The Dangers of a High External Loop Impedance In Your Property (Ze)

When it comes to electrical safety, external loop impedance (Ze) is one of the most important measurements we take as electricians. A high Ze reading can indicate a serious fault with your supply or earthing arrangement — one that could leave your property and its occupants at risk.

But what is Ze, why does it matter, and what are the dangers if it’s too high?

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What Is External Loop Impedance (Ze)? • Ze is the measurement of the resistance in the fault path between your property and the point of supply provided by the Distribution Network Operator (DNO). • In simple terms, it tells us how quickly enough fault current can flow to operate protective devices (like fuses, MCBs, or RCBOs).

A low Ze means fault current will flow easily, so protective devices disconnect supply quickly. A high Ze means fault current struggles to flow, so disconnection may be delayed — leaving the installation unsafe.

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Why Is a High Ze Dangerous? 1. Failure of Protective Devices • If Ze is too high, fuses and MCBs may not trip within the required disconnection times. • This increases the risk of overheating, electric shock, or even fire. 2. Shock Risk • A fault on a live conductor could make exposed metal parts (like an appliance housing or consumer unit) stay live for too long, giving a dangerous electric shock if touched. 3. Non-Compliance with Regulations • The Wiring Regulations (BS 7671:2024) set maximum values for Ze depending on the supply type (TN-S, TN-C-S, or TT). • Anything above these values is unsafe and must be rectified. 4. Potential DNO Supply Issue • Sometimes, a high Ze is caused by a fault in the supply network itself — such as a broken PEN conductor or poor connection at the service head. This requires immediate attention from the DNO.

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Typical Maximum Ze Values (BS 7671:2024) • TN-S system – 0.8 Ω • TN-C-S system (PME) – 0.35 Ω • TT system – No fixed maximum. Disconnection must be achieved by RCDs, with earth electrode resistance low enough to ensure safe operation. • Formula: Ra × IΔn ≤ 50 V • Example: With a 30 mA RCD, Ra should be ≤ 1667 Ω. • In practice: An electrode resistance of 200 Ω or less is recommended for stability and safety.

⚡ Any reading above these limits (for TN systems), or an electrode resistance that prevents RCDs from disconnecting correctly (for TT systems), is classed as unsatisfactory and requires urgent action.

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What Causes a High Ze? • Corroded or loose connections at the service head. • Broken or undersized earth conductors. • Faults in the DNO supply cables. • In TT systems, poor earth electrode resistance.

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What Happens If a High Ze Is Found? 1. Immediate Safety Measures • The electrician may install temporary RCD protection or disconnect circuits if they cannot be left safely energised. 2. Notification • If the fault lies with the DNO supply, it must be reported so the network operator can rectify the issue. 3. Remedial Works • For TT systems, new earth rods or additional electrodes may be installed. • For TN-S/TN-C-S, bonding upgrades or supply-side repairs may be required.

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Real-World Example

Imagine a home with a TN-C-S supply and a high Ze. If a live wire inside a washing machine touches the metal casing, the casing could remain live. Because of the high impedance, the MCB might not trip in time — meaning anyone touching the machine risks a severe or even fatal electric shock.

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Final Thoughts

A high external loop impedance (Ze) is not just a technical fault — it’s a serious safety risk. That’s why it’s always tested during an EICR (Electrical Installation Condition Report) or consumer unit upgrade. If your property records a high Ze, immediate remedial action is essential to bring the installation back to safe standards.

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Switch Electrical & Mechanical Services Ltd carry out professional testing and inspections across Newcastle and the North East. If you’re concerned about your earthing or have been told your Ze is high, our expert electricians can diagnose the issue, liaise with the DNO where necessary, and provide safe, compliant solutions.

📞 0191 9052723 📱 WhatsApp: 07388 439427 ✉️ switchtoday@mail.com

Frequently Asked Questions (North East UK)

What are my legal electrical responsibilities as a Landlord in Newcastle? As a landlord in the North East, you are legally required to ensure that all electrical installations in your rental property are safe when tenants move in and are maintained in a safe condition throughout the duration of the tenancy. This includes obtaining a satisfactory Electrical Installation Condition Report (EICR) every 5 years or at the start of a new tenancy. Switch Electrical & Mechanical Services provides comprehensive EICR audits for landlords across Newcastle, Sunderland, and Gateshead.

How much does an EICR cost for a standard property in the North East? The cost of an EICR depends on the size of the property and the number of circuits. Switch Electrical & Mechanical Services provides highly competitive, transparent pricing for landlords in the North East. We also offer discounted rates for landlords with multiple properties or HMOs requiring block certification.

What happens if my rental property fails an EICR? If an EICR identifies 'C1' (Danger Present) or 'C2' (Potentially Dangerous) codes, the installation is deemed 'Unsatisfactory'. Landlords have exactly 28 days to complete the necessary remedial work. Our team at Switch Electrical & Mechanical Services can provide immediate, fixed-price quotes for all remedial work following an inspection to ensure you remain legally compliant.