ZS,ZE confusion

[pencilpusher]

I have a TNCS system in my ten year old house. I have a distribution circuit running to my detached garage 4 metres away via SWA. The PME is not exported to the garage but it has a separate TT system. My question is:

How is ZS measured at the DB in the garage and how can I check the earth fault loop (Ze) is OK via the TT system? I am aware there is only one Ze and that's at the origin (CU in house). I would normally measure Ze via line and the disconnected earthing conductor. If there is only one "real" Ze at the origin, how is the TT earthing loop checked?

ZS for the db circuit would surely be L/N to the armoured CPC which of course would not be connected at the garage DB but terminated in an appropriate gland in the garage CU but would still be accessible as a CPC for the measurements?

Apologies if this does not make much sense but it is driving me mad.

Thanks in anticipation.

PP

 

[johnboy6083]

your Zs for the circuits supplied from the garage DB will be relying on the rod, but the Zs of the distribution circuit will be using the armour.

 

[pencilpusher]

Thank you johnboy6083. Does that mean that there is no real Ze for the TT system as it is not the "origin" of the installtion and that ZS (garage circuits) will be the measurement of the earth rod suitability?

 

[johnboy6083]

you will need to test the rod, and record its Ra. for small installations, using a normal loop tester, with the earth lead connected to the disconected earthing conductor to the rod.

 

[pencilpusher]

you will need to test the rod, and record its Ra. for small installations, using a normal loop tester, with the earth lead connected to the disconected earthing conductor to the rod.

So although not actaul Ze, you would still test between line (at garage DB) and the disconnected earthing conductor to the rod? This can be taken as Ra? Do I have this right yet? Thanks

 

[johnboy6083]

Yes that's the method. This will be noted on the EIC, and I would record as Zdb.

 

[SPARTYKUS]

Hmmmm.

 

[Deleted member 26818]

Ze, Zs, Zdb and RA, are terms used to help you and anyone else reading your test results, determine where and what the measurements refer to.
Ze refers to the resistance of the earth fault path external to the installation with parallel earth paths disconnected.
Zs is the resistance of the earth fault path including parallel earth paths at the pont where the measurement is taken.
Zdb is the term used for resistance of the earth fault path measured at the DB with parallel earth paths connected.
Ra is the resistance of an earth electrode.
In the situation you describe, you could easilly have two different Ze readings, as the measurement taken at the origin of the installation, and the one taken at the earth rod would both be the resistance of the external earth fault path.
However to avoid confusion, the measurement taken at the earth electrode would normally be termed as Zdb2, as it is presumally the second DB in the installation.
You could, if you wanted to, term the measurement taken at the earth electrode as Ze DB2.
To obtain an acurate measurement or the resistance (ra) of an earth electrode, an earth electrode resistance tester is used.
However, in an installation where an RCD is used to provide earth fault protection, it is allowed (because accuaracy is not required) to use an EFLI tester. In such cases the Ze measurement replaces Ra, this measurement would include the earth conductor.

 

[malcolmsanford]

For the 1st time now the regulations in reg 612.8.1 recognizes that on a TT system, taking an Ra by use of an External Earth Fault Loop Impedance Test (Ze) may be used where the use of an Earth Electrode meter is not practicable.

Have to ask the IQ if that now means the part in GN 3 about only being able to do this if the installation is protected by an RCD will now be removed. Where is our man

 

[pencilpusher]

Ze, Zs, Zdb and RA, are terms used to help you and anyone else reading your test results, determine where and what the measurements refer to.
Ze refers to the resistance of the earth fault path external to the installation with parallel earth paths disconnected.
Zs is the resistance of the earth fault path including parallel earth paths at the pont where the measurement is taken.
Zdb is the term used for resistance of the earth fault path measured at the DB with parallel earth paths connected.
Ra is the resistance of an earth electrode.
In the situation you describe, you could easilly have two different Ze readings, as the measurement taken at the origin of the installation, and the one taken at the earth rod would both be the resistance of the external earth fault path.
However to avoid confusion, the measurement taken at the earth electrode would normally be termed as Zdb2, as it is presumally the second DB in the installation.
You could, if you wanted to, term the measurement taken at the earth electrode as Ze DB2.
To obtain an acurate measurement or the resistance (ra) of an earth electrode, an earth electrode resistance tester is used.
However, in an installation where an RCD is used to provide earth fault protection, it is allowed (because accuaracy is not required) to use an EFLI tester. In such cases the Ze measurement replaces Ra, this measurement would include the earth conductor.

Thank you to all who have tried to explain. Spinlondon - that is a great explanation BUT Zdb, as your explanation above, is measured with all parallel paths connected. However, you state that "to avoid confusion" I could show the TT Ze as Zdb2. But Zdb is with paths connected but external EFLI is measured with paths disconnected?? Thats my confusion. I would rather term it Ze DB2 as you stated at the end. Ze automatically says to me , "parallel earth paths disconnected" I am getting there I think!? PP

 

[WayneL]

I fully agree with Spartykus......hmmmm!

