Multimeter fuses - Page 1
Forum section: electronics / testgear

author: Fungus
author_rank: Super Contributor
timestamp: February 23, 2021, 02:34:11 pm
content: My newFluke 37has two fuses in series on the 320mA range. A standard size 630mA glass fuse and a big-ass 2A HRC fuse.I'm guessing the 20 cent glass fuse is there to protect the expensive HRC fuse from silly mistakes like leaving the leads in the wrong holes. This seems like a really good idea to me, why don't modern meters do it?

author: Hydron
author_rank: Super Contributor
timestamp: February 23, 2021, 03:45:33 pm
content: Some of them kind-of do, at least bench meters. The DMM6500 has little 3A fast-blow glass fuses accessible from the front/rear 3A sockets, while inside it has a 3.5A HRC fuse on the same range. You'd have to void the cal by opening up the unit to get to it, but at least it might avoid serious damage if you f**k up bad. The 10A range (rear panel only) has a HRC fuse only (changeable without opening unit this time), but that kinda makes sense given it's a much lower burden-voltage range, which 2 fuses in series would compromise.

author: Hexley
author_rank: Regular Contributor
timestamp: February 23, 2021, 04:49:45 pm
content: The Keysight 34465A family has series fuses in the 3A path. There is a little 3.14A 500VAC/400VDC time delay fuse that is accessible from the rear panel, in series with the 11A 1000V fast acting fuse inside the case.For minor overloads, the little 3A fuse handles the job and is easily replaced.After my first oopsie, I laid in a supply of the 3A fuses and taped a baggy full of them to the back. That worked like a charm -- haven't blown one since. :-)[Edit: typo]

author: dcac
author_rank: Frequent Contributor
timestamp: February 23, 2021, 05:37:09 pm
content: Quote from: Fungus on February 23, 2021, 02:34:11 pmModern meter manufactures perhaps believe selling expensive HRC fuses is good for business.

author: dcac
author_rank: Frequent Contributor
timestamp: February 23, 2021, 05:47:05 pm
content: This - or similar - could serve the same function.

author: HKJ
author_rank: Super Contributor
timestamp: February 23, 2021, 05:53:13 pm
content: Quote from: dcac on February 23, 2021, 05:47:05 pmNo, it is a safety hazard.

author: bdunham7
author_rank: Super Contributor
timestamp: February 23, 2021, 06:00:14 pm
content: Quote from: HKJ on February 23, 2021, 05:53:13 pmWhy?

author: dcac
author_rank: Frequent Contributor
timestamp: February 23, 2021, 06:22:15 pm
content: Perhaps, one reason I can think of is you might be physically closer to the fuse (i.e. with your hands) in case of it rupturing when blowing in a (very) high energy circuit.

author: joeqsmith
author_rank: Super Contributor
timestamp: February 23, 2021, 06:23:05 pm
content: Quote from: Fungus on February 23, 2021, 02:34:11 pmI looked at an old Fluke that has something similar. I wonder if they arc flash test the meter, and the smaller one breaks down and creates lots of ionized gas, where is the next flash over point. Does the large HRC fuse even come into play every single time or are there unintended paths. I agree that from a layman's standpoint, it makes a lot of sense but I am not sure how it plays out in practice.I am looking at another 40 year old Fluke that also uses this arrangement. There is a small fuse that is user accessible from the front of the meter. Then there are 4Xlarge HRC fuses buried deep inside the meter that your can pull after pretty much taking the meter all the way apart. This board and others use GDTs. This thing is a bench meter and would have been used by scientists, not electricians. It shows the early signs that Fluke was starting to see what it took to protect a meter.

author: Fungus
author_rank: Super Contributor
timestamp: February 23, 2021, 06:50:14 pm
content: Quote from: joeqsmith on February 23, 2021, 06:23:05 pmYep. The small one can arc over. The arc would then reach the HRC fuse.Quote from: joeqsmith on February 23, 2021, 06:23:05 pmFWIW the fuses in the 37 are on a small daughterboard that stands away from the meter's main PCB. Hopefully Fluke will have laid out the fuses appropriately. I guess I could take a pic of that board but I'm not sure if it would prove anything.Quote from: joeqsmith on February 23, 2021, 06:23:05 pmI believe the Fluke 37 was made before CAT ratings were a thing but the manual has this to say on safety:Both of those standards are pay-to-read.

author: dcac
author_rank: Frequent Contributor
timestamp: February 23, 2021, 06:59:15 pm
content: Fluke 45 has this fuse configuration.Where the 100mA input is protected by 500mA 250V whats looks like an ordinary 5x20mm glass fuse - but searching the Fluke part number it costs about $8.00.

