In-house fusible link - design for medium voltage? 1

[TheSwener]

Hi Gents,

I want to design a custom fuse link, and am trying to find resources to help understand the principles and design rules that govern it.

The biggest question I have is: what design parameters determine the fuse voltage rating? Intuitively it seems like the distance bridged by the think (between large conductors) needs to be sufficient to stand off the rated voltage, but does it get more complex than that?

If any of you consult or know of a good resource/consultant that can talk us through fuse design, that would be great. Thanks for the help!


Cheers,

Rob

 

[Skogsgurra]

Medium voltage?

I wouldn't even try that for low voltage. Once, when prospective short circuit currents were moderate, you could use wire fuse links. I would not do that today. And certainly not for medium voltage. What voltage are we discussing? And why on earth do you want to blow your own?

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[TheSwener]

Whoops, meant to add that detail... let's say ~400V

The primary reasons for wanting to go this way are cost and reliability - integrating a purchased fuse link into our system means having several critical electrical connections to attach it, which escalate cost and reliability concerns dramatically.

This fuse is an emergency last-ditch safety effort - if I can just stamp a thin section into our busbar and replace the whole thing if the link blows, that would be much better.

 

[davidbeach]

400V is well under 1000V and therefore is low voltage. Otherwise, I'm sure that Gunnar is dead-on with his assessment.

 

[Skogsgurra]

Busbar. Sounds like hefty currents.

A weak bridge that opens under high current will start an arc. And arcs at 400 V are nasty things.

I repeat: It is a very bad idea. Do not do it!

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[ScottyUK]

TheSwener,

If this is to be connected to a power system of any size then there's no way that you should be thinking about an open fuselink. HRC fuselinks use a high strength ceramic tube containing silica grains around a (typically) silver alloy element. The fusible element ruptures and the resulting arc is quenched as the silica fuses (melts) around it. These things are tested over and over again in a high power test lab delivering typically 80kA or more until their capability to break the fault current is proven. An open link just burns back until the arc hopefully goes out. Which is ok, except when the arc doesn't go out - then you have a problem.

If you want an idea of what an electrical arc running amok in a LV switchboard does, have a look at

http://www.youtube.com/watch?v=YwI48O0MUXY,

or just search Google or YouTube for hundreds of examples. Personally I think this stuff is scary enough that penny-pinching over the cost of an LV fuse is too ludicrous to entertain. But maybe that's just me.


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If we learn from our mistakes I'm getting a great education!

 

[TheSwener]

Hi All,

For starters, we are currently using an open fuse link designed specifically for this application by a major fuse manufacturer. Unless they are risking their entire reputation for our sake, it must be doable with high repeatability.

I simply want to integrate that same fuse shape directly into our custom "busbar" (90A peak rating, really just nickel sheet). This necessitates a change in the fuse material and thickness. Even the cheap-ish open link fuses are a significant percentage of total system cost, so yes it's worth trying to "penny-pinch."

I'm looking for design guidance before we start making custom shapes and attempt to blow them in the lab (yes, with a reliable breaker also in the test circuit). Thanks!

 

[Skogsgurra]

What is your prospective short-circuit current?

When you say bus-bar, I - for one - think high currents. Is that not what you have?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[rbulsara]

TheSwener:

Fuse manufacturing is not a DIY item.

You came here for an advice and you got it from some very knowledgeable and among the wisest members on this forum. Heed their advice. If you knew anything about the hazard involved in what you are asking, you would not have asked the question.

Better yet, you would do well to stay away from any type of electrical work for the sake of your own safety and that of others.

Plus "low voltage" is a misleading the term for those outside electrical power field. 400V system faults can carry a lot of energy and be very damaging and lethal.

Rafiq Bulsara

http://www.srengineersct.com

 

[TheSwener]

Sorry Gunnar, probably need to clean up my terminology when dealing with electrical folk - our group in automotive has different associations to the lingo

Voltage: ~400V (medium to us, low to you)
Current: ~90A (peak normal operating conditions, 60A continuous)
Fuse: ~250A, 3s blow

We are currently getting a custom designed open fuse link that meets these requirements, but integrating it into our circuit as a separate piece creates new weak points. Unless they are welded the reliability of these connections are unacceptable, and se far we haven't found a good enough welding method.

 

[Skogsgurra]

OK. If you can't get more than 90 A, it may be safe. Is it DC or AC? How come that a standard connection fails? Do you have high vibration? Corrosive environment? There are remedies for that.

Anyhow, I would never take the risk of putting my own device there. Lawyers will spot it immediately and you will be a very bad guy if something happens.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[davidbeach]

But, the maximum fault current question has not yet been answered. Has to be well more than 90A, or there is no point to the existing 250A fuse.

