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Do-It-Yourself (DIY) Question & Answer Forum
Zs formula
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<blockquote data-quote="kme" data-source="post: 9629" data-attributes="member: 113"><p>Are we talking Zs or Ze (actually Ra on a TT)?</p><p></p><p>Zs is dependant on the type and rating of protective device on the circuit in question. Therefore you have not provided sufficient information mate!</p><p></p><p>However, Ra ( the rod impedance) is more the question. The formula is:</p><p></p><p>RCD rated tripping current ( in amps) multiplied by Ra must be less than or equal to 50(volts).</p><p></p><p>ergo 300mA (0.3A) X 167 ohms = 50.1</p><p></p><p>The max Ra is actually 166.66 recurring.</p><p></p><p>Now I`ll ask you a question, if I may.</p><p></p><p>What is this TT installation, that is covered by a 300mA RCD? If it contains socket outlets, intended to be used by unskilled persons, the RCD should be 30mA</p><p></p><p>HTH</p><p></p><p>KME</p></blockquote><p></p>
[QUOTE="kme, post: 9629, member: 113"] Are we talking Zs or Ze (actually Ra on a TT)? Zs is dependant on the type and rating of protective device on the circuit in question. Therefore you have not provided sufficient information mate! However, Ra ( the rod impedance) is more the question. The formula is: RCD rated tripping current ( in amps) multiplied by Ra must be less than or equal to 50(volts). ergo 300mA (0.3A) X 167 ohms = 50.1 The max Ra is actually 166.66 recurring. Now I`ll ask you a question, if I may. What is this TT installation, that is covered by a 300mA RCD? If it contains socket outlets, intended to be used by unskilled persons, the RCD should be 30mA HTH KME [/QUOTE]
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