Fusing the main batteries...

SVFlyAway · Sep 30, 2012

This has been debated over and over in many forums for years. So I guess it
is a matter of your personal preference since, as I recall, ABYC exempts
the starter circuit from having a fuse (I'll have to check on that as it
was a long time ago that I took any ABYC courses).

Anyway, a short is a short, all shorts are "big." A group 24 battery which
I think most Vegas use (or something akin to that size), can easily put out
a couple thousand amps with a direct short, probably more than that. So a
100 or 300 amp fuse would take care of it. Make sure you use a fuse in this
application and not a circuit breaker. Put the fuse as close to the battery
terminal as possible. And carry spare fuses (and don't be like me and never
be able to find them!!!!).

tiempo2010 · Sep 30, 2012

ABYC does exempt a dedicated starting battery from fusing, but it is a good
idea. I fuse starting circuits regularly and if the fuse is properly chosen
there should not be any issues with nuisance blows. Because you are fusing
for short circuit and not the load ABYC allows up to 1 1/2 x the wire's
ampacity for sizing. I try to keep within the wire's ampacity. For example
1 awg has an ampacity of 208 in engine spaces so a 200 amp fuse is a good
choice. Either ANL or MRBF fuses should be used in this application as
lesser fuses do not have a high enough AIC rating.

tiempo2010 · Sep 30, 2012

Groundhog

You are fusing the main positive cable from the battery to the bus - the
wire for this should be fully capable of handling starting loads if this
bank can be used for engine starting. That would be a pretty large cable,
probably 1 awg or larger. For battery cabling I use nothing smaller than
1/0 or 2/0 on any boat. Whenever the wire reduces in size after a bus there
should be a smaller fuse to protect that wire. ABYC calls for the main
battery fuse to be within 7" of the positive post, but this isn't always
possible. I try to keep it within 12" or use MRBF fuses on the positive
post. They do add height and may not fit in all battery boxes. MRBF fuses
can also be added to a bus for fusing wires going to different items.

vegatern · Oct 2, 2012

I had posted 2 questions a while back but not sure I ever followed up.

The first was regarding a runaway diesel, being unable to stop the diesel with the fuel shut off. The consensus, I think here as well as elsewhere, was the lifting the decompression lever on the MD7A was OK and will kill it, which makes sense. Blocking the air intake will as well, which I think is the only other way, since it needs the three; air, compression and fuel. A runaway is burning its lubricating oil as fuel so stopping the fuel flow from the tank has no effect. I don't think I want to see this on the boat.

The second question was how to fuse the battery cable, which is suggested by Don Casey and others, and the answer I found was the Marine Rated Battery Fuse, which mounts on the battery terminal and protects the cable from the battery to the switch, at least as I understand it, and since I haven't yet done anything your answer here is perfect. The two questions are only related in my mind in that one day I turned the key on the boat and got the starter stuck "on". The key did not start the engine. (I must have backed off the key when I realized something was wrong) and with the key removed the starter still ran. By the time I got below and figured out what was wrong, and got the main switch off, (not being either mechanically inclined or very bright) the battery terminal had begun to melt. A fuse at the terminal should have prevented this I think. (Squirting the key slot with a corrosion inhibitor periodically would have as well). Had it taken much longer to get the starter turned off I might well have lost the boat.

Terminal Fuse Blocks - Blue Sea Systems

Easily and economically satisfies ABYC 7 circuit protection rule by mounting on a 3/8 battery post, battery switch or bus bar.

bluesea.com

How do I determine what size fuse to buy? I don't want it to blow when I start the engine, but do want to protect the cable from overheating.. Thanks

Craig Tern #1519

tiempo2010 · Oct 2, 2012

Craig

The fuse size relates to the wire gauge. For main battery cabling you are
using probably 1 awg or larger like 1/0 or 2/0. ABYC allows a main battery
fuse to be 150% of the wire's ampacity, but I try to stick to 100% as a
maximum. The olly real problem is when too small a wire gauge is used the
fuse may be limited to a smaller size, which can cause nuisance blows when
starting. I have not had a fuse blow during starting on any diesel start
circuit I have fused, and I use cabling 1/0 or 2/0, never smaller.

