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Old 21-04-2015, 11:01   #16
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Re: Mystery major short circuit

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Originally Posted by Scrimshaw4 View Post
You might want to check around your boat to see if you took a lightning strike. otherwise check your ac and charging circuits. Good luck
I kind of blew this off until remembering my pickup getting fried by lightening. I hope that is not it, It had hi,pot shorts in everything, It did take out the battery fuse link/
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Old 21-04-2015, 11:29   #17
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Re: Mystery major short circuit

One other situation to consider is that maybe your start cct. energized with the fuel shutoff in stop mode. That could have caused the engine start system to run long enough to blow the fuses. Have you ever had that situation occur. It is often corrosion in the start cct. somewhere. That could have blown both fuses if the switch was left in the both position. Just a thought.
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Old 21-04-2015, 12:00   #18
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Re: Mystery major short circuit

If, as the OP states, he only starts from B1 and both battery fuses were blown, one might ask "How and when is the system set to "Both", so that both fuses could blow?


Are the batteries only combined (in this time frame) when the charger/inverter/shore power is running?


Is there some chance a cable has been chafed in or near that equipment, so that it only rarely shorts out as it hits a metal bulkhead or equipment case?
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Old 21-04-2015, 12:28   #19
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Re: Mystery major short circuit

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If, as the OP states, he only starts from B1 and both battery fuses were blown, one might ask "How and when is the system set to "Both", so that both fuses could blow?
I don't know. Just playing 20 questions along with the rest of you.
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Old 21-04-2015, 18:14   #20
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Re: Mystery major short circuit

A lot of great answers here. I am especially re-impressed with the magnitude of the event-seems like a number of you think something big happened.

I am about to go out of town for two weeks so will get back on this thread when I return. You all have concerned me enough that I am going to head down to the boat right now and disconnect everything until my return. I do not need a call from the marina telling me I have burned up my boat and 6 others.

Whatever the answer turns out to be, I'm sure I'll be the cause!

Thanks, all
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Old 21-04-2015, 18:14   #21
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Re: Mystery major short circuit

By the way-we have essentially no lightning at all here in San Diego.
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Old 21-04-2015, 18:40   #22
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Re: Mystery major short circuit

Hey Captain Lee, if you need help when you get back to San Diego, PM me. My boat's in Dana Point, but I work in San Diego.
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Old 21-04-2015, 19:25   #23
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Re: Mystery major short circuit

While others have correctly suggested "checking the cables", I think it is important to state that one should also check the torque on the nuts on battery posts and starter studs, as well as any other screw terminals in the circuit, such as at fuse blocks, terminal strips, etc. Are the terminals properly made up, tinned, with heat shrink and are they free of corrosion? A loose connection , or corrosion, can seriously cause a voltage drop, which in turn will cause amperage to increase. Never place a stainless steel washer between a terminal and the contact surface below the stud, as stainless has very poor conductivity and will also cause a voltage drop. Regularly checking these connections should be on the semi annual maintenance list, in my opinion. Good luck tracing the problem, detective!
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Old 21-04-2015, 20:05   #24
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Re: Mystery major short circuit

I really like this idea. I do have stainless steel washers just above the fuses, and will get rid of them!

Thanks!
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Old 21-04-2015, 20:38   #25
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Re: Mystery major short circuit

MRBF fuse holders come with ss washers, which should be used. The lug sits directly on top of the fuse and the washer is installed above the lug. Diagram below.
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Old 21-04-2015, 22:44   #26
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Re: Mystery major short circuit

OK. All these ideas have really got my thinking going, and with it, my memory.

First, some facts in response to questions:

1. The bilge pump wire from Batt 2 pos has its own fuse within 3 '' of the terminal.
2. My battery switch is a Blue Sea dual circuit combo switch, with the main line to the distribution panel at the output side of the batt 2 switch.
3. Starter cable comes off the batt switch.
4. Current electrical layout not ideal in terms of minimal cable run. This 1972 Ericson has the engine in mid-cabin, battery bank mid port side of engine, but battery switch, AC panel, DC panel all on starboard side at aft end of galley. So two large battery cables and a large starter cable make the trip through bulkheads, around corners, across the boat and back, maybe 25' round trip.
5. All circuits are appropriately breakered to load, and the only line without a fuse is the starter cable from battery switch, output 1, to starter.
6. At the moment I can't recall where alternator comes in.
7. The key to this puzzle may be that this is not a static layout, but a work in progress that looks different every week.

