Education please... or why Neutral?

[ebee]

No I think you are basing a diffence between 0.8 and 0.35 (0.45) ohms as a difference caused by the earthing via true earth thus enhancing the reading (lowering it).
This is not the case.

The declared values are expected maximums.

Around here about 0.25 is fairly average measured max value a lot of the time on either system.

The earth itself is not contributing very much in way of lowering the total.
To a small extent yes but not that much.

 

[NotHimAgain]

The 0.35 figure is, as you say, a maximum. It was based on a typical small transformer and a certain length of radial main.

I think it was 300kVA and 500 m of 95mm² but i haven't checked that.

Edit: That is probably more like 100 m .

 

[ericmark]

Mother earth isn't part of the Ze path in a TN system tho.

TN-C-S also called PME which stands for Protective Multiple Earthing the system does not rely on one earth but many earths so any break in the PEN conductor will not result in a loss of earthing.
Yes I know this is in an ideal world and not all suppliers do as there are supposed to but there is no way "Mother earth isn't part of the Ze path" in a TN-C-S system however you would be correct for a TN-S system.

No I think you are basing a diffence between 0.8 and 0.35 (0.45) ohms as a difference caused by the earthing via true earth thus enhancing the reading (lowering it).
This is not the case.

The declared values are expected maximums.

Around here about 0.25 is fairly average measured max value a lot of the time on either system.

The earth itself is not contributing very much in way of lowering the total.
To a small extent yes but not that much.

Yes of course these figures are expected maximums. Why do you think the expected maximums vary between TN-S and TN-C-S systems?

If you really think having multiple earths does not contributing very much in way of lowering the total I think you should study the effect of parallel resistors.

What I think you are missing there is a big difference in the resistance of the earth to the connecting of a cable to the earth. When we measure 60 ohms on an earth rod it is not the earth that is 60 ohms it is the connection of the rod to the earth which is 60 ohms.

Untitled

• ericmark
• 28 Jun 2009
• 1

The image shows how it is tested and how we work out how big the area is around the earth rod where there is a gradient. After this point the ground is such a good conductor one can't detect a gradient.

 

[Spark123]

Mother earth isn't part of the Ze path in a TN system tho.

TN-C-S also called PME which stands for Protective Multiple Earthing the system does not rely on one earth but many earths so any break in the PEN conductor will not result in a loss of earthing.

Maybe so which is why it is PME, however I doubt that these additional electrodes will make much difference at all to the overall figure obtained for Ze.

Yes I know this is in an ideal world and not all suppliers do as there are supposed to but there is no way "Mother earth isn't part of the Ze path" in a TN-C-S system however you would be correct for a TN-S system.

I'd have thought the amount of current flowing between PME electrodes would negliable compared to the amount of current flowing in the CNE conductor as you have units or tens of ohms resistance in the electrode compared to tenths or hundredths of an ohm in the metal.

 

[ericmark]

the electrode

This is where you have the error should read "electrodes" plural.
Because a single electrode in a house may read 10s of ohms does not mean that an electrode installed with a band jack will also be that high. I would expect there are many rods within one system.
Working on petro-chemical I had the job of installing electrodes and I was fittings rods non stop for 6 months they would be between 2.5m to 10m deep and 8 ohms was maximum they were allowed to read on fitting. Returning to the electrodes and re-measuring after they had been fitted a month the reading were a lot lower.
I know 21 ohm is the reading outside the consumers installation with TT and I would think the same limits would be used for TN-C-S systems.

 

[ebee]

If you`ve managed to get 8 ohms on electodes then yes that would be very much more significant .
I was considering a figure of 20- ohms or just less being the realisitic minimum achieveable in many instances and hence the suppliers stated figure of 21 ohms.
I am aware that the earth is highly resistive but the parallel paths over large distances means that away from the resistance area the figure is low - rather like a massive spiders web thingy.
I`m still struggling to see an impedance to earth being so low with electrodes being the whole the of the difference between the figures given though.