What size battery cabling are you using?

As far as a runaway diesel the decompression lever should not be used to
stop it as you are risking damage to the valve train. Closing off the air
intake is the only safe way.

tiempo2010 · Oct 2, 2012

It could have depended on wire size. Using a wire too small can start a
diesel for years if it starts quickly but if the starter is engaged for a
long time as was the case here the wire and connections could get very hot
quickly.
What size was the wiring?

tiempo2010 · Oct 3, 2012

If the terminal melted the wire had to be hot enough for a fuse to blow, if
sized properly.

n4lbl · Oct 3, 2012

Unencumbered with the facts I\'d guess that the wiring was of adequate gauge
but due to corrosion or a poor crimp and helped with vibration and perhaps
something falling on the terminal the resistance became too high. Think of
it as some failed strands in the wire. The heat generated is a function of I²R
(from P=I²R) and fortunately I goes down as R goes up (from I=E/R). My
speculation is that the engine wasn\'t starting as the starter was too slow
due to inadequate current and the heat built up. Going from wild
speculation to pure guesses, I\'d guess that the current that caused the
terminal to melt was lower than the normal starting current (without the
higher resistance due to a good joint) so you would likely had too large a
fuse to fail at this current.

This is not a suggestion that fusing the battery is a bad idea. I just
suspect that it wouldn\'t have helped with this failure.

Alan

n4lbl · Oct 3, 2012

You are right, but "gobs" is relative. If the wiring is 2/0 gauge then 200
amps for starting (I made that number up) is nothing. If the failed area
of the cable (speculating that that is the problem) makes it\'s cross
sectional area the same as 12 gauge wire then 100 amps is "gobs²". Running
starter current thru (the speculated) 12 gauge wire might as well be a
short circuit.

txsailcouple · Oct 3, 2012

Ok... first off, wiring failure doesn't happen all at once. It builds up over a period of time with one spot getting warm and degrading the connection and creating resistance. The next time a high current flows through that connection the 'spot' becomes more degraded and the resistance gets higher and so on. A simple crimp connection is just a 'spot' waiting for the next current spike. If your engine starts quickly then it probably would not have high current long enough to create damaging heat. But if you need to crank it for more than a minute you either need to fix that problem OR check to see where your wiring has been degraded and not passing enough current to the starter.
If you notice, most battery cable is thin multi-stranded wire due to its high current capacity over solid or heavy wire twisted type wire. But the downside is, unless it is well sealed, grease, moisture and contaminants will find their way inside the crimp and help to degrade it. The best way to prevent this is to use high temp solder and fill the connection as much as possible. Without doing this, current capacity of the actual surface contact between the crimped connector and the wire will be far less than the current capacity of the wire. itself. Fusing is a good concept but every extra connection is just one more possible point of failure.

John Devany
Southern Comfort Too

________________________________
From: Groundhog groundhogyh@...
To: "AlbinVega@yahoogroups.com" AlbinVega@yahoogroups.com
Sent: Wednesday, October 3, 2012 11:54 AM
Subject: Re: [AlbinVega] Fusing the main batteries...

I think melting a large metalic heat sunk
object like a battery terminal is not an easy
task. I would be very concerned about
discovering the exact reason.

I think to produce heat in this spot would
require gobs (read: a giant short circuit somewhere)
of current.

SVFlyAway · Oct 3, 2012

Well a new debate. One should never solder a marine joint. I could go on
and explain in detail, but here is an article written by a fellow USPS and
ABYC member that is excellent. Trust me, I know this guy knows what he is
talking about. I do want to emphasize that stranded wire has to be used
because it can stand the flexing (soldering turns stranded wire into solid
wire by the way). And when crimping the connectors please don't use as $2
crimper, go and buy a really good one (and also use only the best marine
grade connectors). I am always amazed how folks will pay thousands of
dollars for a boat then try to do the repairs and upgrade on the cheap.

Anyway here is the article:

Follow the ABYC Standard in Electrical Wiring and Connections | SailAngle.com

.

www.sailangle.com

txsailcouple · Oct 3, 2012

Actually to quote the document you attached it says:

Solder shall not be the sole means of mechanical connection in any circuit....

