Ze and Zs Readings on TT Installation

[tonywevans]

I have a Ze measurement on a TT installation of 134 ohms but the Zs measurement I'm getting on the ring is 25 ohms.
Measured R1 + R2 on the ring is 0.82 ohms.

If Zs = Ze + (R1 + R2) could someone please explain why my readings do not stack up with the formula?

Have I taken a wrong reading or is this an expected result for a TT installation?

 

[scotsparky]

when you took the ZS did you have all main bonding? equipotential bonding ect disconnected?? probbably not! Its the earth paths you are picking up. what you have to do is transpose the formula and calculate the ZS with Your R1 R2 results

 

[tonywevans]

Ok thanks. The Zs measurement was taken with all the bonding still in place! But I'm still a bit confused. What should I put down as my Zs on the cert? Should I add together Ze and (R1 + R2) = 134.82 ohms and report that as the Zs on the certificate. I would have thought I should state the 25 ohms Zs value as measured.

 

[Marx]

Yeah, that's correct.

 

[the jock]

ze + zs are irrelevent ot TT systems just put n/a in box your 134 is way above the 1.22 or what ever

as long as Ze is less than 200 oms should be less than 100

 

[pevvers]

ze + zs are irrelevent ot TT systems just put n/a in box

as long as Ze is less than 200 oms

So, Ze is relevant then!!

Why would you not fill in a box that you're meant to, I always would even if it was calculated as Ze+(R1+R2) to give the worst case scenario....

That's just me though, I like filling my boxes!!

 

[Monkey1984]

So, Ze is relevant then!!

Why would you not fill in a box that you're meant to, I always would even if it was calculated as Ze+(R1+R2) to give the worst case scenario....

That's just me though, I like filling my boxes!!

I would always write down the value 'Measured' as opposed to by 'Calculation'.

The Zs will include parrallel paths through the equiotential bonding and the Ze is the Earth Fault Loop Impedance on the earth of the rod alone.

As its a TT system the value will fluctuate throughout the year due to the soil drying out in the summer etc.

However your Certificate is the readings obtained on the day you tested. - Therefore your measured value should be used. I would only use calculation if there was no way of obtaining a measured reading for one reason or another.

Maybe people will disagree with me??

 

[tonywevans]

Ok this helps me to understand a bit more. My understanding is that this all centres around whether your RCD will trip at or below the safe touch voltage of 50V. Let's say my measured earth electrode resistance was say 234 Ohms and my Zs was still 25 Ohms. This would mean that my 30 mA RCD would trip below 50V but the Earth electrode resistance could be unstable. But the fact that the Zs reading is so much lower than the earth electrode resistance then the Zs reading is mainly coming from other paths.

Is the Ze reading now irrelevant and I wouldn't need to put another electrode in?

Also the OSG states that Earth Loop Impedance (Zs) should be below 200 Ohms whereas the regs state that the Earth Electrode Resistance should be below 200 Ohms.

What's the right answer?

 

[Des 56]

Is the Ze reading now irrelevant and I wouldn't need to put another electrode in?

Its not irrelevant,it is slightly above the max recommended The parallel paths that give the good ZS reading are not to be relied on,
They could eg be metal water mains converted in the future to plastic
If you wish to bring the reading down,soil conditioning is one method
Sneaking the reading after a wet period of weather is another
Add another rod a few yards away from the original will also improve readings



Also the OSG states that Earth Loop Impedance (Zs) should be below 200 Ohms whereas the regs state that the Earth Electrode Resistance should be below 200 Ohms.

The electrode resistance is the value that we measure,because it is less practical to carry out than an earth loop test ,The loop reading is allowed as a comparison to a proper resistance test

 

[Mark2009]

Ok thanks. The Zs measurement was taken with all the bonding still in place! But I'm still a bit confused. What should I put down as my Zs on the cert? Should I add together Ze and (R1 + R2) = 134.82 ohms and report that as the Zs on the certificate. I would have thought I should state the 25 ohms Zs value as measured.

Hi
I done a loop impedance test today the results via the sockets was 0.28ohms

Thanks

Mark