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

[Paul Moir]

The point is that education saves lives and stopping companies from cheating on their ratings and specifications can save lives.

I agree completely.  I just wanted to point out that it was education that was the principal cause of that root of the incident and not a false rating.  There was no rating in that case. 

Furthermore I would argue that education about the IEC CAT ratings existence is more important than pointing out bad actors who falsify (perhaps not necessarily legally, but definitely morally) their ratings.  I could accept the contrary argument that pointing out the bad actors leads to education. 

[Ericho]

Quote from: Ericho on February 13, 2015, 04:58:03 AM

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.




nice to hear raw and real experiences, would you care to elaborate how your accident happen? hope its not too personal ... could be good for everyone to know 

Sure, no problem. Not proud of it nor ashamed, it was a long time ago when a differend attitude towards safety was all to common.

The 24V incident:

A electricaly started 400kw emergency genset needed testing. De battery bank was empty. We made a pair of long jumper cables out of 70mm² welding cables. I connected to the full 24v bank while my second engineer was 20m away holding the non insulated connections on the other end. He tripped and connectors shorted on the aluminium floor plating. At this moment I was already 6m away from the battery bank. they exploded And the plastic shards hitting me cut me all over,, the acid in the wounds and my eyes did the rest. This was hospitalization due to lack of respect, reclesness and assuming 24v is not dangerous.

380v (or 440V) incident:

4 a clock in the morning a 350A fuse blow in a switch / fuse cabinet with a few 800kW gensets connected to it.
I opened the cabinet, wacked a new fuse in and without thinking, meashuring or closing the door I trew a genset back online, after all I was in a hurry to go back to sleep.
A half shorted amp coil behind the fuse produced a large flame and I got 1 degree burned on my arm, lost some hair to.

This is how I learned that the warning labels / CAT ratings / certificates / safety systems etc etc. are not bullshit. (more then 25 years ago).

I do understand how doing low power electronics can trick you into thinking your home 220Vac net belongs to the same category. It doesn't. It is to be considered high power and treated as such.

I own a VC940, Its ok for ow power stuff. I use basic flukes for high power stuff. I stay away with DMM's from live high voltage (> 440Vac), there are, most of the time other means to do repairs on those systems.

Brgds,
Eric

[Fungus]

If you're regularly sticking bits of metal into household electrical sockets to see if they're live or not then you're doing it wrong.

why doing it wrong ? can you explain ?

Would you let a 4-year-old child do it?

i see no real use for those fancy contact-less phase testers (o.k. those are good for someone who is overly afraid of electricity )

"According to the Electronic Library of Construction Occupational Safety and Health (eLCOSH), exposure to electricity is still a major cause of death among construction workers. Among electricians, the most serious concern is working with or near live wires without enlisting the proper safety procedures. Electrocutions kill an average of 143 construction workers each year. Data from 1992 through 2003 indicates electrical workers suffered the highest number of electrocutions per year (586 or 34 percent of the total deaths caused by electrocution)"

http://www.ecmag.com/section/safety/alarming-statistics

[gildasd]

To Enricho,

You'll be glad to hear that the STCW (Standards of Training, Certification and Watch-keeping http://www.stcw.org/ for you land lubers ARRR) is imposing a high power electricity course on top of all the electrical stuff we already have to do for the Unlimited ticket.

My school (HZS Antwerp) has finished the lab and will be starting this year.

All engineers on ships above 500 tons DWT will have to pass this course over time.

Seeing the increasing prevalence of diesel electrics and the coming usage of batteries while docked (when possible, of course), this makes sense.

[Fungus]

We made a pair of long jumper cables out of 70mm² welding cables. I connected to the full 24v bank while my second engineer was 20m away holding the non insulated connections on the other end. He tripped and connectors shorted on the aluminium floor plating.

Amazing how even a tiny change in protocol can make the difference between life and death.

With hindsight it's obvious that he should have connected his end of the cables first...

...although any electrical fault in the system means that you would have been two feet from the exploding battery when you connected your end!