author: joeqsmith
author_rank: Super Contributor
timestamp: February 23, 2021, 07:01:49 pm
content: Quote from: Fungus on February 23, 2021, 06:50:14 pmThat's the layman's point of view. The reality is the ionized gas cloud isn't confined to area neat the small fuse. Rather it can create a large pressure wave. That expanding gas may cause other areas to break down. The HRC fuse will contain the arc. Imagine having the outside of the fuse surrounded by a cloud of conductive gas.I ran a small experiment once to demonstrate the gas expansion. No where near the energy of an actual arc flash event.

author: HKJ
author_rank: Super Contributor
timestamp: February 23, 2021, 07:04:49 pm
content: Quote from: bdunham7 on February 23, 2021, 06:00:14 pmTwo reason:1) You measure a voltage and it shows 0, you then assume it is safe to work on it, but the reason the meter showed 0 was a blown fuse.2) You have the leads plugged into the 10A socket and put the probes across some high power, you may get a very bad hand from that (I doubt the fuse can break kA).

author: Fungus
author_rank: Super Contributor
timestamp: February 23, 2021, 07:05:05 pm
content: Quote from: joeqsmith on February 23, 2021, 07:01:49 pmMaybe the glass fuse isafterthe HRC fuse.

author: Hydron
author_rank: Super Contributor
timestamp: February 23, 2021, 07:05:48 pm
content: Quote from: bdunham7 on February 23, 2021, 06:00:14 pmUser checks for live mains voltage using fused lead, sees nothing (due to blown fuse), touches stuff, becomes ex-user. (HKJ beat me to it).

author: joeqsmith
author_rank: Super Contributor
timestamp: February 23, 2021, 07:14:26 pm
content: Quote from: Fungus on February 23, 2021, 07:05:05 pmBefore/after, I don't see your point. If the small glass fuse were to rupture from an arc, I don't think you can with out a doubt say what the path is.Let's assume I am full of shit and you and I both agree that this is a 100% safe and controlled method. (I'm not saying that) Ask yourself, why did Fluke stop this practice.I suspect they helped to shape the safety standards we have today.

author: dcac
author_rank: Frequent Contributor
timestamp: February 23, 2021, 07:28:57 pm
content: Quote from: HKJ on February 23, 2021, 07:04:49 pmThese are CAT IV rated fused test leads:http://www.extech.com/products/TL900"(1000V, 11A, 20kA breaking capacity)"But of course these fuses are probably just as expensive to replace as any DMM HRC fuse - but still probably easier accessible.But I agree on your notion that fused test leads could be dangerous for voltage measurements - especially if you don't happen to know they are in fact fused test leads.

author: Fungus
author_rank: Super Contributor
timestamp: February 23, 2021, 07:30:33 pm
content: Quote from: joeqsmith on February 23, 2021, 07:14:26 pmIf it's after the HRC then not enough energy can make it through the HRC fuse to make the glass fuse arc.Edit: Or, at least, not enough tosustainan arc - the arc would have to be fed through the HRC.FWIW I looked at the PCB and the glass fuse is indeed after the HRC:

author: bdunham7
author_rank: Super Contributor
timestamp: February 23, 2021, 07:35:43 pm
content: Quote from: HKJ on February 23, 2021, 07:04:49 pmI suppose anything is a safety hazard if you don't use it right. Scenarios like your #1 can happen with a variety of meter faults, which is why there is a TBT (Touch Before Test) protocol for any type of high energy circuit.Fused leads and probes are made by well-reputed companies and are not a shady product. The leads in question have an IEC CAT-IV 1000V rating, for which the manufacturer states a specific fuse is required.

author: Hydron
author_rank: Super Contributor
timestamp: February 23, 2021, 07:36:33 pm
content: RE: joeqsmith's posts:Thebenchmeter I mentioned (and probably the KS one too) is only CAT II 300V rated, so I guess there is probably less concern about high energy faults (though still enough to include the HRC in series). The fuse is also built into the input jack, so not next to the PCB, though as you say a high enough energy event may not be deterred by an itty little bit of plastic around the fuse (I would also be concerned about the fuse holder leaving the instrument panel at a rate of knots, as amply demonstrated in the video!)Seems to me like a reasonable trade-off using the dual fuse design in the CAT II bench meter which is not going to be poking around in a switchboard but is likely to have a few over-current mistakes made while measure low voltage DC circuits (it is after all the same type that you'd find on the mains input of most equipment). For handhelds you just have to live with the pain of costly fuse replacements in the name of safety, again it seems reasonable given they are much more likely to be used in situations where high fault current is possible (also teaches the use to be careful - I've blown a few glass fuses in the past but haven't popped an HRC yet thankfully!).

