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Old 04-08-2016, 18:32   #31
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Re: Fused battery connector Up to the task?

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Originally Posted by skipmac View Post
The 250 amp fuse on my starter battery terminal would blow before the 00 cable melts so that is certainly acting as a safety device to prevent a fire. Waste of money? Not in my book, especially considering how little it costs.
As a practical matter, a 250A fuse requires a long time (several seconds at least and maybe up to a minute) before it will blow if the current is 250A. I can't think of a practical circumstance where a "short" somewhere along a battery cable could ever draw enough current to blow a 250A fuse in a short period of time. The most likely "short" would be where insulation was chafed away and the wire touched a metal object. This type of event is unlikely to ever blow a 250A fuse even though it might start a fire.

What might blow a 250A fuse is a long cranking session trying to start the engine after replacing a filter. That's exactly when one does not want the fuse to blow.

The best solution is to route the wires carefully. Support them well with non-chafing stainless steel clamps. Then the odds of a short become very small indeed.

For really small engines with oversized batteries and long cable runs then a fuse makes sense.
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Old 04-08-2016, 19:18   #32
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Re: Fused battery connector Up to the task?

Quote:
As a practical matter, a 250A fuse requires a long time (several seconds at least and maybe up to a minute) before it will blow if the current is 250A.
Not really. A 250A fuse will pass 250A indefinitely. (See link below.)

Quote:
I can't think of a practical circumstance where a "short" somewhere along a battery cable could ever draw enough current to blow a 250A fuse in a short period of time.
A "short", by definition is an unwanted connection between B+ (ungrounded) and B- (grounded) terminals, buss bars, etc. A short will instantly draw the maximum current that the source is capable of producing. Empirically, it has been found that the short circuit potential of a lead acid battery is 4.8 x CCA. Therefore an 8D with 1600CCA has a short circuit potential of 7680A. The 250A MRBF curve shows that a load of 1500A will blow the fuse in about 80msec.

Quote:
The most likely "short" would be where insulation was chafed away and the wire touched a metal object.
Agree.

Quote:
This type of event is unlikely to ever blow a 250A fuse even though it might start a fire.
Simply not true. Review the trip curve at: https://www.bluesea.com/products/519...al_Fuse_-_300A

Quote:
What might blow a 250A fuse is a long cranking session trying to start the engine after replacing a filter. That's exactly when one does not want the fuse to blow.
Incorrect. A 250A fuse will pass 250A indefinitely. See link above.

Quote:
The best solution is to route the wires carefully. Support them well with non-chafing stainless steel clamps. Then the odds of a short become very small indeed.
I concur.
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Old 04-08-2016, 19:30   #33
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Re: Fused battery connector Up to the task?

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Originally Posted by transmitterdan View Post
As a practical matter, a 250A fuse requires a long time (several seconds at least and maybe up to a minute) before it will blow if the current is 250A.

Correct. This is why it is very unlikely that normal cranking will ever blow a properly sized fuze.

I can't think of a practical circumstance where a "short" somewhere along a battery cable could ever draw enough current to blow a 250A fuse in a short period of time.

Then you don't understand what happens when a battery is shorted. A short in a starting battery positive to ground give you 1000-2000 amps. This will blow a 250 amp fuse instantly.

This is why an electrical short is a dangerous fire risk, this is why you need fuses in the system.


The most likely "short" would be where insulation was chafed away and the wire touched a metal object. This type of event is unlikely to ever blow a 250A fuse even though it might start a fire.

Again, you are very mistaken. Chafing through the insulation of a wire and shorting the positive to metal that is grounded like the engine block will be a violent event. If a small wire the wire will immediately heat up and probably melt the wire (like a fuse). A larger wire will probably ignite the insulation.

What might blow a 250A fuse is a long cranking session trying to start the engine after replacing a filter. That's exactly when one does not want the fuse to blow.

I recently started my engine for the first time after a seven year layup. This involved new filters, bleeding all the way from the fuel tank to the engine AND at one point, a stuck solenoid resulting in non stop starter operation for about 45 seconds until I could get below and switch off. Still using the same fuse in my Blue Sea holder.

The best solution is to route the wires carefully. Support them well with non-chafing stainless steel clamps. Then the odds of a short become very small indeed.

