Fuse between Battery(s) and Selector Switch

[Chief Engineer]

I have a gentleman who wants to put High amp fuses between his engine/house batteries and the selector switch. Is there a need for this? If so any ideas on the amperage rating the fuse should have?

[never monday]

Originally Posted by Chief Engineer

I have a gentleman who wants to put High amp fuses between his engine/house batteries and the selector switch. Is there a need for this? If so any ideas on the amperage rating the fuse should have?

not needed, starting circuits are exempt. could cause you to not have power to start the engine. Put a fuse between the battery switch and the panel board

[Chief Engineer]

Thank you for the info...I kind of figured that, but I just needed a confirmation/reason to tell the Customer why this wasn't a "hot" (pun intended)
idea. Merry Christmas and Happy New Year.

[hellosailor]

What does the starting circuit have to do with the battery primary circuit to the selector switch?

In fact, modern designs in cars and boats both follow the rule of mounting a heavy power fuse (or fuse block, etc) directly on the battery positive lead, and it SHOULD be there according to multiple codes and regs now, apparently.

This is to ensure that if the battery cable is chafed in the engine spaces, or the starter switch iiself fails, the primary cabling from the battery is protected by a fuse (not a breaker) and that fuse will prevent a battery fire.

By all means, those fuses should be installed. Right on the battery if possible.

Fusing the starter circuit is something else again, but if the starter goes into overload the odds are a fuse directly on the battery will still provide enough protection against that as well.

[never monday]

Originally Posted by hellosailor

What does the starting circuit have to do with the battery primary circuit to the selector switch? The battery cables from the battery to the selector switch carry load to the starter during starting

In fact, modern designs in cars and boats both follow the rule of mounting a heavy power fuse (or fuse block, etc) directly on the battery positive lead, and it SHOULD be there according to multiple codes and regs now, apparently. []It's not in the current ABYC E-11. I believe SOLAS will have a word about this also. With a fuse. one overcurrent event would render and engine unable to start without replacing the fuse.[/i]

This is to ensure that if the battery cable is chafed in the engine spaces, or the starter switch iiself fails, the primary cabling from the battery is protected by a fuse (not a breaker) and that fuse will prevent a battery fire. proper securement and size of cable will minimize this potential

By all means, those fuses should be installed. Right on the battery if possible. I disagree

Fusing the starter circuit is something else again, but if the starter goes into overload the odds are a fuse directly on the battery will still provide enough protection against that as well. and odds are the starter will dislodge and perform another time. BUT, it won't have power due to the fuse.

my comments are in italics above.

[Ex-Calif]

Years ago I had the pleasure of troubleshooting a Convair 440 with P&W R2800s on it that wouldn't crank. When I asked about a primary fuse the salty old boss told me I was an idiot and the primary system never would have a fuse.

A day later after changing the starter (oh joy) we opened a side panel in the wheel well and sure enough we found a honking great 90 amp fuse.

Installed and sized properly there is nothing wrong with fusing the primary to the starter and they definitely would protect against a ground fault in the circuit. Make sure yuo carry spare fuses.

[hellosailor]

Well, as Dan says you should be carrying SPARE fuses if you are using fuses. If not, a length of bare cable makes a good way to bypass a blown fuse in an emergency. We used to use single wire strands, or the aluminized chewing gum stick wrappers, on automotive type fuses when necessary.

This is a place (the battery) for a fuse, not a breaker. Most breakers arc and fuse shut (dead short) around 3000-3500A load rating, and a direct short on the battery can pull that much. And that's assuming the breaker trips at the rating. Yes, you can buy breakers with higher ratings--you'll spend about $100 for one rated nearer to 5000A service. Compared to a $10 fuse and holder and $10 for a couple of spares, that's just not a good place for a breaker--unless you're rich.

If you think a blown fuse is going to ruin your day, consider the alternative: A fire in the engine or battery spaces. Now, THAT'S gonna ruin your day, big time. I'd rather have a blown fuse to deal with.

[GordMay]

I suggest that those unfamiliar with OVERCURRENT PROTECTION read ABYC E-11, paragraph 11.12, beginning page 24, at:
http://www.boatdesign.net/boat-desig...ricalGuide.pdf
and also see
Figures 10 (A & B) pages 41, 42, etc

[hylas49]

This request is fine if you install the correct fuses. Many production boats are coming from the factory in this way already.

[Topazken]

Get rid of the selector switch. Having two house banks is one of the silliest things ever done aboard a boat.

Have one house bank wired directly to the alternator (fuse in-line is fine).

A solar charged, back-up starting battery is suggested in the event that you foolishly discharge your house bank below 12v.

[Paul Elliott]

Originally Posted by Topazken

Get rid of the selector switch. Having two house banks is one of the silliest things ever done aboard a boat.

Have one house bank wired directly to the alternator (fuse in-line is fine).

A solar charged, back-up starting battery is suggested in the event that you foolishly discharge your house bank below 12v.

I agree that a single large house bank is better than two smaller banks, and I have a single large house bank. But, I also have a selector switch that lets me start the engine from the house bank, or run my electronics and navlights from the starter battery if necessary. There are definitely big fuses between the batteries and the selector switch.

[GordMay]

A fuse/breaker is required if the feeder (battery to switch or panel) exceeds 72".
I recommend a fuse in all cases (regardless of length).

DC Circuit Protection from ABYC Section E-11 ~ Table 15
http://www.cruisersforum.com/gallery...r&imageuser=79

[S/V Elusive]

So ... if you get rid of all those switches ... what do you do when one of your batteries decides a cell is going to fail ... let it pull down all the others?

[Topazken]

In the real world, the most common failure is a dead alternator. The number one cause for a failed alternator is the flipping of a switch while charging which oppens the power lead from the alternator: Instant death.

The ABYC guidlines are great but realistically, a direct connection from the alternator to the battery has virtually no chance of failure. Millions of cars are wired like that.

If we did everything as per all of the safety guidlines we would not even leave the dock.

I submit that all of the fuses and switches (all subject to failure) are more likely to cause you hardship than a simple, short, well installed cable.

Oh, if a battery acts up, disconnect it from the system. A switch might be faster than a wrench but only by a minute or so.

[hellosailor]

"So ... if you get rid of all those switches ... what do you do when one of your batteries decides a cell is going to fail ... let it pull down all the others?"
Yes. As Ken says.

The alternative, of keeping the batteries ganged in multiple banks, is that if you are running on "all" they will all be pulled down anyway. If you are running on alternate banks, you have to keep switching them on else you'll deep cycle them twice as deep, resulting in shorter life and higher expenses. And problems switching the alternator sense lead between banks, or inaccurately charging them.

No solution is perfect, the tradeoffs are an individual choice. As long as you have the separate starter battery to get you restarted, you can unbolt and recover from one bad cell in one bad battery. And unless it shorts rather dramatically, you'll spot that before it pulls everything down--if you're attending to monitoring them.