FAQ
TL;DR: A 3.15A glass fuse can still explode during a mains short because fault current can far exceed its safe interrupting limit, and one expert warned that sand-filled fuses "extinguish" the arc faster. This FAQ is for repairers and hobbyists who need to prevent shard injuries, arc damage, and avoidable equipment loss during fuse faults. [#21026920]
Why it matters: A fuse rating in amps alone does not tell you whether the fuse will survive a real short circuit without spraying glass, metal vapor, or ceramic fragments.
| Fuse / setup |
Construction mentioned in thread |
Behavior under high fault current |
Practical consequence |
| Ordinary 3.15A glass fuse |
Glass tube, visible fusible wire |
Wire burns, glass cracks, shards can fly |
Eye-injury risk during mains faults |
| Sand-filled fuse |
Filler identified as silica/quartz sand |
Better arc extinction, less surface flashover |
Limits damage duration and fragmentation |
| Large ceramic BM fuse |
Thick ceramic body with sand filling |
Can still burst under very severe faults |
Ceramic fragments can be sharper than glass |
Key insight: Choose a fuse by breaking capacity and construction, not just current rating. In the thread, both small glass fuses and large sand-filled BM links failed violently when the available fault energy was high enough. [#21026715]
Quick Facts
- Littelfuse fuse breaking capacity was quoted in the thread as 35A to 1500A, with one example rated 10000A at 125V but only 40A at 250V. That shows why voltage rating changes real interruption performance. [#21026715]
- A reported capacitor accident involved 0.17F charged to 120V; molten metal from a dropped screwdriver fused into safety goggles. That is a direct PPE case for face and eye protection. [#21026610]
- One workshop-protection idea was a dedicated branch with its own RCD and B16 overcurrent breaker, so a project fault trips only that circuit instead of blacking out the whole room. [#21027306]
- Extreme DC systems in the thread included 48V / 2500A power plants, 2x1200A battery fuses, and a 24-cell OPzS 2500Ah battery set. Even at low voltage, short-circuit energy was described as explosive. [#21053746]
- Large-fault examples included 630A BM fuse links in switchgear and 400–600A BM links in a harbor LV installation; both produced blast, dust, and flying fragments despite sand filling. [#21027394]
Why can a 3.15A glass fuse explode and throw shards during a mains short circuit instead of just melting safely?
A 3.15A glass fuse can explode because the short-circuit current from the mains can be far above what its body can interrupt safely. In the thread video, the wire first burned open, then cracks spread across the glass tube, and finally shards flew outward. Another participant explained that large fault current vaporizes metal and sustains an arc, which heats the tube instead of ending cleanly. The ampere rating only states normal current; it does not guarantee safe interruption under every fault level.
[#21026812]
What is the breaking capacity of a fuse, and how do voltage and fault current affect it in models like Littelfuse fuses?
Breaking capacity is the maximum fault current a fuse can interrupt without the body melting apart or exploding. One thread example cited Littelfuse parts rated from 35A to 1500A, and one model was quoted at 10000A at 125V but only 40A at 250V. That means the same fuse family can tolerate very different fault conditions depending on voltage. Higher voltage makes arc extinction harder, so safe interrupting current can drop sharply.
[#21026715]
How do sand-filled fuses compare with ordinary glass fuses when interrupting high short-circuit currents?
Sand-filled fuses handle high short-circuit currents more safely because the silica or quartz filling helps quench the arc and shorten current flow. A thread participant warned that replacing such a fuse with an ordinary glass type can increase injury risk from flying glass and can also worsen equipment damage because the arc lasts longer. Another reply added that the metal-vapor flashover seen on glass does not occur in a sand-filled fuse. That makes sand-filled designs the safer choice where fault energy is high.
[#21026920]
What causes metal vapor to deposit on the inside of a blown glass fuse, and how can that lead to cracking or flashover over the glass surface?
Metal vapor deposits on the inside of a blown glass fuse when heavy current vaporizes the fusible wire during interruption. One reply explained that this conductive coating can let voltage break down across the inside surface, so current then tracks over the glass itself. That extra surface current heats the tube and can crack it. In the thread, users also reported many failed glass fuses with a clear metal coating inside after the event.
[#21026812]
What is PU equipment in the context of fuse selection and household power supplies?
