Re: Fuse Wire
- From: "radsett" <sub2@xxxxxxxxxxxxxxxxxxxxx>
- Date: 13 Jan 2006 21:40:35 -0800
Adrian Tuddenham wrote:
> radsett <sub2@xxxxxxxxxxxxxxxxxxxxx> wrote:
>
> > Adrian Tuddenham wrote:
> > > Is there any inductance in the circuit or any of the loads?
> > >
> > > Breaking an inductive DC fault is a different and exciting game. You
> > > may even have to consider magnetic arc blow-outs and arc chutes (see
> > > electric railway and tramway circuit breakers for more details).
> >
> > Some at least since the main loads are motors .
>
> 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.
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.
> 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).
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.
>
> 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.
Actually most of the power stuff I've worked with has been DC, not AC
Robert
.
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