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??

 

[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.