GC,
Sorry I haven't got back to you sooner on your questions above.
The comparisons that I've done using calculation and then measurement of Earth Loop resistance, have at times shown wide variations between the two values, the measured value being the lowest, 99% of the time.
While I agree that motors do add to Fault current levels, the Loop test is only used to verify that the installed wiring and it's connections, back to the point of supply (ie: Transformer), are of high electrical integrity (ie: Low Impedance).

Here is a link to the Kyoritsu 6015 Multi-tester, that I use at work for the majority of my testing:

http://www.kew-ltd.co.jp/en/products/multifunction/6015.html
This thing cost an absolute fortune (NZ$3500), but it was well worth the money spent, considering all the features it has, all the seperate test instruments, would cost a lot more than this, if bought seperately.
Finally, here is a link to a loop tester I found made for the US market:

http://www.megger.com/us/products/ProductDetails.php?ID=141&Description=
[Linked Image]

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[This message has been edited by Trumpy (edited 08-24-2004).]