Testing final ring

[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 

 

[Murdoch]

What’s the OCPD for the circuit?

 How did you get your r1 + r2

 

[Traineeboy]

It’s a 32a MCB type B 

 

[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 

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. 

 

[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

 

[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?

 

[Traineeboy]

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 ? 

 

[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 

 

[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. 

 

[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?

 

[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 

 

[Murdoch]

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 ?

 

[Traineeboy]

The Zdb is 0.15 the Ze is 0.14 

 

[Murdoch]

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

 

[Traineeboy]

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

 

[Murdoch]

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?

 

[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 ? 

 

[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. 

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.

 

[boltonsparky]

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. 

 

[kerching]

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

mine do