Testing final ring

[Traineeboy]

As I said in a previous post.     Do your leads need nulling for Ze /Zs test?

mine do

no they only need nulling on dead testing - continuity. 

That max Zs is the uncorrected value from the big book, better to use the 80% value in GN3 or the OSG, which would be 1.1 ohms (at 10C ambient temp) correct it to the likely testing temp at the moment of 20C and it should be max 1.14 ohms. 

fair point yeah times 0.8. That remind me on the section of the form on schedule of test - where it says maximum permitted Zs do we put the figure out of BS7671 or do we 0.8 x it and then write that figure ? I know you can take it from the on-site guide if you state so. 

 

[kerching]

What make of MFT is it?

 

[SPECIAL LOCATION]

If I wanted to obtain the Zs for a ring final circuit that had a spur ( furthest point )

Just as a side thought, in case you are not aware...

Just because a ring has one (or more) spurs off it...

The furthest point (highest Zs) can still be a socket directly connected to the ring.. not always at a spur..

e.g.

Imagine a ring with 5 sockets equally spaced 10m apart..  

The middle ring socket will have two parallel paths equal to the resistance of 15m worth of cable..  (2x 30m in parallel).

And either of the sockets nearest the supply will have parallel paths via a 10m & 50m cable run..

So resistance will be less than the value of 10m worth of cable due to parallel effect.

If this socket has a spur with 3m of cable the R1+R2 (& Zs)

will be proportional to less than 13m worth of cable resistance..

i.e.  The spur will have a lower Zs than the middle socket directly connected to the ring.

Whilst this may have no relevance to your particular question in hand..

It may be worth remembering so you don't go making any wrong assumptions in the future.

Guinness

 

[Traineeboy]

Just as a side thought, in case you are not aware...

Just because a ring has one (or more) spurs off it...

The furthest point (highest Zs) can still be a socket directly connected to the ring.. not always at a spur..

e.g.

Imagine a ring with 5 sockets equally spaced 10m apart..  

The middle ring socket will have two parallel paths equal to the resistance of 15m worth of cable..  (2x 30m in parallel).

And either of the sockets nearest the supply will have parallel paths via a 10m & 50m cable run..

So resistance will be less than the value of 10m worth of cable due to parallel effect.

If this socket has a spur with 3m of cable the R1+R2 (& Zs)

will be proportional to less than 13m worth of cable resistance..

i.e.  The spur will have a lower Zs than the middle socket directly connected to the ring.

Whilst this may have no relevance to your particular question in hand..

It may be worth remembering so you don't go making any wrong assumptions in the future.

Guinness

Thanks for pointing that  out and yes I understand what your saying. 

 

[Andy™]

Imagine a ring with 5 sockets equally spaced 10m apart..  

The middle ring socket will have two parallel paths equal to the resistance of 15m worth of cable..  (2x 30m in parallel).

And either of the sockets nearest the supply will have parallel paths via a 10m & 50m cable run..

So resistance will be less than the value of 10m worth of cable due to parallel effect.

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

 

[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?

 

[SPECIAL LOCATION]

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  

 

[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..

 

[Andy™]

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...

 

[Sidney]

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.