author: Fungus
author_rank: Super Contributor
timestamp: February 23, 2021, 07:39:21 pm
content: Quote from: joeqsmith on February 23, 2021, 07:14:26 pmPenny pinching? Not enough space to do it inside handhelds?There's other reasons to do things apart from safety.

author: dcac
author_rank: Frequent Contributor
timestamp: February 23, 2021, 07:39:39 pm
content: @FungusIs that fuse at the top of your picture for the high current input? It's contact surfaces looks a bit moldy, what kind of burden voltage are you getting?

author: Fungus
author_rank: Super Contributor
timestamp: February 23, 2021, 07:54:49 pm
content: Quote from: dcac on February 23, 2021, 07:39:39 pmYep, that's the 10A fuse. I hadn't looked closely before but it certainly looks a bit crusty in that photo.I just pulled it out and it smells OK, I don't think it's mold.(seems more like old grease than anything - I gave it a bit of a polish with IPA just in case, it came up a bit shinier)Edit: Here's a pic. I don't think it's the original Fluke fuse.

author: bdunham7
author_rank: Super Contributor
timestamp: February 23, 2021, 08:05:28 pm
content: Quote from: joeqsmith on February 23, 2021, 07:14:26 pmWell I agree that there is no before or after unless we're talking about actual lightning or something--the fuses are in series, so the same current must flow through each, at least at first. You aren't going to rupture the small fuse into an ionized plasma with a small current, say 10 amps. However, that 10 amps will blow the big fuse very quickly and open the circuit. The only way I see a problem is if there were a high voltage DC applied with limited current. Say you had a 1kV DC supply limited to 3 amps--you would blow the small fuse but not the big one and now you have a 3kW arc across the little fuse. There probably are other scenarios I haven't thought of.On my version of your meter, the 8505A, I found two blown fuses and the 39.1K X-Ref input resistors were melted. I've wondered how that could have happened. I looked up those fuses and one thing I noticed is that they are only rated for 170VDC. I replaced them with a a 1kVAC/DC 10kA rated fuse, which I think is better overall protection than 600VAC/170VDC 100kA.https://www.mouser.com/datasheet/2/643/ds-CP-0ADBX-series-1664157.pdfAs far as why Fluke would discontinue the practice (while apparently some other manufacturers continue it) it probably is just a case of consolidating their designs--all of their meters appear to use the same 0.44A/11A pair of fuses--and eliminating both known corner cases and possible unknowns. IOW, they have a known solution (the particular fuses) so why add parts and expense and increase risks just for a user convenience?

author: Fungus
author_rank: Super Contributor
timestamp: February 23, 2021, 08:14:08 pm
content: Quote from: bdunham7 on February 23, 2021, 08:05:28 pmThe only thing we know for sure is that Fluke doesn't worry about how much people have to pay for fuses.

author: Fungus
author_rank: Super Contributor
timestamp: February 23, 2021, 08:26:19 pm
content: Quote from: bdunham7 on February 23, 2021, 08:05:28 pmIn that case you'd be in trouble because even the big fuses are only rated for 600V (in this meter).So really it wouldn't matter if you were limited to 3 amps or not.OK, so we say 600V@3A, what would happen to the glass fuse then? Would that be enough to create a deadly arc?(I don't know).

author: joeqsmith
author_rank: Super Contributor
timestamp: February 23, 2021, 08:33:44 pm
content: Quote from: Fungus on February 23, 2021, 07:30:33 pmSo you feel that if two fuses are in series, the one closest to the input is guaranteed to blow first? Could you explain how you came to that conclusion.That looks scary as all get out. I was thinking the smaller fuse would have been in its own holder to at least try and somewhat contain it but it's right next to the input. That distance is nothing for that pressure wave of ionized gas to travel.Nothing in what you have would suggest "the arc would have to be fed through the HRC". I am not surprised that this sort of practice is no longer in use.

author: joeqsmith
author_rank: Super Contributor
timestamp: February 23, 2021, 08:43:22 pm
content: Quote from: Hydron on February 23, 2021, 07:36:33 pmAgree about the bench meters not typically being used in areas where arc flash would be a concern. I was surprised to find the large HRC fuses in the Fluke bench meter I mentioned. It's physically large (19" rack mount). With the high cost of the meter, I doubt people were using it to look at their mains. Still, maybe some high power physics applications drove them to include the fuses in the design.

