CAT ratings misunderstood - Page 1
Forum section: electronics / testgear

author: 001
author_rank: Super Contributor
timestamp: November 05, 2018, 10:01:59 pm
content: HiTell me what is REAL difference behind CAT ratingsFor exampleI can work with motor at plANT (CAT III) and at lab (CAT I)But motor requieres same energy in both casesVoltage and inrush current are same tooSo what the difference?

author: janoc
author_rank: Super Contributor
timestamp: November 05, 2018, 10:12:33 pm
content: It is not about how much energy the motor requires but how much is potentially available should anything go wrong. In the lab you likely have 10-15A fuses or circuit breakers. At a factory you can have a mains supply on the same voltage but rated for kiloamps (or more). You can guess which one will make a bigger bang should anything go wrong.Then there are transients - e.g. on a factory floor you can have voltage spikes of several kilovolts for various reasons, not likely in a lab. If you use a crappy meter with no/poor input protection and you get a surge of 3-4kV into it it will explode in your face and likely flash over. People have died because of that.Here is a good explanation:https://content.fluke.com/promotions/promo-dmm/0518-dmm-campaign/dmm/fluke_dmm-chfr/files/safetyguidelines.pdf

author: tautech
author_rank: Super Contributor
timestamp: November 06, 2018, 12:27:15 am
content: Quote from: 001 on November 05, 2018, 10:01:59 pmSupply impedance and the addition risk/hazard it offers closer to the supply.

author: 001
author_rank: Super Contributor
timestamp: November 06, 2018, 03:22:44 pm
content: Quote from: tautech on November 06, 2018, 12:27:15 amSoHow I can make my bench rataed to CAT I? Will be ok with simple 16A outlet fuse?

author: HKJ
author_rank: Super Contributor
timestamp: November 06, 2018, 05:25:31 pm
content: Quote from: 001 on November 06, 2018, 03:22:44 pmSupply it from a inverter.

author: simone.pignatti
author_rank: Frequent Contributor
timestamp: November 06, 2018, 05:31:57 pm
content: check this image

author: 001
author_rank: Super Contributor
timestamp: November 06, 2018, 06:19:27 pm
content: Quote from: simone.pignatti on November 06, 2018, 05:31:57 pmNo fuse ratings anywereAnd what is for "1000V CAT I" sign?

author: alm
author_rank: Super Contributor
timestamp: November 06, 2018, 06:36:55 pm
content: Quote from: simone.pignatti on November 06, 2018, 05:31:57 pmThat is a little bit misleading, in that connecting a socket to a CAT III circuit will not magically make it CAT II, but in many (but not all, see below) cases it will identify one.Quote from: 001 on November 06, 2018, 06:19:27 pm1000 V CAT I is a circuit with up to 1000 V nominal voltage, but with transients that are only class I. An inverter for a CCFL backlight in a laptop might be an example: high voltage, but only able to supply very limited energy even in a single fault condition.In general what separates CAT X from CAT X+1 (e.g. CAT I from CAT II) is impedance. For example, a small power transformer like you find in a desktop computer or a phone charger will present a much higher impedance source than a mains socket, and will severely limit voltage spikes and short circuit current. Same with some length of household mains wiring. That's why the Fluke appnote linked above specifies CAT II as "Outlets at more than 10 meters from CAT III source" and "Outlets at more than 20 meters from CAT IV source". A fuse will not present much of an impedance until it blows (which might be a few tenths of a second later).Note that the usual sources are aimed at identifying the overvoltage category of an existing circuit, not at artificially constructing a circuit to meet a certain category. I imagine something like a small inverter of isolation transformer would indeed present a category I source.

author: 001
author_rank: Super Contributor
timestamp: November 06, 2018, 06:46:49 pm
content: I have 4000W isolated transformer with 32A outlet on my bench. I dont know what voltage pikes fly in my outletCan I say what this is CAT I area? Must I add some varistors or overvoltage TVS diodes?

author: tautech
author_rank: Super Contributor
timestamp: November 06, 2018, 07:02:43 pm
content: Quote from: 001 on November 06, 2018, 06:46:49 pmA mains transformer is not optimized for HF pass through and will help to suppress any incoming mains spikes.This is not to say little won't get through but understand all modern appliances have inbuilt mains suppression circuitry.You worry too much.

author: simone.pignatti
author_rank: Frequent Contributor
timestamp: November 06, 2018, 07:15:03 pm
content: I found this document by Fluke which may helpshttps://content.fluke.com/promotions/promo-dmm/0518-dmm-campaign/dmm/fluke_dmm-chfr/files/safetyguidelines.pdf

author: 001
author_rank: Super Contributor
timestamp: November 06, 2018, 07:24:42 pm
content: Is it mean what CAT parameters are not measured and not standartisised anyway? No "affordable" values for voltage, current, mains inductance, wire resistance, etc?

