Author Topic: EEVblog #712 - Uni-T UT71E Multimeter (Why Uni-T Meters Suck)  (Read 156205 times)

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Offline c4757p

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Re: EEVblog #712 - Uni-T UT71E Multimeter (Why Uni-T Meters Suck)
« Reply #225 on: February 08, 2015, 04:42:56 pm »
As much as I thought that most of the opinions he gave in this video were silly (exception being the safety standards bit), anyone bleating about it being illegal to give such opinions really ought to just shut up and let this viewpoint die. They're opinions for fuck's sake. "Dave's blanket statement that Uni-T has a bad reputation" is an opinion, unless there's some objective notion of what makes up a "bad reputation".
No longer active here - try the IRC channel if you just can't be without me :)
 

Offline Pentium100

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Re: EEVblog #712 - Uni-T UT71E Multimeter (Why Uni-T Meters Suck)
« Reply #226 on: February 08, 2015, 05:31:10 pm »
I once shorted out the mains outlet with a really cheap meter (with unfused 10A range). The meter survived, part of the probe pin vaporized though.

And I do not have access to a 10kV line or even a 100A line. Maybe shorting out 400V 100A line could make the meter explode...

Also, most of the devices connected to the mains (except multimeters) have glass fuses rated for 250V. If that fuse is good enough for a radio, PC or an amp, why a multimeter containing the same fuse should not be connected to the mains?

 

Offline rob77

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Re: EEVblog #712 - Uni-T UT71E Multimeter (Why Uni-T Meters Suck)
« Reply #227 on: February 08, 2015, 05:39:36 pm »
I once shorted out the mains outlet with a really cheap meter (with unfused 10A range). The meter survived, part of the probe pin vaporized though.

And I do not have access to a 10kV line or even a 100A line. Maybe shorting out 400V 100A line could make the meter explode...

Also, most of the devices connected to the mains (except multimeters) have glass fuses rated for 250V. If that fuse is good enough for a radio, PC or an amp, why a multimeter containing the same fuse should not be connected to the mains?

you have a circuitbreaker/fuse (usually 6/10/16Amp here in Europe) in the line which is providing the power to that radio.... but in the distribution panel you have lines with 100+ Amp fuses..... that's why there are CAt II, III, IV ratings.. CAT II is good enough to poke around in the house - and a glass fuse is kind of OK for CAT II.  CAT IV must have "giant" HRC fuses to be able to disconnect the circuit safely when the meter is used in the distribution panel.
 

Offline classical

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Re: EEVblog #712 - Uni-T UT71E Multimeter (Why Uni-T Meters Suck)
« Reply #228 on: February 08, 2015, 05:42:36 pm »
Also, most of the devices connected to the mains (except multimeters) have glass fuses rated for 250V. If that fuse is good enough for a radio, PC or an amp, why a multimeter containing the same fuse should not be connected to the mains?
Please read http://en.wikipedia.org/wiki/Measurement_category for CAT definition.
A mains outlet is different from the mains inlet into your house. within your house you have additional breaker which are lower rated and help a lot.
 

Offline Lightages

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Re: EEVblog #712 - Uni-T UT71E Multimeter (Why Uni-T Meters Suck)
« Reply #229 on: February 08, 2015, 05:43:23 pm »
It is a good argument that glass fuses in any equipment connected to the mains is a bad idea. In fact, in England, all equipment connected to mains is supposed to have a specifically designed ceramic fuse.

But the main difference is with a multimeter is that you are intentionally connecting a direct short across two test points. This is much different than providing a backup protection for a piece of equipment that normally runs without shorts.

If you have a multimeter that is rated CATI/250V, then glass fuses are acceptable. If you have a meter rated for CATIII then they are not acceptable.
 

Offline classical

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Re: EEVblog #712 - Uni-T UT71E Multimeter (Why Uni-T Meters Suck)
« Reply #230 on: February 08, 2015, 05:44:17 pm »
Also, most of the devices connected to the mains (except multimeters) have glass fuses rated for 250V. If that fuse is good enough for a radio, PC or an amp, why a multimeter containing the same fuse should not be connected to the mains?
Please read http://en.wikipedia.org/wiki/Measurement_category for CAT definition.
A mains outlet is different from the mains inlet into your house. within your house you have additional breaker which are lower rated and help a lot.
 

