Insulation Resistance Testing

<h2>Purpose of IR testing</h2>
<p>Insulation resistance (IR) testing verifies the integrity of the cable insulation between live conductors and between conductors and earth. Degraded insulation allows leakage current to flow — causing shock risk, fire risk and RCD nuisance tripping.</p>

<h2>Test voltages and minimum values (BS 7671 Table 61)</h2>
<table>
<tr><th>Circuit type</th><th>Test voltage</th><th>Minimum IR</th></tr>
<tr><td>SELV and PELV</td><td>250V DC</td><td>1 MΩ</td></tr>
<tr><td>Up to 500V (all LV)</td><td>500V DC</td><td>1 MΩ</td></tr>
<tr><td>Above 500V</td><td>1000V DC</td><td>1 MΩ</td></tr>
</table>

<h2>Before testing — what to disconnect</h2>
<ul>
<li><strong>Surge protective devices (SPDs)</strong> — will be destroyed by 500V DC</li>
<li><strong>Electronic dimmers</strong> — contain components that cannot withstand test voltage</li>
<li><strong>Sensitive electronic equipment</strong> — computers, controllers, smart home devices</li>
<li><strong>Lamps and luminaires</strong> on lighting circuits (or short L-N at each fitting)</li>
</ul>
<p>Regulation 643.3 requires that precautions are taken to prevent damage to equipment.</p>

<h2>Test method for a complete installation</h2>
<ol>
<li>Isolate the supply and verify dead</li>
<li>Connect all line and neutral conductors together at the consumer unit (or close all MCBs)</li>
<li>Disconnect sensitive equipment</li>
<li>Test between all live conductors connected together and earth</li>
<li>Also test between line and neutral with earth disconnected</li>
</ol>

<h2>Interpreting results</h2>
<ul>
<li><strong>Above 200MΩ</strong> — excellent, new installation</li>
<li><strong>1–200MΩ</strong> — satisfactory</li>
<li><strong>Below 1MΩ</strong> — unsatisfactory — investigate before energising</li>
<li><strong>Very low (less than 0.5MΩ)</strong> — serious fault — do not energise</li>
</ul>

<h2>Key exam points</h2>
<ul>
<li>Test voltage for standard LV circuits: 500V DC</li>
<li>Minimum result: 1MΩ for all circuit types</li>
<li>SPDs MUST be disconnected — they will fail under test voltage</li>
<li>IR testing comes AFTER continuity in the CRIPPER sequence</li>
<li>Low IR = insulation fault, moisture, or connected equipment — investigate all three</li>
</ul>