# disconnect fire



## MDShunk (Jan 7, 2007)

Chargers are funny. They don't necessarily draw anything near nameplate during all points of the charging cycle. I deal with 200amp chargers pretty often, but they only draw 200 amps for about 10 seconds then quickly drop over a few minutes and settle in around 50. Chargers all have some wild and crazy charging routines programmed for battery type and who knows what all else. They don't draw max all the time, we'll just say. 

Disconnects don't really burn up from being overloaded if the fuses are not oversized. Your fuses were, technically, a little undersized. That's not what caused the failure. Loose fuse clips, loose lugs, loose wipers, etc. That's what makes disconnects burn up that otherwise have properly sized (or even undersized) fuses in them.

The guy that tied in the new charger did nothing wrong, in my opinion, that contributed to the disconnect failure. It died a natural death. There's an outside chance that there was a defective fuse, but it is extremely rare for a fuse to destructively fail unless there's a lightning strike associated with the failure. Don't get too shook up over a failed disconnect. It's a pretty normal thing, regrettably. Next time you have an IR guy in to scan the heavy gear, it's worth plugging in a bunch of lifts, popping open the lift charger disco's, and have them peeked at. I'd bet money there are others not far behind. 

EDIT: Just out of curiosity, what was the fuse type? They look like bog standard RK5's, probably, but I was curious to know if maybe they happen to be some crazy boutique fuses somebody had kicking around and slapped them in based on amp rating alone or maybe even had 250 volt rated fuses in a 480 disconnect.


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## allpr0h (Aug 10, 2017)

thanks for reply

the electrician have said that everything in the disconnect was properly tightened nothing was loose.. i also think that the cause is probably more related to something loose than under rated fuses..

im still swiming in a mist of non understanding.


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## MDShunk (Jan 7, 2007)

allpr0h said:


> thanks for reply
> 
> the electrician have said that everything in the disconnect was properly tightened nothing was loose.. i also think that the cause is probably more related to something loose than under rated fuses..
> 
> im still swiming in a mist of non understanding.


after a failure you can't properly diagnose what was or wasn't loose and causing the heating. Fuse clips and disconnect blades are popular culprits. Well, it could be pretty much anything in the disco, but the least likely of them is the fuse itself. :wink:


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## MDShunk (Jan 7, 2007)

You may still be left with the question: How does it get hot enough to melt plastic and make smoke, but not blow the fuse.

Easy. Two main ways:
1) The heating was on the line side of the fuses, like at the disconnect blades, line side lug, or line side fuse clip.
or
2) The heating was caused by current draw at less than the fuse's rating. You can do a lot of destruction with a current draw of less than 15 amps. Throw a fist full of napkins in your toaster if you don't believe that and see what happens.


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## allpr0h (Aug 10, 2017)

thanks guys you must be right

what do you mean "The heating was caused by current draw at less than the fuse's rating" drawing 10amps on a 15amp fuse will generate heat?

as for the fuse they were trs15r 600vac now replaced with trs20r 600vac

you are correct about the IR, the others must be checked


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## MDShunk (Jan 7, 2007)

allpr0h said:


> what do you mean "The heating was caused by current draw at less than the fuse's rating" drawing 10amps on a 15amp fuse will generate heat?


You best believe it, if there's a resistive connection (a fancy word for "something's loose"). In fact, if 10 amps at 480 volts was nothing but heat, it would be 4,800 watts. I think I could melt down that fuse block with 150 watts of heat.


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## allpr0h (Aug 10, 2017)

ok thanks for info


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## splatz (May 23, 2015)

How about a fall of potential test across the terminals to look for bad connections?


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## allpr0h (Aug 10, 2017)

ill have a discussion with the electrician about it
i cant eat that the problem was the under rated fuses, its the official version according to him
i believe hes wrong, not gonna start a fight with him but gonna try to have more infos next week
i dont know where he put the old disconnect.. id like to see it
deffective cutter blades makes sens to me, even tho the disconnect was only 1 year old


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## paulengr (Oct 8, 2017)

Fuses are an electrochemical reaction. They have spots where the electrical path is thin so it heats up to the point that it melts. There is more to it but the arcing and heating is contained 100% within the fuse in low voltage fuses. Fuse ratings are typically at least 20kA or more so it is simply not going to damage anything by itself. An “underrated” fuse simply trips sooner like on rated current.