 

[rbulsara]

Yes, determining the safe fault current for a fuse is one of many important factors. Load current itself does not determine if the setup would be safe.

Start here to see just the list of considerations, never mind the how to manufacture and test those.

http://www.littelfuse.com/data/zh/Product_Selection_Guides/Fuseology.pdf

If an open fuse does not open properly, you can end up with ionized air surrounding it and quickly (read ms) in a fire ball.


Rafiq Bulsara

http://www.srengineersct.com

 

[ScottyUK]

When I look at some of the applications where we use bolted connections in the power industry I'm really surprised that you can't get a reliable connection without welding. I know automotive is a fairly tough environment, but compared to, say, a ladle arc furnace, or a turbo-generator or an open yard substation it begins to look rather tame. Perhaps efforts could be better focussed on trying to get a reliable connection to a commercial fuse? What failure mode are you seeing - vibration damage?

Gunnar has a good point about lawyers. If you do decide to roll your own, get it proof-tested by one of the high power labs. If you tell us where you are we could probably point you toward one.

Breaking a 250A overload isn't the problem you need to solve - it's breaking a dead short across the terminals which you need to worry about: on a 400V battery it could be a good few thousand amps, and the arc just won't want to go out once it strikes. I've worked around industrial batteries for a fair few years now and I am very cautious around big banks of cells. I really suggest you look again at using a DC-qualifed HRC fuse. The cost of a bolt-in fuse just can't be that significant compared to the cost of a traction battery, can it?


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If we learn from our mistakes I'm getting a great education!

 

[TheSwener]

Hi Gunnar, Hi David,

DC battery powered system, ~300A is the max fault current. We are basically just trying to break the circuit in the event of a dead short.

And it's automotive environment, so "yes" to vibration and corrosion. We have multiple fuses in parallel (to protect batteries in parallel), meaning that under certain circumstances we will not be able to detect a single poor connection *and* that connection will have high current pushed through it... so welded connections (or continuous metal) are highly preferred for peace of mind.



Rafiq,

Again, a major fuse manufacturer has provided us with an open fusible link that meets all of these requirements. Would you like to tell them that they would "do well to stay away from any type of electrical work for the sake of their own safety and that of others?" And if not, are you implying that they have mystic knowledge that a helpless professional like myself could not possibly hope to understand? If not, then I am trying to learn and would appreciate your help.

I have designed, qualified and implemented an open link of this type before for a major OEM, but at lower current and in a different design. So for this version I am hoping to understand the basic principles a little better in the hopes of shortening our development time.

If there are fuseology experts out there I am looking for your help to create a robust, integrated, and above all else safe fuse design.


Cheers,

Rob

 

[TheSwener]

Hi Scotty,

We have one HRC fuse across the pack terminals, but in the event of a sub-pack event or even shorting failure of one cell within a parallel group we need localized backup fuses. The Tesla roadster for example has 6821 cells, individually fused... 6821 HRCs would be bigger than the rest of the pack (and more expensive).

 

[ScottyUK]

I'm surprised that the short circuit current is 'only' 300A from a fairly high voltage battery capable of supplying 60A of load current. For my own education, what cell type are they?

I think I understand your connection problem too - you're running mutliple strings of cells in parallel, each string being individually fused?


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If we learn from our mistakes I'm getting a great education!

 

[TheSwener]

Hi Scotty,

I'd prefer not to give too many hints but 300A is a theoretical maximum based on the cell's voltage and internal resistance. At lower state of charge the cell voltage is lower and the peak fault current goes down.

And you are close on the interconnection strategy - cells are connected in parallel first, then in series. This dramatically reduces the number of unique voltages that we have to keep track of.

For most circumstances the fuse only needs to stand off a single cell voltage, but there are some circumstances in which all the fuses of one parallel group could break, and then they would suddenly see the entire pack voltage. Maybe an arc would keep enough current running to blow the HRC, but I'm assuming it wouldn't be wise to rely on that...

 

[ScottyUK]

Could you draw out a simplified sketch of your battery showing the fuse locations? I'm starting to wonder if the fuse we're discussing is a low voltage type such as those designed for electric forklift trucks, which are rated for about 60V.


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If we learn from our mistakes I'm getting a great education!

 

[TheSwener]

Hi Scotty,

Under most circumstances you are right, these fuses will just stand off the voltage of the parallel group they belong to (<5V). HOWEVER, if a massive short breaks all of the fuses in a parallel group AND the contactor + HRC fail to open the circuit, those fuses will stand off full pack voltage. That extremely obscure case is the one I am concerned with.