PU equipment means ordinary household or domestic equipment in this thread’s terminology. A participant explicitly clarified that it stands for common-use equipment and refers to domestic gear and power supplies. In context, the term was used while explaining why some household power supplies use silica-sand fuses instead of ordinary glass ones. That matters because home equipment can still see fault conditions severe enough to justify better arc extinction.
[#21027038]
How do ultrafast aR fuses differ from standard fast-blow fuses in construction and application?
The thread only states that ultrafast aR fuses exist and are distinct from ordinary fast fuses, but it does not describe their internal construction. The author said he had encountered aR fuses and had not been able to verify how their design differed from fast ones. So the only safe conclusion from this discussion is application-level: users recognize them as a separate fuse category used where faster fault interruption matters. The thread provides no verified geometry, filler, or timing data beyond that.
[#21027027]
What safety gear should I use when testing or repairing high-energy circuits so I’m protected from exploding fuses, capacitors, and ceramic fragments?
Use eye protection first, then gloves, and add face and body protection when fault energy is high. The thread includes reports of molten metal embedded in safety goggles, ceramic BM fragments described as sharper than glass, and a user who later refused to open LV switchgear without a mask, gloves, dielectric boots, and protective clothing. One practical rule from the discussion is simple: if the circuit stores or can deliver significant energy, treat blast and shrapnel as credible hazards, not edge cases.
[#21054845]
How should I set up a dedicated workshop circuit with its own RCD and B16 breaker to limit damage when an electronics project fails?
Set up one separate workshop branch so only that branch trips when a project fails. 1. Put the bench outlets on a dedicated circuit. 2. Protect it with its own RCD and a B16 breaker. 3. Keep experimental loads on that branch, not on the room’s general outlets. The thread recommendation aimed to stop a fault from cutting power to the entire room. It is a containment strategy, not a substitute for proper PPE or correct fuse selection.
[#21027306]
What are the main differences between AC and DC fuse testing, especially when it comes to arc extinction and danger level?
DC fuse testing is more dangerous because extinguishing a DC arc is harder once current starts flowing. In the thread, one user proposed an AC-versus-DC comparison specifically to show the arc-extinction problem, and the author replied that DC is “quite a dangerous game” and requires a suitable capacitor bank, a switch that survives several operations, and checks for leftover capacitor voltage. AC naturally passes through zero crossings; DC does not, so the test setup becomes much less forgiving.
[#21027447]
How could I build a simple DC fuse test setup using a bridge rectifier, capacitor, and kettles as a current limiter, and what are the main risks?
One proposed setup was bridge rectifier -> capacitor -> two series kettles as current limiter -> tested fuse in series. The aim was to force current without making a dead short. The author immediately pointed out two main risks: the kettles may have to limit capacitor charging current, and the bridge would need to withstand part of the fault current, so it must be powerful. Residual capacitor voltage after the test is another stated hazard. This idea is experimental and was discussed as risky even before construction details were settled.
[#21027503]
Why can even a large ceramic BM fuse filled with кварtz sand still burst apart under a severe fault current?
A large ceramic BM fuse can still burst because high fault energy can exceed what even a sand-filled body can absorb and interrupt cleanly. The thread describes 630A BM links blown apart in switchgear and 400–600A BM links rupturing so violently that quartz dust filled the room. Sand helps quench the arc, but it does not make the fuse indestructible. As one comment put it, bigger fuses can mean bigger problems because the arc in a “thicker” circuit is also more severe.
[#21054845]
What is a GDT, and why does it reportedly survive DC short-circuit testing better than some other protection components?
“GDT” is a protection component that conducts during an overvoltage event, using a gas discharge path as its key operating element. In the thread, the author said GDTs “stand up well” to DC testing and linked to a demonstration, but he did not provide measured current, voltage, or survival limits in the posts themselves. So the thread supports only a narrow claim: compared with some other tested parts, GDTs were observed to tolerate DC short-circuit-style tests better in that example.
[#21027503]
How dangerous are battery-backed DC systems like 48V telecom power plants, UPS strings, or large VRLA banks during a short circuit?
They are extremely dangerous because low voltage does not mean low energy. The thread cites a 48V, 2500A power plant, 2x1200A battery fuses that did not clear during one event, and a 24-cell OPzS 2500Ah battery string. Users described disappearing metal objects, ringing ears for a week, evaporated cable lugs, and explosive splatter from vaporized metal. Another post warned that DC battery strings also raise shock concerns at higher series voltages, alongside the blast hazard from short circuits.
