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Old 19-06-2024, 10:43   #1
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Join Date: May 2016
Location: Denmark
Boat: Nordship 808
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Should I add main breaker and fuse?

So, my 1984 built boat (Nordship 808) has a rather simple electrical system, maybe too simple?

Drawing attached, but here are the major lines.
In the for-peak the 12v battery is placed (now an LFP 100AH).
From there two thick wires (+ and minus) go to a fusebox with small-ish fuses where some loads are hooked up (Autopilot and minor stuff), and also two wires go from there to power the main breaker panel with 6 breakers and fuses. Typical stuff like Cabin Lights, Navigation lights, Instruments etc.

As you can see, there is no main fuse and no main breaker.

The wires from the battery to the fusebox are about 5 meters long and I think 16mm2.

Bonus info, the battery BMS has BlueTooth where I can disable and enable discharge ("main breaker?).

The BMS also has short circuit protection that will kick in after 58 microseconds (not miliseconds, microseconds i.e. 58 millionths of a second).

So my questions are:
Should I add a main breaker?
If yes, why and where should it be located?

Should I add a main fuse?
If yes, why and where should it be located?

Thank you very much in advance
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Old 19-06-2024, 11:14   #2
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Join Date: Sep 2016
Location: Lake Erie, PA
Boat: Jeanneau Tonic 23
Posts: 544
Re: Should I add main breaker and fuse?

You should have a fuse. I believe ABYC standards requires a fuse within 7 inches of the battery. For a 100 Ah LifePo4, you can use a MRBF type fuse which is cheap and attaches directly to your battery stud. A breaker is convenient as a switch, but if you have a short and your battery BMS melts down, a breaker may not interrupt the massive current a Lifepo4 can produce, thus even with a breaker, you still need a fuse. MRBF are so cheap and easy, just pick your size and pop it on.
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