No wonder rumours and false info go round and round these forums.

Ze is the earth loop impedance external to the installation - this includes the Line conductor, the earth path back to the transformer (depending on system) and the disconnected Earthing conductor.

Zs is the sum of the impedances of the 'circuit' Line & CPC and the Ze (as above), with the earthing conductor connected. (i.e. including parallel paths).

Zdb is a name made up and used by some sparks to describe the Zs of the distribution circuit feeding a remote board......it's still Zs of the distribution circuit, though, whatever they try to call it.

Ra is the resistance of the 'earth rod' to earth - note resistance....not impedance.
Measured with an 'Earth Electrode Resistance Tester', it includes the CPC/Earthing conductor, the 'rod' and it's resistance to earth - it does not include the 'Line' conductor.........this is a dead test for resistance (hence the 'R' in Ra - as opposed to 'Z' in Zs).

I have always argued the fact that if you install a seperate means of 'earthing' (e.g. by TT'ing an outbuilding), then you are creating a seperate installation.
This installation would then have it's own Ze at the outbuilding Db, it's own 'Earthing Conductor' and it's own set of test results.

That's how I see it - there are many who disagree.:smile5:

 

[malcolmsanford]

I think your virtually spot on Wayne.

The Zdb is a term mostly used for commercial/industrial where taking a Ze at the origin would be virtually impossible mainly due to the fact of disconnecting and isolating an entire installation of something like a large factory.

I remember when testing was not as rigorous as it is today the term Zdb being used during my apprenticeship and that was over 30 years ago.

It is something that as overlapped into domestic installations in the last few years and to be frank very few unless they are commercial/industrial trained would have seen it. I think a lot of us, me included, sometimes forget that lads have trained and work purely on domestic and therefore some terms and procedures seem alien.

I personally would not use the term Ze2 as there is only the one Ze and that is at it origin. As Wayne as said once you have TT the garage you can indeed class this as a separate installation and a separate earthing arrangement within that installation and call that your Ze, but you would then need to produce separate certs for the sub main and then the garage install.

This is what I use to do as for me you are not relying on the house earthing system to provide the earth path in that garage you have completely divorced that, all the house earthing system does is protect the sub main to it. so anything within the garage is a totally new arrangement and IMO should be treated as such.

Of course if you had "exported" or extended the equipotential zone from the house to the garage then on the EIC in section for supply characteristics and earthing arrangements you would put your Ze and because it is a domestic installation there should also be no problems of putting the Ze on your SoTR

 

[pencilpusher]

Right, I see. Several differing views here. You can see why I am confused! As I have a separate earthing system (TT) in the garage, I will have two Ze's as such. One for the TNCS arrangement and one for the TT one. The SWA feeding the garage from the main CU will have a ZS which will be measured as any outher circuit, at its remote end. The rest of the garage circuits will have the ZS measurements taken as I would in any other install. The rule seems to be, treat both as seperate installs.

"but you would then need to produce separate certs for the sub main and then the garage install" confuses me too. The submain runs from a 16 amp way in the main CU so surely that SWA is certificated on the main CU cert? I see two installation certs, one for the main CU and one for garage one???

Cheers PP

 

[topquark]

I'm of the view that this is two installations. So, two certs ultimately needed as there are two installations. You'll need one cert to cover the TNCS installation (including the Zs of the distribution circuit protecting the cable). The other cert to cover the garage TT, with the Ze for that CU being the Rod(s)/Cable as measured.

Consider the fault path current when trying to decide if you have two separate installations. Here you have two separate routes to earth, one via the TT rod(s) (from circuit faults in the garage) and one via the TNCS provided main earth (from circuit faults in the house).

Anything extraneous in the garage should also be bonded to the TT earthing system (unless it also enters the house).

The Zs for the dist circuit should be measured via L/N and the armour (as you said). This will not be the same as the Ze (or Zdb) for the garage CU.

 

[pencilpusher]

I'm of the view that this is two installations. So, two certs ultimately needed as there are two installations. You'll need one cert to cover the TNCS installation (including the Zs of the distribution circuit protecting the cable). The other cert to cover the garage TT, with the Ze for that CU being the Rod(s)/Cable as measured.

Consider the fault path current when trying to decide if you have two separate installations. Here you have two separate routes to earth, one via the TT rod(s) (from circuit faults in the garage) and one via the TNCS provided main earth (from circuit faults in the house).

Anything extraneous in the garage should also be bonded to the TT earthing system (unless it also enters the house).

The Zs for the dist circuit should be measured via L/N and the armour (as you said). This will not be the same as the Ze (or Zdb) for the garage CU.

I have it now loud and clear and I am grateful for all those who have tried to help me. Two installations it is then with two earthing arrangements tested seperately. Thanks again. PP