What are the *specific* differences between CAT III and CAT IV? - Page 1
Forum section: electronics / testgear

author: loudaslife
author_rank: Newbie
timestamp: July 08, 2020, 07:09:51 am
content: Many people have noticed that transient overvoltage tests for CAT III ratings are equivalent (in voltage and source impedance) to the next lower CAT IV rating (i.e. CAT III 600V = CAT IV 300V). People also often wonder if a CAT III 600V meter, for example, could be used for CAT IV 300V measurements. The answer is always a resounding "No", which I agree with completely. I don't plan on ever using a meter for something more dangerous than the manufacturer recommends, regardless of what numbers say.However, purely out of curiosity, I started to wonder what the concrete differences were in design requirements and test protocols between CAT III and the equivalent lower tier CAT IV rating. I spent a while searching through the following standards:61010-1Safety requirements for electrical equipment for measurement, control, and laboratory use — Part 1: General requirements61010-2-030Particular Requirements for Equipment Having Testing or Measuring Circuits61010-031Safety requirements for hand-held probe assemblies for electrical measurement and test.61010-2-032Particular requirements for hand-held and hand-manipulated current sensors for electrical test and measurement61010-2-033Particular requirements for hand-held multimeters and other meters for domestic and professional use, capable of measuring mains voltageAnd, to my surprise, I haven't been able to find any meaningful differences. Clearance, creepage, impulse testing, exposed probe tip length, everything for CAT IV seems to either line up with the equivalent higher tier of III, or in some cases, belessstringent for IV. (All of this is excluding CAT IV 1000V of course, which has no equivalent.) Sure, they specify the difference inintendeduse, which is more than enough to keep me from using a CAT III meter for CAT IV measurement. But I really expected to find something more concrete than that. I was only able to get access to older versions of some standards, so maybe the latest versions have changed or clarified this. I also could have just completely overlooked something. It seems like there must be a difference, otherwise almost every CAT III meter would also be certified as IV.

author: 2N3055
author_rank: Super Contributor
timestamp: July 08, 2020, 07:53:35 am
content: Quote from: loudaslife on July 08, 2020, 07:09:51 amSo apart from technical specification, recommended use and safety practice that was enough to fill 400 pages of documents you did't find meaningful differences...So you should read again and keep on reading until you understand differences. It is actually quite understandable, as e those standards are really written by lawyers more than engineers and are really hard to read. But when you get used to it, that is a useful skill too..Just a hint: categories really correspond to available energy disposable at measurement site, equating to damage potential available.Or shall I say simply: categories define what kind of explosion will you survive if you do something stupid with the meter at certain places...So standards will define both voltages and maximum currents available and define how meter should behave in certain circumstances.Going backwards, simplified (just a ballpark) : CAT IV is domain of industrial electricians and people who work on large industrial automation installations. Also your house before main fuse coming directly form distribution transformer, and anything before energy meter.CAT III is your distribution board after the meter and really close circuits and circuits with very hefty wring (like heaters and machines).CAT II and CAT I is your power socket on your wall, depending on current available (and other factors too).As to: is CAT III 600V = CAT IV 300V answer is no. And yes. Do you ask about what is meter certified to do or are the specifications compatible?Whole thing is this: there is almost no use for CAT IV 300V because if you work on CAT IV, you will probably work also on 3-phase installations, and you need to take into account overvoltage transients too... So if your working on CAT IV you need at least CAT IV 600V, but would be smarter to go for CAT IV 1000V.

author: loudaslife
author_rank: Newbie
timestamp: July 08, 2020, 04:18:59 pm
content: I understand the differences in intended use, the available energy at different points in the system, etc. My question was about specific, concrete differences in design requirements and test protocols.

author: Neilm
author_rank: Super Contributor
timestamp: July 08, 2020, 06:09:38 pm
content: The biggest difference is the energy available in a fault. CAT IV systems unprotected - Nothing between your instrument and the power station except the high voltage network. All other CAT locations have at least a fuse in the way.So while there are not too many differences between them (as you have seen) those that are there are pretty important. Note that there are a few very subtle differences that have significant implications. Not all of these are noted in the standards directly. As a for instance, consider what would happen you had a unit that used 2 m test leads and your 10 A fuse opened.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 08, 2020, 07:25:27 pm
content: Quote from: loudaslife on July 08, 2020, 07:09:51 amFault currents are listed in the standards you mention.

author: 2N3055
author_rank: Super Contributor
timestamp: July 08, 2020, 08:12:46 pm
content: Quote from: loudaslife on July 08, 2020, 04:18:59 pmIf I understand correctly what you asking, there are generally not much stipulations as to design details of instrument (there are some, like separation distances and such). Only tests that it has to survive. It is up to manufacturer to devise design that will satisfy the requirements... Again, as Joe said, what is in the standards is in the standards. No more, no less. What detail is there you have to obey, everything else you have to design yourself. And make it so that it survives the test...

