Fuse Wire

Hello David,

And after the fire engines have left the insurance adjuster would certainly have a few questions ...

Regards, Joerg

On Thu, 12 Jan 2006 00:01:09 GMT in sci.electronics.design, Rich Grise wrote,

In your case, sure. In the case of radsett's cheapskate client who is trying to save the cost of a proper fuse, who knows?

Is it possible to fully top up a lead-acid battery and equalize the cells without some amount of electrolysis?

This picture is from a couple of months ago on the street in front of my apartment building. If you look closely, you can see the ends of the jumper cables hanging down the front of the van.

Not my van!!!

From your other posts I now see that the application involves the protection of 48Vdc. This voltage wasn't stated earlier so I was left to guess that 32V might have been sufficient.

When it comes to high current dc you don't have many options other than open style fuses (such as the ANN type) where cost is a major factor. Alternative tubular ceramic links which prevent any molten splatter are even more expensive. The manufacturers you mention are the major players so I don't think you can go past them for solutions.

I would be very wary of using blade type fuses, particularly for industrial uses where the possibility of developing even minute resistance at the blade contact points presents itself. Any resistance at high current can cause the fuse holder to get extremely hot even to the point of charring and eventual arcing and then fire. I think that a major reason why designers have selected a reliable bolt down type fuse and fuse holder for fork lifts.

Personally, in your client's case, I don't think you can do better than what is there already. About all you can do is to find a cheaper source of fuse links (than the Bussmann ANN), as I suggested.

This practice has been fairly common in telecommunications in remote areas of Australia in the distant past when the correct fuse may not be to hand and it may have taken a week to fly replacements in.

Charts showing the fusing current for copper wire have been around for donkeys ages so it is not as uncommon a practice as one might think.

What price is the client looking for?

Personally, if he thinks he can get an approved 150A fuse to fit his existing mounting block for less than USD7.23 ea (probably cheaper in quantity) then he is dreaming.

Sure, a piece of fuse wire will be cheaper, but I doubt his insurance will cover any damage should the use of a non-approved item be shown to have resulted in a fire. I doubt even he would think it was cheaper if this happened?

Hey John,

you have erroneously posted a response direct to me whereas I suspect you meant it for the OP. Don't blame Robert though as I am certain he is trying to do the right thing by his client without alienating him. I feel confident that he will be able to convince his client that he should stick to what he's got, and stay safe.

Ross

Wow, that's a neat table! So if I have some 40 AWG copper wire, it will melt at 1.77A?

Does it matter whether the voltage is 1.5VDC, 12VDC or 110VAC?

Would be a neat experiment to test the "fuse" with some 1.5VDC batteries and a lamp...

It doesn't look like the aftereffects of a hydrogen explosion - it looks like the guy shorted something and started a grease fire.

Thanks, Rich

No, but copper has to get an awful lot hotter to fuse than fuse wire. Fuse wire melts even cooler than solder - it never gets anywhere near red-hot (unless you count the arc, but no 48V is going to maintain a 2" arc!), it just melts, splop. I've seen chargers with a blown 40A or 50A fuse wire link, and it just looks like big solder globs all under where the link was. I doubt if it would even be hot enough to set off hydrogen, assuming there's no platinum in it. ;-)

Cheers! Rich

Maybe it will be cheaper to parallel several fuses of a smaller, but more popular value?

I blew a 15 A fuse in the house this very morning.

I could not find a torch, so intead I took the candle from my breakfast table.

Then holding the candle close to the shelf where the fuse box was buried in crap & verious bottles, I began to clean out some room around the fuse box in oreder to look at it.

In the warm, cosy glow from the candle I then spotted - amongst other goodies - some cleaning petrol, ethyl ether, solvent based contact glue, a plastic container with sulfur, turpentine, some sodium permanganate and some toluene ....

Hmm ... all this stuff .... Oh!

