EEVblog #712 - Uni-T UT71E Multimeter (Why Uni-T Meters Suck)

[DanielS]

come to think of it, it is good if there is more tear downs of these "cheaper" gear. it will show alot of potential buyers if they should take the risks of using lesser protected gear (due to financial limitations) or just for pure show and tell so as to let people know the protection ratings is way over blown (and also whether they are worth the money)  ... MORE  ! MORE !! 

That's how I see it too.

The only way to (hopefully) stop manufacturers from fraudulently mislabeling and specifying their products is to name and shame them when caught so future would-be buyers will be aware of the issues they might be buying into and factor that into the price they are willing to pay for it.

Personally, the fake cat-III/IV ratings on the UT71E (at least as-shown in the tear-down) would knock $50 off what I might be willing to pay for one solely based on principle and the lack of genuine safety rating, regardless of how low, would knock another $20 off. Questionable clearances on the PCB and missing MOVs might knock another $20 off. Lop another $10 off for being useless and grossly inaccurate for low-power measurements and one more $20 for being barely within specs on many ranges with potentially questionable long-term accuracy.

Since leaving safety ratings out altogether is cheaper and safer than faking them, I would prefer if customs seized, inspected and destroyed them outright as hazardous goods - thou shall not fake safety ratings.

[Pentium100]

The problems with mislabeled CATIV ratings apply only to a small number of people. If the meter does not blow up when connected to the mains outlet then most people are safe. If the meter does not blow up when connected inside the breaker panel of a flat (or individual house) then even more people are safe.

So, showing that a meter with the wrong rating can explode when connected inside a substation (especially if it's the 10kV part) will make most people go "well, I'll keep that in mind if I decide to break into a substation to steal the oil".

[TMM]

Personally, the fake cat-III/IV ratings on the UT71E (at least as-shown in the tear-down) would knock $50 off what I might be willing to pay for one solely based on principle and the lack of genuine safety rating, regardless of how low, would knock another $20 off. Questionable clearances on the PCB and missing MOVs might knock another $20 off. Lop another $10 off for being useless and grossly inaccurate for low-power measurements and one more $20 for being barely within specs on many ranges with potentially questionable long-term accuracy.

So, with this thing built to Fluke levels of refinement you'd only pay $300? I'm sorry to say you are still very far off the mark.

[rob77]

The problems with mislabeled CATIV ratings apply only to a small number of people. If the meter does not blow up when connected to the mains outlet then most people are safe. If the meter does not blow up when connected inside the breaker panel of a flat (or individual house) then even more people are safe.

So, showing that a meter with the wrong rating can explode when connected inside a substation (especially if it's the 10kV part) will make most people go "well, I'll keep that in mind if I decide to break into a substation to steal the oil".

you got it wrong... that meter will not explode when you connect it to mains voltage regardless of the place (outlet in a house, distribution panel) , it's "just" 240 or 420VAC (phase-ground or phase-phase). the meter without a proper protection WILL blow up during a transient voltage on the line (kV range) again regardless of the place. another type of "blow-up" is when you accidentally short the mains with a meter on current range - again during normal operation (no transients on mains) the glass fuse will do the job , but during a transient (btw.. transients are pretty common - just shut off a motor nearby) the meter with glass fuse will blow up... the location (outlet in the house, fuse box, distribution panel) is just determining the maximum fault current and therefore determining the "amount of damage"

[Wytnucls]

There is no transient on the A/mA range. The fuse ruptures as soon as the meter shorts the mains supply (250V).
That's why there is no high voltage transient test done on the current ranges for a CAT rating. The test consists of applying double the voltage rating with ruptured fuses in place to check for arcing (creepage and clearances), for one minute.
New IEC regulations do require HRC fuses now, with a voltage rating equal to the highest DMM CAT rating. Glass fuses are out for ever in any new multimeter (in countries which abide by those regulations, like Europe and the USA).
If you see some meters with glass fuses for sale in a local shop, 'educate' the manager and tell him to dump them in the trash.
Small 5$ pocket multimeters with current ranges, equipped with small glass fuses or no fuse at all, are not legal either.

[EEVblog]

The problems with mislabeled CATIV ratings apply only to a small number of people. If the meter does not blow up when connected to the mains outlet then most people are safe.

No. You don't understand what the CAT ratings are for.
Even a $5 meter can measure 240V mains, that's not the point. The point is about transients that can happen on the mains and the energy behind those transients.
Lighting strikes, heavy gear switching, your neighbor doing something stupid etc. High CAT rated meters are designed to safely absorb such transients without blowing your hand off.
If you think the odds of this are low and want to take your chances, then fine, it's your life, but don't say it doesn't apply for anyone else.
These CAT ratings have been developed for a very good reason, your safety.

[electr_peter]

The problems with mislabeled CATIV ratings apply only to a small number of people. If the meter does not blow up when connected to the mains outlet then most people are safe. If the meter does not blow up when connected inside the breaker panel of a flat (or individual house) then even more people are safe.

