Continuity of Protective Conductors

<h2>Why we test continuity</h2>
<p>Every protective conductor (CPC) must form a complete, unbroken path from every point in a circuit back to the main earthing terminal (MET). Without this path, fault current cannot return to operate the protective device — leaving exposed metalwork live and creating a life-threatening shock hazard.</p>
<p>Continuity testing uses a low-resistance ohmmeter (minimum 200mA test current per IET GN3) to verify this path is intact before the installation is energised.</p>

<h2>What to test</h2>
<ul>
<li><strong>CPCs</strong> — from every socket, switch and fitting back to the MET</li>
<li><strong>Main protective bonding</strong> — gas, water, oil, structural steel back to MET</li>
<li><strong>Supplementary bonding</strong> — where required (bathrooms, swimming pools)</li>
<li><strong>Ring final CPC</strong> — three-step test for ring final circuits</li>
</ul>

<h2>The test method</h2>
<p>Connect one lead of the ohmmeter to the MET (or a confirmed earth point). Connect the other lead to the CPC at the furthest point of the circuit. The reading should be low — typically less than 1Ω for most circuits. Record all readings on the schedule of test results.</p>

<h2>Acceptable values</h2>
<p>There is no single maximum value — the reading must be low enough that Zs (Ze + R1+R2) does not exceed the maximum for the protective device. However, any reading above 1Ω should be investigated. Very high readings or open circuits indicate a fault that must be rectified before energising.</p>

<h2>Key exam points</h2>
<ul>
<li>Test current must be at least 200mA to give accurate readings on low resistances</li>
<li>Continuity must be tested BEFORE insulation resistance</li>
<li>R2 (CPC resistance) + R1 (line conductor resistance) = R1+R2, used to calculate Zs</li>
<li>Record readings on the Schedule of Test Results</li>
</ul>