Is it worthwhile to put an inline fuse on the test lead of a cheap DMM (UT120C)?

[3141592]

I'm thinking of buying the UT120C pocket DMM for my father as it seems ideal for a 'kept in the toolbox, just in case' meter. I am however a bit wary about it's input protection and I don't want it to blow up in his hands, so I think it could be made a lot safer, if I were to slap a 6.3x32 inline fuse holder on one if it's leads with a somewhat proper fuse in it.

Would this mod be worth the effort and plus 5 bucks, or am I being dumb about something? What's the reason no one recommends this whenever cheap unsafe DMMs come up?

[ElektronikLabor]

Fuse would affect the Resistance measurement.
But it looks like the meter already has a fuse:
https://www.eevblog.com/forum/testgear/uni-t-ut120c-review-and-tear-down/

So I don't think it make sence to build in a second inline fuse. 

[3141592]

Fuse would affect the Resistance measurement.
But it looks like the meter already has a fuse:
https://www.eevblog.com/forum/testgear/uni-t-ut120c-review-and-tear-down/

So I don't think it make sence to build in a second inline fuse. 

The built-in fuse has a voltage rating of just 250 (the meter says CAT II 600V on it) and the only data i can find says it's Imax to be 10A. So while it does have a fuse, I don't really trust it very much. (http://www.wondhope.com/pptc/WH250.html)

Well a 0.4A fuse e.g.: http://www.tme.eu/en/Document/098eab7e9b963129acdfce5cf7cd57cb/SIBA-GSS-7012540.pdf would be 1.6 Ohm at the rated current if I gather correctly. If I would choose a 2A (wouldn't protect the meter, but the user) one, that would be just 0.33 Ohm. Not ideal sure, but comparable to bad leads, and could be zeroed out. Good point though, thanks!

[Lightages]

Unless you are going to use a full size HRC fuse you will gain no safety. All you will do is move the possible explosion due to misuse from the meter to the test leads in the hands. It also makes the meter bigger and less pocket sized and also ads the problem with adding resistance to the test leads.

[3141592]

Unless you are going to use a full size HRC fuse you will gain no safety. All you will do is move the possible explosion due to misuse from the meter to the test leads in the hands. It also makes the meter bigger and less pocket sized and also ads the problem with adding resistance to the test leads.

I don't quite understand why you say that. I don't get ultimate safety, nor safety that a proper DMM with some big arse HRC fuses provide, but the SIBA (filled ceramic) fuses linked above do state a 50kA / 700V rupture capacity which is comparable to proper HRC ones.
The added <1 Ohm resistance is not that big of a deal in my opinion. It does make it less portable, but I think that also comes second to safety. The explosion (why would such a fuse explode?) is not in the hands of the user, as the inline holder could be placed near the case, not at the pointy end.

[cs.dk]

I don't get it;
In the V range, most multimeters have 10 MOhm input impedance. Even if 1000V is injected, the current draw is 0,1 mA. That little draw won't ever blow a fuse, or am i missing something?

[nidlaX]

It does make it less portable, but I think that also comes second to safety. The explosion (why would such a fuse explode?) is not in the hands of the user, as the inline holder could be placed near the case, not the at the pointy end.

And you're going to make sure your father always keeps that fuse holder away from vulnerable parts of his body? Is the fuse going to be fully encased inside the holder? Part of a multimeter's safety comes with the design of the enclosure which should prevent hazardous debris from escaping the meter in case of a blow out. If you watched Dave's teardown of the Agilent / TDK Lambda PSU, you might think twice about how a ceramic fuse could potentially fail when subjected to high energy.

Which leads back to the question: why not get your dad a UL / Intertek listed meter with the proper safety ratings for his potential applications?

[3141592]

I don't get it;
In the V range, most multimeters have 10 MOhm input impedance. Even if 1000V is injected, the current draw is 0,1 mA. That little draw won't ever blow a fuse, or am i missing something?

In the current range all you have is a low value shunt resistor. This meter doesn't have separate inputs for current and voltage measurement, so with a flick of the switch (or just forget to adjust it) you can basically short out a power source.

[Lightages]

Some you come here, ask for advice on problems, and you got them. If you don't understand the problems then what can we do? There are reasons for the types of uses used in properly designed multimeters. Please take the advice and don't give an untrained person a modified multimeter and give them the impression it is safe.

[nidlaX]

Please take the advice and don't give an untrained person a modified multimeter and give them the impression it is safe.

That's a key sticking point for me. Even if YOU are able to properly modify the meter to enhance its safety in a specific way, I think you can only trust YOURSELF to be aware of and understand your modifications to use the multimeter in a safe fashion.