Your disconnect had some kind of problem where it was loose or an extreme overvoltage where it raced over and blew apart. Normally connection resistances are under 50 microohms and contact resistance is under 1 milliohm. Heat is proportional to the square of current and linear with resistance. So going from 15 to 18 A is a 20% increase in current but a 44% increase in heat. Typically loose connections when they reach failure are exponentially bad so loose connection heating goes up by a factor of 10000% or more. That’s why IR scans work so well. Typically on a given IR PM I’d say half the issues we find are loose fuse clips. A little conductive grease, cleaning corrosion off, or clip replacement fixes it. Sometimes you can just turn the fuses a little to scrape the oxides off.

So no surprise here. Happens more often than most people know unless you do a lot of failure investigations (hundreds).


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## MotoGP1199 (Aug 11, 2014)

Your co-worker is an idiot. A smaller fuse, if anything would have helped the situation. Unfortunately a 15 amp fuse is still large enough to allow a resistive load to create enough heat to melt conductors, fuse block, etc. Anyone who works with motors will tell you they have seen many times wires/disconnects melted before a fuse will blow. The disconnect was either damaged or had a loose connection. The increase in size of the new charger just happened to draw enough current to expose this.


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## allpr0h (Aug 10, 2017)

now i understand everything
thanks all very much for the help


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## paulengr (Oct 8, 2017)

allpr0h said:


> now i understand everything
> thanks all very much for the help













Here’s one in the making that did not trip a fancy GE Multilin relay or the fuses. The bolt wasn’t tight enough on the left lug and the heat was causing discoloration and corrosion. The insulation was burned up about a foot. This is 350 MCM on a 4160, 1500 HP motor inside the peckerhead after I stripped away the boot and all the tape on it. Left alone it would eventually burn up completely. The cable is unshielded 5 kV so probably 1990s vintage. Motor was removed from service after 28 years. This is a wound rotor motor for a chip mill. Here is the motor. It’s a wound rotor. I think this one originally came off a battleship so this will probably be the fourth rewind.










If I could find it I’d post photos of another similar size one where the old Model 3 Square D starter lost a disconnect but I think probably from no lubrication. It failed while under load powering a similar size motor. It had a four foot black streak on the floor and melted and sooted up everything around it. Blew the poles fuses for both the plant and utility but not before it burned up everything.

So...this kind of stuff happens all the time. These are just pictures off my phone from the last month.


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## allpr0h (Aug 10, 2017)

wow, impressive device
yes ok i understand


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## glen1971 (Oct 10, 2012)

paulengr said:


> Here’s one in the making that did not trip a fancy GE Multilin relay or the fuses. The bolt wasn’t tight enough on the left lug and the heat was causing discoloration and corrosion. The insulation was burned up about a foot. This is 350 MCM on a 4160, 1500 HP motor inside the peckerhead after I stripped away the boot and all the tape on it. Left alone it would eventually burn up completely. *The cable is unshielded 5 kV so probably 1990s vintage.* Motor was removed from service after 28 years. This is a wound rotor motor for a chip mill.


Curious how unshielded cable would date it to the 1990's? I've done lots of recent installations with unshielded cable. Or is there other documentation supporting it?


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## paulengr (Oct 8, 2017)

Shielded medium voltage cables became NEC mandated in 2002 or 2005 edition at 2 kV and above. Previously from an engineering perspective was required at 8 kv for all installations and some 5 kv but not at 2.5 kv or below (2300 V systems or 4160 solidly grounded which is 2400 V phase to neutral). So that kind of dates your MV cables. Manufacturers voluntarily pulled all the unshielded stuff off the market except Exar. You might find somewhere somebody did an install later from a stockpile of unshielded cable but the manufacturing date will be before that and the unshielded stuff was so much cheaper then it very quickly got used up. The other key date is around the early 1970s when PILC all but disappeared for good, and the early 2000s when we transitioned to electrical tree resistant XLPE.

You can still get up to 15-25 kv labelled as temporary hookup wire. This is supposed to be for emergency repairs and such for utilities only which is outside NEC. The most popular brand is Exar colored red. If you look at any substation the internals are full of it by necessity (they can't get anything else either). Some mining cables under NEMA specs that are CSA (OSHA accepted) listed as type W or R as well as DLO. Technically DLO for instance is 2 kV and interpretatiob of whether the rule is above or equal to 2 kv is subjective. Some are rated as "2 kv" but the fine print says 2.5 kv so the no man's land that has always existed at 2400 V is still confusing at best. Exar is way overpriced and there are lots of knockoffs. In a substation of course it's sold as an assembly so theoretically again NEC does not apply and UL medium voltage rules are a joke when it comes to Listed assemblies.