author: GeoffreyF
author_rank: Regular Contributor
timestamp: November 06, 2018, 07:26:08 pm
content: My view of Cat ratings is simple - What the testing labs say it means. Any attempt to simplify the standard basically changes what the standard says.However, this is how I think about it.First, any equipment that has any connection to mains voltage could deliver a possibly lethal current due to an unanticipated failure. This includes a circuit powered by a brick, wall wart or whatever.Second, the less things between what you are doing and the step down transformer out on the street, the more possibility of lethality, the higher the Cat rating is appropriate.Third, user mistakes can and do occur. Higher Cat ratings recognize the possibility of user error and the greater need to protect the user. Example old style not guarded pin connectors or banana plug connectors could pull out of the socket and cause injury.Fourth, Higher CAT ratings are protective but don't guarantee safety for an inattentive or careless user.Fifth, the CAT rating of the system is the lowest CAT rating in use. A CAT III meter with Cat II probes is a Cat II system. Carelessness or lack of skill while probing high voltages can lead to a dead CAT no matter the CAT Number. Basic Care is to assume lethal voltage is everywhere until proven differently.You can die of mains electricity and not blow a fuse. Therefore concerns about current ratings of fuses in prior posts are misplaced. Lethality has to do with whether the current, the length of time the current is there and how much can pass through a major organ. Voltage high enough to cause dielectric breakdown of skin, reducing resistance, increase critical currents. Fuses are there to prevent fire. That's a different topic.Nothing that I just wrote is absolute. Read some books. Simplifying creates a real possibility of leaving something out. Here is a nice wiki page.https://en.wikipedia.org/wiki/Electrical_injury

author: GeoffreyF
author_rank: Regular Contributor
timestamp: November 06, 2018, 07:30:39 pm
content: Quote from: 001 on November 06, 2018, 07:24:42 pmNo. It doesn't mean whatever you were trying to mean. "etc" (what you didn't list) can kill you. There is no simple answer, such as you seem to think. I really recommend that you do more reading. You will also enjoy electronics a lot more if you know what it is.

author: Neilm
author_rank: Super Contributor
timestamp: November 06, 2018, 08:16:26 pm
content: Quote from: blueskull on November 06, 2018, 07:31:08 pmWith only a x2 overload, fuses probably will not blow. Any connection to the mains supply would still pretty much count as a CAT II situation. The stand alone supply would reduce that to CAT 0. There isn't a CAT I situation any more.CAT situations are basically as shown in those pictures, CAT IV is the supply outside the building, CAT III is the supply from the distribution board to the sockets and CAT II is from the outside of the sockets to whatever is being powered.Transient wise the expected surge drops with each drop in rating, CAT IV 600 V is 8 kV, CAT III 600 V is 6 kV, CAT II 600 V is 4 kV. Each rating has an expected energy it has to deal with. HRC fuses are used as they deal with a lot of energy when opening. I have seen glass fuses used in meters that claimed CAT III 250 V protection. They exploded when there was a transient.

author: Fungus
author_rank: Super Contributor
timestamp: November 07, 2018, 08:39:04 am
content: Quote from: 001 on November 06, 2018, 06:19:27 pmRight! Fuses will be appropriate for the installed equipment.Quote from: 001 on November 06, 2018, 07:24:42 pmYou've got itcompletelybackwards.CAT ratings aren't a code that needs to be followed during electrical installations.CAT ratings are anattemptto classify the world's existing electrical installations with regards to suitable equipment/procedures for working with them.CAT ratings are a bit vague, yes, because it's impossible to precisely classify every possible electrical installation in the world. There's always going to be some that are a bit in-between.

author: Neilm
author_rank: Super Contributor
timestamp: November 08, 2018, 08:18:54 pm
content: One things CAT ratings do define is the potentil short circuit currents. CAT IV situations are defined as low input impedance, IEC61010 notes a 1 kV CAT IV situation will have a potetial short circuit current of >>25 kA. That figure is from memory, but is in the correct ball park. Any meter that is rated at CAT IV must have some way of dealing with that fault current. This is usually a fuse with enough impedance to reduce the current to a more manageable level.

author: bson
author_rank: Unknown
timestamp: November 08, 2018, 09:05:18 pm
content: Quote from: Neilm on November 06, 2018, 08:16:26 pmI don't think an exploding glass fuse in an enclosed chassis contradicts a CAT rating.

