Main and Supplementary Bonding

<h2>Main protective bonding</h2>
<p>Main protective bonding connects all extraneous conductive parts entering the building to the main earthing terminal (MET). This limits the voltage between different metalwork during a fault, reducing shock risk.</p>

<p><strong>What must be bonded:</strong></p>
<ul>
<li>Gas installation pipework — as near as practicable to the meter</li>
<li>Water installation pipework — as near as practicable to the meter</li>
<li>Oil installation pipework</li>
<li>Structural steelwork</li>
<li>Lightning protection systems</li>
</ul>

<p><strong>Minimum conductor size:</strong> Half the CSA of the earthing conductor, minimum 6mm², maximum 25mm² (copper). Regulation 544.1.</p>

<h2>Supplementary bonding</h2>
<p>Supplementary bonding provides additional protection in special locations (bathrooms, swimming pools) by connecting simultaneously accessible exposed and extraneous conductive parts together. This limits touch voltage to a safe level even if the main earth path is broken.</p>

<p><strong>Supplementary bonding can be omitted in bathrooms if:</strong></p>
<ul>
<li>All circuits serving the location have RCD protection (IΔn ≤ 30mA), AND</li>
<li>All extraneous conductive parts are connected to the protective conductor of the circuits serving the location</li>
</ul>

<h2>Testing bonding continuity</h2>
<p>Use a low-resistance ohmmeter. Resistance should typically be less than 0.05Ω for short bonding conductors. Record on the schedule of test results. Any reading that would result in a touch voltage exceeding 50V during a fault must be investigated.</p>

<h2>Key exam points</h2>
<ul>
<li>Main bonding is to MET — supplementary bonding is local to the special location</li>
<li>Minimum main bonding = 6mm² copper or half earthing conductor CSA</li>
<li>Supplementary bonding can be omitted with 30mA RCD protection on all circuits</li>
<li>Regulation 411.3.1.2 covers main bonding, Section 701 covers bathrooms</li>
</ul>