Offline Jay_Diddy_B

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Re: EEVblog #712 - Uni-T UT71E Multimeter (Why Uni-T Meters Suck)
« Reply #231 on: February 08, 2015, 05:53:16 pm »
I once shorted out the mains outlet with a really cheap meter (with unfused 10A range). The meter survived, part of the probe pin vaporized though.

And I do not have access to a 10kV line or even a 100A line. Maybe shorting out 400V 100A line could make the meter explode...

Also, most of the devices connected to the mains (except multimeters) have glass fuses rated for 250V. If that fuse is good enough for a radio, PC or an amp, why a multimeter containing the same fuse should not be connected to the mains?



The issue is fault currents. Essentially the closer you are the source of the electrical power the higher fault currents. Fuse and breakers are given a interrupting capacity. The danger is that if the fuse cannot interrupt the current a plasma will form which is conductive and by-pass the fuse.

CAT 1 is for circuits that are not directly connected to the mains.

CAT II is for sub-circuits. The wall outlets in your house would normally be included in this category.

CAT III would be the distribution panel in your house. An outlet close the panel may be CAT III

CAT IV is for industrial applications, power entry into building etc.

Most of the catastrophic accidents occur when a multi-meter with inadequate safety is used.

The danger with a DMM is might be set on the wrong range, for example the current range and then connected to a supply with very high fault currents. This can trigger a plasma, like arc welding, and is called an arc-flash event.

A small fuse can protect some appliances because they are connected to circuits with lower fault current ratings.

Jay_Diddy_B
 

Online Fungus

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Re: EEVblog #712 - Uni-T UT71E Multimeter (Why Uni-T Meters Suck)
« Reply #232 on: February 08, 2015, 06:05:46 pm »
I once shorted out the mains outlet with a really cheap meter (with unfused 10A range). The meter survived, part of the probe pin vaporized though.

The issue is fault currents. Essentially the closer you are the source of the electrical power the higher fault currents. Fuse and breakers are given a interrupting capacity. The danger is that if the fuse cannot interrupt the current a plasma will form which is conductive and by-pass the fuse.

This is why HRC fuses are filled with sand - to prevent that plasma from forming.

Glass fuses can also explode with enough force to bend the metal of the fuse holder out of shape and touch other parts of the meter internally (this is why flukes have blast walls all around the fuse area even though they use HRC fuses!)

Here's a cautionary tale of an exploding multimeter that killed two people and badly injured another. The suspected reason? It had glass fuses in it.

http://ecmweb.com/arc-flash/case-deadly-arc-flash

 

Offline Monkeh

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Re: EEVblog #712 - Uni-T UT71E Multimeter (Why Uni-T Meters Suck)
« Reply #233 on: February 08, 2015, 06:05:59 pm »
It is a good argument that glass fuses in any equipment connected to the mains is a bad idea. In fact, in England, all equipment connected to mains is supposed to have a specifically designed ceramic fuse.

Which is not internal to the equipment and has nothing to do with protection of the equipment. Nor is it present on any equipment connected via means other than a BS1363 plug or a fused connection unit designed to replace said plug. There is a substantial amount of this equipment in the home.
 

Offline rob77

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Re: EEVblog #712 - Uni-T UT71E Multimeter (Why Uni-T Meters Suck)
« Reply #234 on: February 08, 2015, 06:11:03 pm »
my 2cents for those who don't know....

