians: Earthing System Checks on Every EICR

Measure Ze at the Origin Every Time

Test Main Bonding on Every EICR

Frequently Asked Questions About Earthing System Fault Finding

How do I test for an open circuit earth in an installation?

What is the maximum acceptable earth electrode resistance?

How does the three-electrode fall-of-potential method work?

What signs indicate a corroded or failing earth electrode?

What is a PME (TN-C-S) supply and why is the lost neutral dangerous?

How is the main earthing terminal inspected and tested?

When should I recommend upgrading from TT to PME earthing?

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