Author Topic: Multimeters that do not appear to meet their safety specs. (updated frequently)  (Read 143131 times)

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Offline Wytnucls

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Ok, so what to do now, if I need measure sbout 380-400V?
If you do it properly, with the probes in COM and Volt/Ohm jacks, there is no problem measuring high voltages, if your meter is rated CAT III 1000V.
 

alm

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If your meter is actually suitable for CAT III 1000V. The lack of suitable fuses does not convince me the designers did their job and actually tested it under the prescribed conditions. Any monkey can put that CAT III stamp on there, that's what this thread is all about.
 

Offline Lightages

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I agree with ALM. When you start getting up to 380V plus you are getting into another whole world of hurt if something goes wrong. I would STRONGLY suggest that you find a meter with the proper design for these voltages/energies.

My personal recommendations:

Anything Fluke with a CATIII or CATIV 600V plus rating
Anything Agilent with a CATIII or CATIV 600V plus rating
Amprobe AM140 or 160
Anything Extech with a CATIII or CATIV 600V plus rating scratch those with the recent recall
UEi DM 391/393/397
Brymen DM85X or DM86X

The above are not guaranteed by me nor am I saying you will be safe no matter how you use the meters I have recommended.

Where safety glasses no matter what meter you are using with high energy circuits. Wear heavy leather (dry and clean) gloves too. Better yet, take a course on electrical safety procedures if you think a UNI-T UT61E is a safe meter for 380V plus....

I have seen too many "electricians" with burn scars on their hands and faces.
« Last Edit: February 12, 2013, 04:45:20 am by Lightages »
 

Offline Wytnucls

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Technically speaking, if you cannot trust your meter CAT ratings, you shouldn't use it for 240V measurements either. High voltage transients are possible at that voltage level too. Stick to battery voltages on electronic circuits.
Buying a meter that has been tested by an independent body is never a bad idea, if you can afford it. If you are going to play with high voltages on a regular basis, you can afford it, even if it means not eating for a week.
Remember that accidents have also happened with the best equipment available. So use your common sense and be sure to understand all the limitations of your instrument.
 

Offline T4P

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Uni-Trend UT61E (all A,B,C,D,E) CATIII/1000V  CATIV/600V   ceramic fuses rated at 250V used on current ranges with 250V voltage warning on meter. (submitted by lightages)
Well... my Uni-T 61E from China, has 1A and 10A ceramic fuses, type BS1362, without any voltage. So I think it's trully 1000V fuses, unless someone proves the opposite.
Are you kidding me?
@Lightages
OK. To use a UT61E for 380V MAINS!!! (V range ONLY. Please.) In normal situations ... mess it up and your hand goes.
1000V may be what they mean by low energy ONLY. Definitely not the 8000V rating CAT III is
Fact is, the fuse is wrong but the sockets can be swapped out
 

alm

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Technically speaking, if you cannot trust your meter CAT ratings, you shouldn't use it for 240V measurements either. High voltage transients are possible at that voltage level too. Stick to battery voltages on electronic circuits.

Agreed. Although most 400 V circuits will be at least CAT III, while many 240 V circuits are CAT II, so transients can be larger on the 400 V circuit.

Remember that accidents have also happened with the best equipment available. So use your common sense and be sure to understand all the limitations of your instrument.

I concur. Even a very reliable tool like a CAT IV 600 V rated Fluke DMM should not give you the sense of absolute safety. Accidents can always happen, especially if you're not careful or focused on the task.
 

Offline Lightages

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Yes that was my thinking as well ALM. It is one thing to play inside the house with wall sockets and with lower energy fused circuits. It is another thing to start playing with power house and heavy motor drive type power that 380V implies.
 

Offline Wytnucls

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Actually, in most of Europe, tri-phase voltages in the order of 380V are quite common in the house. Ovens, heaters and dryers can be connected to a higher voltage than the rest of the household.
« Last Edit: November 02, 2012, 10:25:19 am by Wytnucls »
 

Offline Lightages

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Ah yes I forgot about the big appliances.  I would avoid using many meters on those circuits.  I think I might still use my UT61E for that, but I would prefer anything else in the list I mentioned earlier.
 

Offline Salas

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Everything we should know about fuses.
 

