MRBF fuse on disconnect terminal

[OS2Dude]

There is some debate in ABYC about the Class T fuse and the need for the 20,000 AIC rating. When a new technology comes on the market, standards organization tend to be cautious until the field has more experience with it. They also know that boat owners, builders, and repair techs will cut corners on any standard.

For the DIY among us, the standards are aspirational, where we can we should comply with them. Where we can't, we should try to get as close as we reasonably can.

For your specific example, what is the shortest distance you can make it? My primary lead is 8 or 9" to the fuse. A recent surveyor inspected it and said it was fine. The alternative is to sheath the wire in a flame/fire resistant sheath. The goal is to prevent a fire from starting from a short between the battery and fuse. A short wire is less likely to have something abrade it and cause a short, a much longer wire does have more opportunity, hence the fire proof sheathing.

Read MarineHowTo.com's article on Battery Banks & Over Current Protection, section AIC & Why Its Critically Important. It refers to Section 11.10.1.2 of the ABYC standard, subsection 11.10.1.2.3.2 which states for banks with 2200 CCA or 500Ah they require a fuse with a minimum AIC rating of 20kA at 125V if the battery's Short Circuit Rating exceeds 10kA. Most Lithium batteries do, and a bank surely does. To my knowledge, only a Class-T fuse meets this requirement.

8" vs 7", I would not see it being a reason to fail the inspection. But remember ABYC is the LOWEST standard. you can (and usually should) go above and beyond. In your case you could put a short loom section on your cables, and you would be covered. If you ever have an issue that requires Insurance Company intervention, you may wish you had if they find out you didn't adhear to the ABYC in the smallest respect.

 

[dlochner]

To be clear, my point was that when replacing a fuse you can touch the live 12/24V connector even when touching a ground, unlike when you’re working on a 120V line. Certainly if you short 12V to ground with a wrench you’ll get a nasty spark, and if you short them with too small gauge of a wire you’ll start a fire. But in the case of swapping out a fuse the risk is pretty minimal.

For the small systems that we all use, it doesn't matter much whether the fuse is ahead or behind the switch. The ABYC Committee felt after the fuse was best, it works.

The CFR regs the CG enforces are designed for large commercial battery banks on big ships and I don't think they distinguish between AC and DC systems on this issue. The conditions are different. Since we as recreational sailors don't have to deal with CG inspections, it's not a big deal, just an interesting factoid. Like the prohibition of any current carrying wire being smaller 14ga even for a tiny little LED light.

 

[MFD]

One nice thing about LifePO4 batteries is the BMS.
You can shutdown the charge and discharge circuits on the batteries when you are working on them.
Like when installing a fuse in a BATT<=>FUSE<=>SWITCH situation.

And having a buddy around when doing work on high amperage systems (AC or DC) is also a good idea, for that 0.0001% chance something goes wrong? Doesn't have to be this extreme

 

[colemj]

To my knowledge, only a Class-T fuse meets this requirement.

There are other forms of fuses that meet this requirement, like NH, NHL, Class CC, Class J, etc. Class T is compact and common.

Mark