Source impedance - earth fault loop impedance for protective devices to BS EN 60898 & BS EN 60947-2

It might help to refer to BS 7671's definitions:  Zs - is "earth fault loop impedance" (p53) and in turn "Earth fault loop impedance" is defined as:

The impedance of the earth fault current loop starting and ending at the point of earth
fault. This impedance is denoted by the symbol Zs.
The earth fault loop comprises the following, starting at the point of fault:
(a) the circuit protective conductor; and
(b) the consumer’s earthing terminal and earthing conductor; and
(c) for TN systems, the metallic return path; and
(d) for TT and IT systems, the Earth return path; and
(e) the path through the earthed neutral point of the transformer; and
(f) the transformer winding; and
(g) the line conductor from the transformer to the point of fault.

AJJewsbury said:

(c) for TN systems, the metallic return path; and

I like the way that, for such TN systems, there is no mass of Earth involvement in said Earth fault at all.     

Unless, of course, it's the unlabelled 

AJJewsbury said:

the point of fault

(mentioned twice; start & end... ).

Well the whole point of traditional ADS on TNx - fuses and breakers, is that if any at all, the current through the terra-firma earth, compared to that through the earth wire is small. It's also why we need RCDs or an equivalent if you want to detect humans in the loop, or indeed detect any fault at all on TT.

The slightly scary related system to behold, is single wire earth return HV, common in places with large distances to small loads and a more relaxed approach to HV safety (and of course no earth fault detection) South America and Africa mainly and more remote parts of NZ, where it was developed in the 1930s.  
The terra-firma path is then in the main power loop, and being HV we don't need to care so much about a few hundred volts of drop in the vicinity of the electrodes, as the effect on the LV load side is only a percent or two. You do need to fence the electrodes in however so no-one in bare feet goes near it. 
Mike.

Philip Oakley said:

mentioned twice; start & end...

probably reasonable, given that it's a loop?

  - Andy.

Philip Oakley said:

mentioned twice; start & end...

probably reasonable, given that it's a loop?

  - Andy.

Ah, but a loop has neither beginning nor end. 

I suppose that the logical starting point is the fault. If you do not explicitly state that the end point is also the point of the fault, you might leave the reader wondering whether there is a loop, or something else.

It's an earth fault that isn't a fault to earth, hence the rhetorical comment  ??

Aren't all circuits loops, one way or another...[digs another hole..]