Current in TN-C-S main earth conductor

[JohnD]

The reason may be the need to sink to ground potential any extraneous current that is imported to the site by the gas pipe lines from other gas processing plants.

Are these metal pipes? In contact with the ground?

 

[ericmark]

Working on the gas pipeline 42" in Algera the cadodic protect would only work if not grounded.

 

[JohnW2]

credible being 6mm or 10mm. That size of conductor can carry a lot of current before it melts.

Indeed - and no-one should really be contemplating an earthing conductor smaller than 6mm². However, even the highest current we've been talking about (29A with an 8Ω rod) would not 'melt' even a 1.5mm² conductor.

The reason may be the need to sink to ground potential any extraneous current that is imported to the site by the gas pipe lines from other gas processing plants.

Undoubtedly - but if one puts a "car-sized resistor" in series with it 'to limit the current', that surely defeats such an aim, doesn't it?

Kind Regards, John

 

[Charliecambs]

hi sorry been busy on other jobs. I’ll read through tomorrow but in response to earlier question on number of TT rods I’d guess at 4 spread over the site ( about 7 acres I think. Loads in those areas probably a few hundred watts each at a first thought.
Fortunately from a “quadruped” point of view the. Larger ones ( llamas, ponies etc) don’t have any installed electricity. Those with heating would typically be “small” cats such as servals and jungle cats.

I remember discussing the issues with cows near power distribution sites back in college many years ago.

I think I need to present more detailed layouts and measured results for a better view.

I think it may be interesting to measure the isolated exported earth potential relative to its local TT rod and line/ neutral/ earth current at each sub board although hopefully earth is near zero ( apart from one location)

The major “split” to sub mains is at the incoming board so I think I’ll try current/ voltage there initially.

Thanks to all for the input I my go quiet for a while before I get there to retest.

 

[bernardgreen]

Are these metal pipes? In contact with the ground?

Some pipelines are sheathed with material which is electrically non conductive. I don't know if this was intended for electrical insulation or for chemical separation ( to prevent acidic soil attacking the metal pipe ? )

 

[Charliecambs]

finally had a chance to look at earth current.
previously the on site house was supplied by swa from one of the sub main “consumer units” but prior to leasing the main site both electricity and water were split.
as electricity use has always been high I had checked the connection to this feed and only armour and “earth” core were still connected at our end.
whilst checking with the clamp meter I clamped this swa and saw my high (15amp ish) current which was also in the earth from this unit to the main incoming switch fuse.

I need to speak to the house occupants as an swa runs up their wall and into the house above a window which I assume is this disconnected feed. Their supply appears to be via an external meter box which I assume will be Tnc-s /PME as ours is.

on holiday at moment but why should this current be flowing? Obviously I won’t disconnect the swa earth at our end as it may impact their supply! I hope to check on their end of the swa and their feed next time I’m up but does this sound like DNO need to be involved?

 

[Adam_151]

If I'm reading this right, the house on site is separatly let, and as a result of this has recently had a new supply installed, but previously was supplied on a submain from your installation, but only the L+N have been disconnected, leaving the SWA glanded off and a core still connected as CPC?

If so, its likely that quite a bit of PEN* conductor will exist between these supplies on the DNO side, this PEN conductor will have a voltage drop across it, a voltage drop that is shunted out by the SWA, leading to some of the current flowing that way. Chances are, after inspection, the best thing would probably be to ungland the redudant SWA both ends.

*Some of this will be in your service cable, some of this will be in the house's service cable, and some might be in main feed assuming the service cables are jointed on at separate points. The DNOs realised this was a problem with rows of 'terraced' industrial units, sharing the same bonded steelwork, each with a PME supply originally I don't think it was considered a problem (lots of such arrangements from the early 80s). The rules were then changed to they could only give a PME earth to one, all the rest had to be TT, but of course they had a 'borrowed' PME from the other one that gave them low Zs, but that you couldn't rely on to always remain as it was in someone elses unit. It was normally better to just TT them all. More recently I think the guidence the DNOs work to has changed. They are allowed to provide what we would class as a PME earth (well we would if we could see the whole picture.) to all these units, but the service cables running in must be SNE (so might appear to be TNS at the cutout) but feed in the road can be CNE, and all the joints for units that share the common metalwork have to be at the same location (I have a suspician that this bit gets stretched a little in reality)

 

[Charliecambs]

Yep correct on armour/cpc connected our end ( assume other end too or current wouldn't flow)
When I'm next up I will see if they're OK if I take a look.
I also have a long wander lead and dvm so may check main earth potential difference. Between locations. Knowing swa length etc a quick ohms law should then be valid based upon data for the swa.
Be interesting to look in meter box too!
Thanks for feed back hopefully I'll be able to post the "answer" at some point.

 

[Charliecambs]

actually just thought I have a kt60 psc/loop tester. It's not calibrated but gives sensible readings on my istallation.
(42 ohms on outside TT) and .6 IIRC on in house
a comparison between our site ( we have fixed wiring test report from last year) and the other property would be interesting. Its not too intrusive on residents ( who are usually friendly so far)