Probably the main TT island has an extra  connection to the water mains or something shared with a TN neighbour that you have missed. Less than an ohm  suggests a rather more metallic  connection to the substation than just a very good electrode ;-)

1660 ohms or  so is correct from the point of view of a 50 volt rise at the maximum tripping current, but anything more than a few hundred ohms probably means the electrode has rotted off the end of the wire. A garden fork in the lawn should manage better than 1500 ohms - that would be screwdriver rammed in the flower bed territory or parking on a metal plate.

Mike

Edit update.

If the electrode resistance is more than a hundred ohms or so, consider it worth checking it or connection to it, is not broken and if need be adding a new one in parallel.

mapj1 said:

Probably the main TT island has an extra  connection to the water mains or something shared with a TN neighbour that you have missed. Less than an ohm  suggests rather more metallic  connection to the substation than just a very good electrode ;-)

As Mike says, this is not as unusual as you might think, and this is recognized by the latest versions of GN6 and EIDG .... BUT ...

I think lyledunn  makes a good point, in that Ze at the origin should be conducted between line and the means of earthing only ... no extraneous-conductive-parts coming into play ... so that there are no parallel paths through water pipes etc. Having said that, if the main earth electrode(s) are in extremely close proximity to extraneous-conductive-parts underground, this can also lower the effective earth electrode resistance.

mapj1 said:

Probably the main TT island has an extra  connection to the water mains or something shared with a TN neighbour that you have missed. Less than an ohm  suggests rather more metallic  connection to the substation than just a very good electrode ;-)

As Mike says, this is not as unusual as you might think, and this is recognized by the latest versions of GN6 and EIDG .... BUT ...

I think lyledunn  makes a good point, in that Ze at the origin should be conducted between line and the means of earthing only ... no extraneous-conductive-parts coming into play ... so that there are no parallel paths through water pipes etc. Having said that, if the main earth electrode(s) are in extremely close proximity to extraneous-conductive-parts underground, this can also lower the effective earth electrode resistance.