Is CAT III 1000V always the same as CAT IV 600V?

[ivaylo]

Hi there,

Got myself into a dispute with a eBay seller over safety ratings. The item is Brymen BM857. The documentation which comes with it lists a safety rating of CAT III, 1000V. The seller who advertises it as a CAT IV device referred me to http://www.ni.com/white-paper/5019/en as a proof that CAT III 1000V is the same as CAT IV 600V and he can advertise it as such even though the manufacturers documentation lists it as a CAT III.

Now, without going into the cheap, non-brand, Taiwanese, blah, blah discussion are CAT III 1000V and CAT IV 600V the same? And if so why don't Agilent for example advertise these - http://cp.literature.agilent.com/litweb/pdf/5990-7550EN.pdf as CAT III 600V / CAT IV 300V?

Thank you...

[Bored@Work]

No.

[grenert]

Who's the lying ebay seller?  Name names!   

[Deepak]

Hi there,

Got myself into a dispute with a eBay seller over safety ratings. The item is Brymen BM857. The documentation which comes with it lists a safety rating of CAT III, 1000V. The seller who advertises it as a CAT IV device referred me to http://www.ni.com/white-paper/5019/en as a proof that CAT III 1000V is the same as CAT IV 600V and he can advertise it as such even though the manufacturers documentation lists it as a CAT III.

Now, without going into the cheap, non-brand, Taiwanese, blah, blah discussion are CAT III 1000V and CAT IV 600V the same? And if so why don't Agilent for example advertise these - http://cp.literature.agilent.com/litweb/pdf/5990-7550EN.pdf as CAT III 600V / CAT IV 300V?

Thank you...

No, the quoted whitepaper says as much. Cat III and Cat IV are listed as qualitatively different installations, and this quoted sections gives a clear example that you can't go up a Cat category by derating the voltage:

Channel-to-ground.................................300 V Category II, 600VDC, V ACpk Category I.

Using the chart below we deduce the following: This specification informs the user this module is rated for 300V CAT II and 600V CAT I. In other words, this module can withstand up to 2,500V impulse voltage. This specification additionally informs the user this module must not be connected to MAINs CAT II circuits when operated above 300V. Lastly, the user should not use this module with Category III or IV circuits.

[helloworld922]

From what I understand there are multiple parts to CAT ratings, the biggest being what external protection measures are in place and what magnitude of energy/power you're dealing with.

CAT III 1000V and CAT IV 600V may be rated for the same max impulse voltage, but a CAT IV device is intended to handle much higher power discharges compared to a CAT III devices. For example a CAT IV device would be appropriate for working on power lines where there really isn't anything in the way of power limiting (until transformers start to blow), where a CAT III device is intended for use on medium power circuits like industrial motors where the devices are behind breakers (rather larger breakers compared to CAT II rated devices).

The Wikipedia Article seems to provide a few more specific figures as far as max current/power and what type of external protection measures are in place, though your best bet if you're really concerned about what CAT rating you should get for different situations would be the IEC 61010-20-030 standard (unfortunately not free).

[Neilm]

If testing to IEC61010 (which as it is a multi-meter it should be) the surge test for the 1000V CATIII is the same as 600V CATIV. Generally, a manufacturer will quote the CAT IV rating as it "looks better" on the documentation. Quite often you will see both ratings marked on the instrument and documentation. As a for instance, look here. Fluke have rated this as 1kV CAT II, 600V CATIII. What this means when you look at it is that in CAT II situations you can use the voltmeter up to 1kV, but in CATIII situations you should de-rate it to 600V. Similar argument for the BM857.

My big question is why they would only quote the CATIII rating might be so they can say "our new range of 1000V meters" but that is only a guess.

The one thing that does grab my attention on the datasheet I have just found was that it said "CATIII 1kV AC/DC" CAT ratings are always AC.

Short answer - they are not the same but they are equivalent.

The Wikipedia Article seems to provide a few more specific figures as far as max current/power and what type of external protection measures are in place, though your best bet if you're really concerned about what CAT rating you should get for different situations would be the IEC 61010-20-030 standard (unfortunately not free).

The NI document sums it up pretty well. The explanation in 61010 of the CAT situations isn't much more detailed.

Neil

[saturation]

You can save a lot of legwork by insuring the device is 3rd party tested to be CAT III or IV rated which the Agilent is; they may also stipulate that the rating not be listed as being over III; if the Brymen is only CE certified, its not meaningful.  In addition, safety ratings include 'pollution degree' by which the device will be operated on, which is not mentioned in the Brymen manual.

[Bored@Work]

A CAT III meter is not to be used in a CAT IV environment. End of story. Equivalent test voltages don't matter.

[ivaylo]

Thanks for the help, guys!