 

[jabuzzard]

So I think that earth in fact is a large mass that absorbs excess electrons from faults but does not necessarily return them to where they came from. ............Thoughts on that gents?

I personally know that electrons on a AC system never go very far at all. Think about that one for a minute.

I also know that electrons on a DC system move very slowly, for example in a copper wire of cross-section 0.5 mm2, carrying a current of 5 A, the drift velocity of the electrons is of the order of a millimetre per second.

 

[JohnD]

I personally know that electrons on a AC system never go very far at all.

ha ha

I expect you mean because they go backwards and forwards 50 times a second

However they can go a very long way in 1/100th of a second and there is no reason why they should end up at the same place.

 

[jabuzzard]

I personally know that electrons on a AC system never go very far at all.

ha ha

I expect you mean because they go backwards and forwards 50 times a second

That would be correct.

However they can go a very long way in 1/100th of a second and there is no reason why they should end up at the same place.

No they will travel fractions of a millimeter in 1/100th of a second. Also on average they all end up in the same place. If the did not it would not be an AC current.

Remember also even if it is DC the electrons themselves move *VERY* slowly. The electric field however moves at around 1/10th the speed of light.

Think of it like a very long rod. Push one end and the other moves almost instantly. However the actual atoms in the rod don't move very far at all.

 

[ebee]

Do not rely on this


http://www.youtube.com/watch?v=zFN0wqUIY6o


but it is good fun

 

[Spark123]

the electrode

This is where you have the error should read "electrodes" plural.

No - I was referring to a single electrode's resistance to ground hence singular
These are how your parallel paths are formed, and if these are in your tens of ohms resistance each vs a metal link the parallel path will be negliable in comparison to the metal link.

Because a single electrode in a house may read 10s of ohms does not mean that an electrode installed with a band jack will also be that high. I would expect there are many rods within one system.

There may be many rods, however these all still have a resistance to mother earth which is high in comparison to the metal link back to the transformer.

Working on petro-chemical I had the job of installing electrodes and I was fittings rods non stop for 6 months they would be between 2.5m to 10m deep and 8 ohms was maximum they were allowed to read on fitting. Returning to the electrodes and re-measuring after they had been fitted a month the reading were a lot lower.

I haven't worked on petrochemical however in TT installations the ground conditions will change the overall conductance between the rod and mother earth. GN3 goes on about doing the test in the most onerous condition which in reality isn't going to happen - we have to do all the earth electrodes in the 3 days of sunshine a year

I know 21 ohm is the reading outside the consumers installation with TT and I would think the same limits would be used for TN-C-S systems.

I was thinking around that concept - if you have a shortish TN-CS system with 2 electrodes, one at the sub and one at the remote end each reading 20Ω resistance in contact with earth then the path via earth will be 40Ω. The path via metal is less than 0.35Ω. Parallel the two together and 40Ω doesn't make much difference to 0.35Ω.
Even if you halve the 40Ω it still doesn't make much difference.
Even with few over a network I am not convinced that they make much of a difference.

 

[OilLecky]

Ah yes ebee...now I understand!

 

[NotHimAgain]

I designed an installation for a very large property recently. The supply is via overhead lines but the client was promised PME if he handed over loads of wedge - which he duly did. Its on the end of a line in a village and it is supplied from a fairly small transformer.

Come the day of the big switch on and tests showed Ze = 0.55 ohms - no good for PME then - and probably pretty useless for voltage drop, but we will cross that bridge when we come to it .

The line consists of some 185mm² but it drops down to 95mm² for a considerable distance. This is where I think the DNO may have got their sums wrong before they made the rash promise of PME.

Any way they decided to try and improve things by installing more electrodes (I think they did the same course as Eric ). Many electrodes later and after several retests we are at 0.5 ohms .

I think its going to cost them plenty to replace that section of 95mm² line.

 

[ban-all-sheds]

I think its going to cost them plenty to replace that section of 95mm² line.

Wouldn't it cost them less to give him back his wedge and change their minds about TN-C-S?

 

[Spark123]

They could try it but they have entered into a contract by taking the money to provide the service.