Re: (ET) What size fuse

["Bob Murcek" <rmurcek geisinger edu>]

Would it be good to also incorporate both "last-resort" and running fuses 
into the layering?  For instance use fast but very high current fuses for 
the A's, and slow-blowing but closer to expected current fuses for the 
C's?    I guess this is sorta what they do in cars with the blades and 
circuit breakers for running but a fusible link for disasters.

>>> Robert Adsett <subscriptions aeolusdevelopment com> 1/2/2006 11:43:45 
>>> AM >>>
At 11:15 AM 1/2/06 -0500, Christopher Zach wrote:
>Ok, here is a question for the list: What size and type of crash fuse
>should one use for an Elec-trak?

I'm unfamiliar enough with the current draws to ask some basic questions 
to 
start.

>I'm curious after blowing up that battery pack the other day. I think
>300amp slow blows are not a good idea, and I have two battery strings.
>Then again I use the tiller and the blower, and those can pull way over
>150 amps for brief spikes.

Question:  What are the current draws of all the devices?  Accessories and 
drive.

300A seems way too high.  I see forklifts running with 150A fuses and they 
can have peak draws of several hundreds of amps.

>So what size fuse should I be using? And if I have two batteries should
>I use a fuse at the end of the string, two fuses in the middle at half
>rating, or three fuses; one in each string, and one on the end?

I'd favour a layered approach.

         A - A fuse on each pack.
         B - A fuse on the point where the packs combine feeding into the 
main DC bus.
         C - A fuse block distributing to the various power points, one 
for 
drive, one for PTO (a separate one for each PTO if there is more than 
one), 
one for lift, one for control circuits.

B could be eliminated w/o worry.  It's main purpose in some respects is to 
provide a simple way of disconnecting the DC Bus.

That structure provides protection to each of the main circuits and 
protects the pack separately.  If the packs are reasonably balanced in 
their ability to source current then the A fuses can be ~1/2 the B fuse 
and 
you get some additional overload protection in case one pack physically 
disconnects for some reason.

Now start with a fuse that is rated for about 30% more than the expected 
DC 
current level at each of those points.  The final point to consider is the 
pulse current (and frequency) from startup etc..  That may require raising 
the current rating.  My suspicion is that a 150A fuse might well be 
sufficient but I'd need more info.

See the second littlefuse link I sent.  It deals with the pulse issue.

I'd lean towards sizing the fuses so they had to be replaced every year or 
two due to wearout rather than making them large enough to never need 
replacement.

Robert


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