author: bdunham7
author_rank: Super Contributor
timestamp: February 23, 2021, 08:51:26 pm
content: Quote from: Fungus on February 23, 2021, 08:26:19 pmSo how bright would a 1.8kW light the size of that fuse be?Not arc-flash energy, but I'm sure there would be molten metal bits all over.

author: Fungus
author_rank: Super Contributor
timestamp: February 23, 2021, 09:00:24 pm
content: Quote from: joeqsmith on February 23, 2021, 08:33:44 pmI never said that.The difference is that if the glass fuse is between the HRC fuse and and the leads, the arc from the glass fuse would have more chance to leap around and bypass the HRC fuse before the HRC fuse blows.If the HRC fuse is between the glass fuse and the leads then the arc across the glass fuse would need to be sustained via. the HRC fuse after it's created. If that fuse blows then the arc dies with it.(maybe)Quote from: joeqsmith on February 23, 2021, 08:33:44 pmThe distance between the pads that hold the glass fuse is less than half the distance between the fuse and the terminal. I'd bet that's where the arc would form. Maybe it's even deliberately done that way, like a spark gap?If the arc forms there then the HRC fuse can still do its job.You don't have to run faster than the bear, just faster than the slowest person in the group.

author: Fungus
author_rank: Super Contributor
timestamp: February 23, 2021, 09:02:41 pm
content: FWIW here's the new Fluke 87V max for comparison. The air gaps are about the same as the 37 :-)All that's missing is the PCB slot.(and the 87V is missing the igniter)

author: joeqsmith
author_rank: Super Contributor
timestamp: February 23, 2021, 09:18:27 pm
content: Some good videos on YT of events captured on camera.https://www.ecmag.com/section/safety/lets-blow-it-arc-flash-testingThe Mersen lab has two generators. Each generator is rated 10 megavolt-amperes (MVA) continuous with a short-circuit rating of 68 MVA and is powered by a 4,160-volt (V), 536-horsepower electric motor that is directly connected to the utility. When a test is conducted, the short-circuit current comes from the generator and not the electric utility.Quote from: Fungus on February 23, 2021, 09:00:24 pmQuoteIt's the above statement that leads me to question it. By suggesting the HRC fuse limits the energy to the glass fuse tells me you believe there is an order in which things happen. The small fuse fails. It opens and draws an arc. The HRC fuse then blows and saves the day. Again, thinking about it in laymans terms, I could see why you may think this way. The small fuse may indeed rupture and arc but before the HRC fuse does anything the ionized gas has already bypassed the small fuse along with the HRC fuse. We are left with the shunt across the line with other copper. The HRC fuse may never blow.The fuses do seem to blow outward from the link, right in line with the input. Just crazy they would have even considered this approach but again, my guess is they just didn't know as much back then.From:https://www.vanert.com/page/arc-flash-evaluationArcing from an electrical fault produces incredibly high temperatures. It can easily exceed 30,000 degrees F— hotter than the surface of the sun! Arcing causes air to expand dramatically and metal conductors to vaporize. Copper expands 67,000 times when it is converted from solid to vapor. This rapid expansion of air and metal vapor produce an intensely hot blast.67,000 time expansion isn't going to care much about your little inch or so gap. Then again, I could be full of shit but you asked.

author: joeqsmith
author_rank: Super Contributor
timestamp: February 23, 2021, 09:19:53 pm
content: Quote from: Fungus on February 23, 2021, 09:02:41 pmWhat's missing it the stupid little glass fuse that starts the reaction.

author: joeqsmith
author_rank: Super Contributor
timestamp: February 23, 2021, 09:36:37 pm
content: QuoteOf course, modern meters will typically have a CAT rating and if the meter were marked CAT I or not marked, just ditch the HRC fuse and be done with it. Sadly, modern meters are commonly marked CAT IV 600V and have a 250V glass fuse inside and we allow it.If it were a modern meter, we can assume that the need for the HRC fuse was driven by the intended environment. But, wait, those fuses are expensive so let's save the user some money and shove a lower cost fuse in line with it. In principle, seems like there's some merit to compare human life to the $10 fuse (what ever they cost). Maybe it could be done but somehow you would think that sacrificial fuse would need to be contained. One way to do this would be to make the sacrificial fuse in itself and HRC fuse.

author: Fungus
author_rank: Super Contributor
timestamp: February 23, 2021, 09:46:08 pm
content: Quote from: joeqsmith on February 23, 2021, 09:19:53 pmThe 'igniter'...?

