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.