The big problem is for instance if the cable in the photos was too far gone then I'd have a big problem. It runs in conduit so shielded cable won't fit if I try to pull new and where it runs I won't be able to pull bigger conduit. So either I install an oversized box or a junction box and pull new from there to the motor, or pull all the conduit out and run say three conductor tape shielded MV-105 as long as bend radius works and terminate with tape terminations (half the length of cold shrink) at the ends, or ignore Code and use temporary hookup wire because it's the same stuff just no longer Code. No good options thanks to lawyers and engineers screwing with NEC.

Three other items to note. Prior to the 1970s and especially in the early 1980s when the Toshiba vacuum contactors came out, 2400 V was used even though it was an "off" standard voltage due to availability of 5 kv equipment. Once the DHP breakers and Toshiba contactors and similar stuff came out, 2400 V was all but dropped for good reason.

However in tight spaces type W unshielded running at 2400 V carries more power than the "smaller" 4160 V equivalent so I know of one mining application where they run 4000/2300 V motors at 2300 V for that reason. And GE notoriously grossly undersized peckerheads, wire cavities, and the internal bends so that resorting to DLO is the only viable way to terminate it short of rebuilding everything.

Knowing a little history helps tremendously when working on older plants when it comes to medium voltage. Finding a date helps me know what modetn wiring methods and equipment will work and what won't.

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## glen1971 (Oct 10, 2012)

paulengr said:


> *Shielded medium voltage cables became NEC mandated in 2002 or 2005 edition at 2 kV and above.* Previously from an engineering perspective was required at 8 kv for all installations and some 5 kv but not at 2.5 kv or below (2300 V systems or 4160 solidly grounded which is 2400 V phase to neutral). So that kind of dates your MV cables. Manufacturers voluntarily pulled all the unshielded stuff off the market except Exar. You might find somewhere somebody did an install later from a stockpile of unshielded cable but the manufacturing date will be before that and the unshielded stuff was so much cheaper then it very quickly got used up. The other key date is around the early 1970s when PILC all but disappeared for good, and the early 2000s when we transitioned to electrical tree resistant XLPE.
> 
> You can still get up to 15-25 kv labelled as temporary hookup wire. This is supposed to be for emergency repairs and such for utilities only which is outside NEC. The most popular brand is Exar colored red. If you look at any substation the internals are full of it by necessity (they can't get anything else either). Some mining cables under NEMA specs that are CSA (OSHA accepted) listed as type W or R as well as DLO. Technically DLO for instance is 2 kV and interpretatiob of whether the rule is above or equal to 2 kv is subjective. Some are rated as "2 kv" but the fine print says 2.5 kv so the no man's land that has always existed at 2400 V is still confusing at best. Exar is way overpriced and there are lots of knockoffs. In a substation of course it's sold as an assembly so theoretically again NEC does not apply and UL medium voltage rules are a joke when it comes to Listed assemblies.
> 
> ...


Thanks for the info.. I didn't know that the NEC has that written in.. The CEC doesn't and I guess that's why we can still get unshielded cable..
https://www.texcan.com/teck-cable-3-0-awg-5kv-3c-unshielded


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## paulengr (Oct 8, 2017)

glen1971 said:


> Thanks for the info.. I didn't know that the NEC has that written in.. The CEC doesn't and I guess that's why we can still get unshielded cable..
> https://www.texcan.com/teck-cable-3-0-awg-5kv-3c-unshielded


Maybe that's why most of the unshielded solutions are CSA approved but not UL or maybe it's just that ridiculous UL snooty attitude that refuses to recognize standards or even testing by others whereas Canadian practical attitude shines through in CSA.

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## Wiresmith (Feb 9, 2013)

paulengr said:


> Here’s one in the making that did not trip a fancy GE Multilin relay or the fuses. The bolt wasn’t tight enough on the left lug and the heat was causing discoloration and corrosion.



i recommend Nord-Lock washers for extra insurance when you want it, they are ridiculously more reliable than even any other style of fastener lock


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## paulengr (Oct 8, 2017)

Wiresmith said:


> i recommend Nord-Lock washers for extra insurance when you want it, they are ridiculously more reliable than even any other style of fastener lock




No.

https://ntrs.nasa.gov/search.jsp?R=19900009424

No matter the design lock washers don’t work. Split washers are even worse...testing proves they actually cause looseness. They are supposed to take up slack from expansion but they don’t work in practice. Belleville washers help significantly since it’s a true spring. Lock nuts, safety wiring, and chemical (loc tite) works as far as “locking” (increased friction) is concerned. The lock nuts all use some kind of deformed thread. Nylocks are cheap and the most popular.

