Author Topic: CAT ratings and interpretation  (Read 19274 times)

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Offline Lightages

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CAT ratings and interpretation
« on: October 26, 2012, 02:22:07 pm »
I have started this to discuss CAT ratings as there seems to be confusion as to what they actually mean in relation to multimeter design and protection. I am getting confused where in the past it seemed clear to me. I have been believing that a multimeter must meet its CAT specs on ALL functions and inputs. Others here have been stating that the CAT ratings only apply to the non-current terminals.

After searching through the maze of trying to get clear results searching online I finally came across this quote from the Gossen webpage and I now remember reading something similar quoted here before.

"Above all one specific point (chapter 16.2) must be observed in the EN61010-1 standard:

“Multimeters and similar devices may not be the source of any danger whatsoever, regardless of the utilized combination of specified input voltages, function settings and range selections. The term danger includes electrical shock, fire, sparking and explosion.
Conformity is substantiated by means of the following test:
The highest specified, rated voltage for any of the device's functions is applied to each pair of terminals, one after the other, in every possible function and range combination. ...
No danger may occur either during or after the test.”

Thus every range must now be protected in accordance with the specified measuring category and input voltage (e.g. 600 V CAT IV).
This was not required in the past. It was permissible to reduce the overvoltage strength of individual measuring functions (e.g. resistance measurement)."

Thus from Gossen's interpretation of the rules it would appear that my assumption was correct.
 

Offline EEVblog

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Re: CAT ratings and interpretation
« Reply #1 on: October 26, 2012, 05:07:29 pm »
I recall (I think) Fluke telling me that unlike many multimeter manufacturers, they send their meters for external independent CAT IEC/EN certification rating. They don't simply rely on in-house testing, or worse, in-house verification based simply on design.

Dave.
 

Offline EEVblog

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Offline Lightages

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Re: CAT ratings and interpretation
« Reply #3 on: October 26, 2012, 05:14:12 pm »
Any respectable manufacturer would at least send out a sample of a prototype and then later a sample of the first production run to verify compliance, and then anytime after any changes were made in components or design.

But my question really is does compliance to the EN61010-1 standard require that all terminals (inputs) comply for the whole meter to comply to the CAT rating stated on the meter as asserted by Gossen on their webpage?
 

Offline EEVblog

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Re: CAT ratings and interpretation
« Reply #4 on: October 26, 2012, 05:20:35 pm »
But my question really is does compliance to the EN61010-1 standard require that all terminals (inputs) comply for the whole meter to comply to the CAT rating stated on the meter as asserted by Gossen on their webpage?

No idea, the full standards document is expensive, and I have yet to find a copy online.

Dave.
 

Offline Lightages

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Re: CAT ratings and interpretation
« Reply #5 on: October 26, 2012, 05:29:20 pm »
OK, then hopefully someone who has access to the documents can chip in here. I have based my opinions on certain equipment based on the interpretation that equipment must meet its CAT rating on ALL functions and ALL terminals.
 

Offline Wytnucls

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Re: CAT ratings and interpretation
« Reply #6 on: October 26, 2012, 05:55:16 pm »
As I explained to you in the other thread, connecting your meter in series in a circuit to measure amps, results in very low voltages across the shunt (Ohm's law). So, connecting the meter to 600V or 1000V in series is no big deal, as long as the impedance in the circuit is much higher than the internal impedance of the shunt and fuse, usually about 0.01 Ohm each. Of course, the amps must remain below the breaker's rating (10A usually), or it will blow.
Now, if you put a transient of 8000V with an impedance of 2 Ohms on the A or mA range, all bets are off! It is in essence a short! 8000V and 40-50 amps across the fuse would cause havok on any meter. Very few meters will survive that, depending on the transient time.
The fuse may not even have time to blow, depending on the rise time, frying the traces upstream.
On the mA/uA range, the rectifier bridge and Zener diode would have to take the brunt of it, until the fuse blows.