author: joeqsmith
author_rank: Super Contributor
timestamp: February 23, 2021, 09:52:55 pm
content: Quote from: bdunham7 on February 23, 2021, 08:05:28 pmInteresting about the voltage rating on those fuses. I had not looked at them outside to marvel that they were there along with the GDT. It was quite the surprise.Most of the fault current on the HRC fuses I find in the meters are 10KA and up as well at rated for both AC/DC at 1KV. We are not talking about 10A fault currents but 10's of thousands of amps and enough voltage to do some real damage. But again, if the meter isn't designed to work in this environment who cares. Just don't mark that it is and ditch the HRC fuse.I want to say that little cork gun experiment I ran, the generators source impedance limits the current to around 600A or so. The total energy (assuming we actually short the output) is limited to about 4-600 Joules. Not much but enough to blow the cork out of the tube. Sure, it's enough to kill a human. Not enough to blow the limbs off one and vaporize other parts.

author: joeqsmith
author_rank: Super Contributor
timestamp: February 23, 2021, 09:55:57 pm
content: Quote from: Fungus on February 23, 2021, 09:46:08 pmI believe so. That or a trace opens up. Something to start it. Read that article. They use some small wire. Similar to my cork gun but I bet a lot more fun to watch. At least for me it would be.

author: Fungus
author_rank: Super Contributor
timestamp: February 23, 2021, 10:03:17 pm
content: Quote from: joeqsmith on February 23, 2021, 09:18:27 pmOTOH I'm sure they must have donesometesting. They quote even safety standards in the manual, etc.Whatever, I for one am very happy to have that little fuse in there as I potter around with my unlikely-to-arc circuits. If something looks a bit more exciting then I can always get out my Fluke 101.Quote from: joeqsmith on February 23, 2021, 09:36:37 pmYou've made many of those meters jump around and vaporize copper without resorting to the current input jacks.Quote from: joeqsmith on February 23, 2021, 09:36:37 pmI think the concept could be made to work. There's HRC fuses out there that cost50 centsand could be protecting the $10 Bussmans.

author: Hydron
author_rank: Super Contributor
timestamp: February 23, 2021, 10:20:10 pm
content: Quote from: joeqsmith on February 23, 2021, 09:55:57 pmThe guys in the HV lab at uni had some fun creating plasma conduction paths by blowing up thin wires (the vast majority of the energy going into the plasma, not the wire). I think their record was 60 or 70 meters... (to be fair it was using a40kJ70kJ capacitor bank at afew 10s ofnearly 300kV, but the peak current was within the HRC interruption range, though obviously not the voltage!). I missed seeing the biggest demonstrations, but helped photograph some of the smaller ones (and also saw what happens when your earth inductance is too high and causes the potential of the earth cable to rise high enough to break down and punch a hole in some concrete to arc to the rebar inside!).

author: joeqsmith
author_rank: Super Contributor
timestamp: February 23, 2021, 10:35:50 pm
content: QuoteFungus, I think that the blown fuse test that they are required to run considers that the rest of that area isn't going to breakdown (arc over) under normal conditions. This assumes the fuse contains the arc.Indeed, I have made a few of them jump just as you suggest, using the voltage inputs. The small generators put about about 20J. That in itself isn't enough to make the meters jump but couple it with 4-600 Joules, we can even split a case open.Most meters won't break down (or arc over) internally with the voltage inputs at the 6KV I test at. So I could have 100KJ available and it would not matter. All the energy just dissipates internal to the generators output stage in heat.Of course, when the meters have these cheap fuses in them, I look at it as an opportunity to play with the current inputs as well.A bit off topic.The hobbyist working in CAT I most likely doesn't care about any of this HRC stuff. I imagine most are working on their bench with very low energy circuits and chances of an arc flash are zero. Then we have my favorite, avg ave canadian guy that made a rant video about meter safety all the while with showing they had blow a fuse and not replaced it with the proper safety fuse.Again, with some basic education, you could get away with a lot to the point of bypassing the fuses all together (which is what I typically do on the bench when I am using shunts). At home, I fit this hobbyist group. I don't care about HRC fuses and really, don't care about using the meters to directly read current. But, I also don't want a meter that when I do something stupid, that the meter is damaged. For me, it's not ever been a concern for safety but robustness. IEC 61010, who cares. 61326 is where it's at. AgainWhen I work in CAT III, I use a meter with clamps. There is no fuses. And, the meter is certified.

