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Traineeboy

Testing final ring

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Traineeboy

If I wanted to obtain the Zs for a ring final circuit that had a spur ( furthest point ) Then from what I under I would take the Ze and add the R1 + R2 readings. Ze is 0.15 and R1 + R2 is 0.15 so the measured Zs would be 0.30. 
 

However I was told that I could obtain the Zs I wouldn’t have to do the above I would just need to plug my Megger tester in at the furthest socket (in this case it’s a spur so know it’s furthest point ) Put in on Loop Z and L-PE and Select 2 lead Low setting. I did that and got 1.39. 
 

Those readings a very different it’s convenient fused me as I thought it was Zs = Ze + (R1 + R2) but then I saw the thing about plugging in the tester while the install is live and it tells you the Zs but as I said the readings are very different. 
 

please help me understand this 

* its confused me - not convenient fused lol 

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Murdoch

What’s the OCPD for the circuit?

 

 How did you get your r1 + r2

Edited by Murdoch

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Traineeboy

It’s a 32a MCB type B 

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Traineeboy

The max Permitted for 32a MCB Type B is 1.37 therefore 1.39 would fail if I was to use that measurement but I do it via Calculation ( which is the way I was taught) then I get 0.30 which is totally fine. Would I be getting a much higher Reading , taking the Zs via measurement because of parallel earths ? 
 

as 1.09 difference is a big difference 

12 minutes ago, Murdoch said:

What’s the OCPD for the circuit?

 

 How did you get your r1 + r2


got the r1 + r2 via testing at the board. end to end then Figure of 8. Took highest reading on the circuit. 

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Andy™

also worth noting that some manufacturers max Zs are higher than those in BS7671, so if your doing I&T and a Zs is higher than 7671, check MI's first because it may still be acceptable

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Murdoch

This is a very good example of why dead tests are vital


did you add the spur and if so how is it connected to the ring?

 

Edited by Murdoch

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Traineeboy
Just now, Murdoch said:

This is a very good example of why dead tests are vital

 

 


Yes I have always got the Zs via Zs = Ze + (R1 + R2) not via measurement. 
 

It just confused me when I was told about direct measurement. I did just read something about RCD uplift. Would that affect the Zs Reading via measurement ? 

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Murdoch

Some makes of rcd or RCBO can add to the Zs  

 

i would test at other sockets on the ring to get comparison readings 

Edited by Murdoch

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Traineeboy

Done measurement with tester and other sockets are reading between 1.20 - 1.39 . Other than an RCD uplift I don’t get why my calculated measurement is much lower than the measured reading from the tester. 
 

apart from RCD uplift what could this be ? 
 

I presume that the calculated way is preferred anyway due to the EAWR 1989 reg 14 about working near live parts. 

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Murdoch

You could do the Zs test on the load side of the RCD to see the difference between the Ze and that reading too - which should be negligible 

 

What are the end to end readings of the ring?

 

How is your spur connected to the ring?

Edited by Murdoch

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Traineeboy

End to End 

 

r1 = 0.15

r2 = 0.21

rn = 0.15 

 

the spur is just coming directly out a socket next to it on the ring. 

Also the ring circuit is off a garage board which is then off the house board via a 32a MCB 

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Murdoch
28 minutes ago, Traineeboy said:

 

Also the ring circuit is off a garage board which is then off the house board via a 32a MCB 


 

so what is the Zdb ?

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Traineeboy

The Zdb is 0.15 the Ze is 0.14 

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Murdoch
Just now, Traineeboy said:

The Zdb is 0.15 the Ze is 0.14 


are you sure about the Zdb? 
 

that would suggest it’s almost next to the main CU

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Traineeboy

Yes the garage unit is on the other side of the wall 

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Murdoch
14 minutes ago, Traineeboy said:

Yes the garage unit is on the other side of the wall 


Good answer

 

i would suggest you do a Zs on the load side of the RCD

 

 what make of board is it?

Edited by Murdoch

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Traineeboy

The Garage board is a British General 

Just throw another one in there on testing. With IR testing if you don’t have a piggy back lead , when IR testing I presume you can just do between L-E and then N-E and record the lowest Answer as the Live - Earth value ? 

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boltonsparky

Did you take the R1+R2 at the socket fronts (plug in tester) or at the terminals with the probes and did you use the same method when taking the Zs?

 

Worn out sockets and switches can give high readings but if you're using the same method for both dead and live tests this would be irrelevant. Have you nulled the leads, some testers need the leads nulling for live tests too. 

 

1 hour ago, Traineeboy said:

 

I presume that the calculated way is preferred anyway due to the EAWR 1989 reg 14 about working near live parts. 

 

Calculated Zs is ok if you have a real life measured Ze.

If you do it all dead and get your Ze by enquiry and then add on your R1+R2 to get your Zs you can have no effective earth and be unaware of it. 

Nothing wrong with live testing if you're aware of the risks and take precautions. Live testing on a socket circuit is no more dangerous than plugging in the kettle. Live testing is essential as far as I'm concerned, for the above reason.

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boltonsparky
2 hours ago, Traineeboy said:

The max Permitted for 32a MCB Type B is 1.37 therefore 1.39 would fail if I was to use that measurement

 

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. 

 

Edited by boltonsparky

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kerching

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

mine do

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Traineeboy
1 hour ago, kerching said:

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. 

1 hour ago, boltonsparky said:

 

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. 

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kerching

What make of MFT is it?

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SPECIAL LOCATION
7 hours ago, Traineeboy said:

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

 

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Traineeboy
7 hours ago, SPECIAL LOCATION said:

 

 

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. 

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Andy™
11 hours ago, SPECIAL LOCATION said:

 

 

 

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

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