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EICR Failed Due to High Ze or Impedance Readings. What It Means & How to Fix It
10 November 2025
EICR Faults & Failures
EICR Failed Due to High Ze or Impedance Readings
If your recent EICR report mentions “high Ze” or “impedance readings above limits”, you’re not alone.
These are among the most common causes of EICR failures in London properties, especially older homes or offices with outdated earthing arrangements.
In this 2025 guide, the certified electricians from London EICR Certificates explain what “high Ze” means, why it causes an EICR fail, and how to fix it safely without breaking your budget.
What Is Ze in an Electrical Installation?
The term Ze stands for External Earth Fault Loop Impedance, which measures the resistance of the earth path outside your property, between your consumer unit (fuse board) and the electricity supplier’s earth connection.
A low Ze means electricity can flow safely back to earth in the event of a fault.
A high Ze, however, means the circuit’s fault current may not be strong enough to trip protective devices (like fuses or RCDs), leading to potential electric shock or fire hazards.
Acceptable Ze Values (Typical)
| Earthing Type | Max Ze Value (Ohms) |
|---|---|
| TN-S | 0.8 Ω |
| TN-C-S (PME) | 0.35 Ω |
| TT | 200 Ω (max, but should be much lower in practice) |
If your reading exceeds these limits, it’s a red flag during an EICR inspection.
Why High Ze Causes EICR Failure
When your Ze (earth loop impedance) is too high, the protective devices can’t disconnect the supply quickly enough during a fault.
That delay increases the risk of electrocution or fire; hence, EICR inspectors must code it as a C2 fault (potentially dangerous).
A typical EICR remark might read:
“High external earth loop impedance – disconnection time not met (C2).”
This instantly fails the EICR until the issue is corrected.
Common Causes of High Ze in London Properties
- Corroded or loose earth connections at the supply head or main bonding clamp.
- Outdated or missing main earthing conductor.
- Old TN-S earthing systems where the metallic sheath has degraded.
- Detached PME (TN-C-S) connections in multi-occupancy buildings.
- Poorly installed consumer unit or broken earth bar links.
- Incorrect bonding to gas or water pipes – see our guide on Missing Gas Bonding.
- DIY modifications that interrupt the protective earth path.
In London’s older buildings, worn cables or corroded terminals are the number one cause.
How Electricians Test Ze During an EICR
Your electrician performs an Earth Fault Loop Impedance Test using a multifunction tester (MFT).
It’s done by disconnecting the main earth conductor temporarily and measuring the resistance between line and earth at the incoming supply.
If readings are high, additional tests are done at various points (distribution boards, sockets, lighting circuits) to locate the problem.
The process is quick, safe, and fully compliant with BS7671:2018 (18th Edition Amendment 2) wiring regulations.
How to Fix High Ze or Impedance Readings
Depending on what’s causing the problem, fixing a high Ze can involve:
- Tightening or cleaning the main earth connection at the supply intake.
- Replacing damaged earthing conductors or upgrading to a thicker cable size (often 16mm² or 25mm²).
- Installing a new PME earth if available from the supplier (UKPN or equivalent).
- Adding an earth rod for TT systems to reduce impedance.
- Checking all bonding points (gas, water, structural steel).
In some cases, a new consumer unit upgrade with RCD/RCBO protection might be required, especially if your fuse board is outdated or damaged.
How Much Does It Cost to Fix a High Ze EICR Fault?
In London, costs typically range between £120 – £450, depending on access, earthing type, and remedial complexity.
| Repair Type | Average Cost |
|---|---|
| Reconnect loose earth terminal | £120–£180 |
| Replace or upgrade main earthing conductor | £150–£250 |
| Install new earth rod (TT system) | £200–£350 |
| Full earthing rewire / bonding upgrade | £350–£450 |
You can learn more about pricing on our EICR Certificate Cost page for a transparent breakdown.
Example: Real London Case Study
In early 2025, one of our electricians in Clapham SW4 tested a 1930s terraced property and found a Ze reading of 2.47Ω on a TN-S system (limit 0.8Ω).
We traced the issue to a corroded earth clamp on the incoming water pipe.
After replacing the clamp and upgrading the bonding, the new reading was 0.42Ω, and the property passed its EICR re-inspection.
What Happens If You Ignore a High Ze Fault?
