Experimenting with 32v DC fuses at 48v and 68v

Overclocker

Overclocker

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so i wanted to see if a 32v dc rated fuse is going to work on a 48v ebike battery pack. specifically if the arc would quench, and how quickly. apparently it works just fine. simulated dead short. popped instantly. arc didn't linger.

i also prepared a modded fuse filled with sand taken from a larger fuse

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as expected it worked as well.
 
If my 12S of Lipo (46V nom.) gets shunted it will vaporize one of the 4 mm bullet connectors.
Sounds crude, but it works and is all that is needed.
Don't ask how I know :roll:
 
MagiicHat said:
Excuse my noobishness.... but what does the sand do? reduce the fire hazard?
Click to expand...


the interesting thing here is that the stock fuse seems to have vaporized a longer segment of metal compared to the sand-filled. so i guess that sand does really work...
 
Part of the fuse rating is in the amount of inductance in the circuit that needs to be broken (the energy stored in the inductor has to be cleared by the fuse). If its just a short length of cable between the battery and the caps of the controller then the inductance is low.......although the current is going to be high due to the caps/battery. It could become an issue if you put in too thick a cable, since that will have a role in limiting the peak of the fault current.

Regards Bob
 
DVDRW said:
Why not glass fuses? dirt cheap and 250V AC
Will 250V AC glass fuse work with 90V DC?
Click to expand...


AC current ratings probably don't translate directly to DC but i could be wrong. but definitely the AC voltage ratings don't mean anything when using DC

so to take the guesswork out of translation i'll be sticking w/ DC fuses
 
Overclocker said:
DVDRW said:
Why not glass fuses? dirt cheap and 250V AC
Will 250V AC glass fuse work with 90V DC?
Click to expand...


AC current ratings probably don't translate directly to DC but i could be wrong. but definitely the AC voltage ratings don't mean anything when using DC

so to take the guesswork out of translation i'll be sticking w/ DC fuses
Click to expand...
There's no conversion from AC ratings to DC. If there's not a DC rating on the fuse, don't use it for DC. The reason is that AC changes polarity at 50 or 60Hz. During that change the current drops to zero, which makes extinguishing the arc easier to do.
 
Yes you guys are correct. The act of going through zero current does help in finishing the arc ! Generally most AC fuses do have a DC rating similar in current but much less in voltage. The issues involved with arc clearance are many, but to get the arc "out" you need to be able to cool it. You also need to be able generate voltage across the arc (making the arc longer or splitting it up into lots of little arcs). If the volt generated in the arc is less than the voltage feeding the arc, the arc isn't going to "go away". To make the arc go "out" the arc needs to cool. So if we are talking about straight wire fuses then the longer the length of wire between the posts the better. so a 32mm (1 1/4" ) is much better than a 20 mm fuse. packing the fuse body with "sand" is good since the arc plasma melts the sand and forms a non conducting mass which interrupts and lengthens the arc path and cools the arc. Straight sand will work but the "sand" in the body of a fuse is a bit more special than just plain sand. It would probably be better to research the miniature circuit breakers than can be used to fit in the place of some auto fuses. these have special contacts and often either coils or magnets that stretch the arc out in length.
Bottom line is that the "larger" auto fuses will probable work OK on 48 volts but don't try it with circuits that have larges amounts of inductance in them. One thing to look at if you have a real problem is to connect two fuses of the same type in series ! If the prospective fault current is going to be high ( 1kA plus with these battery's on s/c with the caps as well.) both fuses will rupture and share the voltage. In which case two 30 volt fuses are good for 60 volts. Note that this will only work for high level faults.

Regards Bob
 
The 80V fuse above is an interesting find as it seems like an easy substitute for the similar ones that are only rated for 32VDC.

Putting fuses in series and close to the battery to minimise cable inductance are also helpful suggestions. The latter is good practice anyway, since the fuse protects the battery cable and the closer it is to the battery the more of it is protected.
 
When I was taught how a sand fuse works I was told it quenches the arc by turn a small amount of sand into glass with the heat of the arc helping insulating itself and extinguish the arc thats why a fuse with no sand will melt a large area of conductor compared to the one with sand. The one with out sand relys on the conductivity of air so the higher the voltage you go the larger the spark gap and more of a need for sand.
 
I would just like to point out if you search 80v automotive fuse then there are a array of different size blade fuses or bolt downs and values to fit most purposes
 
Failed a good en by there I've been using them at 16s for light duty and my 5amp seems to work well too.

Only thing the arc seems to have blown the casing apart so could have then shorted to a frame etc, it may be better to use the maxi size holder i use that and mine just melted straight out when overloaded case still complete but it does depend on the type of fault to how bad she blows, but anything over 80vdc and i just use a house hold 230vac plug fuse they operate in range and have sand for extinguish.
 
So what's the verdict here for 32V fuses? My rig is 52V, and I just put a "Max 32V" MAXI-Fuse inline, near the battery, on the positive side... What's this about "arc length"? I'm pretty ignorant and merely a backyard electrical tinkerer.

This is for a BBSHD and the 32V fuse is rated at 50A. I chose 50A to protect my 24AH battery pack (their documentation said something about it being able to handle 50A draw) - should I have chosen 30A because that's all the controller asks for?
 
Mr. Bungle said:
So what's the verdict here for 32V fuses? My rig is 52V, and I just put a "Max 32V" MAXI-Fuse inline, near the battery, on the positive side... What's this about "arc length"?
Click to expand...

Under load, a DC circuit that is disconnected will arc across the gap (this is a part of how brushed motors work at the commutator). If the gap is large enough, the arc will be extinguished. If it is not, it will continue to arc and create intense heat (this is how arc welders work), which can cause a fire inside the fuse, holder and wiring. The gaps are based on the voltage expected in the circuit.

Fuses aren't all rated for high voltage because they have to be physically larger to get a bigger gap, and have thicker and/or better insulation around them, so smaller fuses are rated for lower voltage, and are cheaper due to less material, cheaper insulation, etc etc.

So if you use a fuse or a circuit breaker or a switch, or anything intended to physically disconnect a circuit while it is in operation under maximum load, on a voltage higher than it was designed for, there is no guarantee that it *will* disconnect at all, and arcing across the gap is possible, which means fire is possible.

(and if there was a downstream short in wiring that caused the fuse to blow in the first place, and it doesn't disconnect, then you also have the potential for a fire at the shorted point as well as the fuse!)

It might work fine...but it might not. So if you want a guarantee that it will do it's job, you have to use a fuse rated for the max voltage the system could ever be at during a failure that could cause the fuse to blow.

This is for a BBSHD and the 32V fuse is rated at 50A. I chose 50A to protect my 24AH battery pack (their documentation said something about it being able to handle 50A draw) - should I have chosen 30A because that's all the controller asks for?
Click to expand...

A fuse is chosen for the max current that the source or destination can handle without damage.

IF the wires are the potential failure point in a short circuit (overheating, melting, fire), then fuse against that. (meaning, determine what the max current the wire gauge can handle, and fuse so it can't be exceeded).

If the battery is the potential failure point, then fuse against that. (meaning, determine what the max the cells or internal battery parts (BMS, wiring, etc) can handle, and fuse so it can't be exceeded.

In other words, the fuse should be rated no higher in current than the minimum current that will cause damage to something in the system, and preferably far lower than that, but higher than the worst case current draw the system would normally be able to handle in-use.
 
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