author: loudaslife
author_rank: Newbie
timestamp: July 09, 2020, 03:05:41 am
content: Quote from: joeqsmith on July 08, 2020, 07:25:27 pmI went back through them again and I'm still not seeing anything unique to CAT IV. The overcurrent tests are set at 10 times the device's max rated current, for all categories. The current created during the impulse testing should be the same, since both CAT III and CAT IV are tested with 2 ohm source impedance. Fault current when testing with probes in the incorrect terminals or in the incorrect range should also be the same, since both III and IV are specified to be tested with a 30kVA supply.

author: bluey
author_rank: Regular Contributor
timestamp: July 09, 2020, 12:27:23 pm
content: Maybe the main difference is the price of a certification test???Must cost something significant since the cheap manufacturers don’t bother to pay for third party testing.

author: Wytnucls
author_rank: Super Contributor
timestamp: July 09, 2020, 04:43:08 pm
content: Quote from: loudaslife on July 09, 2020, 03:05:41 amThere is no difference in the testing, as you found out and all meters that can meet CAT III 1000V are also rated for CAT IV 600V.There is only a couple of exceptions that I know of: The Fluke 867B and 787, a limitation possibly imposed by the manufacturer itself.

author: tom66
author_rank: Super Contributor
timestamp: July 09, 2020, 05:49:46 pm
content: Quote from: blueskull on July 09, 2020, 12:30:37 pmUnsure on multimeters, but certainly some devices require testing and continuous industrial monitoring by a notified body, e.g. smoke detectors require all tests to be performed by notified body and factory is surveyed annually, with random product sampling.

author: Neilm
author_rank: Super Contributor
timestamp: July 09, 2020, 07:42:39 pm
content: Quote from: loudaslife on July 09, 2020, 03:05:41 amThat is how it is tested and believe me finding places that can test more than 400 V with a 2 ohm source impedance is pretty hard.The big issue with CAT IV is how much energy is available. Fuses have to be rated correctly, leads have to be correctly rated, some instruments specify minimum lead resistance to ensure that this happens. Many will say use fused test leads to overcome some of the big hazards with CAT IV system. So while a quick look at the standards could give an engineer teh idea that they are the same there are a few gotchas - as usual the devil is in the detail.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 09, 2020, 07:55:39 pm
content: Quote from: loudaslife on July 09, 2020, 03:05:41 amI wasn't referring to surge or burst transient tests but rather the fault currents. There is a reason for those 1KV AC/DC 50KA fuses. Contrary to what some people have posted in various threads, it has nothing to do with getting a transient on the line when attempting to measure current across it. You will see posts about how there countless home appliances have glass fuses and how they have never exploded. These people are clueless. There is a separate standard for the test leads as well.As fun as the IEC surge tests are to perform, arcflash testing would be on a whole different level. While the standards you are looking at provide you with some idea of what sort of fault currents you may expect for the different catagories, I don't remember them going into any details about arcflash testing.https://www.ecmag.com/section/safety/lets-blow-it-arc-flash-testingWhen I was conducting my own private tests people would tell me how unsafe they were, how my high voltage probes were not safe to be used during the tests, how the meters were safe because they didn't explode. The stupidity was endless. Testing at 0-20 Joules is one thing, following the IEC surge test is another but then there is arcflash... I wrote once about having friends who worked with an electrician who was involved with an arcflash incident and how there was very little left of the body.Again, I am not an electrician and have little interest in safety when it comes to handheld DMMs as I just don't use them in that environment. For me, it's far more important that a DMM would pass the EMC standards. I am far more interested in knowing if a meter is robust enough to survive some minimal transients without damage. The fact that I based my open circuit voltage waveform on the IEC surge test doesn't mean that anything I have shown has anything to do with safety.About the closest thing I have ever shown relating to arcflash was to use a bit of copper wire to make a cork gun. Again, it was a very controlled experiment with a small amount of energy but it did provide some idea about the expansion of copper.

author: TRN
author_rank: Unknown
timestamp: August 11, 2020, 12:21:23 pm
content: Neilm,Just out of curiosity;What would actually happen if you were using 2m leads and the 10Amp fuse would open?rgds

author: Fungus
author_rank: Super Contributor
timestamp: August 12, 2020, 05:57:26 am
content: It's nothing to do with voltages or impedance.CAT III should have some sort of local circuit breaker between you and the substation.CAT IV doesn't.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 12, 2020, 11:30:35 am
content: Normally for surge you will have a network to prevent the majority of the transient from back feeding onto the mains. The AC mains are always active and the transients are applied at 0,90,180 & 270 degrees in both polarities. Mains my be derived from a generator or direct. Say for example you are testing 50Hz in the USA, we would create the line.If you were to put your meter into current mode and attempt to run the surge, the fuse will blow as soon as the meter is attached regardless of the transients.While they do require you to test with an open fuse at higher voltage levels, surge is not part of that.*****For my little home experiments, the transients are not sync'ed to the line and there are no provisions to apply AC to the DUT. The generator can apply a full rectified AC signal to the DUT but the generator will disable it automatically at the time the transient is applied. I don't collect any data where I transient test the current inputs. At times I will apply a DC voltage with the fuse pulled to make sure the meter will not break down. If they supply the meter with non-filled glass fuses, then I may take the time to blow them....