... Perhaps I have to relocate the chemical collection ;-)

A large fault current in an *inductive* circuit stands a pretty good chance of sustaining a 2" arc long enough to vapourise enough of the terminals or holder to generate a cloud of ions. After that it will just burn back along the wiring and vapourise anything else in the vicinity.

The name Paul Jablochkoff comes to mind; as does a certain incident with a tramcar full of passengers in Shettlestone Road, Glasgow, being cut in half in less than 5 minutes by fault current (although the open circuit voltage was a lot higher than 48 volts on that occasion) .

Wot's Your Real Problem?

a 150 amp arc is something to behold if you happen to be close by.

If the system is properly designed the fuse will never blow and if it someone does something silly you will really want the best protection you can get. Tell the customer to spend the few pennies and stop being a skinflint.

Some at least since the main loads are motors . Also of course the power wiring will introduce a little. It is, however, partially bucked out but the capacitance in the controllers. The ANN style fuses are standard forklift equipment and they usually have higer inductances so I'm not too worried about them.

The inductance in contactor coils is probably much higher than in the motors, but then again so is the resistance.

On a side note when I was involved with forklift DC motor controllers I had a call from an end user who complained that he drew a 6 foot arc plugging the battery pack into the controller. I do believe he saw a spark when he plugged it in from charging the caps but a six fort arc, no.

Robert

Sorry Ross I did not mean to do that.

Actually Ericsson uses "rebuildable" fuses in their AXE phone exchange power plants. Theres a plug in holder with a couple of screw terminals in it with the fuse wire stretched between them. A spring loaded pin is held in place by the wire, if it burns the pin gets pushed into the fuse panel where it trips the fuse alarm.

A couple of rolls of different sized fuse wire are part of the standard spares package.

H.

You need properly designed cartridge fuses or even a DC circuit breaker.

Have you calculated the maximum prospective fault current? Have you calculated or measured the voltage spike which will occur when the circuit is broken at that current? Fuses and breakers have to handle fault conditions, not normal running conditions.

A piece of fuse wire on a charrable board is probably as bad as no fuses at all because there will be enough energy in a circuit like that to vapourise a spot on the surface of the fuse terminals. Once that has happened it will sustain an arc which will eat back into the fuse board. If your customer insists on using rewireable fuses, tell him he will need long porcelain fuseholders to cope with the inductive surge voltage and the explosion of metal vapour (and that will work out more expensive than cartridge fuses).

We have become used to AC power technology, which is relatively benign. Go and look at some of the DC electrical engineering textbooks (railway/tramway design) from around 1900 - 1904 and you will soon get the picture.

Yes, Inductance and the size of the short is important, I do know this :)

Take the battery pack. Nominally 48V at approx. 40 millOhms before considering the interconnecting wires and especially the connections. That gives you a 1200A fault current and that's an overestimate since at that point there would be no voltage drop across the short itself and the fuse. Also before that point the resistance in the battery pack would rise considerably. That's well within an ANN's capability.

The inductance's for the high current loads are small. They have to be otherwise the motors wouldn't react quickly enough and if it's not going through the motors it's smaller still. As a matter of field experience the ANN's are almost universally used in forklifts with series wound fields. The field winding in these will have fairly high inductances campared to the motors I'm dealing with and also higher peak currents.

I don't think the intent was to place it on a PCB. When I asked the question it was because I was unfamiliar with the construction and specific limitations of fuse wire. I was concerned that whatever covering it had migh actually provide enough of a conductive pathway to sustain an arc. Since I couldn't find any specifications on the materiaI I asked for some pointers. It hadn't occured to me that it might actually be bare wire. Now that I've been enlighted I've a few directions in which to head. Thanks guys.

Actually most of the power stuff I've worked with has been DC, not AC

Robert

It is also exceedingly easy for some unauthorised person to replace your carefully specified piece of wire with a bit of anything that comes to hand. That's another reason for preferring cartridge fuses or breakers (they can be knobbled too, but not quite as easily).

Sorry , it appears I was 'teaching my grandmother to suck eggs'.