I think for the most part we are agreeing.
"I agree with you in that over current will vaporize the fuse. ... if you're saying that the over current situation can exist for 5 or 10 seconds, I have to disagree. "Short time periods," in this context, means milliseconds. "
Well, let's say you have a 40A ATO fuse and you pull 60 amps through it. It will not blow in milliseconds. It may take some heat damage and partly melt but I suspect it will hold for 5 seconds, and it will take repeated overloads at 60A to blow it. (Without looking up the overload tables, I know they're available and i really should.)
I'm also not saying an ATO would make a good main battery fuse for a boat's battery banks. But let's say you have a 26' boat and other than the starter, a 60A or 80A fuse is all you need. An ATO would cover that, and as we agree, if you crowbar the battery cable, it IS going to vaporize, IRC rating to the contrary or not. I think there's something those IRC ratings are not saying, because I really can't believe an ATO, or an AGC or glass cart for that matter, isn't going to blow up if I try to put 3000 amps through it.
ANL fuses, btw, may be a bargain. IF you can find a car audio supplier that actually has the things in stock, as low as $2.00 for a 100A fuse, $5.00 for a fuse block. In theory, anyway. Plus shipping of course, or the chandlery stock fee. I don't recall ever seeing such low prices for them before, maybe the car audio market is migrating over to them.
"A starter can, and should be capable of turning for up to 30 second bursts, with a 60 second rest (33%, or less, duty cycle)."
There we gotta disagree. Most of my starter experience is with "automotive" grade starters, as opposed to commercial ones such as you'd find on locomotive engines. The typical boat starter is not a full-blown commercial starter, it is more of the automotive type. And folks like Delco, who have made the damned things for ages, tell me that the designed duty cycle is more like "20 seconds on, 20 minutes OFF" at 20C and above. Rule of thumb.
Starter motors, all of them, are "high impulse motors" They are designed to be small, light, and cheap while putting out a very high amount of power in a very short burst, to start an engine once in a while. When you exceed the duty cycle, the heat created during normal operation does not get a chance to dissipate, so the coil windings on the rotor expand and scrape against the coil windings on the frame. In order to get power, they are normally constructed with near zero clearance, and just thin varnish or paper insulation, which is destroyed when they expand and grind themselves up.
The ratings may vary but "15 seconds on, 15 minutes off" 15-20-30...pick a number, I've heard many similar ones but they are all similar. The exception being when you're stuck in a snow drift and cold soaked.
Yes, you can exceed those ratings quite often--but the point is that if you exceed them at all, you'll find the starter failing, sooner or later, from grinding itself internally.
There's a word for small engines that won't start in under ten seconds cold. "MEDIC!" Take it away and fix it, please.
Last edited by hellosailor; 02-01-2011 at 02:22 PM.