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Student & Learning Zone - City & Guilds
Testing final ring
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<blockquote data-quote="SPECIAL LOCATION" data-source="post: 500691" data-attributes="member: 250"><p>I think you will find that is only applicable whilst doing your Cross-Over "Line-CPC" measurements where they are substantially the same...</p><p></p><p>But once you un-cross the cables the R1+R2 will increase to the furthest socket on the ring..</p><p></p><p>Consider the calculation for just the CPC element on a 60m ring back to origin..</p><p></p><p>Furthest point; 2x 30m in parallel will be equivalent to 1x 15m</p><p></p><p>whereas 1x 10m in parallel with 1x 50m will be equivalent to less than 10m </p><p></p><p>same will apply for the R1</p><p></p><p>Hence also why ZS readings are not identical at every point if testing a live ring.</p><p></p><p>Although a spur is generally the highest point of any circuit..</p><p></p><p>If you have a 2m spur very close to the circuit origin.. R1+R2 (&amp; Zs) it will be lower than at socket on the far end of a 60m ring circuit..</p><p></p><p>(or even a spur connected to a ring at a CU!!) </p><p></p><p>Draw it out and do the maths.</p><p></p><p>As the earth fault loop will take the shortest path of least resistance and not go all the way round the full ring.</p><p></p><p>Which is why you should take account of where the spur connects to the circuit..</p><p></p><p>Not just assume its a spur it must be the largest value!</p><p></p><p> :coffee </p></blockquote><p></p>
[QUOTE="SPECIAL LOCATION, post: 500691, member: 250"] I think you will find that is only applicable whilst doing your Cross-Over "Line-CPC" measurements where they are substantially the same... But once you un-cross the cables the R1+R2 will increase to the furthest socket on the ring.. Consider the calculation for just the CPC element on a 60m ring back to origin.. Furthest point; 2x 30m in parallel will be equivalent to 1x 15m whereas 1x 10m in parallel with 1x 50m will be equivalent to less than 10m same will apply for the R1 Hence also why ZS readings are not identical at every point if testing a live ring. Although a spur is generally the highest point of any circuit.. If you have a 2m spur very close to the circuit origin.. R1+R2 (& Zs) it will be lower than at socket on the far end of a 60m ring circuit.. (or even a spur connected to a ring at a CU!!) Draw it out and do the maths. As the earth fault loop will take the shortest path of least resistance and not go all the way round the full ring. Which is why you should take account of where the spur connects to the circuit.. Not just assume its a spur it must be the largest value! :coffee [/QUOTE]
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Testing final ring
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