# Main Pack Fuse Selection



## kennybobby (Aug 10, 2012)

The primary function of a fuse is to prevent a fire in the event of an overcurrent condition such as a short circuit. Usually the wiring is the weakest link and would burn first, but in some cases it is the 'box' that requires protection. It must be large enough to handle the normal operating and inrush and transient currents to avoid nuisance blows, yet fuse without fire when an overcurrent occurs. There are also time-delay features to consider. Here is a good link about selecting fuses: http://www.schurterinc.com/content/download/339609/11198688/file/Guide_to_Fuse_Selection.pdf


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## EVfun (Mar 14, 2010)

You need a suitable *DC* voltage rating and the ability to break the maximum current you pack will dump into a short circuit. The Zilla manual suggests using Ferraz Shawmut A30QS. These have a 300 volt DC rating at are rated to interrupt 100,000 amps (!).

For the 153.5 volt pack that is feeding 1300 amps peak controller current, and for the 144 volt pack with a Raptor 1200, I'd pick an A30QS700. That is a 700 amp fuse that will take about 3 minutes to blow at 1300 amps. My Zilla manual recommend an A30QS500 for the 1000 amp model and an A30QS800 for the 2000 amp model. 

For the 115 volt pack feeding a 650 amp controller I'd suggest A15QS500 because you don't need the higher voltage rating of the A30 series. I may move down to a A15QS250 with my Zilla because my pack is under 150 volts and I have the controller current set to 300 amps, with a long hang time at 300 amps because the motor current is set to 900 amps. I'm currently using the standard A30QS500.


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## evmetro (Apr 9, 2012)

Thanks for the replies kenney and evfun. I checked out that pdf and some others on the web, and it seems challenging to find much on big DC fuse selection. One thing that I did notice, was that the DC volt rating tends to be about half of the AC volt ratings for the same fuse. My head is swimming with all the fuse approvals, IEC ratings, UL characteristics, trip characteristics and such. I wish that there was a translation of all this stuff that was broken down into a simple guide for the DIY electric car guy to make a good choice and know why it is a good choice. I will keep studying and maybe I can write a fuse guide one day that is targeted at the DIYer EV folks who have day jobs that have nothing to do with converting EVs.


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## EVfun (Mar 14, 2010)

Here is a quick link to the manufacturers information about the A30QS line of fuses. They list the ratings and have charts to blow times at different overloads.


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## evmetro (Apr 9, 2012)

What is the reason for melting time? If your controller is rated for 500 amps max, why would you not just have the fuse blow the moment the circuit exceeds 500 amps? I assume that there is a reason that the shawmut fuses are popular in EVs, is there something that these semiconductor fuses are so often selected over others? I have read that an ANL fuse could still conduct electricity after they blow due to arcing across the blow... What should I look for in a fuse that will ensure that it blows correctly?


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## major (Apr 4, 2008)

evmetro said:


> What is the reason for melting time? If your controller is rated for 500 amps max, why would you not just have the fuse blow the moment the circuit exceeds 500 amps? I assume that there is a reason that the shawmut fuses are popular in EVs, is there something that these semiconductor fuses are so often selected over others? I have read that an ANL fuse could still conduct electricity after they blow due to arcing across the blow... What should I look for in a fuse that will ensure that it blows correctly?


Everything takes time  Define moment. A second? Millisecond? Nanosecond? And it is not a fuse made from semiconductor; it is a semiconductor protection fuse. The fuse contains an element which will melt in a very predictable manner. Its characteristic is often described by using I²t. The fuse manufacturer will have more accurate charts showing when the fuse will blow depending on the current. You select the fuse to best suit your application.

If you actually need a device to protect by opening the circuit exactly at a set current (500A) instantly (milliseconds), then you would need to use a contactor or switch (transistor) with a current sensing circuit instead of a fuse or circuit breaker which rely on a thermal element. And good fuses (like Shawmut) are expensive but worth it IMO. Company reputation and product quality go a long way in the decision process I use for personnel and property protection.