Offline Wytnucls

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Let me try to dispel some misconceptions about high voltage measurement.
Let's forget about high voltage transients for a moment, as these are unlikely in the home environment, if you can discount lightning strikes and such on the supply side. (These would be just as lethal whether you're measuring 115V or 380V, by the way)
Let's also assume that the meter is used properly, with the probes in the Volts/Ohms and COM jacks and the range selector on Volts.
Let me also remind you that all protection devices (PTCs, MOVs and GDTs) only come in action for voltages higher than 1000V.
Because of the high impedance (usually 10M Ohms on the Volt range and 2.5G Ohms on the mV range), the amps flowing through the meter have a very low value (about 100uA) and the high 1000V voltage is reduced straight away to 10 mV (assuming an impedance of 100 Ohms for the upstream circuit), as the current flows immediately through a network of beefy step-down resistors before hitting the measuring circuit. So the traces can be very thin on the Volts range and they usually are.
So, in that situation, all CAT-rated multimeters are essentially the same and the main safety concern should be for the operator not to come in contact with the high voltage being measured.
That brings us to the probes, their insulation and meter connections. Make sure that those have the correct CAT ratings and inspect your probe and meter connections. If you don't trust your probes, do what I did and buy a nice set of Fluke TL175 probes ($20.00), for peace of mind.  ;)
« Last Edit: November 02, 2012, 10:42:03 pm by Wytnucls »
 

Offline Wytnucls

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Now, in the case of the UT-71, as Lightages pointed out, there is a very small clearance between the Volts trace and the 9V battery wires on the PCB (0.6mm on mine). This is what could happen if a short ever develops:



I modified mine, with a proper PCB connection, extra insulation and a heat guard and got rid of the routing through the PCB.
If you decide to modify your Uni-T, make the heat guard a lot shorter, otherwise it will interfere with the battery compartment seating.
Here is also the original set-up for reference:


« Last Edit: November 03, 2012, 01:09:42 am by Wytnucls »
 

Offline Lightages

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So, in that situation, all CAT-rated multimeters are essentially the same and the main safety concern should be for the operator not to come in contact with the high voltage being measured.


If your meter is actually suitable for CAT III 1000V. The lack of suitable fuses does not convince me the designers did their job and actually tested it under the prescribed conditions. Any monkey can put that CAT III stamp on there, that's what this thread is all about.

As ALM said. Although I agree that the chances of harm are pretty slim, there is always the concern that if the designer/manufacturer of a meter cut corners in one place then they might have in many others too. In all reality the UT61E looks well designed and I guess on second thought I probably was over stating the danger with that meter. It probably is fine for almost anything anyone would use it for in a home. Its biggest problem would arise from someone forgetting to put the leads in the wrong right terminals and blowing up a fuse inside.
« Last Edit: November 03, 2012, 07:01:29 am by Lightages »
 

Offline Wytnucls

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I think you meant to write "the right terminals"  :D
Talking about right terminals, the 71 actually beeps madly at you if you have the probes in the amps jacks with the range switch out of the A or mA/uA positions. Has the 61 got that safety feature?

Here is the 71B with modified battery connections and a SIBA HRC 10A 1000V 30kA fuse installed:
« Last Edit: November 03, 2012, 07:13:08 am by Wytnucls »
 

Offline Soertier

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Talking about right terminals, the 71 actually beeps madly at you if you have the probes in the amps jacks with the range switch out of the A or mA/uA positions. Has the 61 got that safety feature?


It does not have that feature.
 

Offline Wytnucls

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Here is a picture of what the UT-71 should have been and may become, if the new regulations are respected:
1000V HRC fuses and safe battery connections.
The 500mA fuse is in a tight space. It's not touching anything right now, but I'm not too happy with the clearances. I will eventually install it into a small enclosure to prevent spark-overs.
The 500mA fuse is a very fast SIBA HRC 1000V 50kA (part number:7017240)
Ideally a 1000V gas discharged tube should be fitted behind the PTC, as the PTC is very slow to react to high transients.
A MOV would do the job too, but they tend to catch alight under stress.
« Last Edit: November 03, 2012, 10:04:57 am by Wytnucls »
 

Uncle Vernon

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Even a very reliable tool like a CAT IV 600 V rated Fluke DMM should not give you the sense of absolute safety. Accidents can always happen, especially if you're not careful or focused on the task.

