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Topic Title: Strange Zs reading on TT install.
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Created On: 08 February 2013 07:57 PM
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 08 February 2013 07:57 PM
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alanblaby

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I've been doing some work on a TT supplied installation today.
Before I started, I did both a Ze and Zs (at a socket outlet).
The Ze was 1451 Ohms, not surprised at that seeing the state of the rod.
Zs was 5.6 Ohms. Ok, it's earthing would be through the water pipe bonding.
Then, I went to see where the water pipe was bonded, and found the cable not connected at all.
So, I was getting a 5.6Ohms reading with no apparent means of earthing, apart from the high 1451 ohm main earthing conductor.

So, my query, how is the Zs so low?
My only thought must be it was being earthed via all the steel socket boxes, and things such as the boiler, which would have a direct contact between the CPC and water pipe, making acircuit to earth.

Any other ideas?

Ta
Alan.

Edited: 08 February 2013 at 08:13 PM by alanblaby
 08 February 2013 08:02 PM
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daveparry1

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And the gas pipe Alan?

Dave.
 08 February 2013 08:12 PM
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alanblaby

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No gas. Oil was there, but not bonded.
 08 February 2013 08:16 PM
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daveparry1

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I see, but is the oil feed pipe metal and does it go through the ground anywhere.
 08 February 2013 08:19 PM
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alanblaby

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No, through the wall, direct to the oil tank at the back of the kitchen wall. It wasnt even touching the ground, but clearly was touching the walls as it goes through.
 08 February 2013 08:20 PM
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daveparry1

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Another thing Alan, did you unplug everyting from socket outlets? eg, computer connected to telephone line and getting some earth from there.
 08 February 2013 08:23 PM
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mikejumper

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Originally posted by: alanblaby
So, my query, how is the Zs so low?

My only thought must be it was being earthed via all the steel socket boxes, and things such as the boiler, which would have a direct contact between the CPC and water pipe, making acircuit to earth.

Any other ideas?

If adjacent or nearby properties are PME'd then you could be finding a path via your water pipe (assuming it's not plastic) to a PME terminal.
 08 February 2013 08:25 PM
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daveparry1

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He did say he found that the water bonding wasn't connected Mike.
 08 February 2013 09:07 PM
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mikejumper

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Originally posted by: daveparry1

He did say he found that the water bonding wasn't connected Mike.


He did indeed.
But he also said that the cpc to the boiler could be making contact with the water pipe (via it's connection to the boiler chassis and associated pipework).
 08 February 2013 09:28 PM
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dbullard

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What type of construction was the house ?? Cobb walls, stone directly built on the mud ??? if so it may well have been possible if damp and the oil pipe was in contact with building fabric then maybe a possible answer.

Regards


Daren

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 09 February 2013 01:05 AM
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weirdbeard

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Originally posted by: alanblaby


So, my query, how is the Zs so low?

My only thought must be it was being earthed via all the steel socket boxes, and things such as the boiler, which would have a direct contact between the CPC and water pipe, making acircuit to earth.



Any other ideas?

.


No, seems like you have covered all bases - if the water pipe is metal there could be all sorts of connections - immersions, pumps, supplementary bonds in the bathroom, airing cupboard, kitchen sink, etc.
 09 February 2013 06:21 AM
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ebee

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As an aside, II think this tells us one thing though.

In theory, an electrical system that has no earth reference could be considered safer than an earthed system.
Bathroom shaver socket being a small example of this, you`d need to touch two conductors of different polarity to get a shock rather than one conductor and something earthed.

With one item such as one shaver there are not many problems.

However to run an entire installation on such a system it would be difficult to ensure that one conductor is not earth referenced thereby making other conductors live with respect to this.

To deliberately have an earth reference and to design safety around it
becomes more manageable as we move away from one single item to an entire installation.

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Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 10 February 2013 09:24 AM
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AJJewsbury

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Or a N-PE fault somewhere?
- Andy.
 10 February 2013 11:04 AM
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ebee

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Aboslutely AJJ,

Rather than risk a Pole becoming earthed via a fault which will make all all poles of opposing polarities from becoming "phase" in respect to it, we deliberately do so and declare it N .

Here`s a thought. L & N are both live conductors on the input side of a transformer (example bathroom shaver socket) are the outgoing terminals Live or merely just phase conductors of opposing polarity.
That is do they only become Live if they are referenced to earth but not if they are floating (no reference to earth) or is the fact that they are straight forward normally current carriers enough to declare them live?

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Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 10 February 2013 12:52 PM
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daveparry1

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I'd say they are both live with respect to each other Ebee, (not to earth of course) so as there's no "polarity" as such because they're AC I can't see that either one could be called "live" on it's own,

Dave.
 10 February 2013 02:05 PM
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ArthurHall

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Same as the output of a site transformer it two phases with a centre tap to earth, thats why we always sleeve the blue conductor in grey, dont we!
Earth free systems are used but are not allowed to be connected to public distribution systems. They are used in hospital operating rooms. on ships and on drilling rigs.
Rigs are normaly 600V phase to phase. You will still get a shock/fault current from an unearthed (IT) system due to capacitance of the conductors. Normaly a system of earth monitoring is used which sounds an alarm if it detects an earth fault.
 10 February 2013 02:11 PM
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mikejumper

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Originally posted by: ebee
Here`s a thought. L & N are both live conductors on the input side of a transformer (example bathroom shaver socket) are the outgoing terminals Live or merely just phase conductors of opposing polarity.

I'd say 'live' in the sense that it is active and has the potential to do work
 10 February 2013 02:22 PM
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Jaymack

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Originally posted by: ebee
In theory, an electrical system that has no earth reference could be considered safer than an earthed system.

A widespread system used until fairly recently in the USA, OK until you have 2 earth faults on the same line!

Regards
 10 February 2013 04:44 PM
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ebee

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Two on the same line being worse than one on a line - OK I`ll think about that one.

I was thinking a earth fault on one wouldn`t always make it unsafe but would make the other polarity now less safe.

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Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 11 February 2013 01:22 PM
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AJJewsbury

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or is the fact that they are straight forward normally current carriers enough to declare them live?

I think that's the usual definition.

Two on the same line being worse than one on a line - OK I`ll think about that one.

Humm. Two faults, one from each line, is normally regarded as the problem - unless all the exposed-conductive-parts are connected together then you can have two at different potentials, no ADS, just waiting for someone to touch both and complete the circuit...

Another, lesser, problem is that if you had several such systems and opposite poles on each became inadvertently earthed, the voltage between the two unearthed poles of the two systems is then the sum of the two system voltages - something the insulation may or may not have been designed to cope with.

- Andy.
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