Ignoring a high Ze issue can lead to:
- Failed compliance under landlord regulations
- Increased risk of electric shock or fire
- Potential insurance invalidation
- Civil penalties up to £30,000 for unsafe rental conditions
If you’re a landlord, you’re legally required to maintain safe electrical installations . See our full guide on EICR Certificates for Landlords in London.
Preventing Future EICR Failures
To avoid high impedance readings in the future:
- Schedule routine electrical inspections every 5 years (or sooner for rentals).
- Keep your main bonding points accessible and corrosion-free.
- Upgrade to a modern consumer unit with RCD/RCBOs.
- Always use a registered NICEIC or NAPIT electrician. You can book an EICR inspection online here.
High Ze or impedance readings can sound intimidating, but they’re a straightforward fix for a qualified electrician.
The key is addressing the issue early before it compromises safety or leads to costly fines.
At London EICR Certificates, we handle diagnostics, repairs, and re-testing across all London postcodes from Chelsea to Canary Wharf.
✅ Book your EICR remedial inspection today and ensure your property meets 2025 electrical safety standards.
London Areas We Frequently See This Fault
Our engineers often find this issue in:
Explore our location pages:
👉 EICR East London
👉 EICR Central London
👉 EICR North London
👉 EICR South London
👉 EICR West London
FAQ: Understanding and Fixing High Ze or Impedance Readings on EICR Reports
1. What does high Ze mean on an EICR report?
High Ze means that the external earth fault loop impedance of your electrical installation is above the safe limits set by the wiring regulations. In simple terms, electricity cannot flow back to earth efficiently during a fault, increasing the risk of electric shock or fire.
2. Why does high Ze cause an EICR to fail?
When the Ze reading is too high, it means that protective devices such as fuses or RCDs may not trip fast enough to disconnect the power during a fault. Because of this, inspectors are required to issue a C2 fault code, meaning “potentially dangerous,” which automatically fails your EICR.
3. What is the acceptable Ze value in the UK?
Acceptable Ze values vary based on your earthing type. For TN-S systems, it should not exceed 0.8 ohms. For TN-C-S (PME) systems, the limit is 0.35 ohms. For TT systems, it must be under 200 ohms, though lower is better. Any value higher than these limits indicates excessive resistance in the earthing path.
4. Can a high Ze be fixed easily?
Yes, in most cases it’s straightforward. A qualified electrician can tighten or clean the main earth connections, upgrade bonding to gas and water pipes, or install a new earth rod if required. For more complex issues, such as old earthing systems, a new connection or consumer unit upgrade may be needed.
5. How much does it cost to fix a high Ze fault in London?
The average cost in London ranges from £120 to £450, depending on the cause. Minor fixes like reconnecting an earth clamp are cheaper, while replacing bonding or installing an earth electrode costs more. You can find full details on our EICR Certificate Cost page at londoneicrcertificates.co.uk/eicr-certificate-cost.
6. Can a high Ze reading damage electrical appliances?
High Ze itself doesn’t directly damage appliances, but it can lead to delayed fault disconnection and unsafe voltage conditions, which may stress sensitive electronics and increase the risk of overheating or electrical failure over time.
7. What causes high Ze readings in older London homes?
Older London properties often have corroded or degraded earthing cables, missing main bonding, or worn metallic sheaths on TN-S systems. In multi-occupancy buildings, shared earth paths can also be damaged or disconnected, leading to unstable Ze values.
8. Do I need a new fuse board if my EICR failed due to high Ze?
Not always. If the main earth connection is sound, a simple repair may be enough. However, if your consumer unit lacks RCD or RCBO protection, upgrading it can significantly improve safety and ensure compliance during your next EICR reinspection. Learn more about remedial upgrades at londoneicrcertificates.co.uk/remedial-work-for-failed-eicr-certificates.
9. How long does it take to repair a high Ze issue?
Most high Ze faults can be repaired in one to two hours, depending on accessibility and the cause. After the repair, your electrician will perform a retest to verify that the readings now fall within safe limits.
10. Can I still pass an EICR with a high Ze if RCD protection is installed?
RCDs offer an additional layer of protection, but they don’t replace proper earthing. If the Ze value exceeds the maximum allowed by the regulations, the installation may still fail. An electrician will decide whether it’s a C2 (fail) or C3 (advisory) based on the actual readings and system design.
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