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  #21  
Old 02-01-2011
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  #22  
Old 02-01-2011
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Some fuses can sustain an overcurrent situation for a great deal more than "milliseconds". The blow point of ANL fuses are shown in the chart I posted above. The 100 amp ANL will blow at 175 amps of current at 500 seconds - over 8 minutes. But in the case of a dead short it will blow quickly.

Below (from Blue Seas) is the trip delay for the MRBF terminal fuse showing the time/amperage it will sustain. The chart is for 30,40, and 50 amp fuses. Plenty of margin for starter fusing.
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  #23  
Old 02-01-2011
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Mitempo - Excellent chart!

Let me try to interpret, and put it into practice; (and feel free to correct me)

The curves are all close, so let's assume a 50A fuse.
  • Any 50A fuse will carry 50A for an infinite time period.
  • However the typical 50A fuse will carry 150% (this is hard to read on the chart. The scale on this axis is linear. However, it could be as low as 120%) for under 500 seconds (again, hard to read on the chart. The scale on this axis is non-linear). So, the typical 50A fuse could carry 75A for 8.3 min. before it melts. According to the chart, however, any given 50A fuse could last as long as 33 minutes at 75A, while another could could last only 5 seconds and both of these would meet the manufacturer's spec.
  • At 200% of the rated capacity, or 100A, the typical 50A fuse will melt in 2˝ seconds. The limits at 100A are 0.2 seconds, or as much as 14 seconds.

The problem is that we need 135A to 150A for a starter motor. Can we do this with a 50A fuse? Well the third data point for the 50A fuse says that we can support 600% of rated current, or 300A. However it will only last for between 0.1 to 2.0 seconds. By inference, I can see that I could get 300%, or 150A for as long as between 0.4 to 3 seconds.

Your engine must start more quickly than mine... More realistically, I want to crank the engine for let's say - worst case, 1 minute. The only way that I can RELIABLY get 1 minute of 150A is to use a 150A fuse.

I believe that these curves are meant to provide the manufacturer with some documentation of the "wiggle room" in the case that some component was destroyed because it drew too much amperage, and was not protected by the fuse.

Good discussion!
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  #24  
Old 02-01-2011
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I just happened to pick the first chart on that part of Blue Seas site. There are charts for every range of terminal fuse size.

I would never use a 50 amp fuse for a starter. Below is another part of Blue Seas site. It lists many types of fuse but not all have a high enough AIC rating for a starter fuse. The terminal fuses, ANL fuses and T class fuses would all work for a starter fuse. For any starter a fairly large wire will be used and the fuse size can be large enough that there is no chance of it blowing in normal use. I have fused diesels of 30 or 40 hp with fuses rated at 200 amps and wire sized at 1 awg or 1/0 which are both rated above 200 amps capability. I usually use ANL fuses and a 200 amp ANL will withstand 160% of its rating (320 amps) for 500 seconds. I have never had a problem with nuisance blows.
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Last edited by mitiempo; 02-01-2011 at 01:09 PM. Reason: add
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  #25  
Old 02-01-2011
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eh-
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.
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  #26  
Old 02-01-2011
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I think we are drifting a bit. We are (were) talking about main battery fusing, for starter and possibly house loads. The fuses I have posted about are Blue Seas terminal fuses as well as ANL fuses and I also mentioned T class fuses.
Nowhere did I post about ATO fuses. And nowhere does it say that an ATO fuse will handle 3000 amps. Nowhere does it say a terminal fuse or ANL fuse will handle 3000 amps either. An ANL will handle about 150% to 160% of its rating for several minutes which is plenty for engine start times and amperages assuming the wire size is properly chosen as well.
For house loads, lets say all loads possible (everything on at once) reach 50 amps. You choose a feed wire that can handle say 100 amps or so and fuse at about 60 amps everything should be fine. The fuse will blow before the wire gets hot which is the goal.
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Old 02-01-2011
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Slow down, Brian.
"And nowhere does it say that an ATO fuse will handle 3000 amps. Nowhere does it say a terminal fuse or ANL fuse will handle 3000 amps either"
You're misreading things. I'm the one who is saying these fuses will not carry a crowbar load. I'm saying they will as you say handle it [sic] the way that any fuse should handle a crowbar load--by near instantly disintegrating and opening the circuit!
Not by carrying 3000 amps, but by vaporizing in the presence of it. Which is pretty much what you'd expect from any fuse seeing a 30x-60x overload.
Not talking about proptecting wires versus devices. Not talking about properly sizing fuses at all. Simply about whether these "unsuitable" fuses are in fact perfectly suitable for the job of OPENING without fail when subject to extreme overloads.
Blue Seas and ABYC seem to think the fuses will not open. Some of us think they will, rather certainly, based on experience.
So are BS and ABYC full of it? Trying to hype overpriced $20 per fuse solutions? Or is there some reason these fuses won't vaporize and safe the circuit from an overload, contrary to what some of us have seen them do?
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Blue Seas and ABYC think that there is a "possibility" the fuse will melt and basically weld closed. This is certainly true of breakers that don't have an AIC rating high enough (what I think you mean by IRC rating) An ATO should vaporize in any short circuit condition - the fuseable link is pretty small. And probably an ANL as well.
All I could find on Blue Seas site, with quotes from ABYC, refers to circuit breakers, not fuses. See below.
But if we are only talking about a few dollars, for what is just about the least expensive item on a boat, I would (and do) spend the extra. Isn't doing otherwise a bit like buying a parachute based on low bid.
I think spending a few extra dollars to be sure is good practice. It's not like you have to buy many. It will probably only ever blow when that battery breaks free of its mounts or a feed wire chafes through - pretty seldom on a properly wired boat.
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Old 02-01-2011
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You guys might be interested to read this account of some real-world testing of fuse and circuit breaker performance under extremely high current conditions:

http://www.ibiblio.org/london/altern...s/27/27p26.txt

Don't try this at home, kids :-)

Bill
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Old 02-01-2011
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Interesting reading. And no I won't try it at home - which is a boat!
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