Surely the answer to why it's presented like this in the OSG is in BS 7671 itself? Regulation 643.7.1 b) tells you what is to be measured to verify ADS where RCDs are used for this in TT systems:

(1) measurement of resistance of earth electrode (with option to take Z_e reading ... not Z_s ... instead of measuring R_A): limit therefore 200 Ω (but this is Z_e not Z_s) ... provided that for downstream of the RCD, R₂ is confirmed (but this is a required Dead Test in any case).

(2) verification of effectiveness of protective device. With an RCD visual inspection (or characteristic type) and RCD testing (1x test: 300 ms max for non-delay, 130 ms min and 500 ms max for S type time-delay).

Isn't this about Note 2 to Table 41.5? 1667 Ω is the theoretical maximum resistance, but 200 Ω is the practical maximum. That is to say that provided the measured resistance when the rod is inserted is around 200 Ω or less, it is highly unlikely that it will ever rise beyond 1667 Ω when soil conditions change.

On a slight tangent, the symbol Ze can be confusing on TT systems. Technically it's the loop impedance external to the installation, and as the consumer's electrode is part of the consumer's installation, Ze does NOT include the resistance of the consumer's electrode. That makes sense when official documents quote a maximum Ze for TT systems as 21Ω for example.

On the other hand a lot of people who are perhaps more used to TN systems often refer to the loop impedance at the origin as Ze and then try to do things like Zs = Ze + R1 + R2, whereas technically it should be Zs = Ze + R_A + R1.  (not Zs = Ze + R_A + R1 + R2, since R2 is included in R_A).

Maybe we could do with a symbol for the earth fault loop impedance at the origin - Zo perhaps?

   - Andy.

Whichever you choose, 643.7.1 permits you to either ignore R₁, and/or you can ignore R_EE (the actual consumer earth electrode resistance) and use Z_e (not including R_EE ... but then how do you measure what you termed Z₀, terminology which incidentally is also sometimes used in guidance).

but then how do you measure what you termed Z₀

Exactly as you would for Ze in a TN system - e.g. loop test from L to the disconnected end of the earthing conductor.- just call it Zo (or whatever) rather than Ze to avoid confusion with the 21Ω figure etc.

   - Andy.

Chris Pearson said:

Isn't this about Note 2 to Table 41.5? 1667 Ω is the theoretical maximum resistance, but 200 Ω is the practical maximum.

Thanks for clarifying Andy, I've see 'Z₀' confused with what you've said 'Z_e' is taken to include in a TT system.

Regulation 643.7.2 confuses matters by using the term R_A for earth electrode resistance alone (which I would term R_EE) ... I think it assumes the earth electrode is there because it's a TT system, but in fact it might be there for other reasons (e.g. means of earthing for TN-S source or transformer if you don't take your supply from the distributor's LV network, or additional earth electrode in TN system, etc.).

I think it's generally cleared up by the fact that, usually you are relying on RCDs for ADS so (Z_e+R_A) >> R1, and in fact in a lot of installations, but most importantly, the condition we are trying to achieve is in 411.5.3, basically trying to achieve touch-voltage maximum 50 V for residual currents (notionally flowing down R₂ and R_EE, i.e. R_A)  up-to and including I_Δn.

Chris Pearson said:

Isn't this about Note 2 to Table 41.5? 1667 Ω is the theoretical maximum resistance, but 200 Ω is the practical maximum.

Absolutely ... Table 41.5 is actually aimed at Z_S being the 'earth electrode resistance' substitute ... (if you read Note 2 to Regulation to 411.5.3).

So, basically, Note 2 to 411.5.3, along with Table 41.5, is saying that if Z_S < 200 Ω then  R_A < 200 Ω, simply because Z_S = Z_e+R₁+R_A... but it's really R_A we are after.

AJJewsbury said:

Exactly as you would for Ze in a TN system - e.g. loop test from L to the disconnected end of the earthing conductor.- just call it Zo (or whatever) rather than Ze to avoid confusion with the 21Ω figure etc.

I would have thought that Ze of a TT installation was a simple contemplation of Ra and Re in series. The line part of the measurement is irrelevant as it is so small in comparison. 
In any event, the regs require confirmation that the RCD will operate within the required time so measuring Ra is only half the battle. Using the standard Ze technique will also confirm that all is good at the DNO tx. 
As I understand it, the 21 ohm value is maximum Re of 20 plus an arbitrary 1 ohm for connections. I do not see the need to introduce anther symbol like Zo to describe something that is already perfectly clear.

I would have thought that Ze of a TT installation was a simple contemplation of Ra and Re in series.

The definitions differ though - the consumer's electrode is part of the installation, not External to it, hence its resistance isn't included in Ze. Hence we can have simultaneously Ze ≤ 21Ω and Zo ≤ 200 Ω.

   - Andy.

Isn't this about Note 2 to Table 41.5? 1667 Ω is the theoretical maximum resistance, but 200 Ω is the practical maximum.

Agreed - or to put it another way 1667Ω  is needed to satisfy section 411, but something substantially lower is needed to satisfy 542.2.4 (may vary depending on local conditions but 200Ω might be a reasonable starting point).

   - Andy.