I haven't seen the actual required procedure to test meters. I suspect that all that is required on the amp range, is to verify that in series in a circuit with a 600/1000V power supply with high impedance, there is not leakage or shorts in the meter. I don't see the point of blowing the fuses with anything above 10A/1A on the Amps ranges.
So, in essence, most meters are fine with 600/1000V on the amps range and are not limited at all by the voltage rating of the fuses, which is just an indication of their ability to contain the arc and energy, when a short occurs.
« Last Edit: October 26, 2012, 05:58:06 pm by Wytnucls »
 

Offline Lightages

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Re: CAT ratings and interpretation
« Reply #7 on: October 26, 2012, 06:12:59 pm »
Your suspicions, and my understanding of what is the actual case, is what this thread is about. I have cited a specific manufacturer which is regarded by many as being one of the best manufacturers in this field. They have made the assertion and have quoted directly from the documents relating to the safety requirements mandated. They have access to the documents and would have great problems if they outright lied about what is required.

Feelings and suspicions are a mute point.

I hope to put to bed the speculation everyone has about these specifications.

Until proven otherwise by the actual full documents or an assertion by a participant in these specifications, I can only fall back on the reputation of Gossen for what has been asserted by them. To whit, and I paraphrase: All equipment that claims to be rated and complies to the standards stated in EN61010-1 must meet these requirements on all terminals (inputs or outputs) for all functions.

If this is the truth, then many meters I have seen, seen in tear downs, seen in reviews, and have seen in advertising, are not meeting these requirements and are lies or at best marketing spin that has no basis in the truth in the final product.
« Last Edit: October 26, 2012, 06:14:44 pm by Lightages »
 

Offline Wytnucls

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Re: CAT ratings and interpretation
« Reply #8 on: October 26, 2012, 06:32:41 pm »
Gossen didn't say that a high voltage transient with low impedance has to be applied to the the amps range.
Gossen meters might not survive that either! All the documents I have seen so far, only mention high transients on the other ranges.
But the crux of the matter is that you thought the low voltage rating of the fuse (250V), meant that the meter didn't meet its CAT rating and that is not the case at all.
 

Offline Wytnucls

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Re: CAT ratings and interpretation
« Reply #9 on: October 26, 2012, 06:41:58 pm »
The highest specified, rated voltage for any of the device's functions is applied to each pair of terminals, one after the other, in every possible function and range combination. ...
No danger may occur either during or after the test.”

1000/600V on the amps range is no big deal with most meters. That's probably the easiest CAT rating test of them all.
No high voltage transients here...
 

Offline Lightages

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Re: CAT ratings and interpretation
« Reply #10 on: October 26, 2012, 06:48:03 pm »
Your opinion is not fact without the documents to back it up.

Gossen has unequivocally stated "The highest specified, rated voltage for any of the device's functions is applied to each pair of terminals, one after the other, in every possible function and range combination. ...
No danger may occur either during or after the test." This is directly opposed to your assertion.

Please demonstrate by way of another document derived from the EN61010-1 standard, or the EN61010-1 standard itself, or from a statement from an actual official or inspector of the standard that this is not the case. Without this, Gossen is to be believed and anything else is just personal opinion or speculation.
 

Offline Lightages

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Re: CAT ratings and interpretation
« Reply #11 on: October 26, 2012, 06:58:31 pm »
1000/600V on the amps range is no big deal with most meters. That's probably the easiest CAT rating test of them all.
No high voltage transients here...

Dump a CAT IV 600V 8000V peak impulse transient 2 Ohm source across a 250V glass fuse and you will see quite the nice reactive high voltage high energy blast of energy. There is no doubt that it will result in a dangerous situation. Where does this not involve high voltage? It is in the specification by your own assertion!
 

Offline Wytnucls

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Re: CAT ratings and interpretation
« Reply #12 on: October 26, 2012, 07:17:46 pm »
I don't dispute the Gossen statement at all.
The highest voltages for CAT III/IV ratings are either 600V or 1000V. Easy test for most meters on the amps range.

High voltages transients with low impedance on the amps range would fry most meters, regardless of the fuse in place, so that's why I'm pretty sure it is not part of the test, but you're right, we need to see corroborating evidence.
Up to now, I can only refer you to Fluke's documents and others , where there is no mention of the amps range at all, while applying high transients.