author: bdunham7
author_rank: Super Contributor
timestamp: February 23, 2021, 10:40:13 pm
content: Quote from: joeqsmith on February 23, 2021, 09:55:57 pmThis is all a bit theoretical and the various demonstrations, while spectacular, seem a bit rigged. Is there anything using something like an actual AGX-2 glass fuse? And keep in mind that the impedance characteristics of the meter and leads are going to keep the initial fault current to a certain level--perhaps 1000A max--unless you exceed the rated input voltage. So if we're worried about that additional fuse causing otherwise avoidable mayhem, you'd have to show that the fuse can be blown up and plasmatized before the HRC blowsat an input voltage and current that the HRC would have otherwise been able to interrupt.In fact, I think the HRC might actually blow first at very high currents.I don't know. The only high fault-current test source I have is my 200A service panel. I have some spare SC-3 fuses, so if Fungus wants to send me his Fluke 37, I'll give it a try.

author: joeqsmith
author_rank: Super Contributor
timestamp: February 23, 2021, 10:45:44 pm
content: "OTOH I'm sure they must have done some testing. "Yeah, we seem to like cars. I would imaging how we crash tested cars in the 70s isn't how we crash test them today. Side load testing, what's that? Don't cars always hit head on? Seems obvious now. I went to a test track once and person I was working with was telling me how they started out simulating a road with vehicles running in two directions. A few crashes and then they sorted out that maybe that wasn't such a good idea. We are not always as smart as we think we are. We know we are in trouble when we think we actually know a lot. I'm turning white and still don't know jack.The standards change as we learn more. Don't kid yourself in thinking that they have no evolved in the time that crazy fuse approach was introduced.

author: joeqsmith
author_rank: Super Contributor
timestamp: February 23, 2021, 11:07:12 pm
content: Quote from: bdunham7 on February 23, 2021, 10:40:13 pmRigged? It's not a carnival game.

author: Fungus
author_rank: Super Contributor
timestamp: February 23, 2021, 11:21:19 pm
content: Quote from: joeqsmith on February 23, 2021, 09:52:55 pmYep. Anybody who works in that environment probably owns a pair of test leads that come on long sticks and some PPE.The Fluke 37 has a tiny little sticker on the bottom:So I guess that's what the CSA thought of it at the time, where it should be used, and by whom.

author: nightfire
author_rank: Frequent Contributor
timestamp: February 23, 2021, 11:45:26 pm
content: Quote from: Hydron on February 23, 2021, 07:05:48 pmThis exact scenario is the reason why only a two-pole voltage tester (brand name: Duspol) is allowed in germany to check that a circuit is safe to work on...

author: nightfire
author_rank: Frequent Contributor
timestamp: February 24, 2021, 12:05:06 am
content: Regarding short-circuit currents: Some years ago I learned how to calculate those currents, and one thing was memorizable and stuck in my head as a rule of thumb:Regardless of what short-circuit-capability your transformer and the high-voltage network is, 3 meter of 1,5mm² (somewhere between 15 and 16 AWG) singlephase 230V AC could only produce 2000 Amps short-circuit current due to the dampening effect of the thin copper...And as long a bench DMM is used only for probing voltages withing CAT II devices (and not on special generators) those fuses should be able to do their job. An ordinary 5x20mm glass fuse can break about 200A (IIRC, would have to look that up in the datasheet) and then, if a short circuit is passing through, the HRC fuse should go into action. Depending on the characteristics of the HRC fuse, if it is a rapid blow type, it can act fast enough to prevent the little glass fuse from venting plasma...But here one has to check the datasheet and type of that fuse to be sure.A 10 A rapid type can basically begin to arc faster than a comparatively slow acting piece of wire in that glass fuse will begin to act, therefore absorbing lots of energy (cut-off-current is lower than prospective current) and therefore eventually preventing the small fuse from exploding.BTW: The fuses depicted above is some ancient type with "gl" characteristics, that nowadays is rated as gG (general use, usually used to protect to ordinary circuits up to 400/500 Volts AC). So it is probably exchanged by the former owner.My Fluke 45 has a ceramic 500mA fast BUSS fuse in the milliamp input, just checked.

author: nightfire
author_rank: Frequent Contributor
timestamp: February 24, 2021, 12:17:01 am
content: Quote from: Fungus on February 23, 2021, 07:54:49 pmThats a standard 10x38mm fuse used mostly in industrial applications for 230V/400V 3phase circuits in special holders. Therefore the 16A (1,5mm² copper wire) rated current and the quite high 100 kA breaking capacity.To be honest: Thats a fuse that should not be there, but in normal circumstances, as long as you probe household devices and short them, this fuse should protect you quite well...Ok, its old, gL is the old designation, nowadays it is gG. (L was for Line, G is "General"- according to some ancient VDE standards, the difference was so academical that they did not mentioned the few slight differences between them even in a standard...)

