Fungus, I think if you wanted to figure this out, the place to start is by writing one or more manufactures and seeing what they have to say. Certainly they are the experts.
It should be obvious that the higher the CAT rating, the more energy available. Assuming you would understand why they would derate the meter at higher CAT ratings and you really are just asking if any CAT III rated meter is automatically rated to CAT IV 600 and if not, what is the difference.
Again, turn to the experts but the first thing I would consider (a guess on my part) is that the fuses used would be rated to break a higher energy circuit for CAT IV than CAT III. Maybe for a CAT III environment for example, they use a 1KV AC/DC 10KA rated fuses. For CAT IV 600, they may require 20KA and CAT IV 1000 maybe 30KA. Again, ask the experts. I am just guessing.
How did your relay drop test ever work out? Did you ever buy any and try to get them to change states? I have not heard any more from Gossen but I expect they don't move very fast.
EVS-EN 61010-2-033:2012
Table AA.1
The short-circuit current is calculated for a 1000 V line-to-neutral voltage and the minimum loop impedance. The values of loop impedances (installation impedances) do
not take into account the resistance of the probe assemblies and impedances internal to the measuring equipment. These short-circuit currents vary, depending on the characteristics of the installation.
For CAT II, they say less than 10kA typical. CAT III, less than 50kA. CAT IV, much greater than 50kA. Wow, 50kA!! Dave's ASTM 11A fuses are marked 30kA. I wonder if these would be fine for a CAT IV environment.
I don't think the test lab knew how to check the Gossen Metrawatt I have for this condition:
16.101 Over-range indication
If a HAZARD could arise from an OPERATOR'S reliance on the value (for example, voltage)
displayed by the equipment, the display shall give an unambiguous indication whenever the
value is above the maximum positive value or below the minimum negative value of the range
to which the equipment is set.
NOTE Examples of ambiguous indications include the following, unless there is a separate unambiguous
indication of an over-range value:
a) analogue METERS with stops at the exact ends of the range;
b) digital METERS which show a low value when the true value is above the range maximum (for example
1 001,5 V displayed as 001,5 V).
Conformity is checked by inspection and by provoking an over-range condition.