For really small engines with oversized batteries and long cable runs then a fuse makes sense.
Maybe I am misunderstanding what you're saying but if not, I am really surprised that you don't understand what happens when a battery wire is dead shorted. It is really quite dangerous, generates big sparks, almost explosively, and a lot of heat really, really fast. I had a friend accidentally short his metal watch band from the starter + terminal to ground and it instantly welded the band to the starter and in a few seconds melted several links of the SS band.
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Old 04-08-2016, 19:37   #34
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Re: Fused battery connector Up to the task?

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Originally Posted by sharpey View Post
The issue I have is a height one... I don't think i'll be able to fit a bluesea one on...

Forgive me for being silly, but the spark issue when the fuse blows... how nasty can it be? considering battery is not near engine, engine is diesel etc...
Batteries give off very explosive hydrogen gas during charge AND discharge.
A boat's batteries are usually in a container /enclosed space that may collect hydrogen gas. A spark or hot terminal may set it off.

Auto/truck batteries are usually more open & also have the radiator fan to get rid of H2 gas.

That H2 gas is about the only explosive thing around a diesel.

As for that automotive fuse-if it blows will it spark or glow?
I don't know. That would be my only personal concern in using it long term.

Hope this helps / Len
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Old 04-08-2016, 20:16   #35
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Re: Fused battery connector Up to the task?

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Originally Posted by skipmac View Post
Maybe I am misunderstanding what you're saying but if not, I am really surprised that you don't understand what happens when a battery wire is dead shorted. It is really quite dangerous, generates big sparks, almost explosively, and a lot of heat really, really fast. I had a friend accidentally short his metal watch band from the starter + terminal to ground and it instantly welded the band to the starter and in a few seconds melted several links of the SS band.
I understand full well what happens. The reality is that in order to blow a 250A fuse with a 12V battery requires about 500A to have it blow pretty quickly. That requires a 24milliohm total circuit resistance. It's unlikely that some casual contact from the wire to metal can create that low resistance. The point of contact will instantly get hot, oxidize the copper wire and the resistance will go up. It is incredibly difficult to have a wire touch another wire and draw hundreds to thousands of amperes. It takes serious pressure compressing the wire to get the required low resistance. That's why big wire requires hydraulic crimpers to get a good low loss connection.

The theoretical calculations being given in this thread do not take into account the real world. A 250A fuse might give a false sense of safety if you think it is guaranteed to prevent a fire caused by a chafed wire touching ground somewhere. It won't.
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Old 04-08-2016, 20:31   #36
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Re: Fused battery connector Up to the task?

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Originally Posted by rwidman View Post
I agree.

The ABYC does not require circuit protection on the engine starting circuit because the fuse would have to have such a high rating that it would be ineffective.

In practice, there could be a time when you need to start your engine right now and you don't have time to search for and install a replacement fuse.
ABYC was likly written before the awesome MRBF and larger ANF fuses were made. today there is no reason to not fuse an engine cable.

they make ANL's up to 750a so even engines the size of cars can be fused.


the only way a properlly sized engine fuse would blow would be if the starter cable is shorted or the started is f*cked. either way you are not starting the engine and moving.

so you have 2 choices. fuse it. and not be able to start your boat when something fails

or B, not fuse it, and not be able to start your boat as it burns to the ground.

atleast at B you'll be setting off smoke signals to alert the boat that's heading towards you... so it can move instead of you.
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Old 04-08-2016, 20:37   #37
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Re: Fused battery connector Up to the task?

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Originally Posted by sharpey View Post
The thing is... the start battery is a good distance away from the engine (Beta 38), so am fitting 2/0 cable to cope with voltage drop. Not sure how that might affect the discussion, since a 150a fuse would blow well before the cable started to sweat (starter draws 100a)...

Again, am relatively inexperienced with all this, but am a quick study...

Again, thanks folks.

James
I would fuse it at 250 or 300a since it's 2/0. then you will never have a false trip and still be totally safe.
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Old 04-08-2016, 20:38   #38
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Re: Fused battery connector Up to the task?

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Originally Posted by smac999 View Post
ABYC was likly written before the awesome MRBF and larger ANF fuses were made. today there is no reason to not fuse an engine cable.

they make ANL's up to 750a so even engines the size of cars can be fused.


the only way a properlly sized engine fuse would blow would be if the starter cable is shorted or the started is f*cked. either way you are not starting the engine and moving.

so you have 2 choices. fuse it. and not be able to start your boat when something fails

or B, not fuse it, and not be able to start your boat as it burns to the ground.

atleast at B you'll be setting off smoke signals to alert the boat that's heading towards you... so it can move instead of you.
+1

I know three people (and have taken courses taught by them) who have been on the ABYC E-11 technical committee and all three believe the starting battery should be fuse protected .... unfortunately they were out voted.
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Old 04-08-2016, 20:45   #39
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Re: Fused battery connector Up to the task?