[3141592]

Some you come here, ask for advice on problems, and you got them. If you don't understand the problems then what can we do? There are reasons for the types of uses used in properly designed multimeters. Please take the advice and don't give an untrained person a modified multimeter and give them the impression it is safe.

I went through and replied to every point of your reply (for which I am thankful). It never crossed my mind that a slapped-on fuse would make this crap CATIII or II even. That I would recommend to him that it's safe to use for high power applications is nonsense I can assure you. If you think I don't understand a problem, then please explain it, I came here with a question to learn about.

And you're going to make sure your father always keeps that fuse holder away from vulnerable parts of his body? Is the fuse going to be fully encased inside the holder? Part of a multimeter's safety comes with the design of the enclosure which should prevent hazardous debris from escaping the meter in case of a blow out. If you watched Dave's teardown of the Agilent / TDK Lambda PSU, you might think twice about how a ceramic fuse could potentially fail when subjected to high energy.

Which leads back to the question: why not get your dad a UL / Intertek listed meter with the proper safety ratings for his potential applications?

Fair points that I'll think about, thank you for that. Price and portability are the main reasons, the meter would see little use and even that would be in the home.

[jwm_]

I don't get it;
In the V range, most multimeters have 10 MOhm input impedance. Even if 1000V is injected, the current draw is 0,1 mA. That little draw won't ever blow a fuse, or am i missing something?

In the current range all you have is a low value shunt resistor. This meter doesn't have separate inputs for current and voltage measurement, so with a flick of the switch (or just forget to adjust it) you can basically short out a power source.

Yikes. I would consider a multimeter without separate inputs for voltage and current broken by design.

If you want a simple, cheap, and extremely safe meter, the fluke 101 can't be beat if safety is your number one priority. It takes the sane option of not having current measurement, if you can't do it safely at a price point, don't do it at all. The 107 isn't much more and has current measurement.

 http://www.fluke.com/fluke/inen/digital-multimeters/compact-multimeters/fluke-101.htm?pid=77003

[AG6QR]

If considering fused leads, be aware that they can cause their own subtle dangers.  There's always the chance that the fuse has blown.  After the fuse blows, the meter will indicate no voltage, and/or open circuit, no matter what it's connected to.  So if you want to verify that power has been disconnected from a circuit, you have to put the meter in voltage mode, measure the circuit and see that it has no voltage, then measure a known non-zero voltage source to verify that the fuse hasn't blown.

I suppose any meter could fail, so it's always a good idea to measure a known voltage before and after verifying that something has been disconnected, but fused leads give an easy mechanism for the leads to fail open circuit, disabling measurement of voltage as well as current.  On the other hand, an open fuse in the current path of a meter will only disable current measurement.

There's so much more to meter safety than just the fuse.  I'd also echo the sentiments suggesting buying a relatively low-end but safe meter by one of the good companies when dealing with anything that has mains-level energy or higher.

[Tim F]

There's no guarantee that the HRC fuse in the lead opens the circuit before the fuse in the meter explodes. You'd probably have to defeat the fuse in the meter by replacing it with a shorted link and then it would be really unsafe if anyone were to unintentionally use it with non-fused leads.

In-line fuse holders rated for 600V+ are not exactly cheap either.

[3141592]

If considering fused leads, be aware that they can cause their own subtle dangers.  There's always the chance that the fuse has blown.  After the fuse blows, the meter will indicate no voltage, and/or open circuit, no matter what it's connected to.

This is a very good counter-argument and more likely danger than a 50kA fuse blowing up. I'll scrap this idea, just because of this. It's probable that someone sticks this DMM in current mode in to the mains, the fuse goes quietly, switches it back to voltage and sees a false 0V reading. Thank you for bringing this up!

There's no guarantee that the HRC fuse in the lead opens the circuit before the fuse in the meter explodes.

I'd guess one could look at the time it takes for the fuse to open and calculate the max energy that it lets through. I'd also guess that it's quite limited, but you are quite right that it would have to be checked, thanks!

If you want a simple, cheap, and extremely safe meter, the fluke 101 can't be beat if safety is your number one priority.

That's my candidate now, one could argue that a 400mA current range is not that useful anyway, if the problem is not in sensitive electronics.

[dom0]

+1 to the Fluke 101. A shunt current range doesn't do any good in electrical installations anyway (industrial would be different).

I don't get it;
In the V range, most multimeters have 10 MOhm input impedance. Even if 1000V is injected, the current draw is 0,1 mA. That little draw won't ever blow a fuse, or am i missing something?

Some meters don't  have separate inputs for the current shunts. So that's an issue. Another issue are surges, which can arc over in the meter. In that case the input impedance isn't 10 M? anymore, so a lot of current can flow. In a typical household you might get short-circuit currents of 200-300 A, while in industrial installations you can easily get into the kA range. At these currents stuff starts to burn and explode rather quickly; a high-rupturing capability fuse is used as a countermeasure to both, because it can break these high currents quickly and safely.