So, showing that a meter with the wrong rating can explode when connected inside a substation (especially if it's the 10kV part) will make most people go "well, I'll keep that in mind if I decide to break into a substation to steal the oil".

I don't agree.
Technically your statement is partly true (if you measuring only low level voltages, CAT ratings are not that important), but the whole point of measuring is to find out unknown value. If you already know in advance about mains level, why do you measure it? It is because you don't know what reading will show. And this is the problem - you are measuring unknown voltage source while assuming that it should be safe (which might not be true). Reasons for potential dangers were well explained in other posts.

My point is that in almost all cases where injuries occurred (or were close calls) person measuring the voltage assumed (but did not knew exactly) that both meter and voltage levels are safe for work. See posts above - "I thought it is OK", "it is only low voltage", "I have done it before", "I will show other guys how to do it", "meter looked good enough", etc. If one always goes with such logic, one day serious accident can occur.

The other point is a lie about CAT ratings. Would you like a lift with fake maximum capacity rating/plane with false max cargo weight/building with false load rating/etc.?

[george graves]

No. You don't understand what the CAT ratings are for.
Even a $5 meter can measure 240V mains, that's not the point. The point is about transients that can happen on the mains and the energy behind those transients.
Lighting strikes, heavy gear switching, your neighbor doing something stupid etc. High CAT rated meters are designed to safely absorb such transients without blowing your hand off.
If you think the odds of this are low and want to take your chances, then fine, it's your life, but don't say it doesn't apply for anyone else.
These CAT ratings have been developed for a very good reason, your safety.

Energy.  Yes, we(I) understand that.  That's college/University freshman year stuff.  I don't think you're giving your viewers/forum peeps the credit they deserve.

Yes, CAT ratings are important.  If your working on high voltage, high energy stuff.  And those that do....know to look for it.  And would never use a Chinese DMM for work. 

For the 99.999% of that don't.  We know a chinesse multimeter is going to fib on it's CAT ratings.  Duh.

As far as a lightening strike while I measure mains voltage?  Well.  That's just plain silly.  Everyone knows during a lighting strike you need to fly a kite with a key on the line.

Should Uni_T be slapped on the wrist for faking CAT ratings?  Sure.  That sounds like a great idea for a video for you!

[Tepe]

No. You don't understand what the CAT ratings are for.
Even a $5 meter can measure 240V mains, that's not the point. The point is about transients that can happen on the mains and the energy behind those transients.

A risk people also run with electric hand mixers, curling irons, power drills, etc., etc. That said, exaggerated CAT ratings are of course despicable. I'd go as far as saying that not having a rating at all is much better than having one that is a lie.

[Wytnucls]

It would be difficult to prove that it doesn't meet its CAT rating.
It now has the right HRC fuses.
Regulations don't impose the fitting of MOVs, PTCs or high power current limiting resistors on the PCB.
They only require adequate clearances and a minimum of 8mm creepage distances (for CAT III 1000V) between, presumably, not all traces but only traces with potential high voltage differentials.
Internal damage is allowed, as long as it doesn't escape the meter and cause injury.

[Pentium100]

If you already know in advance about mains level, why do you measure it? It is because you don't know what reading will show. And this is the problem - you are measuring unknown voltage source while assuming that it should be safe (which might not be true). Reasons for potential dangers were well explained in other posts.

I may want to measure the voltage to find out if the value is not too low. If the voltage is too high, but only slightly (270V instead of 230) then the meter will not explode. If the voltage is 800V instead of 230V then the meter might explode, but so will all devices connected to the mains. I may also want to measure just to see if the voltage is there and whether the wire is safe to touch.

A cheap way to reduce the risk might be to connect a lightbulb in series with the probes when measuring voltage. Then even if the meter is shorted, the lightbulb will limit the current to below the explosion level (and provide indication that there is a problem) while the resistance of the cold filament will not affect the voltage reading.

They have fuses in the plugs, the multimeter has a fuse but it's in your hand, and in these models there isn't enough clearance and the fuse isn't a suitable type.

I have seen a fuse in a plug once, in a British plug. The other devices do not have fuses in the plugs, including my electric shaver (which runs on 230V and not batteries).

[Ericho]

I worked on high power systems part of my career (mostly on ships) personally up to 4000kw electrical systems up to 1100v able to handle >2000A
I’ve seen the inside of a hospital twice and got plenty of near misses and burn scares.
Those accidents I got were on a 220/380 net and on a 24v dc net (both able to handle a few hundred amps)
So not the 660v 1100v stuff
Lack of respect for high power systems was always part of it. (I was young and ….. back then)
You cannot just poke anything just into anything. It ends badly, and yes explosions happen easily when equipment fails. (never seen a dmm explode thou)
Cat ratings and safety guidelines should be taken seriously. Don’t go assuming that it is not so, you may hurt yourself or someone else.

[Quarlo Klobrigney]

The naysayers must understand this one statement & question:
Electricity travels at the speed of light. Do you think that your reflexes do? 
Then there is no hope for you. Move along, there is nothing here to see.
Perhaps we will read about you in the local news or a trade magazine.
Please look up Arc Flash, and understand that before you can pull away or even manage a yelp, it's over.
BTW so are you.

