The Offshore Voyaging Reference Site

Another Danger With MRBF Fuses

The great discussion on my recent article about proper battery fusing and particularly the potential danger of MRBF fuses failing closed, instead of opening the circuit in the event of a short circuit, reminded me about another big downside of these kinds of fuses that actually negates the supposed advantage of being able to install them on the battery terminal.

Take a look at the graphic above. As you can see, the fuse comes with a little rubber cap that I bet is there to satisfy the ABYC requirement to insulate all battery positive terminals from possible shorts.

First off, having used these fuses on the small lead-acid bank on our J/109, I can tell you that little cap is pretty useless. It refuses to stay in place and does not properly cover the exposed parts of the battery lug.

Wait, it gets worse: The other problem is how the heck can we insulate the metal part that bolts to the battery terminal?

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Jesse Falsone

Begs the question why doesn’t a manufacturer just offer a proper boot?

Terence Thatcher

Last year I finally installed terminal fuses on all 5 of my batteries, after your warnings about dead shorts on the positives. But I missed the warning that these fuses can fail open. I may not have room in my battery locker for the T fuses, however. But I will try to figure out a way to follow your advice, as I regularly do. I certainly found the rubber covers difficult to use effectively. You might also look at these, which I have not personally measured for the job yet.
Thanks for never ceasing to learn yourself and then teaching the rest of us.

Terence Thatcher

Further, I now see the major problem is lithium, not my small, in comparison, Lead Acid batteries. Nonetheless, I will seek better boots. I have used zip ties on my terminal fuse covers, but never felt satisfied with the result.


“ even a small group 31 lead acid can deliver enough amps when shorted to melt a metal tool,”

When my raw water pump repair developed a syphon up through the Heat exchanger, I discovered the awesome power of a group 27 battery covered by salt water.
The images of my 2/0 battery cable connections just gone. The BlueSea fuses and connections a disfigured lumps of metal makes you a believer in the damage a short circuit can do.

I’ve use the battery terminal covers indicated by Terrence. I zip tie the covers to the wire, otherwise they can be knocked off the terminal. You can cut slits in the side to handle additional wires attached to the terminal and still leave the terminal covered. (No more than 3 connections to a terminal i.e. bilge pump) .

The issue of the MRBF lack of cover is still an issue to properly protect the terminal from accidental damage. The tolerance of the MRBF fuse to manage surge power mitigating incidental replacements is both a positive and a negative.


Looking at the Class-T rated 200Amp fuses on the BlueSea site, does it matter if they are not Ignition Protected. I note that the ANL and the MRBF fuses both say they are Ignition Protected and approved for Marnine Use. Is this just a case of not having the fuse tested or is this a factor?