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Student & Learning Zone - City & Guilds
What fault can turn a TT into a TNCS and disable the RCD?
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<blockquote data-quote="ProDave" data-source="post: 263451" data-attributes="member: 6969"><p>That explains why the RCD does not trip.</p><p></p><p>Now the second part. Why has it "disabled" the RCD?</p><p></p><p>Well lets assume we now get a second fault, a faulty appliance plugged in that has a 1Kohm fault from L to E.</p><p></p><p>So that 1Kohm fault will cause a fault current of 0.23A to flow.</p><p></p><p>Without the N-E fault, that 0.23A fault would be seen as an imbalance by the RCD and it would trip.</p><p></p><p>But now, thanks to our N-E fault, the second L-E fault current will share the same two paths back to the star point, so will be split again in the same 1:500 ratio. So of that 0.23A fault current, only 0.46mA will flow down the earth lead, the rest will flow via the neutral, back through the RCD, maintaining an almost balanced load as seen by the RCD.</p><p></p><p>Hence on a second real L-E fault, the RCD won't trip.</p></blockquote><p></p>
[QUOTE="ProDave, post: 263451, member: 6969"] That explains why the RCD does not trip. Now the second part. Why has it "disabled" the RCD? Well lets assume we now get a second fault, a faulty appliance plugged in that has a 1Kohm fault from L to E. So that 1Kohm fault will cause a fault current of 0.23A to flow. Without the N-E fault, that 0.23A fault would be seen as an imbalance by the RCD and it would trip. But now, thanks to our N-E fault, the second L-E fault current will share the same two paths back to the star point, so will be split again in the same 1:500 ratio. So of that 0.23A fault current, only 0.46mA will flow down the earth lead, the rest will flow via the neutral, back through the RCD, maintaining an almost balanced load as seen by the RCD. Hence on a second real L-E fault, the RCD won't trip. [/QUOTE]
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Student & Learning Zone - City & Guilds
What fault can turn a TT into a TNCS and disable the RCD?
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