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Testing final ring

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Sidney

If your dead tests are all good at the spur then I would suspect that the low test on the MFT can be a significant cause. This setting puts out a lower current to test the Ze so isn't as accurate, couple with that possible loose connections by the socket clamps then you can see unexpected results.

 

Have you taken the Zs from the terminations on the back of the socket rather than plugging in?

Edited by Sidney

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SPECIAL LOCATION
2 hours ago, Andy™ said:

 

eh? on a ring, your R1R2 should be the same at every socket. youve still got the exact same length of copper at every point. so a spur would be higher

 

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 

    

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apprentice87

The massive difference is because there is an RCD in circuit.. This has had me not so much puzzeled as courgetteed about a few times.. Bypass the RCD and try the test again..

 

john..

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Andy™
4 hours ago, SPECIAL LOCATION said:

 

I think you will find that is only applicable whilst doing your Cross-Over "Line-CPC" measurements where they are substantially the same...

 

 

:coffee 

    

 

yep, for some reason i was thinking of figure 8. no idea where that came from. probably skipping parts and not fully reading others...

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Sidney
1 hour ago, apprentice87 said:

The massive difference is because there is an RCD in circuit.. This has had me not so much puzzeled as courgetteed about a few times.. Bypass the RCD and try the test again..

 

john..

Good point, I’d forgotten about RCD uplift. 

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