Designation as HRC Fuse and Selection

[rstor22]

Would all ceramic fuses be considered an HRC fuse? To my understanding, the ability of a fuse to with withstand a high current fault without exploding/arcing can be considered HRC. If this is the case, would this high current fault be the fuses interrupt rating (aka breaking capacity) ? If so, what is the interrupt rating value for such a fuse to be considered HRC (i.e. 1.5kA? 10kA?) ?

Let's say you have a 50mA ceramic fuse (Little fuse mfg part# 216.050) connected to residential mains 120V (Canada) with about 8mA passing through. Would this specific fuse be safe to use under these conditions?

The interrupt rating of this specific fuse is 1.5kA at 250VAC. Under short circuit conditions, would the rating of 1.5kA be exceeded? I would think that the main house breaker would trip at 15A however there may be a short amount of time that this current would be exceeded, what would this value be?

[Cliff Matthews]

All ceramic fuses are not High Rupture and massive quantities of heartless mfg's will fake anything.. and breakers are slow blow when compared to certified HRC fuses. Joe Smith has a playlist of fuse entertainment "Fun with Fuses" (as always, mixed in with factoids everywhere).
https://www.youtube.com/playlist?list=PLZSS2ajxhiQAk2Q57Obdy-2yVNF-m_huu

[rstor22]

Thank you Cliff, I watched some of the videos by Joe and was surprised how the glass fuses held up when connected directly across the mains.

I emailed Littelfuse with regards to some of my questions, below are their responses in red:

Would all ceramic fuses be considered a HRC fuse? No. To my understanding, the ability of a fuse to with withstand a high current fault without exploding/arcing can be considered HRC. No. A fuse operating safely within its interrupting capability is not necessarily considered to be HRC. If this is the case, would this high current fault be the fuses interrupt rating (aka breaking capacity) ? If so, what is the interrupt rating value for such a fuse to be considered HRC (i.e. 1.5kA? 10kA?) ? A High Rupture Capacity fuse is defined by the specification that was used for its design. For European IEC 60127 miniature fuses, HRC is defined as 1500A at 250VAC. This is the interrupt rating of the fuse.

As an electronics hobbyist sometimes I need to select a fuse that will be connected to the mains (120V AC). As part of the decision on selecting the fuse how do I decide on the correct interrupt rating. Some of my Little Fuses that I have specify an interrupt rating of 1.5kA at 250VAC. From my understanding the interrupt rating of the fuse should be higher than the prospective short circuit current:https://en.wikipedia.org/wiki/Prospective_short-circuit_current Yes, that is correct. If the short circuit current exceeds the fuse AIC (AmpInterrupting Capacity) rating, the fuse body might crack or shatter.

In this case I am assuming that my fuses interrupt rating needs to be rated higher than the prospective short circuit current that could occur at my residance before the circuit breaker trips? That is true, but residential wiring, switches, plugs, etc.,  normally has substantial resistance that will limit available short circuit current. If this is the case, would you know what the prospective short circuit current is for the general house in the United States? Not really sure. Much depends upon the physical location of the delivery point.

Follow up email:

I always thought only ceramic fuses could be HRC. Technically, any material can be used but ceramic is the usual material chosen. Fiberglass and Melamine also used. Based on the information you provided I understand now that HRC  is "defined by the specification that was used for its design"

If I understand correctly, if there is a glass miniature fuse with an interrupt rating of 1500A at 250VAC then it too would be defined as an HRC fuse per IEC 60127 ? Yes, but there is no glass that can withstand the abuse so ceramic is normally used with a quartz sand filling

[rstor22]

Based on the above, how does the hobbyist decide upon choosing a ceramic vs glass fuse for their application (e.g. building something for personal use / not for sale) ? In one of my earlier posts where I was trying to determine the proper fuse to use for a power supply I was building nctnico pointed out that fuses “… are intended to protect against fire by being the weakest link instead of wires catching fire”. With this in mind, I want to take into account the worse case situation of a mains short.

In this case I would need to consider the prospective short circuit current of my residence to determine the appropriate interrupting current for my fuse. Does anyone have a general ball park of what this would be? My circuit breakers are listed at having an interrupting current of 10kA so I am assuming that my fuse doesn’t need to be rated higher than this.

For one of my projects I used the following IEC connector with built in fuse holder, such as this:

http://www.ampmaker.com/store/Integrated-IEC-and-fuseholder.html

These hold 5x20 fuses and searching on Digikey the highest interrupt rating for ceramic 5x20 fuses that I could find was around 1.5kA @ 250V. There are glass varieties available for 10kA.

What should I choose in this case? Glass with higher interrupt rating or ceramic with lower interrupt rating or something else? Does that fact that the fuse is contained within the IEC connector have any bearing on the decision? Am I going about this the correct way?