HRC fuse has sand in it, and it's bigger.... that's the whole point - if the fault current (in kilo-Amps range) is blowing a fuse, the wire in the fuse is instantly vaporized - the sand will simply extinguish the arc by cooling the plasma (former wire) very quickly - combined with the longer distance for the arc it yields a very good disconnect capability.

small glass fuse - once the wire is vaporized - the air / gas in the fuse is not able to cool the plasma (former wire) quick enough and the plasma becomes a short - letting even more current to flow, heating up even further till the whole multimeter explodes, PCB traces are vaporised - providing even more plasma to conduct across the wires - this will go like avalanche - it will go like that till wires are melted and the distance between the wires will be large enough to extinguish the arc.

however with small fault currents the glass fuse is just ok - because it can't heat up so rapidly (less energy blows the fuse).
 

Offline Lightages

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Re: EEVblog #712 - Uni-T UT71E Multimeter (Why Uni-T Meters Suck)
« Reply #235 on: February 08, 2015, 06:12:23 pm »
It is a good argument that glass fuses in any equipment connected to the mains is a bad idea. In fact, in England, all equipment connected to mains is supposed to have a specifically designed ceramic fuse.

Which is not internal to the equipment and has nothing to do with protection of the equipment. Nor is it present on any equipment connected via means other than a BS1363 plug or a fused connection unit designed to replace said plug. There is a substantial amount of this equipment in the home.

I stand corrected. I thought that all equipment was required to have the BS1363 fuse.
 

Offline DanielS

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Re: EEVblog #712 - Uni-T UT71E Multimeter (Why Uni-T Meters Suck)
« Reply #236 on: February 08, 2015, 06:13:45 pm »
I once shorted out the mains outlet with a really cheap meter (with unfused 10A range). The meter survived, part of the probe pin vaporized though.

And I do not have access to a 10kV line or even a 100A line. Maybe shorting out 400V 100A line could make the meter explode...
If you want/need to poke around inside a breaker box, the fuse/breaker bus-bars can carry 100-200A depending on what the entrance breaker/fuses are, more if you account for surge ratings. If you probe the mains entrance terminals, then you have a completely unprotected connection directly to the local low-voltage distribution transformer which can provide a short-circuit current well over 1kA, enough to make your probe leads explode if nothing else blows first.
 

Offline Monkeh

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Re: EEVblog #712 - Uni-T UT71E Multimeter (Why Uni-T Meters Suck)
« Reply #237 on: February 08, 2015, 06:16:58 pm »
It is a good argument that glass fuses in any equipment connected to the mains is a bad idea. In fact, in England, all equipment connected to mains is supposed to have a specifically designed ceramic fuse.

Which is not internal to the equipment and has nothing to do with protection of the equipment. Nor is it present on any equipment connected via means other than a BS1363 plug or a fused connection unit designed to replace said plug. There is a substantial amount of this equipment in the home.

I stand corrected. I thought that all equipment was required to have the BS1363 fuse.

Absolutely not. Not only does it have nothing at all to do with the equipment (it protects the cable) the fuse (A BS1362 fuse..) is only required to protect said cables from the excessive continuous current available on a 30/32A ring final circuit. This means: Every switch, every light fitting, every smoke alarm, any equipment over 13A (and most over 10A), and anything not fitted for a BS1363 plug (substantial amounts of industrial and commercial equipment, most hardwired equipment) has no contact with a BS1362 fuse.
 

Offline Pentium100

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Re: EEVblog #712 - Uni-T UT71E Multimeter (Why Uni-T Meters Suck)
« Reply #238 on: February 08, 2015, 06:41:13 pm »
If you want/need to poke around inside a breaker box, the fuse/breaker bus-bars can carry 100-200A depending on what the entrance breaker/fuses are, more if you account for surge ratings. If you probe the mains entrance terminals, then you have a completely unprotected connection directly to the local low-voltage distribution transformer which can provide a short-circuit current well over 1kA, enough to make your probe leads explode if nothing else blows first.

In which case the multimeter is good enough. The power to my house comes from an outside panel with the meter and a 3x16A breaker. I do not have access to that panel, it belongs solely to the power company. The breaker panel inside my house is thus current limited by the breaker. So, unless I break the law and either open the panel with the meter or break into a substation, I probably would not be able to make the meter explode by accident.
 