author: bdunham7
author_rank: Super Contributor
timestamp: February 24, 2021, 12:19:05 am
content: Quote from: joeqsmith on February 23, 2021, 11:07:12 pmBefore I would condemn the two-fuse system as problematic, I'd want to know if any of the proposed scenarios actually are plausible in real life. Exploding things and generating arc flashes in a lab certainly demonstrates that the energy involved can do spectacular things, but not that the fuse pair would actually behave that way. I'd like to know how many joules of energy it takes to actually rupture an AGX-2 fuse. And if there is an issue, perhaps the fix is the one you suggested--updated fuse technology for the smaller fuse. Here's a candidate with sufficient specs and it fits right in the same slot as the AGX-2.https://www.mouser.com/datasheet/2/701/P404900rB_drawing-1633768.pdf

author: joeqsmith
author_rank: Super Contributor
timestamp: February 24, 2021, 02:30:11 am
content: Quote from: bdunham7 on February 24, 2021, 12:19:05 amReal life gets difficult. It's very unpredictable, well from what I have seen anyway.I suspect you could have unlimited energy at a very low voltage and not explode any fuse. This is why arc flash ratings are voltage dependent.

author: joeqsmith
author_rank: Super Contributor
timestamp: February 24, 2021, 02:36:14 am
content: Quote from: Fungus on February 23, 2021, 11:21:19 pmI told that story here about an electrician I met that had a meter explode while he held it. Two man job. One with poles, the other with the meter. Full suits. He said that changed his whole outlook on PPE. They were lucky.

author: tautech
author_rank: Super Contributor
timestamp: February 24, 2021, 02:47:07 am
content: Quote from: bdunham7 on February 24, 2021, 12:19:05 am2 series fuses are a practical solution for protection of a measurement system where one might be inside the instrument and not readily accessible yet the other in an external cartridge type fuse holder at the rear of the instrument where it is one readily accessible and 2 a lesser rating to the internal fuse.Either can be contained so not to blow plasma around delicate instrumentation when users make dumb errors.Staging series fuse breaking characteristics is not difficult these days by using rapid or time delay fuses of slightly different amperages.SDM3000 bench meters employ this strategy with internal 12A and external 10A slow and fast blow fuses.

author: Fungus
author_rank: Super Contributor
timestamp: February 24, 2021, 07:48:55 am
content: Quote from: nightfire on February 24, 2021, 12:05:06 amYep. That's basically the difference between CAT II and CAT III, i.e. the distance to the fuse box.

author: Fungus
author_rank: Super Contributor
timestamp: February 24, 2021, 10:53:09 am
content: Quote from: bdunham7 on February 24, 2021, 12:19:05 am

author: joeqsmith
author_rank: Super Contributor
timestamp: February 24, 2021, 12:56:51 pm
content: Quote from: tautech on February 24, 2021, 02:47:07 amAssuming we are still talking cheap glass fuses used to protect a more expensive safety fuse so attempt to save a few bucks.....I've looked a some pretty cheap meters, many with glass fuses (marked CAT III 600V and up). One meter I looked at had a blue condom over it. Maybe an attempt to contain the blast. I wonder with an actual arc flash test, could you actually contain one reliably.Better just stop embellishing the meter's ratings. With counties allowing the import of the shit, the markings basically are worthless anyway. I'm actually surprised that top level companies don't lobby for this. The hobbyist could still buy their properly CAT I rated meter with glass fuses for a couple of bucks. Still what we have today isn't so bad. I would imagine few industrial techs, electricians and the companies who hire them, are not running out to buy these low end products. And the hobbyist who doesn't care about such things as arc flash can buy what ever meter they want and jump the fuses with wire to save a few dollars. Life goes on....

author: tautech
author_rank: Super Contributor
timestamp: February 24, 2021, 07:21:30 pm
content: Quote from: joeqsmith on February 24, 2021, 12:56:51 pmA mixture and with different speed ratings:SDM3000 family:Rear panel fuse: 5x20 glass LittlefuseInternal PCB fuse: 5x20 HRC LittlefusePic of SDS3045X showing covered internal HRC fuse bottom left and external accessible fuse holder bottom right:

author: joeqsmith
author_rank: Super Contributor
timestamp: February 24, 2021, 08:53:00 pm
content: CAT II 300V, no problem. It's made for the lab not for an electrician. Now is "... 2 series fuses are a practical solution for protection of a measurement system ..." for this particular meter actually true? They claim 61326 and 61010. Can you provide both reports?