Quote:
Originally Posted by transmitterdan View Post
The theoretical calculations being given in this thread do not take into account the real world. A 250A fuse might give a false sense of safety if you think it is guaranteed to prevent a fire caused by a chafed wire touching ground somewhere. It won't.
It will. If you're fusing a large cable like a 00 battery cable, it will take more than 250 amps to melt the wire but before that happens, the fuse blows.

Fuse size is chosen to protect the wire. Choose a size that will blow before the wire exceeds it's current carrying capacity.



Quote:
Originally Posted by transmitterdan View Post
I understand full well what happens. The reality is that in order to blow a 250A fuse with a 12V battery requires about 500A to have it blow pretty quickly. That requires a 24milliohm total circuit resistance. It's unlikely that some casual contact from the wire to metal can create that low resistance. The point of contact will instantly get hot, oxidize the copper wire and the resistance will go up. It is incredibly difficult to have a wire touch another wire and draw hundreds to thousands of amperes. It takes serious pressure compressing the wire to get the required low resistance. That's why big wire requires hydraulic crimpers to get a good low loss connection.
All correct IF that is how the short occurs. But sometimes stuff happens. Bought a new boat once that had a main, unfused cable to the battery that was caught under a motor mount when the boat was built. First time in rough water the insulation chafed through and the wiring caught on fire. Fuse would have prevented that.

Bottom line, fusing a starting circuit is very, very cheap insurance with almost zero down side. You admit that it takes significant over current to blow the fuse so why not use one? In 50 years of messing with electrical and electronics I can only remember a handful of fuses that blew without some problem in the circuit.

I prefer to take the extremely small risk that the fuse might blow over the also small risk of a dangerous short. Downside to the first, I take 30 seconds to replace the fuse or 5 seconds to switch in the house bank to start the engine. Downside to the second is a possible fire.

Sounds like a no brainer to me.
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Old 05-08-2016, 09:01   #40
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Re: Fused battery connector Up to the task?

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Originally Posted by skipmac View Post
It will. If you're fusing a large cable like a 00 battery cable, it will take more than 250 amps to melt the wire but before that happens, the fuse blows.

Fuse size is chosen to protect the wire. Choose a size that will blow before the wire exceeds it's current carrying capacity.





All correct IF that is how the short occurs. But sometimes stuff happens. Bought a new boat once that had a main, unfused cable to the battery that was caught under a motor mount when the boat was built. First time in rough water the insulation chafed through and the wiring caught on fire. Fuse would have prevented that.

Bottom line, fusing a starting circuit is very, very cheap insurance with almost zero down side. You admit that it takes significant over current to blow the fuse so why not use one? In 50 years of messing with electrical and electronics I can only remember a handful of fuses that blew without some problem in the circuit.

I prefer to take the extremely small risk that the fuse might blow over the also small risk of a dangerous short. Downside to the first, I take 30 seconds to replace the fuse or 5 seconds to switch in the house bank to start the engine. Downside to the second is a possible fire.

Sounds like a no brainer to me.
.

There is no such thing as a "no brainer" decision except one that has no thought applied, which yours obviously has.

Notwithstanding, I completely concur that the risks of not fusing a cranking battery far outweigh the risks of fusing one.
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Old 05-08-2016, 09:12   #41
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Re: Fused battery connector Up to the task?

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Originally Posted by ramblinrod View Post
.

There is no such thing as a "no brainer" decision except one that has no thought applied, which yours obviously has.

Notwithstanding, I completely concur that the risks of not fusing a cranking battery far outweigh the risks of fusing one.
Well you are quite correct in that I did think out the process, weigh the options, the potential up and down sides to each, the likelihood of each occurrence so I guess I did at least attempt to use my brain.

But in the local vernacular, no brainer is taken to mean an answer so obvious that no brain power required to make the decision. Which side of Lake Ontario are you on? Maybe it translates a little differently if you're on the Canadian side.
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Old 05-08-2016, 09:47   #42
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Re: Fused battery connector Up to the task?

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Originally Posted by transmitterdan View Post
A 250A fuse might give a false sense of safety if you think it is guaranteed to prevent a fire caused by a chafed wire touching ground somewhere. It won't.

