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Topic Title: Lets have another debate about an old subject
Topic Summary: Ze & Zs
Created On: 06 March 2013 09:52 PM
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 06 March 2013 09:52 PM
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Thripster

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Ok - so you all disconnect the main earthing conductor from the board when conducting a Ze measurement (or don't you?). Then, when it comes to measuring Zs, you disconnect the main Bonding Conductors and any other means by which there is a path to earth (or don't you?). When detailing your methods, how do you reconcile one set of results with the other (given R1+R2 in between for non reactive ccts)?


Regards
 06 March 2013 09:59 PM
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daveparry1

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No, we don't disconnect the main bonding when doing Zs tests!


Dave.
 06 March 2013 10:19 PM
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Dale76uk

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Thripster, why would you disconnect Parallel earth paths when carrying out Earth Fault Loops????
 06 March 2013 10:31 PM
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Thripster

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Hi Dale, not saying that I do but some advocate that you should on the basis that you get a true reading without parallel earth paths (Hinsley for one). My point is that, for the readings to correlate, then what you do for Zs you should do for Ze.
 06 March 2013 10:40 PM
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daveparry1

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No because you want to know the earth loop readings under working conditions, ie with all parallel paths in place.
 06 March 2013 10:46 PM
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Legh

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Ze would be to test the integrity of the supply earth without parallel paths as they may not exist when a new round of gas and water updating takes place

Zs confirms the present state of the earth path for each circuit.and is a standard indicator when conducting EICRs

An initial R1+R2 (tabulated) +Ze (measured) will give the reference point and where your subsequent Zs measurements would be compared to.

The problem lies with a Zs test energizing a circuit, for a short time where the obvious parallel paths are disconnected. This may cause some metalwork, not disconnected to appear live.

Legh

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 06 March 2013 11:41 PM
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ebee

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Under Lab conditions in a safe environment we might indeed do that.
But in reallife we C Y A and reconnect bonds before live Zs testing.
Which often makes Zs a lower figure than it would otherwise be

-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 07 March 2013 06:49 AM
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Thripster

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Why not (a) verify earth connection (b) meaure Ze and Zs with all earthing conductors / bonding conductors in place (reflecting the true operating conditions) which would allow you to examine results and expect to see a summation of the component parts of Zs.

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 07 March 2013 07:06 AM
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ebee

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You measure Ze with system powered down to check that Ze by itself is adequate and does actually exist.
If bonds etc were connected then the E might be coming solely from from the bonds and not from the supply.
Measure or calculate R1 +R2 and add to Ze then this should give an indication of what Zs should be.
Under ideal conditions you might be safe to measure Zs to see if adds up.
However most installations are not safe to do this so we connect the bonds and usually this brings our measured Zs reading down somewhat.

That`s why we can do the equation
Zs = Ze + R1 + R2
but never
Ze = Zs - R1 - R2 or
R1 + R2 = Zs - Ze

-------------------------
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Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 07 March 2013 07:21 AM
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Thripster

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Hi Ebee - do not think that I am explaining myself very well. Understand the current tests and why. Am proposing (a) verify earth connection (b) measure Zs and Ze with earthing conductor bonding conductors in place. You could then expect measured results to tie up exactly - whereas at the moment there is an element of interpretation.

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 07 March 2013 07:45 AM
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Richard64

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If we're talking a new installation, then it can't be energised before carrying out an earth continuity check (ie R1 + R2)
As has been stated already, adding this figure to the measured Ze (Bonds removed) will give the most onerous result of Zs. which is the one that we want.
If you are going to energise a system with the bonds off, especially while pumping a Zs fault current down it, make sure you have a good solicitor.
 07 March 2013 07:50 AM
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Thripster

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Hi Richard, read my posts again and try to understand what I am saying. If you then have a question, I'll attempt to answer it.

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 07 March 2013 07:53 AM
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ebee

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Yeah.
The problem is with bonds on is that there might be another parallel path to earth via pipework/other circuits so your perceived R1 + R2 might well include a R2 that has been lowered by this.
No problem in real life because it makes it a better earth path but we are not allowed to rely on it.
Sometimes with seperate lighting circuits the cpcs may be coupled at the odd place or two.
If we measure the R2 for each circuit separately we get the figure that we rely upon whereas once they are both(all) coupled the R2 for each circuit goes down (no bad thing and often adds a bit of redundancy against breaks) but we are actually aiming for the R2 on each circuit individually to achieve compliance

-------------------------
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Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 07 March 2013 07:54 AM
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John Peckham

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I would suggest you put Hinsley's book in the re-cycling bin and buy yourself a copy of IET Guidance Note 3.