Could not agree more, any instrument is only as good as the way it is used and cared for. Often I've seen Flukes and other top shelf instruments belted about in a canvas tool bag and covered in oil and solvents. No device could be expected to retain absolute integrity in these situations.

Similarly in high demand situations it is common to see any manner of cobbled together, broken or cheap and cheerful replacement test leads. The nonsense being that OH&S bureaucracy having many of these meters routinely being sent for expensive test and calibration sans leads. (note also that in every case when I've enquired there was minimal testing and no calibration involved in these OH&S tests. Merely a few automated tests against a reference source, an external only visual test and a very expensive sticker attached)

In another thread here we have a bunch of apprentice brain surgeons extolling the suitability of the $5 meter. There is no way any of the lesser instruments stand up to the rigours of industry and and tribute to the Flukes etc above that they are remain operational. Equally there is no way of ensuring these cheap bit of rubbish are not at some times used on high energy circuits.

This list is pointing out a lot of product that are overstating their safety and abilities, it is no guarantee that other correctly rated instruments are capable of their full ratings unless carefully maintained and handled. What we should do is also compile a list of instruments that do (when in good condition) deliver to their claimed specification particularly the lower priced examples.
 

Offline Lightages

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What we should do is also compile a list of instruments that do (when in good condition) deliver to their claimed specification particularly the lower priced examples.

I am not sure that would be such a good idea. If we provide a list of "approved" meters, the $5 advocates might think they can do anything with them and have no danger. Someone might also find themselves the subject of a lawsuit when someone harms themselves using an "approved" meter that then fails. I would rather err on the side of pessimism when it comes to this. I have, however, provided a recommended list as I see it.
 

Uncle Vernon

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What we should do is also compile a list of instruments that do (when in good condition) deliver to their claimed specification particularly the lower priced examples.

I am not sure that would be such a good idea. If we provide a list of "approved" meters, the $5 advocates might think they can do anything with them and have no danger. Someone might also find themselves the subject of a lawsuit when someone harms themselves using an "approved" meter that then fails. I would rather err on the side of pessimism when it comes to this. I have, however, provided a recommended list as I see it.

I can see where you are coming from, and it's a pity the fanboys and those prone to jump towards litigation make it so difficult to offer and share helpful advice.  Some up to date pass/fail/recommended lists in the wiki would be a valuable resource particularly for newcomers looking for advice, but I fear it would end as a fanboy mire where individuals seek to justify their death-trap of choice.
 

Offline Wytnucls

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Extech is recalling selected EX612, EX613, EX622, EX623 digital clamp meters, as well as EX540, EX542 and EX570 multimeters, due to erroneous voltage readings with depleted battery.
http://www.extech.com/instruments/recall2010.asp



« Last Edit: November 04, 2012, 05:37:21 pm by Wytnucls »
 

Offline Lightages

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This is exactly the kind of thing that discourages me from trying to make a list of multimeters that DO seem to meet their safety ratings.
 

Offline LaurenceW

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Wow.

So is the simple advice: "If you love life, buy a Fluke/Agilent, and look after it"?  ;D
If you don't measure, you don't get.
 

Offline Wytnucls

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Wow.

So is the simple advice: "If you love life, buy a Fluke/Agilent, and look after it"?  ;D
No, most of the accidents have happened with the best equipment available, often used incorrectly, in an industrial environment.
The simple advice is "Know the limitations of your multimeter and be aware of the potential dangers before you connect your probes to anything"
 

Offline nctnico

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I couldn't agree more. In (semi) industrial environments the circumstances are totally different. High voltage outlets, dangerous chemicals, etc. Over here it is mandatory to have a safety license before you can work in such environments. And still you may not get into every factory. Many years ago I had to work in a chemical factory. The least toxic stuff they had in there was Chlorine. Before anyone was allowed on the premises they had to take a safety course (covering handling toxic chemical spills, entering rooms, locking out parts of the plant, etc) and an exam. After taking the exam succesfully (score >90%) you'd get a safety passport which was valid for one year.
« Last Edit: December 27, 2012, 03:41:31 am by nctnico »
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline grenert

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If you want to check if a multimeter's (or any other electric product's) UL certification is for real, you can search at UL:
http://database.ul.com/cgi-bin/XYV/template/LISEXT/1FRAME/index.html
 


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