Maybe you should connect one cable at both ends then connect the other cable to the battery while the other guy holds his end of that cable safely/securely. That way you can be as far away as possible from the boiling sulfuric acid when you make the final connection.

[ShaunO]

Dave (and forum members),

Enjoyed the video, as well as many others. A friend recently (last 6 months) purchased a UNI-T UT61E for roughly $130 AUD. I did a mini tear-down of it to see if looked as questionable as the one you featured. I've attached some images should the be of some use to the community. Needless to say, I won't be trusting this multimeter.

Keep up the great work.

Regards,

ShaunO

[Wytnucls]

Well I am sure at 2500 V a fluke will survive this . In most Fluke I know there are ptc s and movs and when the transient will occur the movs will decrease the resistance and so current will flow and the ptc heats up and increase the resistance . The energy is divided by the movs and the ptc and the chance that the fluke will survive is much higher than a uni t that will definitively be scrapped at 2500 V , also at 1000 V . Look at youtube . There is a video where a brymen 869 is tested with 2000 V and it survived without any problems . The 869 has 2 ptc s and 3 movs and not whimpy ones

The PTCs play no part in protecting against high voltage transients. The spurious transients don't last long enough for the current to heat up the PTC to its Curie point. You should revisit Dave's video on input protection.
The IEC 8,000V series of test pulses are 100uS long, with a steep rise time of 1.2uS and a decay to 50% of voltage after 50uS. (I seem to remember 6 pulses in a series, but I could be wrong) (Correction: Fluke notes say 20 repetitions)
PTCs can take a few seconds to heat up to 60c~80c, with a current limit of a few milliamps.
Incidentally, PTCs should never be touched with bare fingers, to prevent leaving a leakage current path behind.

http://www.vishay.com/docs/29072/29072.pdf

[FrontSideBus]

I love the way they use BS1363 plug fuses in these

[pinchies]

Thanks for the review Dave... but its a bit late for me!

Paid $91 for this meter (UT71E) unused and new in box on flea-bay late last year. Great meter, has both the precision, and fast update rate modes, the only thing that is really annoying is the slow continuity meter. But thats what I have another cheap ($5) meter for! Can't beat it for value, but I agree with Dave that its overselling itself (CAT 4???) when it really didn't need to. And now I can put the money saved towards a MSO!

[Wytnucls]

From a safety point of view, there is nothing special about CAT IV.
The IEC test for CAT IV 600V is the same as CAT III 1000V. If a meter passes the 8000V 2 Ohm pulse test, it gets both ratings.
Now, as a hobbyist, you should seriously ponder if it is wise to use any meter in a CAT IV or CAT III environment. That's certified electrician territory, who should know what kind of meter to use and what protective gear to wear.

[TheEPROM9]

Dave (and forum members),

Enjoyed the video, as well as many others. A friend recently (last 6 months) purchased a UNI-T UT61E for roughly $130 AUD. I did a mini tear-down of it to see if looked as questionable as the one you featured. I've attached some images should the be of some use to the community. Needless to say, I won't be trusting this multimeter.

Keep up the great work.

Regards,

ShaunO

Love how the fuses are the same as you find in standard UK plugs. These are dirt cheap HRC fuses.

It is still very true that you get what you pay for.

[Wytnucls]

It is rather unrealistic to expect 20$ 1000V HRC fuses in a 50$ meter. However, there is a GS approved version for Europe (61D), equipped with the right fuses, but it costs quite a bit more.

[G7PSK]

A BS1363 fuse is designed to work inside a cat111 area so surely it is OK in a cat111 meter, certainly ok for a meter that is on a hobbyist bench, my scope is marked cat11 1400 volts so if that is good enough for bench work those meters should be as well. I am not trying to apologise for any meter manufacturer but a bit of realism has to be shown in these matters, and to the best of my knowledge no one has been hurt by let alone killed by a BS1363 fuse exploding, after all it is designed to work on 250 volt circuits and stop high current excursions. But then unlike the US the UK domestic circuit is not directly tied to the 11Kvolt line the transformers isolate the low from the high tension unlike the US pole pigs.

