Export earth or not?

[skenk]

I need to put in a supply for a shed/workshop about 60m from the incoming TNS service, which appears to be fed by a dedicated (only serves this one house) pole-mounted transformer, Ze=0.23ohm. No extraneous conductive parts in the shed/workshop (wooden construction and plastic water supply).

Normally I work in the city so 60m away outbuildings are new to me.

Exporting the earth using a 3-core SWA will probably cost less than TTing (after concreting in an earth pit and whatnot, despite the cheaper and lighter 2-core cable) and mean lower EFLI's, so that would be my preferred option.

I have just received a copy of GN8 but flicking through it doesn't seem to give much/any guidance on when a TT supply is preferred over using the distributors facility. My understanding is that the only issue would be if the earth potential at the workshop is significantly different to the suppliers earth potential, is this correct? and is it possible to test the difference in earth potential over 60m? (how?)

The workshop is on the other side of a small valley and stream to the house/incoming service.

Any thoughts?

Thanks

 

[ban-all-sheds]

With TNS there will always be a difference between ground potential and the suppliers earth, because the latter is at the potential where the transformer is. How far is that from the house?

In this case the pole transformer is probably fairly close, but typically substations can be 100's of m from the houses they serve - can't think of any reason why you shouldn't export it to go another 60m.

Maybe Bernard will suggest a bizarre one.

 

[bernardgreen]

Transformer feeding one house, transformer's ground rod close to the house, transformer primary between two HV ( 11 kV ) phases and no HV neutral to the pole / transformer.

That suggests that exporting the Earth shouldn't create any hazards.

the only issue would be if the earth potential at the workshop is significantly different to the suppliers earth potential,

That is the main issue.

Maybe Bernard will suggest a bizarre one.

If there is an old electrified railway line nearby then return current through the ground from the 25 kV traction supply may create significant potential gradients in the ground when a train passes by. ( modern and up graded lines have a return conductor along the track which reduces stray ground current to near zero ).

 

[RF Lighting]

I prefer to export the earth when ever it's possible just because it's more reliable, and gives you more options with the RCD setup.

 

[Adam_151]

I prefer to export the earth when ever it's possible just because it's more reliable, and gives you more options with the RCD setup.

Some times I think we worry a little too much over differences in earth potential with PME supplies. Obviously there are cases where its justified, site cabins, caravans, etc but sometimes it pays to ask yourself what the risk is. I've never actually seen anyone complaining of tingles from PME earthed metalwork, but I have seen quite a few instances of shocks when RCD protection wasn't correctly provided or had failed on a TT system

 

[RF Lighting]

Maybe I'm old fashioned but I'd much rather rely on a solid metal conductor right back to the transformer than an earth rod and an RCD.

Obviously there are cases where this can't be avoided, but for me it's a last resort used only where absolutely necessary, and I also like to see TT earthing with much lower values than BS7671 requires.

Whilst neither were domestic installations, the last two TT installations I did had Ze's of 1Ω and 4Ωs. It takes a lot more work and materials to achieve this, but I can walk away knowing the installation will be safe for years to come.

 

[bernardgreen]

Maybe I'm old fashioned but I'd much rather rely on a solid metal conductor right back to the transformer than an earth rod and an RCD.

So would old fashioned me...... BUT with metal thieves going for the "safe" Neutral and Ground conductors one can no longer rely on that solid metal conductor.

And I am also not keen on Neutral currents and Earth fault currents sharing the same conductor back to the transformer.

Inside the PME supplied house and it's equipotential zone where the "earth" potential is the same as the potential on the Neutral there is no hazard created from a Neutral that has bounced up to many volts above ground. The hazard only comes when something conductive ( or someone ) breaches the perimeter of the equipotential zone during a Neutral bounce and creates a path between the "Earth" inside the house's equipotential zone and the Ground outside the house. With the exceptions of the outside water tap, the gas meter and any "earthed" external lights it is difficult in the average house to breach the perimeter of the house's equipotential zone. ( Using electric tools with earthed metal bodies in the garden is another way to breach the perimeter and is therefore not reccommended ). But in a garden shed with an equipotential zone "earthed" by an "earth" exported from the house it might be a lot easier to breach the perimeter. Damp wooden wall could import Ground potential into the shed's equipotential "earth" zone.

The fault ( or metal theft ) that causes the Neutral to bounce could be a mile away from the house and hence the people in the house have no indication that touching "earth" inside the house and Ground at the same time could result in an electric shock, disconnected Neutral where the network is well balance across the three phases would probably only result is a mild shock, maybe not noticable. But if the fault ( theft ) meant the Neutral was pulled up to high voltage than the "earth" to Ground potential could be high enough to give a severe shock.

 

[Adam_151]

Whilst neither were domestic installations, the last two TT installations I did had Ze's of 1Ω and 4Ωs. It takes a lot more work and materials to achieve this, but I can walk away knowing the installation will be safe for years to come.

Last TT I got involved in had a true Ze of about 7ohms. However with the earth link back in place and a parralell path from the water service it ended up about 2 ohms.

Let me guess, the ones you got those figures on were probably rows of steel framed joined together industrial units that the DNO refused to supply a an earth to on grounds of currents in the steel work?

 

[RF Lighting]

They were both farm jobs. One had 40m² of buried steel mesh plus 4no. 1200mm rods

The other had 8no. 2400mm rods at 3 meter intervals.

 

[securespark]

Bet that lot was fun to install!

 

[ban-all-sheds]

The other had 8no. 2400mm rods at 3 meter intervals.

I thought they were supposed to be a minimum of twice their length apart?