Hope someone can shed a little light on my earth rod query: I'm looking at the earthing layout drawings on my site and basically there is a 70mm bare underground earth cable (ring) around the site with numerous cad-weld joints with leads terminating at vessels, skids, pipe-racks, motors etc. The ring terminates at the secondary side, centre point, of a HV\LV transformer. Throughout this ring are also numerous 3.5m earth rods? My question is this, are the rods necessary as there is a solid earth path back to the transformers star point and earth wires to all extraneous points on the site are connected to the earth ring.

Your installation sounds correct to me.
As well as a return path for LV faults the earth mat could serve other functions; It is a HV earth, a fault on the HV side of the transformer will flow to earth then back to the HV source which could be remote from your site and fed by overhead lines. It could be an electrode for the lightning conductors. It ties the site metalwork to true earth reducing the chances of shocks from metalwork.
with earthing the deeper the better so rods count for more than just bare conductor.
If the transformers have an earth terminal it will almost certainly be the HV earth terminal. The LV earth is usually connected to the neutral in the LV cabinate/fuseboard/switchboard. Although all the text books show the earth connected at the transformer star point in reality this is very rare.

I concur with Arthur, and agree the rods are necessary. The IEEE have the green book, which deals with grounding, sorry earthing. The grid around the tranny, is to give a good solid reference to the general mass of earth, as conditions vary, with location.
In there Canadian north, the Canadian shield or bedrock is just below the surface, which is a nightmare for creating a good connection to ground. Which often also means a bentonite backfill is required, to expand the moisture holding capacity of the ground, and hence reduce resistivity.
I found recently 2 installations where the star point(X0 as it's known over here)had not been connected to earth, and the Pscc, was extremely low. The same regime of testing as per the uk is not done here to our detriment.

This was the usual procedure for new petrochemical and other industrial installations in South Africa, that I was involved in for many years as a free lance project engineer.

These arrangements were probably adopted from USA and Continental practice in Europe by companies such as Haliburton, Brown and Root, Badger, Foster Wheeler and Lurgi etc.

A 70mm² G/Y cable was laid on both sides of main cable trenches throughout the site, with cadwelded spurs, from these to plant equipment and foundations etc., the ends were terminated in the main earth bar at the substation. This arrangement was in addition to the standard CPC's in lighting and power cables.


I haven't experienced these earth rods and don't consider them necessary. However, separate earth rods were sometimes installed, for lightning protection at lighting masts and steel building columns etc.

Local standards and regulations, should obviously be consulted.

Regards

Not in my experience, but I wouldn't remove them!

Regards

Thanks for the reply's

"A 70mm² G/Y cable was laid on both sides of main cable trenches throughout the site,"

Would that be known as an insulated earth?

Regards

BOD

Yes, I should have mentioned that. The cable was insulated throughout the route. Main equipment such as distribution transformers, switchgear, MCC's, large motors; steel structures, vessels, and piperacks had 2 points of connection at extreme ends. Smaller electrical equipment such as panels, distribution boards and motors had one point of connection.

Regards

Silly question maybe but would the earth rods be for earthing a circuit or system independently from the MET.

Would there be some reason why the circuits connected to the rods are not connected to MET?

Why is the cable insulated, wouldn't it have been better to lay an uninsulated cable thus creating a better overall connection to the ground?



That surprises me, I always thought the star point was earthed.

Edited: 09 July 2012 at 07:51 PM by postman

Sorry to bring this up again but if the transformer is not earthed at the star point for LV what is holding the neutral at 0 Volts?

I thought the whole idea of earthing the LV side was to stop the neutral voltage from rising above true earth and indeed the earth or CPC itself from rising above true earth.

I was taught that the earthing the star point stabilises the voltage and to not earth the LV side was a very bad idea.

Can any shed some light on why in practice the LV side of a TX is not earthed?

Thanks.

I don't think there's any suggestion that LV N isn't earthed - just that the connection is physically made a few metres away from the transformer (probably to make it easier to keep the LV and HV earths separate so that HV earth faults aren't imposed on the LV system). Perhaps think of it of the transformer windings continuing beyond the tranformer casing for a bit. Have a search for PNB (protective neutral bonding) for a bit more background.
- Andy.

How are your LV and HV earths done; are they separate or interconnected? On our site we use an interconnected HV and LV earthing system because the combined earth resistance is low (around 0.3 Ohms).

We use insulated earth cable in the ground to protect the copper from corrosion - you can test and replace the rods individually when necessary.

Funny about PNB, the books say that it was used mostly for pole mounted transformer systems but it's common in other installations I've seen. Most of our site is PNB even though we have ground standing xfmrs but this is to ensure there isn't multiple neutral earth links when we have a switchboard fed from 2 xfmrs/generators at each end.

Tim

-------------------------
Everyone loves a fireman - but hates the fire inspector.

All the text books show the star point directly earthed but in practice its diferent. At the very least the star point is taken out through an insulated bushing. On pole mounted transformers The LV earth may be connected directly to this bushing but will be an insulated conductor to avoid contact with the HV earth. HV and LV earths normaly have to be seperated by at least 8 meters to stop HV rise of potential being imposed on the LV system. If the combined resistance of the HV and LV earths is lower than one ohm then they may be combined, but you have to install and measure them first to find out. So the first LV electrode/ bare conductor should be at least 8 meters away from the transformer. The normal practice is to earth the neutral at the LV board, using a neutal earth link. This is a TNS supply.
PNB is a system used by the DNO's where there is one customer fed by overhead line from a pole mounted transformer. The LV earth is fitted at the customers cutout and as far as the customers instalation is concerned this is a TNCS supply.

Hi Arthur,
So are you saying PNB should only mean the the N/E link is at the customers end, when he is the sole user of the xfmr?

You wouldn't use the term PNB for say a N/E link in a main switchboard in the substation even though that was the only connection of neutral to earth?

Regards, Tim

-------------------------
Everyone loves a fireman - but hates the fire inspector.

Correct Tim
In my experiance both working for a DNO and on the other side of the meter PNB is for a single customer fed by overheads. The N/E link is at the customers terminals and as far as the customers electrician is concerned it is a TNCS supply.
A dedicated or private transformer feeding a LV board with the N/E link at the board is TNS. In this case the LV neutral / earth may be linked to other supplies or generators.