Earth Electrodes and TT Systems

<h2>What is an earth electrode?</h2>
<p>In a TT system, the installation earth is provided by a local earth electrode driven into the ground — there is no metallic connection back to the supply neutral. The electrode provides the return path for fault current through the general mass of earth.</p>

<h2>Types of earth electrode</h2>
<ul>
<li><strong>Rods or pipes</strong> — most common for domestic TT systems. Minimum 1.2m long, typically 16mm diameter copper-bonded steel.</li>
<li><strong>Tapes or wires</strong> — buried horizontally</li>
<li><strong>Plates</strong> — buried vertically or horizontally</li>
<li><strong>Earth mats</strong> — for large installations</li>
<li><strong>Cable sheaths</strong> — where permitted</li>
</ul>

<h2>Measuring earth electrode resistance (Ra)</h2>
<p>The resistance of a TT earth electrode is measured using a dedicated earth electrode resistance tester using the three-electrode (fall of potential) method:</p>
<ol>
<li>Drive two additional test electrodes — current electrode (C) at least 10× the electrode length away, potential electrode (P) midway</li>
<li>Pass AC test current between C and the installation electrode</li>
<li>Measure voltage drop at P</li>
<li>Ra = V/I</li>
</ol>

<h2>RCDs on TT systems</h2>
<p>On TT systems, Zs will typically be too high (50–200Ω) for overcurrent devices to operate quickly enough. RCDs are essential for fault protection on TT systems. The required condition is:</p>
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Ra × IΔn ≤ 50V
</div>
<p>For a 30mA RCD: Ra must be ≤ 50 / 0.03 = 1667Ω — easily achievable. Regulation 411.5.3.</p>

<h2>Key exam points</h2>
<ul>
<li>TT system: no metallic earth return — uses local electrode</li>
<li>Measure Ra using fall of potential method</li>
<li>RCDs essential on TT — overcurrent devices cannot provide fault protection</li>
<li>Ra × IΔn must not exceed 50V</li>
<li>Typical domestic TT electrode: 1667Ω maximum with 30mA RCD</li>
</ul>