author: 2N3055
author_rank: Super Contributor
timestamp: November 08, 2018, 09:19:28 pm
content: CAT ratings are classification of how much energy is available if something goes wrong. And how much of overvoltage is possible if there is a spike on mains.Or to put it simply: It is a categorization of how big explosion will be if something goes wrong while you hold meter in your hand. And how big overvoltage will be if it happens.If you are measuring voltage on building mains fuse that is connected directly to distribution transformer with cable as thick as your hand, explosion will be huge (lethal). If you are measuring after meters and meters of cable, fuses and protection devices, you will have less energy available and explosion might be just loud pop. And also sharp voltage spikes will be attenuated... So less boom, and less likely to damage something...

author: Fungus
author_rank: Super Contributor
timestamp: November 09, 2018, 07:46:12 am
content: Quote from: Neilm on November 06, 2018, 08:16:26 pma) 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:Quote from: Neilm on November 08, 2018, 08:18:54 pmI'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.

author: rf-loop
author_rank: Super Contributor
timestamp: November 09, 2018, 07:59:28 am
content: In some situations it need also take care about maximum current what fuse can break.In positions where impedance is low there may exist cases that short circuit current is so high that fuse can not break it.Long time ago I work in place where this happen. Oh well there was lot of vaporized copper rods.Lucky I was not inside this room where this happen. (because most danger thing is vaporized copper in air and breathing there ... after some seconds human can not rescue) But after hour (switch room ventilation) we need start carefully repair this disaster and install new copper rods and so on. (because factory need urgently get electric back. Place was short distance from one power plant so impedance was really low. (Later installed some big inductor for reduce shortcircuit current so that this never happen again)Reason for accident was wrong istalled copper rods what bends and bend too near each others and spark-plasma make short when high temperature change in high load situation.Typically big fuses maximum break capacity is around 100kA. Small fuses of course less. And many times peoples forget to think Fuse max break current.What is CAT level in this kind of places.. where max short current may exceed 100kA

author: Fungus
author_rank: Super Contributor
timestamp: November 09, 2018, 08:11:34 am
content: Quote from: rf-loop on November 09, 2018, 07:59:28 amCAT has it's limits. It doesn't try to cover high-energy installations.If I was using a multimeter in places like that I'd want one with no in-circuit current ranges. That's what clamp meters and current clamps are for.

author: vk6zgo
author_rank: Super Contributor
timestamp: November 09, 2018, 08:31:43 am
content: Quote from: blueskull on November 06, 2018, 07:31:08 pmIf a transient is a few microseconds long, the energy of that transient is limited by its low duty cycle.If during its existence, it is able to cause a breakdown of some protective system, such that it creates a current path for the normal supply voltage, that is where the hazard lies.

author: rf-loop
author_rank: Super Contributor
timestamp: November 09, 2018, 10:00:46 am
content: Quote from: Fungus on November 09, 2018, 08:11:34 amOf course.(and of course normal people have nothing to do in this kind of places. We who had licence to enter, we know exactly what and how to do things there, and specially what must not do. Even order how you switch things on and off there is very important. Many times need do service and inspections things without power break.Without adequate training, there is nothing to do with such rooms. In some places in certain jobs also need work in pairs, other do and other watch. Workers both had to also have full first aid resuscitation training (emergency intubation and defibrillator use). But it was old times when I did both industrial electrics, automation and electronics. I do not know today practices there - I'm retired.)

author: Neilm
author_rank: Super Contributor
timestamp: November 09, 2018, 06:01:29 pm
content: Quote from: Fungus on November 09, 2018, 07:46:12 amThe 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.

author: HKJ
author_rank: Super Contributor
timestamp: November 09, 2018, 06:07:12 pm
content: 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.

author: Fungus
author_rank: Super Contributor
timestamp: November 09, 2018, 06:12:34 pm
content: Quote from: HKJ on November 09, 2018, 06:07:12 pmThe trick is NOT to do that.

author: HKJ
author_rank: Super Contributor
timestamp: November 09, 2018, 06:22:49 pm
content: Quote from: Fungus on November 09, 2018, 06:12:34 pmOf 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.

author: Fungus
author_rank: Super Contributor
timestamp: November 10, 2018, 11:28:07 am
content: Quote from: HKJ on November 09, 2018, 06:22:49 pmIf 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.

author: janoc
author_rank: Super Contributor
timestamp: November 10, 2018, 07:18:58 pm
content: Quote from: bson on November 08, 2018, 09:05:18 pmThere 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