Author Topic: Cat 4 DMM with Polyfuse  (Read 857 times)

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Offline zerobrain

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Cat 4 DMM with Polyfuse
« on: July 16, 2018, 01:46:26 pm »
Hi,

i came accross the new Owon DMM and was puzzled it uses only (!) a polyfuse for the mA current input.
From my understanding this may work on a hobbyist´s workbench, but this DMM is advertised as "CAT3 and CAT4".
Any ideas anybody?

best regards
 

Online Fungus

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Re: Cat 4 DMM with Polyfuse
« Reply #1 on: July 16, 2018, 02:59:55 pm »
Which meter is it, exactly?

It's not impossible for it to be a low voltage CAT IV, just unlikely.

It wouldn't be the first meter with a fake rating.
 

Offline zerobrain

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Re: Cat 4 DMM with Polyfuse
« Reply #2 on: July 16, 2018, 03:10:45 pm »
It´s an  Owon OW18B.
 

Offline joeqsmith

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Re: Cat 4 DMM with Polyfuse
« Reply #3 on: July 16, 2018, 05:06:18 pm »
I think Tekpower was selling that same meter.  Maybe.   

It wouldn't be the first meter with a fake rating.
You don't say.  Why would a company ever misrepresent their product?
How electrically robust is your meter?? https://www.youtube.com/channel/UCsK99WXk9VhcghnAauTBsbg
 

Offline Kleinstein

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Re: Cat 4 DMM with Polyfuse
« Reply #4 on: July 16, 2018, 06:14:33 pm »
A lot of the cheap Chinese DMM have fake ratings. I don't know why, but that is what is observed.

The odd thing is that for some meters (some UniT) there is a version with fake ratings and one for the EU with an at least reasonable rating that is considerable lower despite being essentially the same meter, if not higher grade fuses.

It looks like there is a different meaning to CAT... in China. More like the Cat number corresponds to the number of  :-DD and  :palm:.  Maybe for China or India they get away with this and those cheap units find there way to others too.  AT least most of the fakes are rather obvious. I am still waiting to see the a CAT V rating on a handheld.

To have any trust in the CAT ratings it needs additional labels from test institutes (e.g. UL, GS, ...) - however these might be fakes too.
 

Online Fungus

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Re: Cat 4 DMM with Polyfuse
« Reply #5 on: July 16, 2018, 08:08:31 pm »
AT least most of the fakes are rather obvious. I am still waiting to see the a CAT V rating on a handheld.

CAT V? You might have to wait a while for that (at least until they invent that category :popcorn: )

The Amprobe HD160C is CAT IV 1000V. Check out the fuse:



More info: https://www.eevblog.com/forum/testgear/any-experience-with-an-amprobe-hd160c/
 

Offline stj

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Re: Cat 4 DMM with Polyfuse
« Reply #6 on: July 16, 2018, 09:43:20 pm »
hey, the cat rating is about saving the user and not the meter.
if you do something stupid - and i'v seen enough demonstrations of that recently on utube,

if the meter turns to ash internally and the case contains it all, then i think it's a pass!!!
every meter i'v had has imposed voltage limits on the current range that is far less than the maximum in V range - so why does the fuse - polymer,ceramic or whatever have to withstand 1000v??
it doesnt, it's there to protect the tracks and shunt from overcurrent.
 

Offline joeqsmith

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Re: Cat 4 DMM with Polyfuse
« Reply #7 on: July 16, 2018, 11:18:17 pm »
hey, the cat rating is about saving the user and not the meter.
if you do something stupid - and i'v seen enough demonstrations of that recently on utube,

if the meter turns to ash internally and the case contains it all, then i think it's a pass!!!

I've seen people post what you have just wrote about the tests that I run on them.  That's how disconnected some people are.   Just crazy.   From a safety standpoint, it would be great to see an independent lab running meters.   

Quote
every meter i'v had has imposed voltage limits on the current range that is far less than the maximum in V range - so why does the fuse - polymer,ceramic or whatever have to withstand 1000v??
it doesnt, it's there to protect the tracks and shunt from overcurrent.

I think it's because the the standards require it.   They are also not rated for 10KA break current and up to save the meter.   It's all about safety.       
How electrically robust is your meter?? https://www.youtube.com/channel/UCsK99WXk9VhcghnAauTBsbg
 

Online blueskull

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Re: Cat 4 DMM with Polyfuse
« Reply #8 on: July 17, 2018, 12:23:32 am »
That meter has more problems. No FCCID for a BT device alone means it's illegal to sell in US, probably also Canada.
The transistor jack means there's no way the meter can withstand CAT4 600V -- that requires 12kV transient handling.
The distance between socket contact to surface of meter needs to be at least 0.5cm to block 12kV, and I don't think the meter is constructed as such.
The PCB doesn't have a UL file number nor a flame retarding rating or the EU equivalent, which makes it illegal for a board handling mains voltage.
The 2 MELF resistors are certainly not 12kV rated. Nor the PTC judging by the size. I didn't see the MOV at all.
The 5*20 fuse is apparently not an HRC one.
The clearance between metal screw and input jack is only ~1cm, which can't reliably block 12kV under moisture or high elevation. The screw post is NOT shrouded.
The BT module is placed at a really bad location without proper carrier board cutout, and the antenna is definitely detuned.
The board doesn't have any BT logo and device ID, so I would guess they didn't pay BT license fee as well.
 

Online Fungus

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Re: Cat 4 DMM with Polyfuse
« Reply #9 on: July 17, 2018, 01:02:45 am »
every meter i'v had has imposed voltage limits on the current range that is far less than the maximum in V range - so why does the fuse - polymer,ceramic or whatever have to withstand 1000v??

Because real people make mistakes.

Real people put the leads on the wrong input jacks, etc.
 


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