[IanB]

You might consider the specifications and potential use for your applications of British Standard BS1362 plug fuses. These 1/4 inch x 1 inch (6.3 mm x 25 mm) fuses are found in every mains plug in the UK in values between 3 A and 13 A, are designed to interrupt short circuits in domestic 240 V mains supplies, and are manufactured in such large quantity that they cost only a few cents each. I think you would have no worries about them having a suitable interrupt rating.

Here's a typical data sheet:

http://www.cooperindustries.com/content/dam/public/bussmann/Electrical/Resources/product-datasheets-a/Bus_Ele_DS_2042_TDC180.pdf

[rstor22]

You might consider the specifications and potential use for your applications of British Standard BS1362 plug fuses. These 1/4 inch x 1 inch (6.3 mm x 25 mm) fuses are found in every mains plug in the UK in values between 3 A and 13 A, are designed to interrupt short circuits in domestic 240 V mains supplies, and are manufactured in such large quantity that they cost only a few cents each. I think you would have no worries about them having a suitable interrupt rating.

Here's a typical data sheet:

http://www.cooperindustries.com/content/dam/public/bussmann/Electrical/Resources/product-datasheets-a/Bus_Ele_DS_2042_TDC180.pdf

Looking at the above datasheet it mentions an interrupting current of 6kA. I'm still stuck with the question of what is a sufficient interrupting current for residential mains? 1.5kA, 6kA, 10kA, more?

Also, the following IEC connectors with integrated fuse holders such as these: http://www.qualtekusa.com/Catalog/AC_Receptacles/pdfs/719w0003.pdf fit 5x20mm fuses. I am assuming such fuses integrated into the mains IEC connector are also meant to interrupt short circuits up to mains voltage. That is why they are designed into the IEC connector (right?) If this is the case, the question I am still pondering is what fuse is the right one to use in this case? Please correct me if I am wrong, but I would think that there would be some 5x20mm fuses that are available to protect against mains short circuit conditions?

[jmelson]

Your residential panel breakers have a 10,000 A interrupting rating.  That means they will successfully interrupt current up to 10K Amps without exploding.  They may vent flames when tested to this level.

There's no way an 18 Gauge zip cord can deliver more than 1500 A to a faulted little gadget.  So, a 1500 AIC fuse should certainly be OK.  Now, if you have something beefier, such as a ham transmitter powered through a #8 Gauge power cord off 240 V, it is possible there could be more than 1500 A into a short, so a beefier fuse might be warranted.

Jon

[IanB]

Looking at the above datasheet it mentions an interrupting current of 6kA. I'm still stuck with the question of what is a sufficient interrupting current for residential mains?

But as I explained above, these fuses are specified by a national standard for use with the residential mains, and are found protecting every single mains appliance in the UK, without exception. Every UK appliance plug has one of these fuses in it. There are millions upon billions of them in current use on the residential mains in the UK, usually on the end of a 240 V / 32 A circuit. (And if as an experiment you short them out across the mains supply they do not explode.)

If you are unable to take that as an answer to your question, I am not sure what would satisfy you.

[richard.cs]

A few  points to note:
1) Whilst uncommon you can buy the UK BS1362 fuses in ratings lower than 3 A, some suppliers do 1 A and 2 A still with a 6 kA breaking capacity.
2) In the UK fault currents are typically higher that most other countries because of the combination of 240 V supplies, large single-phase supplies for domestic use (100 A rather than 3x 40 A as might be typical in much of Europe), and an LV network based around few large transformers. Typical fault currents at the origin of the installation range from 1 kA to 16 kA, my house measures about 2.5 kA. Canada follows US practice with many small transformers so I suspect your fault currents even phase to phase are considerably lower. The fault current available L-N at the end of a 120 V branch circuit, hundreds of milliohms past the supply origin will be much lower, almost certainly under 1000 A for a 15 A circuit and possibly as little as 100 A.
3) Generally it is safe to have a lower breaking capacity fuse backed up by another with higher breaking capacity. In the rare event our 6 kA BS1362 fuses or similarly (6-10 kA usually) rated MCBs fail to break correctly there is a much higher rated fuse that will blow and cut the power (BS88 and similar) so fire and explosion are prevented.
4) For normal domestic use at the end of branch circuits, especially with only 120 V, I would expect 1.5 kA fuses to break perfectly on a dead short because the supply impedance (the sum of the appliance cord, the house wiring, the cable to the transformer, and the transformer itself*) will add up to way more than the 100 mOhm needed to limit the current to under 1.5 kA. 20x5 mm ceramic HRC fuses should be fine here.

*Technically this also includes the transformed-down impedance of the upstream high voltage network but no-one ever bothers because it is transformed as the square of the turns ratio so ends up negligable.

[rstor22]

IanB > My confusion was that since these IEC connectors exist which accept 5x20mm fuses I figured there should be fuses in that form factor available with an adequate interrupting current rating to handle a mains short circuit (and I didn't see many 5x20mm ceramic rated above 1.5kA).

