Depends on the rcd protecting individual circuits. If 30m/a then yes.

Gary

Hi Gary. Yes all circuits are on an RCBO 

How do you get 0.38 Ω at the origin, but 20 Ω 35 m downstream? Your tester's results are inconsistent.

"Sub-board 35 meters away, with it's own earth stake."

I did wonder if the main installation was actually TN rather than TT though - 0.38Ω does seem implausibly low for a TT system (or the test had been done other than with consumer's end of the earthing conductor connected to nothing but the meter).

   - Andy.

Yeah i know what you mean. Definitely TT though

Did the Ze with a high current 2 lead test and main earth removed at origin

Tested board in the outbuilding with everything connected 3 lead low current and got 20 ohms.

I just thought the Ze  was a better reading because of the high current test

Either the neutral and earth leads have been mixed up on the three lead test or you have put the green lead on neutral. Three lead tests are less accurate but not by that much. Alternatively, the outhouse is TT and TN something at intake. 

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.

More than wanting an explanation for the low Ze, I was curious to know why there was such a big difference when r1 + r2 of the distribution circuit must be of the order of one tenth of an Ohm.

As I read it, the remote DB has its own independent earthing arrangement ("it's own earth stake") - so the loop isn't going through the distribution circuit's R2 or indeed the main installation's earthing arrangement (whatever it turns out to be).

    - Andy.