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Adding an Earth Rod on a TN system and BS7671
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<blockquote data-quote="davetheglitz" data-source="post: 116126" data-attributes="member: 173"><p>I'm voting for ambiguous - but whilst I can't see the harm in it - I really can't see any point.</p><p></p><p>Consider a worst case of a TNCS system with an earth rod in a house. How would this be any different to a TNCS with a bonded water pipe? Could be done with no harm.</p><p></p><p>Consider a TNCS supplying an outbuilding with no added services - which is allowed - an earth rod would be just a 500 ohm impedance to the local earth - so again no harm.</p><p></p><p>Now consider a TNCS system supplying an outbuilding with services - if the earth is beefed up and the services are locally bonded - then the earth rod acts in the same way as the bonding for the most part - so again no harm.</p><p></p><p>Now in the light of the above why is TNCS prohibited in camp sites? Without an earth rod the loss of a neutral by any means would leave all metalwork at line potential. Now assuming a 16A load - and earth rod at say 500 ohms to the local earth, there is now a potential divider to the metalwork of approx 14 ohms to 500 - so the voltage at the metalwork is now 223v. Still pretty damn high!!</p><p></p><p>Overall the conclusion is that you could add an earth rod - but what is the point! Second conclusion is that you would be far safer if you TT'd the job anyway!</p></blockquote><p></p>
[QUOTE="davetheglitz, post: 116126, member: 173"] I'm voting for ambiguous - but whilst I can't see the harm in it - I really can't see any point. Consider a worst case of a TNCS system with an earth rod in a house. How would this be any different to a TNCS with a bonded water pipe? Could be done with no harm. Consider a TNCS supplying an outbuilding with no added services - which is allowed - an earth rod would be just a 500 ohm impedance to the local earth - so again no harm. Now consider a TNCS system supplying an outbuilding with services - if the earth is beefed up and the services are locally bonded - then the earth rod acts in the same way as the bonding for the most part - so again no harm. Now in the light of the above why is TNCS prohibited in camp sites? Without an earth rod the loss of a neutral by any means would leave all metalwork at line potential. Now assuming a 16A load - and earth rod at say 500 ohms to the local earth, there is now a potential divider to the metalwork of approx 14 ohms to 500 - so the voltage at the metalwork is now 223v. Still pretty damn high!! Overall the conclusion is that you could add an earth rod - but what is the point! Second conclusion is that you would be far safer if you TT'd the job anyway! [/QUOTE]
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Adding an Earth Rod on a TN system and BS7671
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