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??

 

[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.