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John Peckham

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 07 March 2013 08:28 AM
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ebee

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I don`t have Hinsley`s book John!

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Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 07 March 2013 09:19 AM
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tattyinengland

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Originally posted by: Thripster

(a) verify earth connection (b) measure Zs and Ze with earthing conductor bonding conductors in place.



HI Thripster: I'll try to put it another way and in answer to:

(a) Verify earth connection: if by this you mean Ze - the only way to verify this is as a live test - the supply does not belong to us. To do this you must switch off the distribution system (eg: the DB) and test only the external earth path - ie: disconnect the main earth from the earth bar and test only this cable.

(b) measure Zs and Ze with earthing conductor bonding conductors in place - this where I think there is a big misunderstanding on your part; You cannot measure Ze with bonding in place (otherwise it is called Zs) they are effectively two different things.

Ze is a measurement of the external part of the electrical supply - outside the household or prmisis. and

Zs is the measurement of both outside AND INSIDE including parrell earth paths such as gas and water lines, structural metal work (in industry you can immagine the vast change this would make to a Zs reading) etc etc.

Regards
 07 March 2013 09:21 AM
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AJJewsbury

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Bonds removed

I'm maybe stating the obvious, but disconnecting main bonding is no guarantee you're removing connections between the MET and extraneous-conductive-parts (and hence parallel paths to earth), since many c.p.c.s will connect to class 1 equipment that's in contact with extraneous-conductive-parts - CH boilers, pumps, motorized valves, immersion heaters, old bathroom circuits via supplementary bonding etc. So when measuring Ze (i.e. checking a reliable earth connection exists and is of sufficiently low impedance by itself), you could disconnect all c.p.c.s as well as main bonding, but it's probably a lot easier just to disconnect the earthing conductor and test to the bare end of that (and carries a much lower risk of unsatisfactory reassembly).

Parallel paths can exist in all sorts of ways (a c.p.c. may be connected to extraneous conductive parts or other c.p.c.s at several points within the one circuit), so even simple R2 reading (whether by long lead or part of R1+R2) is never going to be a reliable reading of the c.p.c. resistance alone. We need to accept that and be able to interpret readings accordingly - i.e. check that measurements do not exceed what would be expected, rather than exactly match.

The follow-on point is that we can't rely on measurements alone to prove the goodness of an installation - so don't leave your Mk1 eyeball at home.

- Andy.
 07 March 2013 09:38 AM
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Richard64

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Originally posted by: Thripster

Hi Richard, read my posts again and try to understand what I am saying. If you then have a question, I'll attempt to answer it.



Regards


Yes I have reread your posts.
I'm only stating what others have,
The Ze should be taken directly onto the suppliers earth. This should (note JP - should) give as true a reading as can be without including any parallell paths external to the property. So any removal of these at a later date, by others shouldn't (again - shouldn't) affect your initial reading.
The R1 + R2 will include internal parallell paths - GN3 and most diagrams I've seen consist of a link from the earth bar to the Live conductor - not removing the cpc. However these will be part of the system, ie containment, that will always be in place.
There should be no need to do a live Zs test, and as far as I am aware the NIC prefer you not to.
As far as livening up a system with the bonds removed, that's a massive no no.
Sorry to repeat what others have already said.
 07 March 2013 10:00 AM
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daveparry1

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I agree with all of that Richard except the "no live Zs test" part, i'm a confirmed live Zs test man myself and no niceic assessor has ever criticised me for it. (I think that's something for the regs book anoraks and pen-pushers, can't beat a Zs test fo see the actual working condition of a circuit!)

Dave.
 07 March 2013 10:17 AM
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Legh

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Firstly, most EICRs are required to prove the integrity of the installation. Why would we need to to them if we could be assured that the installation was safe before we started the tests?

As Ebee states, Zs testing a single circuit without any parallel PE paths is more of a laboratory test and the same principles cannot be applied and safety assured on an installation that is ripe for a periodic I&T.

I suppose, if you could design a test that would guarantee a safe working procedure when disconnecting all 'superfluous' R2 connections then I'm sure that we, TH and JP would be interested.

and as for Mk1 eyeballs. I met a bloke who has the Mk2 edition, (Mk1 refurbed) although he doesn't use them for periodic inspections of electrical installations, but selling me duff stuff on the trade counter.....

Legh

-------------------------
Why do we need Vernier Calipers when we have container ships?

http://www.leghrichardson.co.uk

"Science has overcome time and space. Well, Harvey has overcome not only time and space - but any objections."
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