Offline Yago

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Re: EEVblog #712 - Uni-T UT71E Multimeter (Why Uni-T Meters Suck)
« Reply #239 on: February 08, 2015, 06:54:25 pm »
Huh, I posted a question in here a few minutes ago... oops.

I was asking about the length of cable run from the high energy source providing an inductance that resists the initial current surge.
So the closer you are to the high energy source the higher the danger.
This also being part of CAT ratings and fuse requirements.
 

Offline yaputya

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Re: EEVblog #712 - Uni-T UT71E Multimeter (Why Uni-T Meters Suck)
« Reply #240 on: February 08, 2015, 07:04:37 pm »
The weird ass adapter looks like it has the classic European mains sockets (well, most of Europe)

It's the standard Euro plug alright - called a pig's nose for obvious reasons. The socket is recessed about 2cm with the two holes at the bottom and two springs on the sides for earth, they look pretty strange compared to the flat Aussie type. Also they are not polarised, so Active and Neutral can be on either pin. The socket probably supports the plug mechanically much better than Aussie ones though.
 

Offline rob77

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Re: EEVblog #712 - Uni-T UT71E Multimeter (Why Uni-T Meters Suck)
« Reply #241 on: February 08, 2015, 07:14:17 pm »
The weird ass adapter looks like it has the classic European mains sockets (well, most of Europe)

It's the standard Euro plug alright - called a pig's nose for obvious reasons. The socket is recessed about 2cm with the two holes at the bottom and two springs on the sides for earth, they look pretty strange compared to the flat Aussie type. Also they are not polarised, so Active and Neutral can be on either pin. The socket probably supports the plug mechanically much better than Aussie ones though.

actually it's the Schuko socket  - one of the standard sockets in Europe ;)  the other standard is the French type - but both of the sockets are accepting the Euro-plug.
 

Offline plexus

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Re: EEVblog #712 - Uni-T UT71E Multimeter (Why Uni-T Meters Suck)
« Reply #242 on: February 08, 2015, 07:16:51 pm »
I appreciated the review. The board looks ok but yes it was a weird to see all the trimmers and lack of precision parts. seems like they are dialing in the precision. the challenge with that is linearity. but the lack of claimed rating infrastructure nullifies consideration for the product. add in its $190 (is that AUS?) and well you can do well better or even just spend the money on a quality meter with less features.

Thanks to dave for pointing out the features vs quality on this meter. too many companies are padding up the feature set and comprimising quality. the quality aspect is harder to tell unless you open it up and test it.

I don't feel dave was out of line on his comments about this meter. it should irritate you when companies try and pass off a POS like this by padding it with crappy features and charge so much for it. its a shame we have to dig into these things to validate the claims so thanks to dave for doing that for us.

FAIL
 

Offline DanielS

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Re: EEVblog #712 - Uni-T UT71E Multimeter (Why Uni-T Meters Suck)
« Reply #243 on: February 08, 2015, 07:52:11 pm »
In which case the multimeter is good enough. The power to my house comes from an outside panel with the meter and a 3x16A breaker. I do not have access to that panel, it belongs solely to the power company. The breaker panel inside my house is thus current limited by the breaker. So, unless I break the law and either open the panel with the meter or break into a substation, I probably would not be able to make the meter explode by accident.
In my case, the building has 100A cartridge fuses for my apartment and while investigating the relatively low outlet voltage, I went in the utility room to measure line voltage directly at the service disconnect / entrance fuse box to rule out building wiring. I did the same thing at my previous apartment but that was to file a complaint with the power company for weak/floating neutral which killed both of my CRT monitors. AFAIK, those fuses are intended to be user-serviceable, so no law is broken by me accessing those terminals.

A few years ago, one of my friends called me in the middle of the night because his mother called him about power freaking out in her apartment with lights flickering wildly on half the circuits whenever the fridge started. It turned out there was a 20-50V drop across one of the 100A entrance fuses. After removing the fuse, it read open (>100Meg) on the ohm range - I definitely was not expecting that from a fuse that can still pass enough current to (sort of) run a fridge and some lights.