[kwass]

Consider the Uni-T UT210E instead:

 http://www.amazon.com/Uni-T-UT210E-Current-Meters-Capacitance/dp/B00O1Q2HOQ

If you don't need to measure current below 1mA.  It's very safe (also has NCV),  inexpensive and perfect for a tool box as it's small, rugged and comes with its own case. 

[3141592]

Consider the Uni-T UT210E instead:

 http://www.amazon.com/Uni-T-UT210E-Current-Meters-Capacitance/dp/B00O1Q2HOQ

If you don't need to measure current below 1mA.  It's very safe (also has NCV),  inexpensive and perfect for a tool box as it's small, rugged and comes with its own case.

Thank you! It does seem pretty impressive, although a bit awkward to use too... but it does have everything that's needed for a toolbox (if it can take 250V in all ranges).

[joeqsmith]

I have yet to see any Uni-T meters I would consider robust.  I have looked at four different ones now and they have all failed very early in the tests I have ran.   There may be Uni-T meters that are more robust but I have no idea how you would figure that out.   

Personally, if I were going to gift a pocket meter to someone I knew had no electrical background and wanted to minimize the risk that they could damage the meter or themselves, I would go with the Fluke 101.   

[3141592]

I have yet to see any Uni-T meters I would consider robust.  I have looked at four different ones now and they have all failed very early in the tests I have ran.   There may be Uni-T meters that are more robust but I have no idea how you would figure that out.   

Personally, if I were going to gift a pocket meter to someone I knew had no electrical background and wanted to minimize the risk that they could damage the meter or themselves, I would go with the Fluke 101.

I have seen your transient tests I think, that is some impressive work both quality and quantity wise, that you have done! It was great to see that Brymen holding up well!

I'm still looking for tests about that Uni-T, if I can't find someone proving that it is safe at least to some degree, I'll go with the Fluke or a lower end Brymen. The PCB pictures didn't seem that bad though, but I wouldn't consider myself to be an expert.

[joeqsmith]

Thanks. It's really opened my eyes on just how hit and miss these products can be.   

I have only tested that one Brymen.   If the rest of their products hold up as well as it did, they are doing a very good job.  Without testing them, I have no idea.   I would never gift the 869s to a non-electrical person anyway.  There are so many features that I would need to spend countless hours answering questions about how to use it.   

Both my 101 and 5ky's 107 have held up extremely well compared with every other meter I have looked at.   Besides how poorly all of the 87V tests went,  I really can't say much about Fluke's other products.   Really though,  even the 107 is a little too complex for people I would give it to for a gift.

[kwass]

I'm still looking for tests about that Uni-T, if I can't find someone proving that it is safe at least to some degree, I'll go with the Fluke or a lower end Brymen. The PCB pictures didn't seem that bad though, but I wouldn't consider myself to be an expert.

I haven't tested mine at more than 120 VAC but here's a comment from someone that apparently regularly uses it for higher voltage work:  http://www.doityourself.com/forum/air-conditioning-cooling-systems/550619-outside-unit-makes-clicking-noise-fan-not-turning-smells-like-electrical-burn.html#post2433064

[joeqsmith]

I wouldn't be too concerned about the 110 and 220 V.  It's the crap that comes down the line with it.

[Fungus]

If you want a simple, cheap, and extremely safe meter, the fluke 101 can't be beat if safety is your number one priority. It takes the sane option of not having current measurement, if you can't do it safely at a price point, don't do it at all. The 107 isn't much more and has current measurement.

If he's at all likely to connect it to the mains then THIS (x1000).

A 101 only costs about $42, delivered. There's no real excuse for using anything less safe than that.

You could also consider this: http://en-us.fluke.com/products/electrical-testers/fluke-1ac-ii-a1-electrical-tester.html

They're about $19.

[3141592]

I've ordered the Fluke 106, it has everything that's needed around the house, although it seems a bit artificially dumbed down. The BM805 would be a much better value (and quite likely similarly safe) IMO, but he wouldn't benefit from it and it's less portable. The UT210E seems great, but a bit awkward to use and that means he would never use it. I'll still buy a UT120C/UT210E for myself, these just seem ideal for some tasks I have. Thank you guys for all the help! 

You could also consider this: http://en-us.fluke.com/products/electrical-testers/fluke-1ac-ii-a1-electrical-tester.html
They're about $19.

I don't think I could ever talk him down from those neon sticks he's been using for the past 40 years. 

Thanks. It's really opened my eyes on just how hit and miss these products can be.   

You mean that products from one manufacturer can behave very differently?