CAT III or CAT IV?

[paulengr 44]

What's the difference between "Category III" and "Category IV" for meters? I don't mean "outdoor" vs. "indoor". What do the testing standards actually mean? For instance, NEMA 3(x) is "outdoor" and NEMA 1 is indoor. If you have operators using fire hoses to clean up, better break out the NEMA 4 enclosures. And just try that NEMA 3(x) on board in a marine application! What is the REAL (as in the testing requirement) difference between Cat III and Cat IV?

[Clay B. 1]

Cat III and CatIV are protection levels. The higher the number the larger the voltage the meter can handle. Look here at Fluke's definitions http://us.fluke.com/usen/support/Glossary.htm#C

[rpraveenkum 5]

see this table

[paulengr 44]

None of the meter companies actually list the specs. They just use vague descriptive words such as the reference above. Those terms are purely descriptive. For instance, does that mean that a medium voltage distribution system in a plant is CAT III or CAT IV? If you use poles between your buildings for distribution, does that make your internal distribution systems "service entrance" grade? That's my basic problem with the above definitions...they are so vague that they are useless. I finally did find what I was looking for. CAT II/1000V = 6000V test voltage @ 2 ohms CAT III/600 V = 6000V test voltage @ 2 ohms CAT III/1000 V= 8000V test voltage @ 2 ohms CAT IV/600 V=8000V test voltage @ 2 ohms Things like "water", "distribution", "outdoor", etc. are meaningless unless you realize that the example that these terms are based on the example which is a strictly commercial/residential application. Once you jump to 480V distribution and/or you have a medium voltage bus nearby, the descriptive terms are meaningless. My reasoning on this one is that the highest transient I can reasonably expect under any scenario of various phases opening in certain orders with or without grounding effects would be to create a transient of twice the base voltage. If lots of capacitors are involved, then there really isn't an upper limit. But under normal circumstances, this wouldn't happen. If it did, then a full transient analysis is in order. The second line of reasoning is that there's a chance that something goes wrong in the medium voltage distribution system which would cause the medium voltage to appear on a 480V bus. Nasty I know but I've seen things like this happen before. So that puts most plants at 4160X2 ~ 8000V. Since the medium voltage in the plant where I work is only 2400V, I'm still good at 6000V. There's no mention of water, etc. in the actual specifications. As to the safety category stuff...does not apply in this situation. This is strictly a meter standard, not a control standard.

[robh 12]

Answer I got from a PE awhile back when I asked the same question: In response to your question concerning the Cat. III rating on your multimeter. I asked around about the cat. III rating on your fluke. This rating is an over voltage rating that can be found in EIC 1010-1 and ANSI/ISA S82.01-1994. Basically put, the over voltage surges in a system vary with respect to where in the system you are. For example the over voltage surges that can be expected from a duplex outlet are much smaller than the over voltage surges that could occur at the main service entrance conductors due to the available fault currents, proximity to the utility line, and conductor sizing in the system. For this reason meters are rated according to the locations that they expect to be used. A cat. I rated meter is not rated to be utilized for voltage measurements at the service entrance conductors however, a cat. III is. If you took a facility and mapped it out on a piece of paper, there would be a region surrounding the main entrance conductors and main switchboard that was cat. III, another region encompassing the panel boards downstream (Cat. II) and a third region that would extend down to everything else (cat. I).

[TConnolly 13]

Don't forget to include your test leads. You should use whichever rating for your meter and your leads is lowest. Eg, if you have CAT II leads plugged into your CAT III meter, then you are CAT II. This includes those cheap alligator clips techs so often use.

[Clay B. 1]

Actually saw a guy using a set of leads so cheap that the insulation slid off the bare metal probe. Not sure what rating they had but I am betting it was not very high. When it comes to leads or meters you really do get what you pay for. Ask any electrician and they can tell you horro stories about meters exploding or leads frying.

[robh 12]

We had a guy here who used one of those pocket meters that looks about the size of a small calculator. Has its own little plastic/vinyl case. I don't know if it even had a UL stamp on it. There are two rules about meters I always follow- 1-I never buy a "cheap" meter. I will always try to buy a Fluke, Harbor Freight has a lot of neat stuff, just not meters. 2- I never let people borrow my meters (unless they are working in my direct line of site helping me). I had a teacher who was working on an air conditioning system who got stuck to a ladder and ac unit while testing the fan motor. He had loaned out his meter and the guy who borrowed it made some "internal adjustments", thus basically creating a dead short from the motor leads through his meter to the frame he and his ladder were touching. One of the best all around meters I have found and will not go anywhere with out is the Fluke T5-1000. Very nice for the price at around $100. I think that the nicest feature is the AC/DC automatic selection.

[panic mode 246]

Agreed, you need something dependable. That's decent and affordable meter, nice to be able to measure current without interrupting circuit. Too bad it can't measure DC current (not even mA). I'm just packing away 1587, it has insulation tester (there are cheaper models woth this feature like 1503 and 1507) Praveen It seams you have attached wrong table.

[rpraveenkum 5]

Table for general idea about Cat 3 & 4. sorry i didn't read the post in full

[TConnolly 13]

Reminds me of a story. Quite a few years back a cow-orker who was a student studying electrical engineering at a nearby university came by and wanted to borrow a meter that could measure current. So I handed him a current clamp. He looked at it and then told me he had been to maintenance first and asked the same thing and they handed him a current clamp as well. He didn't know what it was or how to use it. So I explained how to use it. He said that wouldnt' work, so I asked him what he was doing. He said he wanted to find out how much current was available at an outlet. I told him that every outlet had an ID that would identify the panel where he could find the breaker and determine if it was a 15 or 20 amp circuit. He said, "Can't I just borrow your multimeter and stick the probes in the outlet." Naturally, I refused, and attempted to explain why that wouldn't work. About an hour later I walked through the area where this kid worked and saw one of those pocket meters sitting on a desk. It had been blown to pieces, the leads were completely burnt, the front panel was completely blown off. I looked around at the nearby outlets and saw the carbon scoring on one of them. Luckily the kid wasn't hurt. So I explained it again to him and he learned a very lucky lesson the hard way.