Those are the sorts of occasions where someone might end up probing a effectively unprotected circuits without being in an industrial environment.
 

Offline Pentium100

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Re: EEVblog #712 - Uni-T UT71E Multimeter (Why Uni-T Meters Suck)
« Reply #244 on: February 08, 2015, 08:05:10 pm »
AFAIK, those fuses are intended to be user-serviceable, so no law is broken by me accessing those terminals.
Different places, different rules. The panel with the meter and a breaker near my house has a seal on it. Also, breakers are used everywhere instead of fuses (fuses can be found in old installations only). This was done probably because nobody replaced blown fuses - just put a piece of wire on the fuse after it blew.
 

Online Fungus

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Re: EEVblog #712 - Uni-T UT71E Multimeter (Why Uni-T Meters Suck)
« Reply #245 on: February 08, 2015, 09:05:49 pm »
Also, breakers are used everywhere instead of fuses (fuses can be found in old installations only). This was done probably because nobody replaced blown fuses - just put a piece of wire on the fuse after it blew.

I thought the traditional 'fix' was a .22 bullet ... they're about the same size as a standard fuse.
 

Offline Pentium100

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Re: EEVblog #712 - Uni-T UT71E Multimeter (Why Uni-T Meters Suck)
« Reply #246 on: February 08, 2015, 09:19:39 pm »
I thought the traditional 'fix' was a .22 bullet ... they're about the same size as a standard fuse.
Except that a bullet is way more difficult to find than the fuse, not to mention a piece of wire. It was easier to find bullets during and immediately after WW2, but WW2 happened a log time ago.
 

Offline G7PSK

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Re: EEVblog #712 - Uni-T UT71E Multimeter (Why Uni-T Meters Suck)
« Reply #247 on: February 08, 2015, 09:27:08 pm »
There is a good case for banning DVM's in high energy environments and having meters that only read volts and separate meters for amps of the clamp variety, looking through the Fluke range I notice that they do have meters without the amp range.
 

Offline jancumps

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Re: EEVblog #712 - Uni-T UT71E Multimeter (Why Uni-T Meters Suck)
« Reply #248 on: February 08, 2015, 10:11:30 pm »
AFAIK, those fuses are intended to be user-serviceable, so no law is broken by me accessing those terminals.
Different places, different rules. The panel with the meter and a breaker near my house has a seal on it. Also, breakers are used everywhere instead of fuses (fuses can be found in old installations only). This was done probably because nobody replaced blown fuses - just put a piece of wire on the fuse after it blew.
We're not discussing your apartment here. We're talking about a meter that claims it is safe to be used at points where you personally  are not allowed to go in your apartment.
 

Offline Quarlo Klobrigney

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Re: EEVblog #712 - Uni-T UT71E Multimeter (Why Uni-T Meters Suck)
« Reply #249 on: February 08, 2015, 10:33:08 pm »
Slightly off topic,
There is a great invention to thwart dodgy Chinese electronics plugged in to the mains socket, be they glass or ceramic fused, from burning down your house, and it's the:
AFCI http://en.wikipedia.org/wiki/Arc-fault_circuit_interrupter
With these, they detect an arc in its infancy and trip the breaker. I don't know if they are used in the EU & Australia, but they sure do work in here in the US.
Many states specify the use of them in "living areas" as being a mandatory installation. I just got done updating all the wiring from 1939 and up, in my "new" home, and put them on all branch circuits regardless of the mandatory locations.
All wiring was gutted out for scrap and MC armoured cable installed. The MC cable http://www.afcweb.com/product-category/mc-metal-clad-cables/ keeps the critters from chewing on the Romex and see line one above. A little pricy on the refit, but safety first.
The original wiring had seen too many do-it-yourselfers without the simplest of knowledge, including a single circuit that was fed from 2 different breakers in a "loop" configuration. :clap:
My point is if it can be done in a shoddy manner, it will be. China as of late the greatest offender. At least there's a remedy for it in the AFCI. :blah:
« Last Edit: February 08, 2015, 10:50:09 pm by Quarlo Klobrigney »
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