That's still doesn't explain why you thought that my meter couldn't measure amps above 250V and therefore didn't meet its CAT III/IV rating.  ;)

« Last Edit: October 26, 2012, 07:32:44 pm by Wytnucls »
 

Offline Lightages

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Re: CAT ratings and interpretation
« Reply #13 on: October 26, 2012, 07:41:23 pm »
That's still doesn't explain why you thought that my meter couldn't measure amps above 250V and didn't meet its CAT III/IV rating.  ;)

I never said that the meter referenced could not measure current above 250V. The problem occurs when there is a fault causing the fuse to blow when connected beyond its rating. It will be dangerous if happens during CAT III/IV conditions. I am sure that no one here would argue that this is not a dangerous condition.

I am making the claim that a 250V glass fuse is not safe for breaking a current under a 8000V 2ohm source fault. By definition the fuse is not rated to do this. Please demonstrate that the fuse in your meter exceeds its 250V rating by placing a 8000V pulse from a 2 ohm source. This of course assumes that Gossen is not lying and that all CAT ratings apply to all terminals on the meter. I for one will not question their knowledge on that assertion without a higher authority.

If Gossen is correct, that by definition a meter that does not have sufficient protection on all its terminals to meet the CAT rating  is not CAT rated correctly, then yes, your meter does not meet its claim of safety.

 

Offline Wytnucls

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Re: CAT ratings and interpretation
« Reply #14 on: October 26, 2012, 08:12:08 pm »
Your words:
'I agree that my UT71E could be a lemon as far as accuracy goes, but it is clear that the current measurements are not made for more than 250V and thus do not meet their CAT ratings'.

Where did you get that 250V figure from, if not from the fuse?
 

Offline EEVblog

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Re: CAT ratings and interpretation
« Reply #15 on: October 26, 2012, 09:29:14 pm »
High voltages transients with low impedance on the amps range would fry most meters, regardless of the fuse in place, so that's why I'm pretty sure it is not part of the test, but you're right, we need to see corroborating evidence.
Up to now, I can only refer you to Fluke's documents and others , where there is no mention of the amps range at all, while applying high transients.

There can easily be high voltage transients on the amps range, due to inductance in the leads and system under test when the fuse blows open.
Think of back-emf when relays coils switch off for example.
So it would seem obvious that the current range has to survive those transients, and likely that the standard might simply use the existing voltage transients source standard as per the volts range for the sake of simplicity. The diode protection in this case should be more than adequate, as evidenced by the Fluke meters not having high energy MOV's on the amps measurement input circuit.

Dave.
« Last Edit: October 26, 2012, 09:33:19 pm by EEVblog »
 

Offline Wytnucls

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Re: CAT ratings and interpretation
« Reply #16 on: October 26, 2012, 11:08:06 pm »
Yes, I agree, transients can happen while measuring amps, but I'm still sceptical about the testing procedure.
For CAT IV, the test transient voltage would consist of a 12,000V pulse with generator internal resistance of 2 Ohms, applied across the shunt and fuse (0.02 Ohms), for approximately 100 uS. Rise time is 1.2uS with a decay to 50% after 50us.
If my calculations are correct, using Ohm's law, assuming DC voltage, that would put about 120V and 6000A across the shunt at peak, unless the fuse can open faster. With voltage rising high and arcing after that, I doubt most meters would survive such a test.
« Last Edit: October 26, 2012, 11:11:28 pm by Wytnucls »
 

Offline Wytnucls

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Re: CAT ratings and interpretation
« Reply #17 on: October 26, 2012, 11:41:21 pm »
There is also a good chance that the fuse casing would rupture, with random damage in the meter. How would they decide if it meets specifications? Install a new fuse and see if the meter still works? Are the manufacturers allowed several tests?
All this testing is done in-house I guess. Perhaps, independent testing should be required.
What if the meter passes CAT IV 1000V on the Volts/Ohms range with flying colours, but fails miserably Cat II on the Amps range. Can it be labelled CAT IV on the Ohms range and CAT I on the Amp range?  ;D
 

Offline EEVblog

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Re: CAT ratings and interpretation
« Reply #18 on: October 26, 2012, 11:55:38 pm »
Yes, I agree, transients can happen while measuring amps, but I'm still sceptical about the testing procedure.
For CAT IV, the test transient voltage would consist of a 12,000V pulse with generator internal resistance of 2 Ohms, applied across the shunt and fuse (0.02 Ohms), for approximately 100 uS. Rise time is 1.2uS with a decay to 50% after 50us.
If my calculations are correct, using Ohm's law, assuming DC voltage, that would put about 120V and 6000A across the shunt at peak, unless the fuse can open faster. With voltage rising high and arcing after that, I doubt most meters would survive such a test.