But the major issue isn’t self loosening. This is caused by the Jost effect when it happens and only happens under severe vibration with shearing motion, something that doesn’t happen if the bolt fits the hole properly since it is the shear or sideways motion that’s the issue. You can google that one too.

But the majority of the problem is we don’t get it tight.

http://digitaleditions.walsworthprintgroup.com/publication/index.php?i=40316&m=&l=&p=12&pre=&ver=swf

So basically we suck at gauging tightness.

Thermal expansion is a real issue. Belleville washers are standard and recommended in bus bar connections.

Torque wrenches are slowly taking over the mechanical world. Never Seez and loctite help lubricate the joint so you get more consistent torque. It doesn’t affect electrical properties or increase torque but it does make torque more consistent. That’s the best you can do today.

I’ve tried finding torque indicating washers fir electrical use but they all seem to be strictly for structural use only. Those give the most consistent torque of all since instead of relying on thread friction they actually measure the joint tension.

There is also an ultrasound system that measures the bolt stretch. The trouble is it can’t handle really small fasteners and the probe won’t fit in tight spaces like peckerheads. So it’s good on lots of mechanical power transmission equipment but not most electrical jobs.


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## Wiresmith (Feb 9, 2013)

paulengr said:


> No.
> 
> https://ntrs.nasa.gov/search.jsp?R=19900009424
> 
> ...


does that talk about nord lock?






you may be interested in watching the testing in this video


torquing a fastener with a torque wrench isn't very accurate either for critical joints


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## MechanicalDVR (Dec 29, 2007)

allpr0h said:


> *ill have a discussion with the electrician about it*
> i cant eat that the problem was the under rated fuses, its the official version according to him
> i believe hes wrong, not gonna start a fight with him but gonna try to have more infos next week
> i dont know where he put the old disconnect.. id like to see it
> deffective cutter blades makes sens to me, even tho the disconnect was only 1 year old


What do you do there if not electrical work?


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## MDShunk (Jan 7, 2007)

MechanicalDVR said:


> What do you do there if not electrical work?


I think he's gone, but my assumption was that he was either the boss or one of the regular wrenches trying to learn something.


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## MechanicalDVR (Dec 29, 2007)

MDShunk said:


> I think he's gone, but my assumption was that he was either the boss or one of the regular wrenches trying to learn something.


ElectricianTalk.com is designed to be a pros only community serving professional electricians, apprentices, and related professional electrical fields. Anyone who works as a professional electrician, apprentice or in a related professional electrical field is welcome to join the site and participate in our forums.

In addition to professional electricians we also allow Manufacturers, Vendors, and Service Providers who work with electricians or in the electrical industry, on a full time basis, to join the site and join in the conversation. We do however require that they follow our Advertising Rules when doing so. Failure to do so will mean that your account may be banned from the site.


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## MDShunk (Jan 7, 2007)

Oh, stop.


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## MechanicalDVR (Dec 29, 2007)

MDShunk said:


> Oh, stop.


LOL, not my rules as I didn't write them.

But I believe in going by them.


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## MDShunk (Jan 7, 2007)

MechanicalDVR said:


> LOL, not my rules as I didn't write them.
> 
> But I believe in going by them.


What's your badge number, officer? :wink:


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## MechanicalDVR (Dec 29, 2007)

mdshunk said:


> what's your badge number, officer? :wink:


1034


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## allpr0h (Aug 10, 2017)

i am electromechanics
i work on machines and add up my electrical hours to one day do the exam to get my industrial electrician certification

i might wonder one more thing, does a fuse generate heat?
i mean i went measure the charger, charging a dead battery it pulls 17 amps at the disconnect.

the fuses were 15amp, could it be possible the fuses at 15amp not blow and heat because they let 17amp through? would the 15amp fuse generate heat at 14amps lets say?
would a 15amp


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## allpr0h (Aug 10, 2017)

please not consider last words "would a 15 amp".. wish i found how to edit the post


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## MikeFL (Apr 16, 2016)

Anything with current going through it and resistance will have heat. So yes, it's normal to be warm(er than ambient) with a load applied. 

That 15A fuse should sustain a 15A load, not open at 15A. It may open around 20A or more. In a best case scenario you'll never know!


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## allpr0h (Aug 10, 2017)

what I wonder is if a 15A fuse generate enough heat at 17A to be a fire hazard, melt the plastic holder?


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## MikeFL (Apr 16, 2016)

allpr0h said:


> what I wonder is if a 15A fuse generate enough heat at 17A to be a fire hazard, melt the plastic holder?


Not unless its a defective fuse. A fuse's job is to prevent fires. That's what it's for.

