Hi Allan,

Its a compound or dry powder called Bentonite. You can purchase it in bags from most electrical suppliers or even specilist earthing contractors.

I have used it on TT earthing systems where we have had to drill bore holes due to ground conditions/geology and fill they hole with Bentonite, try and pack it then drive in the earth rod.

Basically it is moisture-retaining clay that is commonly used as a backfill for earth rods installed in drilled holes, or as a layer encapsulating horizontal earth conductors buried in a trench.

Its main advantage as far as earthing is concerned is that it has the ability to retain moisture in the immediate vicinity of an earth rod, and hold it there for a considerable period of time. This reduces contact resistance and increases the effective size of earth electrodes.

Hope that helps, but its easy to purchase and not uncommon.

Cheers GTB

carbonaceous backfills are routinely used in cathodic protection anode groundbeds.  There is a UK manufacturer, so it is easy to get hold of.  If you are using it in a DC application you must consider the consumption rate of the backfill when sizing the installation.  As a rule-of-thumb you can estimate that 1 kg of backfill will be consumed per amp per year.  If you are relying on the backfill as a low resistance earth then you might consider a mixed metal oxide electrode to minimize the consumption rate. Hope this helps.

Hi can IK just confirm by DC you mean 'direct current'? Thanks

"Bentonite (/ˈbɛntənaɪt/ BEN-tə-nyte) is an absorbent swelling clay consisting mostly of montmorillonite (a type of smectite) which can either be Na-montmorillonite or Ca-montmorillonite. Na-montmorillonite has a considerably greater swelling capacity than Ca-montmorillonite."

I need a babel fish  Zaphod  :-)

(At least I've learned something new too.)

Yes.  Direct Current.  

Yes.  Direct Current.