CAT ratings misunderstood

[Neilm]

I have seen glass fuses used in meters that claimed CAT III 250 V protection. They exploded when there was a transient.

a) There's no such category as "CAT III 250V"
b) See the million threads about false CAT ratings on multimeters.
c) The meter is allowed to die, all that's required in the CAT system is to protect the user.

CAT ratings chart:

IEC61010 notes a 1 kV CAT IV situation will have a potetial short circuit current of >>25 kA.

I'm not sure where that number comes from.

1kV with 2 Ohms impedance is 500A. A 12kV transient with 2 Ohms impedance is 6kA for the duration.

The resistance in that chart is the requirements for the test equipment when doing the test. The >>25 kA is the predicted fault current expected at a CAT IV situation.

[HKJ]

The transients are relevant in the voltage range, the fuses are in the current range and if you put the probe across voltage when meter is in current the source impedance is much lower than 2ohm.

[Fungus]

if you put the probe across voltage when meter is in current

The trick is NOT to do that.

[HKJ]

if you put the probe across voltage when meter is in current

The trick is NOT to do that.

Of course it is, but I prefer a meter where you do not get an instant suntan (or worse) if you make the mistake. I.e. the fuses have a 10kA+ breaking rating.

[Fungus]

if you put the probe across voltage when meter is in current

The trick is NOT to do that.

Of course it is, but I prefer a meter where you do not get an instant suntan (or worse) if you make the mistake. I.e. the fuses have a 10kA+ breaking rating.

If I was working in those conditions I'd use a meter with no current ranges, or at least take the fuses out of the meter for the duration of the job.

[janoc]

I have seen glass fuses used in meters that claimed CAT III 250 V protection. They exploded when there was a transient.

I don't think an exploding glass fuse in an enclosed chassis contradicts a CAT rating.

There is a documented death of two people thanks to a glass fuse in a cheap meter. And there are no glass HRC fuses - the body of the meter is not sufficient there. The fuse needs to safely interrupt the potentially huge current without arcing and spewing shrapnel everywhere.

https://www.ecmweb.com/arc-flash/case-deadly-arc-flash