The Difference Between 'Earth Fault Loop Impedance' and 'Ground Resistance' — They're Not the Same
Earth fault loop impedance (Zs) and ground/earth resistance (R_E) are frequently confused, but they measure entirely different things. Zs is the full fault circuit path that determines protection clearing time. R_E is the electrode-to-earth resistance that determines touch voltage. Mixing them up is dangerous.
Two Measurements, Two Purposes
These two values are routinely conflated in site reports, and the confusion can kill. They measure fundamentally different things:
| Parameter | Earth Fault Loop Impedance (Zs) | Earth/Ground Resistance (RE) |
|---|---|---|
| What it measures | Full circuit: source → line conductor → fault → PE conductor → back to source | Electrode to general mass of earth only |
| Unit | Ohms (Ω) | Ohms (Ω) |
| Purpose | Determines if protection device operates fast enough under fault | Determines touch voltage in TT/IT systems |
| Standard reference | BS 7671, Regulation 411.4.4 | BS 7671, Regulation 411.5.3 |
| Typical value | 0.2–2.0 Ω (TN systems) | 2–200 Ω (depends on soil) |
| Test instrument | Loop impedance tester | Fall-of-potential earth tester |
Why Confusing Them Is Dangerous
In a TN system, the earth fault return path is through the metallic PE conductor back to the transformer neutral. Zs is low (typically under 1 Ω), fault current is high, and the MCB or fuse operates in its fast region. RE is largely irrelevant to protection operation — the fault current does not flow through the earth electrode.
In a TT system, the fault return path goes through the installation’s earth electrode, through the general mass of earth, and back via the supply authority’s earth electrode. Zs is dominated by RE, which may be 20–200 Ω. Fault current is low (often 1–10A), and overcurrent devices will not operate fast enough — which is why BS 7671, Regulation 411.5.2 requires an RCD for TT systems.
The dangerous confusion: an engineer measures RE = 8 Ω at a TT installation and concludes “the earthing is fine.” But 8 Ω means the fault current is only 230/8 = 29A. A 32A MCB will not trip on 29A — certainly not within the 0.4s required by Regulation 411.3.2.2. The installation needs an RCD, regardless of how good the earth electrode is.
Know Which One You Need
Before you test, know what you are testing for:
- Verifying overcurrent protection operates fast enough? → Measure Zs (loop impedance).
- Verifying touch voltage is within limits? → Measure RE (earth resistance).
- Verifying RCD protection in TT systems? → You need both: RE to confirm touch voltage, and RCD test to confirm trip time.