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## evmetro (Apr 9, 2012)

Major, I can't define "moment" at this moment, and is one of the things that I would like to understand. What is the most appropriate "moment" for a high voltage pack? Are there reasons in an EV packs that dictate how long we would want for a fuse to blow? I have seen the manufacturer charts of specs for various fuses, but to select a fuse and understand the charts I need to already know what length of time I need for it to blow as well as a number of other things. I have seen many threads here where it has been advised to not use ANL fuses, and I have even seen an occasional fuse suggestion, but not anything about how to determine how slow or fast you want your fuse to blow.


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## TEV (Nov 25, 2011)

evmetro said:


> What is the reason for melting time? If your controller is rated for 500 amps max, why would you not just have the fuse blow the moment the circuit exceeds 500 amps? I assume that there is a reason that the shawmut fuses are popular in EVs, is there something that these semiconductor fuses are so often selected over others? I have read that an ANL fuse could still conduct electricity after they blow due to arcing across the blow... What should I look for in a fuse that will ensure that it blows correctly?


Just my .02 , 
the fuse is not there to protect your controller, the fuse is there to protect your wiring from overheating and caching on fire in case of an overload. If your controller is not smart enough to protect itself than it will almost always burn before your fuse. So , the fuse will be calibrated in direct relation with the wire size, and the wire size will be calibrated in relation with the controller and battery pack specifications .
Because of that, no controller manufacturer will list a fuse value in the controller manual.
And again the fuse is not there to protect the controller, more likely is there to protect your wires when the controller blows and make a short circuit.


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## evmetro (Apr 9, 2012)

Thanks. I listed controller size as an example since it is the biggest load that the pack sees, and I figured that if the pack sees more load than that, the fuse should blow. If I understand correctly, the smaller the fuse rating, the more protection, but too much protection could mean blowing the fuse during normal use? This would mean that if a controller draws 500 amps max, the fuse should be higher tham 500 amps? Say you have a 500 amp controller, and say a 100 volt 100ah pack, and say 20 feet of 0/1 cable. What would you do with these numbers to select an appropriate fuse? Are there other things besides these basic parameters that should be considered? How fast or slow would you want it to blow, and why that speed?


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## major (Apr 4, 2008)

evmetro said:


> What is the most appropriate "moment" for a high voltage pack?


Unless you have a good reason to do otherwise, use the Shawmut as recommended by several of us and the link provided by EVfun or a quality fuse from a trusted source which is equivalent to those specs. If you really want to know more, seek out a fuse engineer from Shawmut, Buss or Littlefuse. I've worked with those guys years ago. You might find these companies displaying at industry trade shows. Or maybe some expertise at local electrical supply companies, but maybe not for DC rated stuff, although it is becoming more common with solar PV installs.


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## evmetro (Apr 9, 2012)

I have shawmut fuses in all my builds, but my reasoning for this is that it is what I see in other high end builds, and not because I made an educated decision. I just threw out the parallel ANL fuses from my lead to lithium conversion (and a bunch of cardboard and duct tape that came with the dead lead sled when I bought it) and am replacing it with a shawmut. From what I can tell, the fuses in many other member rides are shawmuts, but nobody really knows for sure why, other than that they are nice quality...


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## evmetro (Apr 9, 2012)

After watching that video that paul posted on a related thread about arc gap, it makes me want to know without a doubt why I select a fuse.

http://www.youtube.com/watch?v=HaLSTnJE44o

If you select the wrong kind of fuse, you risk your entire car and your life, so I am surprised that there is not more info on this site about selecting the right fuse and knowing WHY that fuse was selected. I will keep studying, and will post anything that I might find out about how to properly select the correct fuse. For now, I will do as I do and not as I say... I will get some shawmuts because they are great fuses and that's what people use around here.