author: tautech
author_rank: Super Contributor
timestamp: February 24, 2021, 09:45:35 pm
content: Quote from: joeqsmith on February 24, 2021, 08:53:00 pmMaybe the factory has them on file as they no apparently longer list them on the websites but they do state this:Information regarding product compliance is available on the datasheet for each product.Contact your local Siglent office for more details if you have trouble finding what you need.You can also write sales@siglent.comOtherwise only this from the datasheet:EMC Conforming to EMC (2004/108/EC) and EN 61326-1:2013Safety IEC 61010-1; EN 61010-1; UL 61010-1; CAN/CSA-C22.2 No. 61010-1 Measurement CAT I 1000 V/CAT II 600 V

author: bdunham7
author_rank: Super Contributor
timestamp: February 24, 2021, 10:55:37 pm
content: Quote from: joeqsmith on February 24, 2021, 12:56:51 pmVery true, but IMO some of the markings are 'worthless' in a different sense as a result of the obsession with mains power, transients and arc-flash. A completely non-mains derived source would appear to be CAT 1, but what is the hazard level of something like an 800 volt electric car battery or a 950 volt DC solar array? Does CAT 1/1000V sound sufficient? The fuses in the meters that I actually use -- including the ones I put in my old 8505A-- would appear to be sufficiently rated to not go nuclear on the DC solar array, but the battery would have me wondering what the maximum fault current was. It would certainly give the 10kA or 20kA DC interrupt rating a good verification test.For the vast, vast majority of these meters there is little to no chance of them actually going near any actual arc-flash hazard as defined by IEEE-1584. And that's probably a good thing.

author: bdunham7
author_rank: Super Contributor
timestamp: February 24, 2021, 11:01:44 pm
content: Quote from: tautech on February 24, 2021, 07:21:30 pmIs that the way the SDS3065X is too, with a CAT 2/600V rating? Because I don't think there are any 5x20 fuses like that with a 600 volt rating. That and the interrupt ratings on that size are pretty low. I'd like to see a 1kV 10kA fuse even on a CAT 1/1000V meter.

author: joeqsmith
author_rank: Super Contributor
timestamp: February 25, 2021, 01:12:17 am
content: Quote from: tautech on February 24, 2021, 09:45:35 pmYeah I read the data sheet which is why I asked for the report. I don't have that level of interest to try and hunt it down but thought that with your comment about it being a practical solution for protection of a measurement system, that you had data (the reports) to back up that statement was all. If it were a handheld device, I may take a swing at it.

author: joeqsmith
author_rank: Super Contributor
timestamp: February 25, 2021, 01:33:40 am
content: Quote from: bdunham7 on February 24, 2021, 10:55:37 pmNot being a safety buff, I don't have the background required to answer your questions. My guess is that when the standards were being developed, very little was being done with DC. At least not to the scale we had with AC and that's were the concern was. That said, I don't think they make those HRC fuses both AC and DC rated for the fun of it. I would imagine designing a fuse for 1000V DC with a 30KA fault current came with a bit of research and cost.I would imagine anyone working with high voltages and high energy levels (AC or DC) knows what is required and has the proper training. Myself, I am not an electrician and its rare I am involved with anything that's a concern. Again, when I do, I use a meter with no fuses. Clamp only. For home, I just want the a meter that is robust enough that when I do something stupid, the meter doesn't go to the recycle bin. This is why I test the meters at very low levels.

author: Kosmic
author_rank: Super Contributor
timestamp: February 25, 2021, 01:53:07 am
content: Quote from: joeqsmith on February 24, 2021, 12:56:51 pmMy old HP 974a (Rebranded Yokogawa) has something like that on the mA range. Always weird to see that they were trying to save a bit of money on a expensive meter like that. Then again it was mostly targeted at lab application where it's always CATII and lower.

author: nightfire
author_rank: Frequent Contributor
timestamp: February 25, 2021, 07:37:07 pm
content: I was browsing the recent SIBA catalogue and noted some explanations in the preface section:"4.1 The glass tubeIf the maximum expected short-circuit current (prospective cur-rent Ip) is not greater than 35 A or max. 10 Irat, then a miniaturefuse-link with an unfilled glass tube is used. This fuse-link has theadvantage of a relatively low voltage drop"So it seems that maybe this version has been chosen to provide for a low voltage drop and therefore better accuracy/lower burden.

author: nightfire
author_rank: Frequent Contributor
timestamp: February 25, 2021, 08:11:00 pm
content: Uhh- just looked a bit further with the 5x20mm fuses, and in short: the simple glass-tube fuses have a way lower resistance (and therefore heat dissipation) than their sand-filled ceramic counterparts. We are talking a factor of 3 or 4 here...