[Wytnucls]

On the practical side, you may be right , but legally, the meter can't be sold in Europe anymore.

[Fungus]

Love how the fuses are the same as you find in standard UK plugs. These are dirt cheap HRC fuses.

Is there anything wrong with them?

Presumably they can prevent electrical arcing. What's the advantage over one of those big fat $15 fuses?

[Wytnucls]

BS1363 fuses are only rated for 240V. In Europe, the fuses must now have the same voltage rating as the highest CAT rating of the meter.

[Fungus]

BS1363 fuses are only rated for 250V.

OK, you need physically longer fuses for higher voltages...but do they really need to cost $15 each?

[gildasd]

BS1363 fuses are only rated for 250V.

OK, you need physically longer fuses for higher voltages...but do they really need to cost $15 each?

The fuel tank on the Yacht is not going to be filled up by low priced fuses.

[Wytnucls]

Small volumes, manufacturing tolerances and required certification may explain the cost.

[SeanB]

The difference between the standard plug fuse and the 1kV rated one is the materials used. In both cases they use pressed tinned brass for the end caps ( or pressed tinned steel in some cases, it is immaterial in use) and a ceramic body, with a fine silica sand filling mix and a fuse element made from a wire alloy.

The difference between the plug fuse and the 1kV one is that the plug fuse uses a wire that will break at around 1.2 to 10 times the rated current ( depends on time of the pulse, temperature and such) and the arc that forms will be eventually extinguished in the sand fill in under typically 10 cycles, or around 300ms, and the deposited metal vapour and carbon inside the fuse will not explode it from overpressure and will not conduct electricity at 500VDC applied to the blown fuse.

The 1kV rated fuse uses a similar wire, with similar breaking characteristics, but it is filled with both a finer silica blend and with additive oxides such that it will stop the arc sooner, and that the fill will isolate the particles and the soot so that even at 2kV DC applied it will under no circumstances pass any current. the case is also stronger, as the higher energy available generates a much higher pressure inside the housing, and it must withstand it without exploding or emitting any vapour, and must absorb all the heat energy of the fuse operation.

Place a glass fuse directly across the mains ( in a thick housing) and you might find the end caps if you are lucky still in the fuse holder. A BS1363 fuse will blow, but should not disintegrate. The HRD fuse will blow as well without damage externally. Do the same with a 1kV line and the BS1363 fuse will explode like the glass fuse, but the 1kV one will just blow. At this voltage the glass fuse pieces will simply arc over until the holder melts.

[Wytnucls]

The M205 glass fuse doesn't necessarily explode, unless its rating has been exceeded (100A for a 10A fuse and 35A for a 1A fuse).
The Fluke fuse enclosures are made of glass fiber:
http://mrmodemhead.com/blog/inside-a-multimeter-fuse/

[SeanB]

True, but the most common cause for a multimeter fuse blowing is the meter being inadvertently placed across a power source with the leads set in the current range. Thus the current limiting is basically the lead resistance and the supply impedance. With mains that essentially devolves to lead resistance in most cases. More than enough to detonate a glass fuse, I have seen them explode when a power supply bridge rectifier shorts and the primary side mains fuse shatters. Secondary fuse just melted, as it was a 25A fuse ( basically a solder slug in glass).

[Wytnucls]

The house wall wiring impedance comes into play as well, reducing some of the current. I had trouble rupturing a couple of medium blow fuses, as the circuit breaker kept on tripping on the panel first. Eventually, I found a mains outlet where they blew with a whimper. Of course, if you're very close to the source, anything can happen. In theory, a lead impedance below 2.5 Ohm would be necessary to explode a multimeter 10A glass fuse on 250V mains and about 1 Ohm on a 110V supply.

[G7PSK]

So if these meters with the BS 1363 fuses were rated for 250 volts RMS maximum they would be OK or not?
 

[Lightages]

If a meter with the BS1363 fuses was rated at CATIII/250V, it would be legal.