This customer was very happy he had a 300A fuse on the starter circuit when this happened.. The cause? A plastic wire tie failed. Beyond that I have seen numerous chafed wires short and blow properly sized fuses, including cranking circuits, but never had one fail due to cranking, if properly sized...




I was alerted to this mess because the owner kept tripping a 200A breaker because the crumbling motor created an internal short.


Also over the years I have eaten many a Class T, ANL or MRBF by not being as careful with tools as I should be. Just a quick touch between positive & negative and the amperage spikes to fuse trip levels..

If I had to add it up I would guess I have well in excess of tens of thousands of starts on properly sized fused starter circuits, but not a single nuisance trip. Not all engines can be easily protected with fuses though and bigger engines require protective conduit...
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Old 05-08-2016, 10:16   #43
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Re: Fused battery connector Up to the task?

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Originally Posted by skipmac View Post
But the fuse is NOT going to blow unless there's some kind of problem with the starting system so this is a non issue.

If there is a short that would cause the fuse to blow then you aren't going to be starting the engine anyway.
Fuses are subject to vibration. As I said, not having one on the engine battery cable is JMHO
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Old 05-08-2016, 10:40   #44
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Re: Fused battery connector Up to the task?

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Fuses on starter to battery runs are pretty much a waste of $ and not required by ABYC. A fuse from the alternator to the battery might be prudent depending on how the charging wire is routed and the length. Alternators automatically limit output current.

A fuse between battery and starter has to be so large as to be of little use as a safety device.

Fuses from the battery to DC loads or busses are ABYC mandated and should be selected and installed properly. If you don't know the proper type and installation rules get an ABYC trained technician to do it.
ABYC exempts fusing on dedicated starter circuits. If you have a 1/2/both switch you do not have a dedicated starter circuit and need to fuse both banks to be compliant.

In my opinion as well as others (like Mainesail) every wire attached to a battery should be fused. Sized properly the fuse will not blow unless there is a short circuit. A 300 amp fuse will blow in a split second if there is a short. It will handle much more engine cranking than you starter ever could at one time.

I routinely fuse starter circuits on 2 and 3 cylinder diesels and there has never been a fuse blow in normal use. I have actually fused the start circuit on a 6 cylinder diesel in a Cape Dory trawler and it has worked flawlessly for over a year now.
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Old 05-08-2016, 10:46   #45
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Re: Fused battery connector Up to the task?

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I understand full well what happens.
It really does not appear, by reading what you are writing, that you have any understanding of the phenomena being discussed.

Quote:
The reality is that in order to blow a 250A fuse with a 12V battery requires about 500A to have it blow pretty quickly. That requires a 24milliohm total circuit resistance. It's unlikely that some casual contact from the wire to metal can create that low resistance.
1. Actually, looking at the time-current curve cited in post #32, you will clearly see that a 500A current will blow the 250A MRBF in about 10 seconds.
2. As MaineSail points out, "casual contact" will blow an MRBF, ANL or Class T immediately.

Quote:
The point of contact will instantly get hot, oxidize the copper wire and the resistance will go up. It is incredibly difficult to have a wire touch another wire and draw hundreds to thousands of amperes.
This is simply not true. Quite the contrary, it is incredibly easy to blow or trip an OCPD when an ungrounded conductor (B+) comes in contact, even a brushing contact, with a grounded surface (engine block, steel hull, etc.) or the uninsulated portion of a grounded conductor. BTW: This is not "theoretical".

Quote:
The theoretical calculations being given in this thread do not take into account the real world. A 250A fuse might give a false sense of safety if you think it is guaranteed to prevent a fire caused by a chafed wire touching ground somewhere. It won't.
This is very dangerous advice and should be discounted by all who read it.

Before the thread was highjacked the OP was asking about the suitability of using an MRBF in, what appears to be, a non standard MRBF fuse holder.

My original comments still stand:
1. The MRBF shown does not have the standard Bussmann markings and I suspect, but do not know, that it is a counterfeit. I recommend not using it.
2. The MRBF fuse holder is built into a battery terminal lug. It is a clever piece of kit but before a fuse is approved for its intended use, it must be tested, with its fuse holder, by UL or a third party testing organization that can provide a CE mark. There is no indication that this quality assurance has been achieved on this component. This objection is a far second place to the suspected counterfeit MRBF.

The ABYC continues to review the subject of requiring OCP in the starting conductor, and the concept may be gaining support. However, they also try very hard to not create a solution for a problem that does not exist.
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