Last week, I delivered a review of the Keysight U1461 Insulation Resistance Test Multimeter. It's a lovely unit, and as with all new and wonderful tools, it's become my new "go to" unit when it comes to experiments. In my review, I commended it for the convenience feature of being able to detect an open fuse without actually taking it out and testing it by using the Earth Bond test selection as the manual said ...


Had I been a little more awake this morning, I might not have had to use this feature ... and subsequently feel like an absolute fool.


The Story


Just last night, while I was sleeping, a thought came to me that I should probably investigate the old E-I core laminated transformer I pulled out of some gear a while ago. Its condition was unknown, but it was an unremarkable step-down transformer from an old discarded piece of equipment, maybe even 30 years old. It was pretty small, no more than 10VA at the most. I was thinking of using it in series as a current limiting choke for some AC experiments with a miniscule load, as we know, the transformer should be perfectly safe just hooked across the mains anyway, so in case of a short, we shouldn't be getting dangerous levels of current. After all, this is basically the same principle behind an old-fashioned magnetic/iron fluorescent tube ballast.


Being ever cautious, of course, I don't like playing with mains directly, and I don't like the massive fault currents that can arise. I don't even like using mains through an isolation transformer, fused. Instead, I opted for my go-to - a pure sine wave inverter driven by a lab benchtop supply. As the inverter is "insulated" and the benchtop supplies are "floating", there is no likelihood of me tripping RCDs or having current flow through me and ground - it's purely isolated. Better still, as it's a wimpy 300W inverter, the fault currents are limited too. For an additional safety measure, the power has to go through a 1A Variac/autotransformer (so I can vary the voltage) and it's fused too.



With this sort of arrangement, I should be able to measure the magnetizing current of the transformer to see what it's quiescent losses are (and hence, the current it will "let through" on a worst case scenario). Simple enough right? Not quite.


I did have a thought about whether such an arrangement would be good - there's likely some form of inrush (despite the small transformer) current which may be large enough to stress the fuse (although I doubt it). Of course, with 10VA, the quiescent current should be a lot smaller than 440mA, so I thought, what the heck, lets just get the measurement before I get breakfast. Bad move.


Whoops, I did it again ...


I flipped the switch on the lab supply and immediately, something didn't feel right. There wasn't any sound, but the meter was reading an infinitesimal amount of current when I was expecting something in the order of at least milli-amps. I check my wiring, and it's fine. I check the output voltage on the Variac, all good.


Then I decided to flip the U1461A over to Earth Bond mode ... and the bad news was there ...


That would explain it. A fuse open, meaning the circuit had opened inside the meter and somehow we've exceeded 440mA for long enough to blow the HRC fuse inside. Damn, I feel like a fool now. I blew a fuse when reviewing the Tektronix PA1000 due to inrush currents. You would've thought I'd have learnt my lesson by now.


Interestingly, it wasn't due to inrush currents. Further investigation with the resistance showed what appeared to be a shorted primary turn, meaning that the transformer was busted anyway. Why didn't I check that before I powered it up? It's so obvious in hindsight. Lesson learned ... maybe.


The Fuse


The fuse is housed in the bottom panel section on the U1461A and requires two screws to be removed to get to it. The rear panel markings state the specifications for the fuse. It is to be a 440mA fuse, with a working voltage of 1000V and an interruption rating of 10kA minimum. The size isn't specified, but since I have to replace it anyway, it gives me a reason to pull it out.


The fuse used is a type specially used in digital multimeters, and is a fast, high-rupture capacity fuse with a large size. Levering it out of the clips took the use of some screwdrivers.


The original fuse comes from SIBA - a reputable German manufacturer of fuses. It is designated as a DMI fuse. It has a length of 35mm. The unit exceeds the requisite 10kA interruption capacity, and has 30kA interruption rating.


The fuse has a diameter of 10mm. With this information, lets scour the element14 Australia catalogue for some replacements.


Replacement Fuses


After looking for some replacement fuses, did the actual sense of what I had done finally sink in. I wished I could just say "sorry" and everything would be forgiven. Okay, it's not quite that big of a mistake, but if I were to do it again, I wouldn't be too happy. There are several options for replacements:


As a result, there are plenty of options if you're looking to replace the fuse, but none of them particularly cheap. That being said, I'll probably go with the Fluke unit, as it's the cheapest one that meets specs. If you're looking to be most authentic, the SIBA one is available at about twice the price. None of the other options seems to be in the realm of reasonableness.




In the end, it was a case of mea culpa - I didn't check the winding resistance and shoved power right onto the winding which happened to be shorted. I didn't think carefully about the discrimination of the fuses - I'm just used to the 1A fast blow fuse in the Variac going before anything significant happens, but when your meter has a 440mA fast blow as well, that will be the "weak" spot which goes first. It's a good thing that it worked for the safety of the meter. I suppose this is why I wished the U1461A also had a high current range, which might be fused less aggressively. At least it's not a permanent mistake.


The fuse is a relatively expensive part, and after you blow a few, you'll do your darned best not to blow any more. That being said, it's an integral part of the safety of the meter, and your safety as well, so it's critical to replace it with a part that meets specifications - regardless of the cost. If you're looking for a replacement, it just so happens that element14 stocks several alternatives, so that's nice.


Maybe next time, I should have breakfast first before I try doing anything.