I should rephrase my earlier statement as 6kA would be sufficient per your earlier post -- I was trying to find out if a lower rating would be fine. 

Based on the replies it looks like I'm good with using a fuse with a 1.5kA interrupt rating.

richard.cs > when you indicated that at your installation for your house you measure about 2.5kA is that value obtained using a device such as this? :
http://www.fluke.com/fluke/sgen/installation-testers/fluke-1652c-multifunction-installation-tester.htm?pid=72320

If someone wanted to measure their prospective short circuit current at an outlet would they use such a device linked above?

3) Generally it is safe to have a lower breaking capacity fuse backed up by another with higher breaking capacity. In the rare event our 6 kA BS1362 fuses or similarly (6-10 kA usually) rated MCBs fail to break correctly there is a much higher rated fuse that will blow and cut the power (BS88 and similar) so fire and explosion are prevented.

In the UK is the fuse with the higher breaking capacity, such as the BS88 you described located in the breaker box?

4) For normal domestic use at the end of branch circuits, especially with only 120 V, I would expect 1.5 kA fuses to break perfectly on a dead short because the supply impedance (the sum of the appliance cord, the house wiring, the cable to the transformer, and the transformer itself*) will add up to way more than the 100 mOhm needed to limit the current to under 1.5 kA. 20x5 mm ceramic HRC fuses should be fine here.

In multimeter reviews I always heard how ceramic fuses were critical so anytime I needed a fuse I would go for the ceramic over glass. Reading online I find that ceramic has advantages over glass in terms handling fault conditions. As someone from Littelfuse mentioned that "A High Rupture Capacity fuse is defined by the specification that was used for its design. For European IEC 60127 miniature fuses, HRC is defined as 1500A at 250VAC. This is the interrupt rating of the fuse." does this mean that if I select a glass fuse that has a high enough interrupting current it is also fine (in this case to protect against mains short circuit)? I've seen 5x20 glass fuses rated at 10kA for example. Based on the above definition I would understand that such a glass fuse would also be classified as HRC. Would ceramic be called for in cases were a higher interrupting current rating is required at even higher voltages?

[richard.cs]

Based on the replies it looks like I'm good with using a fuse with a 1.5kA interrupt rating.

I would agree, for applications powered from normal outlets. This is good because it means the people who chose 1.5 kA as the HRC specification knew what they were doing :-)
For reference on normal outlets around my house the prospective short circuit current is much lower than at the breaker box. I think the highest was about 1.2 kA (shortest cable) and the lowest was maybe 450 A.

richard.cs > when you indicated that at your installation for your house you measure about 2.5kA is that value obtained using a device such as this? :
http://www.fluke.com/fluke/sgen/installation-testers/fluke-1652c-multifunction-installation-tester.htm?pid=72320

That device does the measurement in question in the "PFC / PSC" mode. The loop impedance mode is essentially the same it just displays the same information as impedance rather than current. Meters that only do PFC are availiable and cheaper, mine is more like this: https://instrotech.com/robin-kmp4116dl.html?gclid=EAIaIQobChMIkb7Xj9yg2gIVYrftCh3SzQQiEAQYASABEgJgifD_BwE
The test essentially puts a moderately large load on the mains (perhaps 10 Ohms) and measures the voltage drop caused. Some modern ones have a "no trip" mode that uses various techniques to measure the live-earth loop without tripping an RCD, those modes often give unreliable readings.

In the UK is the fuse with the higher breaking capacity, such as the BS88 you described located in the breaker box?

In the UK it is commonly part of the electricity suppliers equipment, before the breaker box. See this photo of a typical UK domestic install:

In this photo the supply cable is the black one coming in from bottom centre, the grey enclosure contains a fuse (BS88, BC1361 type 2, or similar) of 60 to 100 A rating and a very high breaking capacity.

In multimeter reviews I always heard how ceramic fuses were critical so anytime I needed a fuse I would go for the ceramic over glass. Reading online I find that ceramic has advantages over glass in terms handling fault conditions. As someone from Littelfuse mentioned that "A High Rupture Capacity fuse is defined by the specification that was used for its design. For European IEC 60127 miniature fuses, HRC is defined as 1500A at 250VAC. This is the interrupt rating of the fuse." does this mean that if I select a glass fuse that has a high enough interrupting current it is also fine (in this case to protect against mains short circuit)? I've seen 5x20 glass fuses rated at 10kA for example. Based on the above definition I would understand that such a glass fuse would also be classified as HRC. Would ceramic be called for in cases were a higher interrupting current rating is required at even higher voltages?

If the required interrupt rating is met by a glass fuse (or any other type) then it should be safe to use. It is however uncommon to find glass fuses with high interrupt ratings - I don't think I've ever seen one as high as 1 kA let alone 10 kA, but if there is one that meets the HRC spec then it would be fine to use.