Loop impedance (Zs) is the total resistance of the path that fault current takes from the supply transformer through your wiring and back. It directly determines whether your circuit breaker or RCD will disconnect quickly enough in a fault.
Danger Level: Low (Informational)
Informational - understand the issue and when to seek advice.
The Symptom
"The electrician said my loop impedance is too high - what does that mean? Is my house safe?" Customers may hear this after an EICR or during a consumer unit upgrade. High Zs means the protective device may not trip quickly enough in a fault, leaving exposed metal live for longer.
The Detective Work
The electrician measures Zs at each circuit using a certified earth-fault loop impedance tester. The reading is compared against BS 7671 Tables (e.g. Table 41.3 for MCBs, Table 41.5 for RCDs). If Zs is too high, possible causes include: long cable runs, undersized protective conductors, loose connections, or poor earthing. Ze (external loop impedance) is measured at the origin to separate supply-side from installation resistance. Continuity of protective conductors is checked. Remediation might involve upgrading earthing, bonding, or replacing long or undersized cables.
Thanet Grid Note
PME (Protective Multiple Earthing) is common across Thanet. Ze values are usually low, but in some older or rural parts of Margate and Broadstairs, TN-S or TT earthing may apply. TT installations (earth rod) often have higher Ze and rely on RCDs for protection. Coastal soil can vary - sandy areas may give higher rod resistance. Always verify earthing type before interpreting Zs.
Want Your Loop Impedance Checked?
EICRs and circuit testing include Zs measurement. We provide clear explanations and certificates for Margate, Ramsgate, and Broadstairs.
Professional Safety Checklist
- Isolate power at the consumer unit before any electrical work
- Use a voltage tester to confirm circuits are dead
- Ensure all work complies with BS 7671 (IET Wiring Regulations)
- Keep a record of any modifications for future reference
- When in doubt, consult a qualified electrician
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Expert Electrical Advice in Thanet
Still have questions? Contact Roy at Thanet Electrical for a free consultation in Margate, Ramsgate, or Broadstairs.
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