Earth Fault Loop Impedance — Theory and Measurement

<h2>What is earth fault loop impedance?</h2>
<p>When a line conductor touches exposed metalwork (a fault), fault current flows from the supply, through the line conductor, through the fault, back through the CPC and main earth, and through the supply network back to the source. The total impedance of this path is called the <strong>earth fault loop impedance (Zs)</strong>.</p>

<h2>The Zs formula</h2>
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Zs = Ze + (R1 + R2)
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<ul>
<li><strong>Ze</strong> — External impedance (supply network side, measured at origin with main switch open)</li>
<li><strong>R1</strong> — Resistance of line conductor from origin to furthest point</li>
<li><strong>R2</strong> — Resistance of CPC from furthest point back to MET</li>
</ul>

<h2>Why Zs must not exceed the maximum</h2>
<p>The higher the Zs, the lower the fault current (I = V/Z). If Zs is too high, fault current may be insufficient to operate the protective device within the required disconnection time — leaving metalwork live for too long.</p>
<p>BS 7671 Table 41.3 gives maximum Zs values for each type and rating of protective device.</p>

<h2>Temperature correction</h2>
<p>The Zs values in Table 41.3 assume conductors at <strong>70°C</strong> (maximum operating temperature). Tests are carried out at ambient temperature (typically 10–20°C). Conductor resistance increases with temperature, so:</p>
<ul>
<li>Multiply measured Zs by 1.2 to estimate the value at 70°C, OR</li>
<li>Compare measured Zs against <strong>80% of the tabulated maximum</strong></li>
</ul>

<h2>Measuring Ze</h2>
<p>Ze is measured at the origin of the installation with the main switch open (to exclude the installation conductors). Alternatively, it can be obtained from the Distribution Network Operator (DNO). Typical values: TN-S 0.2–0.4Ω, TN-C-S 0.1–0.35Ω, TT varies.</p>

<h2>Key exam points</h2>
<ul>
<li>Zs = Ze + (R1+R2) — must know this formula</li>
<li>Max Zs: 32A Type B = 1.44Ω at 70°C, compare measured against 1.15Ω at ambient</li>
<li>Max Zs: 16A Type B = 2.87Ω at 70°C, compare measured against 2.30Ω at ambient</li>
<li>Apply 0.8 correction to tabulated value before comparing with measured result</li>
<li>Ze measured at origin with main switch open</li>
</ul>