Zs reading with main earth disconnected

[dtec dave]

Morning, just checking a lighting circuit on a house, I have the main earth disconnected and still get a zs reading of 2.2 ohms. No evidence of an earth spike and bonding goes direct to henley block.

Any suggestions why this could happen??

 

[Charlie_]

What conditions would they be?
I can't think of any reason to have an installation which relies on ADS energised without its connection to the earthing system.

Ever been to a property that has an earth fault due to the neighbours or for other extraneous reasons?

 

[davesparks]

Ever been to a property that has an earth fault due to the neighbours or for other extraneous reasons?

I don't believe I have, but I don't see how that requires the installation to be energised without being connected to earth?

 

[Silly Sausage]

I don't believe I have, but I don't see how that requires the installation to be energised without being connected to earth?

As long as there are no members of the public, what have you, present, does it matter?

 

[davesparks]

As long as there are no members of the public, what have you, present, does it matter?

How on earth would you risk assess it as safe to do to start with?

 

[Charlie_]

I have also disconnected the main earth to demonstrate to apprentices how parallel earths can appear to drag down the Ze or Zs readings of a TT installation.
I have also disconnected the main neutral to demonstrate how this can be dangerous..

 

[Charlie_]

How on earth would you risk assess it as safe to do to start with?

The same way you would for any other earth fault that could potentially give you an electric shock.

 

[Wilko]

The same way you would for any other earth fault that could potentially give you an electric shock.

Ha - I start by safely isolating the supply, not the earth .

 

[Charlie_]

Ha - I start by safely isolating the supply, not the earth .

And if you can’t safely isolate the supply?
Do you check the earth before you touch the metal board?

 

[Wilko]

And if you can’t safely isolate the supply?
Do you check the earth before you touch the metal board?

Maybe, in my dreams .

 

[Lucien Nunes]

Whatever the justification for energising some hypothetical system with the earth disconnected for demonstration or analytical purposes, the OP had a specific task in hand which was to trace a connection to a section of CPC that did not seem to start at the origin. As such, it's a simple continuity test to the MET, other CPCs or mass of earth, that does not require the system to be energised to trace. There is no justification here to work with the system energised.

 

[Charlie_]

Maybe, in my dreams .

You will do once you’ve had your 1st zap from a metal board

 

[dancat2022]

did you finally find the root cause of your problem ? if yes i would appreciate you share it. i have the same. w / wo main Earth wire connected i read same Zs 1.5 ohms. ( TT system) . thank you

 

[Risteard]

Neutral/Earth fault causing the reading. But Earth fault loop impedance testing with the Earthing conductor disconnected is stupid and dangerous.

OOPS... Sorry ANCIENT thread!

 

[timhoward]

did you finally find the root cause of your problem ? if yes i would appreciate you share it. i have the same. w / wo main Earth wire connected i read same Zs 1.5 ohms. ( TT system) . thank you

I’d start a new thread if I were you and explain why you are doing this in the first place. Only disconnect the main earth to get a Ze reading. Then do dead tests to investigate anything.
A Zs test with the main earth disconnected should never happen.

 

[dancat2022]

I’d start a new thread if I were you and explain why you are doing this in the first place. Only disconnect the main earth to get a Ze reading. Then do dead tests to investigate anything.
A Zs test with the main earth disconnected should never happen.

I have a TT system ( earth not connected anywhere in any panel with neutral ) and the result i git for Zs was too good to be true . I understood that for a TT system you can not get less than 10 ohms but in my case was 1.5 ohms. I just tested w/o main earth connected to see result..like trying to understand the phenomenon

 

[timhoward]

I have a TT system ( earth not connected anywhere in any panel with neutral ) and the result i git for Zs was too good to be true . I understood that for a TT system you can not get less than 10 ohms but in my case was 1.5 ohms. I just tested w/o main earth connected to see result..like trying to understand the phenomenon

I understand what you are asking - you are saying how can the earth path be this good.
It isn't impossible to get a TT Ze of 1.5 ohms, but I agree it's unlikely most of the time.

So to understand what's going on I'd measure the rod specifically by turning off, and disconnecting main earth and loop testing from Live to the wire going to the earth rod.
If you get a better Zs (when it's all back together) then it's usually down to bonded copper service pipes as they make quite good electrodes, and if one of them is connected to a house which isn't TT then there's actually a low resistance path back to the sub-station that way.
Obviously, this can't be relied on, so it's important to make sure the rod itself has a reasonable impedance.

I've had a Zs at the board of 0.4 ohms on a TT system before now (due to bonded services), but the rod was measuring over 400 ohms which I decided was too high so put a new one in.

 

[pc1966]

Unless you have quite salty ground and wet I would be very surprised if a normal rod would be as low as 1.5 ohms. However, if the rod ran close to a buried service pipe then it would be far easier to understand.

 

[davesparks]

. I understood that for a TT system you can not get less than 10 ohms but in my case was 1.5 ohms

Yes you can, you can get much less than a 10ohm Ze.