[#21054522]
Why do some telecom and radio systems use positive ground or connect the + supply to the chassis, and what practical advantages does that have?
The thread says some telecom and radio systems use positive ground because that arrangement is less sensitive to interference and emits less interference. One participant also stated that older vehicles with positive ground corroded less and suffered less electro-erosion at electrical contacts. The same discussion notes that this convention survives in some licensed-band radio equipment, where the positive supply remains bonded to the chassis. The thread does not add measured EMC figures, but it clearly presents noise reduction and corrosion behavior as the practical advantages.
[#21056397]
What protective covers or fuse holders for 5x20 PCB fuses help reduce the risk of shrapnel and accidental contact in electronic devices?
The thread mentions rubber protective sleeves fitted over some 5x20 PCB fuse mounts. One participant noted that these covers reduce the risk of accidental touch and also limit shrapnel if the fuse ruptures. That makes them a simple secondary barrier around a fragile glass fuse. They do not replace correct fuse selection, but they can reduce injury and contamination inside the device when a small fuse fails violently.
[#21037568]
Generated by the language model.
Comments
I once had an explosion of a capacitor straight in the face : / Not to mention the pyrotechnic effects that occurred afterwards, I was setting the rest current in my DIY power amplifier, powered by 1.5... [Read more]
There happened to be an uncontrolled shorting of a set of 0.17F capacitors charged to 120V. The bang and flash is quite impressive, I smoked a cigarette with very trembling hands. Metal droplets from a... [Read more]
There is such a parameter of a fuse as the breaking capacity. It tells you the maximum current it can disconnect without melting or exploding the case. Littelfuse makes fuses with a breaking capacity of... [Read more]
Something like this happened to me. I accidentally applied 220V to the TG2 transistor (green) (I touched the wrong place with the wire). It banged like a shot from a KBKS and a hole a few mm deep appeared... [Read more]
Thanks for the feedback, at high energies indeed many components can explode, not only the fuse, diode or transistor but also mechanical switching elements can wreak considerable havoc. I am puzzled... [Read more]
I in the army accidentally put a buzzer from razor blades on the interphase. The water disappeared together with the cup and the razor blades.The shells looked from the inside just like the inside of the... [Read more]
When a large current is switched off, the metal from the wire vaporises and deposits on the glass, and this causes a voltage breakdown and current flow over the glass. This is likely to cause heating and... [Read more]
Let me add one more comment to the last words of the Author's statement in the video - There is a reason why silica sand fuses are used in some PU equipment power supplies. When replacing it with an ordinary... [Read more]
What is PU equipment? Sometimes I have also come across ultrafast (aR) fuses, but how their design differed from fast ones I have not been able to check. [Read more]
Equipment P common U use -. means domestic... ;) (I didn't make that up). [Read more]
Eye protection when running anything with higher power is an absolute must. Plus a dedicated circuit in the workshop with its own differential and a B16 overcurrent, so that in the event of a malfunction,... [Read more]
@techekspert What if you did a similar demonstration but for DC? And compared? This would show experimentally the differences between AC and DC and what the problem with DC arc extinguishing is. By the... [Read more]
I had a neon light explode because I didn't give a resistor and connected directly to 230V. It just exploded! The glass shattered into tiny pieces and it's a miracle it didn't hit my eye. Now I wear corrective... [Read more]
I was thinking of DC testing however I don't have a suitable capacitor bank, DC is quite a dangerous game, you would also need a switch that can withstand a few of these switching on and to check what... [Read more]
I was thinking of a simple circuit; Bridge->capacitor->two series connected kettles in the role of current limiter (and forcing it)->serial tested patient (fuse). [Read more]
The kettle would have to limit the capacitor charging current? If the bridge were to participate in the short-circuit current then it would have to be powerful. I see that the GDTs stand up well to... [Read more]
No, although it can be done that way. I was referring to forcing the current to flow but so that it would not be a short circuit. [Read more]
a colleague was replacing a live switchboard (because the ZE was only due to arrive in a week's time to disconnect the transformer substation) and a pair of pliers slipped out of his hand while he was... [Read more]
I wasn't doing any electrical work in this case, I was setting up scaffolding for painting and installing cable trays for the new RG lighting installation. There was a mains socket in one cell in the distribution... [Read more]