If you take that to mean never use solder on marine equipment then it would be tough to challenge. But I take that to mean that you don't tin a wire and wrap it around a post for a connection, and don't twist two wires together and solder them, which I would also agree is wrong. But that's just me with my 40 years experience in electronics.

Yes, stranded wire should always be used for many reasons including flexibility and current carrying cabibilities. There are many ways to protect high current conductors from damage due to overheating. Filling the voids created when crimping a large guage wire is essential to prevent conductor movement within the connector and deny corrosion gaps to form in.

And if you are really keen on tools I'll bet you've already tossed out that mechanical wire stripper in favor of the hot wire insulation removal method, but then maybe not.
More often than not those old mechanical cutters will cut or weaken conductor strands as you struggle to pull the insulation from the bare conductor.

Just my two cents.

John Devany, MSgt, USAF, Ret
Southern Comfort Too

________________________________
From: Chris Brown svflyaway@...
To: AlbinVega@yahoogroups.com
Sent: Wednesday, October 3, 2012 6:00 PM
Subject: Re: [AlbinVega] Fusing the main batteries...

Well a new debate. One should never solder a marine joint. I could go on
and explain in detail, but here is an article written by a fellow USPS and
ABYC member that is excellent. Trust me, I know this guy knows what he is
talking about. I do want to emphasize that stranded wire has to be used
because it can stand the flexing (soldering turns stranded wire into solid
wire by the way). And when crimping the connectors please don't use as $2
crimper, go and buy a really good one (and also use only the best marine
grade connectors). I am always amazed how folks will pay thousands of
dollars for a boat then try to do the repairs and upgrade on the cheap.

Anyway here is the article:

Follow the ABYC Standard in Electrical Wiring and Connections | SailAngle.com

.

www.sailangle.com

tiempo2010 · Oct 3, 2012

A proper crimp made with the correct crimpers has no voids between the
strands - in effect you get a cold weld. Inferior crimpers will likely
leave voids.

This link describes the proper way to terminate wires.

Marine Wire Termination by Compass Marine How To

www.pbase.com

This link shows how to make battery cables.

Making Your Own Battery Cables by Compass Marine How To

www.pbase.com

I do not solder any electrical connections on the boats I work on.

By the way ABYC exempts battery cabling from the solder only rule as long
as the solder contact length is not less than 1 1/2 times the conductor
diameter.

vegatern · Oct 4, 2012

The wire gauge is #2, I believe. The gauge for that cable is not in my "boat book" and I don't see any markings on the covering. Since the cable is not marked I wonder now if it is marine cable. It measures 12mm diameter with the insulation. It is possible that resistance at the lug contributed some to the heat build up. I wouldn't consider the terminal corroded, but it has a nice patina, and since the key slot was needing attention it can be assumed other parts of the boat do as well. The real issue is that it took me too long to figure out what was going on and what to do about it. I am posting a photo of the terminal in Tern's album. It was the mostly the battery case that had begun to melt but the metal on the post had burned as well. The links you shared are useful, thanks.

Reaching the breather on the Volvo is a bit of trick in the best of times and it makes sense that I would be doing damage with the decompression lever. That pretty much was where my original question was going. If push came to shove though . . . I guess the question was mostly academic anyway since I don't expect the Volvo to take off on its own .

Craig

tiempo2010 · Oct 4, 2012

2 awg can handle 210 amps outside and 178 amps inside engine compartments.
A 200 amp ANL or MRBF fuse will work well to protect the wire and you
should not have any starting issues with a fuse of this size. On an
overload basis a 200 amp ANL can handle 320 amps of current for 500 seconds
(8.3 minutes). In a short circuit it will blow instantly.

If the wire is not tinned it isn't considered as boat cable currently,
although in years past tinned was rarely used. 2 awg is smaller than I
would use but better than many boats that had 4 awg for battery cabling -
Catalina for example. If you do change the wire I would go a bit larger. A
starting circuit benefits from minimal voltage drop. Often I replace
battery cables with heavier wire - 1/0 or 2/0 - and find that the starting
time is reduced dramatically. On my own boat my starting time went from
about 12 - 15 seconds to less than 5 seconds after a wire upgrade.