[Lesterwyatt]

The naysayers must understand this one statement & question:
Electricity travels at the speed of light. Do you think that your reflexes do? 
Then there is no hope for you. Move along, there is nothing here to see.
Perhaps we will read about you in the local news or a trade magazine.
Please look up Arc Flash, and understand that before you can pull away or even manage a yelp, it's over.
BTW so are you.

Not quite the speed of light. Electricity (Electrons)travels (1/100) the speed of light. Anyhow it still moves very fast; faster than our reflexes!

[electr_peter]

The naysayers must understand this one statement & question:
Electricity travels at the speed of light. Do you think that your reflexes do? 
Then there is no hope for you. Move along, there is nothing here to see.
Perhaps we will read about you in the local news or a trade magazine.
Please look up Arc Flash, and understand that before you can pull away or even manage a yelp, it's over.
BTW so are you.

Nice said and rhymed.

[DanielS]

So, with this thing built to Fluke levels of refinement you'd only pay $300? I'm sorry to say you are still very far off the mark.

You got it backwards. What I meant is that with the UT71E built the way it is, I would not be willing to pay more than maybe half its current street price.

Most of the safety issues come from basic layout and mechanical design flaws that cost almost $0 to fix... or simply remove the CAT-III/IV rating claims - nobody will complain that a CAT-II multimeter is built like a CAT-II multimeter.

[david77]

A risk people also run with electric hand mixers, curling irons, power drills, etc., etc.

They have fuses in the plugs, the multimeter has a fuse but it's in your hand, and in these models there isn't enough clearance and the fuse isn't a suitable type.

They only do in case of the BS1363 plugs. Are there any other fused mains plugs around?

[Fungus]

A risk people also run with electric hand mixers, curling irons, power drills, etc., etc.

People only plug those into the "CAT II" area of the house...

[Tepe]

People only plug those into the "CAT II" area of the house...

And I assumed that that was the area that Dave's "The point is about transients that can happen on the mains" alluded to.

[DanielS]

A risk people also run with electric hand mixers, curling irons, power drills, etc., etc.

People only plug those into the "CAT II" area of the house...

Just a by-the-way: outlets that are within 10 meters copper distance from the breaker box may also be considered CAT-III - potentially not enough wiring inductance and resistance to limit fault current to CAT-II rating.

[rsjsouza]

A risk people also run with electric hand mixers, curling irons, power drills, etc., etc.

Don't forget there are bad and good quality appliances as well! The bad quality ones will blow up in your face/hand due to poor manufacturing, transients, etc. 

Not quite the speed of light. Electricity (Electrons)travels (1/100) the speed of light. Anyhow it still moves very fast; faster than our reflexes!

Don't confuse the speed of moving electrons with speed of the electronic charge. Electrons move very slowly as they hop from atom to atom, but the energy released and absorbed in the process (the electronic charge) travels at a speed comparable to the speed of light in vacuum.

[Lightages]

I am so tired of hearing the argument: "I stick my tongue in on the wires on the power poles out side and nothing ever happens to me!" Well I have driven the same distance as that from the Earth to the moon, twice, in my lifetime, and I have not died in a car accident. WTF does that have to do with the millions of people who have died from car accidents? Safety rules and regulations have been put in place because of those who have had problems, not those who have never had problems. If you have never had a problem it could be because of all the other people who have been injured or killed before you.

Go ahead, take the risk. I DON'T CARE IF YOU DON'T. I care about the people who are uneducated and care to learn. If you don't want to learn, then stop complaining and go away.

[Hydrawerk]

Any opinion on CEM multimeters? Are they better than Unitrend? http://www.cem-instruments.com/en/pro/sort7-1.html

[Lightages]

They can be  better. They are the OEM for some meters sold by Extech, specifically the EX series. They are also the OEM for many other brands. They seem to designed fairly well but the quality control is all over the place. Dave did his $50 shootout and his $100 shootout with Extech EX series meters included. One was his best buy, the other was a piece of laughable garbage. Other people have complained about the poor build quality of the Extech EX series. Personally I would stay away from anything CEM or its re-brands.

[Yago]

I am so tired of hearing the argument: "I stick my tongue in on the wires on the power poles out side and nothing ever happens to me!" Well I have driven the same distance as that from the Earth to the moon, twice, in my lifetime, and I have not died in a car accident. WTF does that have to do with the millions of people who have died from car accidents? Safety rules and regulations have been put in place because of those who have had problems, not those who have never had problems. If you have never had a problem it could be because of all the other people who have been injured or killed before you.

Go ahead, take the risk. I DON'T CARE IF YOU DON'T. I care about the people who are uneducated and care to learn. If you don't want to learn, then stop complaining and go away.

Me too Light, me too.
Had a gut full of it on EEV youTube vid, people telling me I'm full of it for stating correct Cat safety must be adhered to.