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## evmetro (Apr 9, 2012)

From what I have read so far, it seems that in an EV, we would want slow blow, which apparently means that it can withstand higher current than it is rated for for a while, but the more current you apply over the rated, the faster it blows, up to a point where it can become a fast blow. Just thinking out loud here as I learn... From a budget standpoint, it would seem that a slow blow fuse rated for less current than a controller max would make sense since it can take a little "burst" current. Kinda like our cells are rated for continuous and burst. From an efficiency standpoint, it would seem that since a fuse is a resister in a sense, that maybe way bigger would present less resistance and therefore less watts hrs per mile? Maybe the bigger fuse would give a trivial amount of better efficiency, but it will cost more... The primary concern is to make sure that we protect against a short in the pack circuit, so the minimum standard would be to have a fast blow fuse that solves the problem prior to damaging the wiring or cells or anything other than the fuse, so the fuse must be the weakest link. The closer in current handling the fuse is to the second weakest link the more it will cost, and the higher the efficiency. The closer it is to barely covering the largest load, the less expensive and less efficient it will be as far as watt hours consumption it will be. 

OK, I thought out loud for too long, sorry. I will get back to the studying.


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## major (Apr 4, 2008)

evmetro said:


> I have shawmut fuses in all my builds, but my reasoning for this is that it is what I see in other high end builds, and not because I made an educated decision. I just threw out the parallel ANL fuses from my lead to lithium conversion (and a bunch of cardboard and duct tape that came with the dead lead sled when I bought it) and am replacing it with a shawmut. From what I can tell, the fuses in many other member rides are shawmuts, but nobody really knows for sure why, other than that they are nice quality...


I can tell you that the Shawmuts do work, from experience. They do open when you get a over current fault situation. This has been up to 500A fuses on 350Vdc and 300A on a 480Vac rectified bus, so about 700Vdc. Like was mentioned, the fuse did not save the IGBTs, but I was real thankful to have the system go dead after the fault which essentially short circuited the source. The _t_ in the I²t goes by really fast when the current is in the 1000's and going towards the 10,000's of Amperes 

ANL fuses are typically 32V. I don't have much use for those. I do use the ANN fuses rated at 80V on my lower voltage stuff which is mostly 48V. 

Use the proper fuse with the proper fuse holder


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## kennybobby (Aug 10, 2012)

For the case you asked about, battery pack of 100V at 100A-hr with 20' of 0/1 gauge wire and a 500A controller.

Maybe this could help you determine a time factor to use in the I2-t calculation for interrupt rating. You will need to know the wire resistance and inductance, and the [EDIT] pack internal resistance as Major indicated below.

Let Tau equal the ratio of inductance to resistance, Tau = L/R. This is a measure of how much time it will take for the current to climb to its maximum value--it is usually considered to take from 3 to 5 x Tau to reach the peak.

Look up R (and fusing current) from wire gauge tables:
http://en.wikibooks.org/wiki/Engineering_Tables/American_Wire_Gauge

Calculate L for a single wire based upon diameter and length:
http://www.consultrsr.com/resources/eis/induct5.htm

So R=0.001922 ohms for 20' of wire, and L=8.83 uH.

Internal resistance of the pack, e.g. 33 calb ca and using Davide's SDT from Elithion, Ri=1.11mOhm, x 33 gives Rp=36.6 mohms. Wire plus pack R=38.55 mohms. 

and so Tau=L/R = 229 usec. 

The short circuit current will be on the order of 100V/R = 2593 Amps, or 26C, which can be seen in UT videos heating 18mm spanners. 

From the fuse guide, IEC rated fuses can operate at 100% of the rating, UL is 75%.
http://www.schurterinc.com/content/download/339609/11198688/file/Guide_to_Fuse_Selection.pdf

The Shawmut 300VDC clearing formula appears to be 1/500 x Ip^2 xTau., not sure what melting formula they used. 
Looking at the A30QS charts the UL rated 700 Amp fuse can operate at 75% = 525 Amps, which should allow the controller to operate without nuisance blows. It has a max clearing I2t of 200E3 A2-sec for an L/R= 10msec at 300vdc for 100kA.


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## major (Apr 4, 2008)

kennybobby said:


> So R=0.001922 ohms, L=8.83 uH, and Tau=4.5 msec.
> 
> Now calculate a theoretical maximum short circuit current of the entire pack thru the wire, 100V/R= 52,029 Amps,


kb,

You need to include the internal resistance of the battery.


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