Prospective Fault Current Calculation

<h2>What is prospective fault current?</h2>
<p>Prospective fault current (PFC) is the maximum current that would flow under fault conditions. It must be calculated to verify that protective devices have adequate breaking capacity — a device that cannot interrupt fault current may explode, causing fire or injury.</p>

<h2>PFC at the origin</h2>
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PFC = Uo / Ze
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<p>Where Uo = nominal line-to-earth voltage (230V) and Ze = external earth fault loop impedance.</p>

<h2>Worked example</h2>
<p>Ze measured at origin = 0.28Ω</p>
<p>PFC = 230 / 0.28 = <strong>821A</strong></p>
<p>The MCB or fuse at the origin must have a rated breaking capacity (Icc) of at least 821A. Standard domestic MCBs are rated 6kA — more than sufficient.</p>

<h2>Short circuit current between line and neutral</h2>
<p>For a bolted short circuit between line and neutral:</p>
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PSCC = Uo / (R1 + Rn)
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<p>This is used to verify protective devices at distribution boards and sub-boards.</p>

<h2>Why PFC matters on site</h2>
<ul>
<li>A device with too low a breaking capacity may not interrupt the fault current</li>
<li>This can cause arcing, fire, and catastrophic failure of the protective device</li>
<li>Always check device Icc rating against calculated PFC</li>
</ul>

<h2>Key exam points</h2>
<ul>
<li>PFC = Uo / Ze where Uo = 230V</li>
<li>PFC at Ze = 0.35Ω: 230 / 0.35 = 657A</li>
<li>PFC at Ze = 0.20Ω: 230 / 0.20 = 1150A</li>
<li>Standard MCBs rated 6kA — adequate for all domestic supplies</li>
<li>Record PFC on schedule of test results</li>
</ul>