Do you have an IR thermometer? How hot is it?

Is there melting plastic? If so, then it's too hot. Maybe you should be on a 20A circuit? I haven't read this entire thread. I just found the thread when I opened the question you asked which I first responded to.

And now I have to drive to Miami, Ft. Lauderdale, Palm Beach and Jupiter. So I'm outta here! I'm sure someone else will come along. What is this load?


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## splatz (May 23, 2015)

A fuse has very little resistance so it normally should generate very little heat. 

Good connections have very little resistance, barely more than the conductors, so they generate very little heat. 

Bad connections have more resistance, so they do generate more heat. 

A picture is worth a thousand words, an infrared camera lets you take pictures of heat, so take a look at some of the pictures in these articles 

http://www.fluke.com/fluke/sgen/community/fluke-news-plus/articlecategories/ti/iradvantage 










https://www.irinfo.org/05-01-2011-brady/


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## allpr0h (Aug 10, 2017)

thanks again for replies it helps

ill ask for a thermal imaging fluke device, well see what they say

yes ok, either something loose or deffective fuse


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## paulengr (Oct 8, 2017)

Fluke IR sucks. It's a cheap low end one. Get a Seek if you want that. FLIR is the big name in IR.

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## paulengr (Oct 8, 2017)

splatz said:


> A fuse has very little resistance so it normally should generate very little heat.
> 
> Good connections have very little resistance, barely more than the conductors, so they generate very little heat.
> 
> ...


Funny the picture shows a blown fuse or two overheated fuse holders!

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## allpr0h (Aug 10, 2017)

yea exactly awesome thanks for the link picture is exactly what i was missing
it does overheat
fuse 15A used at 17A is higher than 100% of capacity therefore it will overheat...
how much is it able to take it? is what happened, the fuseholder melting, a consequence of a fuse used over its capacity 1.13 times or is it because the fuse was faulty?

like a good fuse can take 150% no fire?
i don't know but ok yea the under sized fuses are culprit


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## paulengr (Oct 8, 2017)

allpr0h said:


> yea exactly awesome thanks for the link picture is exactly what i was missing
> it does overheat
> fuse 15A used at 17A is higher than 100% of capacity therefore it will overheat...
> how much is it able to take it? is what happened, the fuseholder melting, a consequence of a fuse used over its capacity 1.13 times or is it because the fuse was faulty?
> ...




No to most of your questions. A fuse works SAFELY and more reliably than any circuit breaker with two exceptions. Either you overvoltage it so it arcs around or through it or you exceed its trip
rating which is typically 100,000 A or something very high like that. For the most part fuses work by a constrictive force. When you have a narrow bus bar, the resistance is low below a certain current. Once you exceed it, the resistance goes exponential, the copper melts, and mixes with sand. The sand and copper melting absorb heat harmlessly disconnecting the circuit. This happens at multiple spots in the fuse simultaneously and melting is endothermic (absorbs heat). If it wasn’t the fuse body would burst which is what you get with expulsion fuses used in outdoor cutouts.

A given fuse package can be from say 10 to 150 A. Within that range all the fuses are exactly the same size but the busbar strips are cut with different thicknesses to create different ratings. The overall fuse package is typically rated fir say 35,009 A. So if you hit it with say 50,000 A the fuse would still interrupt but it would explode and the fuse holder would be damaged, too. This is way different from say a 15 vs. 20 A rating. The short circuit rating is an engineering calculation but you can easily estimate a worst case number with just a transformer name plate and the infinite bus assumption. Take kva x 1000 divided by secondary voltage. Divide by 1.732 if it’s 3 phase. Divide by the impedance (%Z) as a fraction so if it’s say 3%, divide by 0.03. This gives short circuit current if there is infinite available current at the primary and the transformer dead shorts across the terminals. Real short circuit current will be a lot less but it’s easy to calculate and something engineers use to decide whether or not the complicated math is even needed.

Finally it’s an electro-thermal-chemical reaction. Fuses typically half +/-10-15% accuracy. That exceeds your math by itself. A 15 A fuse will never trip at 15 A. It will eventually trip at around say 15.5 A maybe in hours. It will trip in several minutes at say 20 A. But at say 500 A (dead short) it trips in 4 milliseconds where the fastest breakers are 15-20.

This is why you are way off base...you are getting worked up about numbers like 15 vs. 17 A but the real concern needs thousands of amps.

For years Bussmann publishes really good handbooks on how fuses work and explaining how to use them in easy terms.

http://www.cooperindustries.com/con...cal-literature/bus-ele-br-10757-fuseology.pdf


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