Bingo.
The devices may not necessarily be designed to survive these tests. Survival may not be a requirement, but "failing safe" certainly would be.
Why do you think meters have 20,000A interrupting current HRC fuses?

Dave.
 

Offline EEVblog

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Re: CAT ratings and interpretation
« Reply #19 on: October 27, 2012, 12:05:33 am »
Notice how the meter in this Gossen video is being tested on the 10A current jack.
http://www.gossenmetrawatt.com/english/seiten/cautiondangerousmultimeters.htm

And some info here:
http://www.gossenmetrawatt.com/gmc/resources/marcom/fachartikel/306_tt1_gb_cat_iv_multimeter.pdf

Whatever the standard requires for testing the A jack, whether is the same as the voltage jack (for convenience or whatever), or another value entirely, you can bet it's a big energy pulse.

Dave.
 

Offline Wytnucls

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Re: CAT ratings and interpretation
« Reply #20 on: October 27, 2012, 12:38:36 am »
Hey, that yellow thing looks just like my meter!  :(
That high energy pulse was waaaay too long, Gossen! 100 microseconds, no more, ok?

Yes, your meter has 20,000A fuses, not mine, unfortunately.
I had read the Gossen publication, but was none the wiser about the actual testing on the Amp range.

I have found the right pdf about multimeter testing, but it doesn't download properly.
Here is the link if anybody wants to try:
http://bbs.dianyuan.com/bbs/u/58/1455811193738104.pdf (Chinese site)
I can't believe that the official document is for sale at such a high price! (About USD150.00)
« Last Edit: October 27, 2012, 12:50:34 am by Wytnucls »
 

Offline Chasm

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Re: CAT ratings and interpretation
« Reply #21 on: October 27, 2012, 01:16:39 am »
If you apply such  test voltages (including transients) to the amps jacks your meter will no longer work.

It is not expected to work any longer, at the very least the fuse will have blown. Maybe without internal damage to the meter, given the energy content of some tests probably with. The question this test tries to answer is: Does it blow up, or electrocute, the operator too?

Its a destructive test, taking the meter way out of its operational specifications. (100x to 1000x the current rating! Followed by a really™ high voltage spike once the fuse tries to blow.)
Again, the question is how does it fail, not if it fails.

 

Offline Lightages

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Re: CAT ratings and interpretation
« Reply #22 on: October 27, 2012, 01:21:47 am »
I was able to download the document and will look through it and report back later.

From what I understand, the intent is not to protect the meter, but the user from harm. The meter can be completely non-functional after the test but nothing can occur that might harm the user.

Sure during the short period of time the fuse is intact there might be much less than 8000V across the leads, but as soon as that fuse lets go, almost anything goes as far as the voltage. The voltage could even end up higher due to inductance if the conditions are correct.

 

Online Monkeh

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Re: CAT ratings and interpretation
« Reply #23 on: October 27, 2012, 01:43:20 am »
But my question really is does compliance to the EN61010-1 standard require that all terminals (inputs) comply for the whole meter to comply to the CAT rating stated on the meter as asserted by Gossen on their webpage?

No idea, the full standards document is expensive, and I have yet to find a copy online.

Dave.

Will email you a copy shortly.
 

Offline Neilm

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Re: CAT ratings and interpretation
« Reply #24 on: October 27, 2012, 03:09:24 am »
Multimeters are covered by 61010 part 1 (general requirements), I think that part 030 is the part that covers multimeter. I believe that it only calls for basic mis-connections as the rated voltage without the transient - i.e. if it is a 600V CATIV  then you connect your current terminals to a low impedance 600V supply.

Neil
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