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??

 

[APE37]

Parallel earth path with bonding still in place?

 

[telectrix]

pics. would help. is you 2.2.ohms a reading from the bonding?

 

[dtec dave]

Bonding not connected, it goes direct into Henley block. Main earth comes from henley block but is disconnected.

 

[static zap]

Outside lights (parallel path)?

 

[Strima]

Parallel path via CPC to boiler circuit CPC to boiler manifold to pipework within the fabric of the building?

Also why is the installation energised with the main earth disconnected?

 

[APE37]

Good point. Regarding the installation energised with main earth disconnected.

 

[buzzlightyear]

Also why is the installation energised with the main earth disconnected

stupid thing any body can do ,op where you going to rely on the bonding .

 

[westward10]

I don't understand what you are doing. Zs at board with earthing conductor disconnected?

 

[Lucien Nunes]

I don't understand either. And why would you expect the CPC to be insulated from earth throughout the installation? It could take any value from infinity to zero without indicating anything abnormal. I can see why it might be useful to know for forensic analysis but not for normal testing.

 

[7029 dave]

If you have main earth disconnected , surely it would be ZE not ZS.? (at source)

 

[DPG]

If you have main earth disconnected , surely it would be ZE not ZS.? (at source)

He says he's checking a lighting circuit though, not at the source.

 

[dtec dave]

Parallel path via CPC to boiler circuit CPC to boiler manifold to pipework within the fabric of the building?

Also why is the installation energised with the main earth disconnected?

 

[dtec dave]

So to answer why I had the circuit energised with the main earthing disconnected..

The lighting circuit connected in board had no CPC so I checked the Zs at a ceiling rose.

I had a reading of 2.2 ohms so I then tried to find out where the earth was coming from.

I disconnected all the CPC's from the other circuit in the board, checked Zs again and still had 2.2 ohms.

Just try and eliminate things and to see if I was picking up earth from elsewhere I then disconnected the main earth and still had a reading of 2.2 ohms.

 

[dtec dave]

Parallel path via CPC to boiler circuit CPC to boiler manifold to pipework within the fabric of the building?

Also why is the installation energised with the main earth disconnected?
[/QUOTE

 

[Charlie_]

Many years ago I used to disconnect the main earth and carry out various tests to help me understand parallel earths in electrical installations.
Done lots of other experimental testing in a controlled environment...

 

[westward10]

Disconnecting the earthing conductor for an energised system isn't the wisest thing to do. You are better to try dead tests by way of a continuity tester.

 

[buzzlightyear]

Many years ago I used to disconnect the main earth and carry out various tests to help me understand parallel earths in electrical installations.
Done lots of other experimental testing in a controlled environment...

working on the HIGGS BOSON PROJECT.

 

[Baddegg]

Many years ago I used to disconnect the main earth and carry out various tests to help me understand parallel earths in electrical installations.
Done lots of other experimental testing in a controlled environment...

You can’t make an omelette without breaking a few eggs.....

 

[davesparks]

Just try and eliminate things and to see if I was picking up earth from elsewhere I then disconnected the main earth and still had a reading of 2.2 ohms.

Why do this as a live test when that means doing something as stupid as energising the installation with the earthing conductor disconnected?

Electricians are supposed to understand the fundamental principles of electrical safety!

 

[dtec dave]

Why do this as a live test when that means doing something as stupid as energising the installation with the earthing conductor disconnected?

Electricians are supposed to understand the fundamental principles of electrical safety!

Thanks for that post really helpful!

 

[The Ghost]

Are you doing a L-N Zs or L-PE Zs as I know my kewtech can do both. What system is it? i.e. TN-C-S or?

 

[westward10]

Zs is line to earth external impedance, it stands for nothing else.

 

[Lucien Nunes]

So you've got a circuit with a CPC present in at least one fitting, but no CPC present at the DB, and you want to find out where that CPC is getting its earth from, to decide whether it's trustworthy?

If it were me, I'd turn the board off, maybe separate out the CPCs, null out a nice long wander lead, clip the end of it on the CPC and probe various likely contenders around the house with the meter on low ohms, to see which gives the lowest reading and hence is the likely tap off point. I can't see any advantage investigating this live, but many disadvantages.

Or, since it's at least a bit of a bodge, maybe a lot of a bodge, declare the circuit CPC-less and treat accordingly.

 

[The Ghost]

Zs is line to earth external impedance, it stands for nothing else.

I understand that but my MFT has those two choices. The KT63 has a piggy back lead that you place the earth lead into to test across L-N I am not truly sure of the purpose of this but there it is. Perhaps you could shed some light on that?

 

[The Ghost]

Of course there is PSCC and PEFC which I also understand before you go there. I know I should have said loop test. Or PFC PSC even

 

[Wilko]

Another vote here for never ever dis the earth on a live installation. Dead test all you want .

 

[Charlie_]

Depending on circumstances, main earth may need to be disconnected to verify certain fault conditions..
All comes down to knowledge, understanding what is going on and being in control of the location.

 

[davesparks]

Depending on circumstances, main earth may need to be disconnected to verify certain fault conditions..
All comes down to knowledge, understanding what is going on and being in control of the location.

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.

 

[davesparks]

Of course there is PSCC and PEFC which I also understand before you go there. I know I should have said loop test. Or PFC PSC even

Yes it is a loop impedance test which is carried out.
PFC and PSC are not tested or measured, they are calculated by the meter based on the loop impedance and voltage measured at the time.

 

[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.