Somewhat surprising they seemed to show the same resistance at any current
That's not surprising to me since what I've been saying is that I don't believe the instruments we're using can necesssarily measure low Ohms as well as we should expect them to and more importantly, perhaps, there appears to be a flaw in your argument.
Theory dictates that as a resistor heats up and being predominantly a metallic element or an alloy it's resistance will tend to increase (or even decrease for some alloys) not remain the same - it will also expand of course, however, the effects of expansion on it's dimensions can be ignored since it's resistivity that's mainly influenced by temperature changes, particularly in copper, i.e. a cable.
Comparing resistance changes in a resistor of one metal, designed to operate over a range of temperatures, possibly with heatsinking or dissipative cladding and much lower coefficients of resistance (as do metals & alloys used in power resistor construction), to a length of copper strands wrapped in a PVC sheath may not be a particularly useful comparison Graham. Anyhow, without getting into too much theory, I'd expect a resistor (or a cable in particular) to warm up a tad more, hence change it's resistance more, with 25A flowing through it , from room temparature, than I would with 1A flowing through it for the same time under the same conditions. Thus being a simple lad, at heart, I'd expect the initially identical temperatures, thus resistance, to be different after time, t, not the same.
Using a simple formula just to illustrate what I'm getting at: R = R0[1+alpha(T-Tref)] where R0 is initial resistance, aplha the temperature cooefficient of resistance for Copper at 20C of 0.004041, Tref at 20c and T at 75C. Ignoring expansion, if a 0.1Ohm
Copper cable heats up to 75C from 20C, across it's length due to a significant current passing through it (thus power dissipated in it), then its resistance should change by up to
22% by my simplistic reckoning.
On the other hand power wire-wound resistors with a low temperature coefficient, of less than ±20×10-6/K, use a resistive element made of Constantan (Nickel and Copper) or Nichrome (Nickel and Chromium). Again, to illustrate, Constantan is used for lower value power resistors with an alpha of -0.000074/K. Repeating the calculation above assuming an equivalent
Constantan cable or wire is 0.1Ohm at 20C heated up to 75C, ie. the same conditions to apply, then the variation in resistance is in the region of
-0.5%.
Should we expect to see a significant change in resistance due to temperature in a wire-wound resitor, for example, Graham? I think not - no surprise there then. What I'm saying is the change in resistance of Copper with temperature (and over the same time with different values of constant current dissipated in it) is much more than that of Constantan (or most other alloys used in resistor manufacture in fact).
Apply different currents for the same time through a copper conductor at the same initial temperature under the same condtions and I'd expect the higher current to produce heating in the cable and more importantly an observable change in resistance (if the test instrument is up to it). Especially Copper cables - that's just physics. Since the effect is expected to be significant, in practical terms, I'd expect a measuring instrument to pick up on this if it's not infuenced by changing test conditions or poor design. Personally I think that it's difficult to comment on resistance measurement performance, at any current, until we can make accurate and repeatable resistance measurements in the milliOhm range over the range of currents we're interested in using.
What I've stated is that I think we've got to ensure that the instruments (and methods, of course) we're making comparisons with are up to making those comparisons before we can progress to deciding what test current to use - since the current obviously has effects in conductors that are well documented in theory - nevermind "wetting currents". Let's make accurate resistance measurements first, using established methods - we need to have repeatable and accurate, i.e. meaningful measurements that are comparable, if we're to investigate wetting currents and suchlike. Why would Rick's test at 1A fail and then pass at 25A if the tester wasn't detecting a change in apparent resistance? What's the difference between a low-ohms resistor and a multi-strand wire? Quite a bit in my opinion, as I've tried to illustrate.
Personally speaking I do think that the earth-bond test current may have a significant "wetting effect" but at the end of the day, if you're measuring resistance, passing a constant current through a resistance then you should obtain a voltage proportional to this. My attitude is that I'd be willing to let someone who knows what they're doing loose with a 25A test current.
However I have seen problems with failure of medical systems, i.e. interconnected class 1 devices, that I've attributed to poorly designed earth-bonding methods damaged by "excessive" current - not my test methods (naturally).
What's really at issue, with regard to these so-called recommendations on test current, is whether we can rely on individuals to perform tests and be fully aware of the potential issues associated with pushing high currents through devices during testing - we should not assume that all operators are highly skilled or that everyone's aware when they damage an earth conductor or earth-referenced functional path.
I'd prefer 10A, considering that "wetting current" may still be an issue, since I think 25A is too high for the devices that I typically and routinely test. Most devices are connected with 10A cable so maybe 10-15A is more than adequate for our purposes, i.e routine testing.
Incidentally were you using "big resistors" with values down in the region of 0.1Ohm Graham? In my view the capabilities indicated by the specifications for testers I've briefly examined when evaluating them may give rise to significant variations in performance as the resistance we're looking at changes - so we need to be comparing like performance in the range of resistances we're interested in measuring as well as comparable methods.
