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Student & Learning Zone - City & Guilds
ProDave Problem, Lamp becomes live when CU switched Off!
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<blockquote data-quote="ProDave" data-source="post: 262185" data-attributes="member: 6969"><p>I think that's close enough so here's what actually happened.</p><p></p><p>It was a split load CU with one side just from the main switch and the other side RCD protected.</p><p></p><p>The CU was either assembled wrong in the factory, or reconfigured by the installer and he got it wrong.</p><p></p><p>The problem was the L feed to the RCD was connected to the incoming side of the main switch. The neutral feed to the RCD was correctly wired to the output side of the main switch. So when the main switch was turned off, the circuits appeared to go dead but the line was still connected to the circuits on the RCD side.</p><p></p><p>Closing my cutters around the damaged cable, bridged E to N in that cable and "completed" the circuit.</p><p></p><p>See the attached diagram to show what happened and where the current flowed:</p><p></p><p>What lessons can we learn from this?</p><p></p><p>NEVER assume turning a switch off isolates everything.</p><p></p><p>ALWAYS test for dead.</p><p></p><p>In a case like this, even if you are not expecting to do any work inside the CU, take it's cover off to actually make sure ALL circuits are dead. You would have either visually spotted this fault, or have found the RCD side to still be live had you opened the CU and tested.</p><p></p><p>Even though I normally hate them, a quick swipe of the cable I was about to cut with a volt stick would have shown it to be live, so I would have gone looking for the fault before cutting it.</p><p></p><p>My old age and to some extend complacency made me wrongly assume everything was dead because the main switch was off. Don't follow my example, do it properly and test first.</p><p></p><p>And lastly, this shows whoever did the original install and original inspection and testing, failed big time. this was an obvious fault as soon as you opened the CU so it makes you wonder if the installation was ever tested properly.</p><p></p><p>[ATTACH]2650[/ATTACH]</p></blockquote><p></p>
[QUOTE="ProDave, post: 262185, member: 6969"] I think that's close enough so here's what actually happened. It was a split load CU with one side just from the main switch and the other side RCD protected. The CU was either assembled wrong in the factory, or reconfigured by the installer and he got it wrong. The problem was the L feed to the RCD was connected to the incoming side of the main switch. The neutral feed to the RCD was correctly wired to the output side of the main switch. So when the main switch was turned off, the circuits appeared to go dead but the line was still connected to the circuits on the RCD side. Closing my cutters around the damaged cable, bridged E to N in that cable and "completed" the circuit. See the attached diagram to show what happened and where the current flowed: What lessons can we learn from this? NEVER assume turning a switch off isolates everything. ALWAYS test for dead. In a case like this, even if you are not expecting to do any work inside the CU, take it's cover off to actually make sure ALL circuits are dead. You would have either visually spotted this fault, or have found the RCD side to still be live had you opened the CU and tested. Even though I normally hate them, a quick swipe of the cable I was about to cut with a volt stick would have shown it to be live, so I would have gone looking for the fault before cutting it. My old age and to some extend complacency made me wrongly assume everything was dead because the main switch was off. Don't follow my example, do it properly and test first. And lastly, this shows whoever did the original install and original inspection and testing, failed big time. this was an obvious fault as soon as you opened the CU so it makes you wonder if the installation was ever tested properly. [ATTACH]2650._xfImport[/ATTACH] [/QUOTE]
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ProDave Problem, Lamp becomes live when CU switched Off!
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