Here's my logic, and my tentative answer:

1. From Maine Sail, Bill, Stu Jackson, W3GAC and others: The answer has to involve current flow through the large diameter cables. Otherwise, there would be some evidence, somewhere, of heat destruction in a circuit, which I have not found yet.

2. The fact that both fuses are blown does not necessarily mean they both went at the same instant.

3. If nothing makes sense, find the human factor that caused the problem.

Additional points that I am only now thinking to give you:

4. This rewiring project has been a part time effort for almost a year. During that whole time, the boat has not sailed, but I have run the engine periodically.

5. I've only had the new DC panel with its new battery switch in for about a week.

6. Prior to that, without proper DC wiring, I decided I needed to run the engine for at least a short time to keep it lubricated, etc. My quick solution 2 weeks ago, was to take the loose end terminals of each of the two positive battery cables and the starter cable, bolt the three together, turn the ignition key, and push the starter. Engine fired right up.

7. I have the DC negative, AC ground, battery negative, all bolted to the engine block as a common ground. The shore power fitting by the marina is 1 year old and probably working well.

My theory: The 100 A fuses are too small for a 190 CCA 20 HP diesel engine. There has been at least this one time, maybe others I don't recall, where battery 2 as well as battery 1 was part of a high current starter event.

Maybe the fuse link melted all the way through on batt 2, only most of the way through on batt 1 during one or more of these episodes. I would not have known that one fuse was out because I have no monitor for that, and had no DC circuits operational yet (lights, blower, etc.) The second one then completed its death at some random moment, like while I was turning battery switch on and off while troubleshooting.

If any of this is true, then from all of you my steps going forward should be:
1. Get larger fuses for the starter circuit, maybe 250-300A at the batter terminal.
2. Run larger cables to match the larger fuses.
3. Do a very thorough check for a dead short somewhere, starting with the old large cables, particularly where they are hidden.

Have fun with my theories, and I appreciate all the great ideas!
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Old 21-04-2015, 22:45   #27
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Re: Mystery major short circuit

Oh, OK! My setup looks exactly like that! (Response to mitiempo photo of MRBF fuse)
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Old 21-04-2015, 22:48   #28
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Re: Mystery major short circuit

I suppose this discussion of fuses in starting circuits will bring out some who think that circuit is simpler if left unfused. I am pretty sure I want to fuse protect all my wiring, even the big stuff.
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Old 21-04-2015, 22:54   #29
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Re: Mystery major short circuit

(In response to long post by W3GAC):

I like all these ideas, W3, and will check them all out one by one. I do have a multimeter and a current clamp. Also, the batteries currently read 12.4 V each. I will do hydrometer testing each cell, but expect will be good.
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Old 22-04-2015, 03:50   #30
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Re: Mystery major short circuit

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Originally Posted by Stu Jackson View Post
Maine Sail already explained it. The fuses were too small.
Not if he didn't start the engine.

Also fuses are more of a black art than a science. A 100amp fuse is designed to take 100amps continuously. The short term rating is much greater than this so whilst you might expect the occasional failure of a 100amp fuse starting a small diesel it would probably be OK for quiet a long time. In this case if the switch was on both then the currrent would be shared between batteries and whilst sharing would restrict the total current rating to less than twice the fuse rating (2 fuses) it is unlikely to be a problem here.

I am with those who say go for the big battery cables. On cars I have come across insulation worn away by vibration - this could happen round the engine on a boat.

I would start with putting a fuse on only one battery and test everything out before connecting the second battery.

Forgive me for asking but you haven't accidentally reversed the connections to one of your batteries when you were doing the work have you?
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