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Methods to attach MRBF fuse holder to SAE battery post


New Member
Oct 9, 2023
London, United Kingdom
I've got a battery with SAE lead battery terminal posts that I want to protect with MRBF "cube" fuses. Since the primary purpose of these is to reduce the risk of a metal tool short-circuiting the battery terminals, I intend to fuse both the positive and negative terminals.

"B" end of the tool contacts unfused conductor"B" end of the tool contacts fused conductor
"A" end of the tool contacts unfused conductorcatastrophica fuse will blow
"A" end of the tool contacts fused conductora fuse will blowone of the fuses will blow

Obviously, I want to minimize the case where a tool touches two different unfused conducting surfaces - so I want to reduce those exposed surfaces as much as possible.

I will tell you what I do not like about these MRBF designs. The fuse holders come with silly little rubber caps for the insulated post, which is already fused. But, there is no safety for the big unfused busbar that runs the length of the device.

I'm thinking of the various ways the MRBF fuse assembly can be attached to battery posts, and how any available rubber boots can help as well. My palette includes:
It seems to me it's possible to install these fuse holders "right side up", "upside down", doubled up, and if necessary, "backward" in terms of electricity flow as well. I attached some diagrams, with Option 1, Option 2 and Option 3... although those aren't exhaustive. Several of my options could apply a lever stress onto the lead posts which I do not like.

My two questions are:

1. I think the minimum exposed unfused surface is Option 3, where I attach the clamp relatively high up on the post, and then rotate the MRBF holder back over the top of the post. Is there any inherent risk in doing that?

2. Considering the different possibilities, which rubber boots are available that could maximally protect the unfused surfaces? I didn't find any rubber boot designed for these things.


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Is this type of stacking even possible?

Note: the product photo is doctored to show how fuses might be stacked. I tried to visually estimate the height of that threaded post and marked it with the red line.


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    MRBF Stacking Option 5.png
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Is this type of stacking even possible?

Note: the product photo is doctored to show how fuses might be stacked. I tried to visually estimate the height of that threaded post and marked it with the red line.
Not sure if you saw this previous thread:
My home-made mount (solid copper) was cheap, easy and works well. I was able to drill exact size holes needed for each bolt (1/4 and 5/16"). I expect the copper would bolt on to your SAE battery clamp easily enough
No rubber boot, but I am not too worried as all is usually enclosed and I switch the battery off with the BMS if working on it. I could also stack fuses with a longer bolt if required.
Thank you very much for your reply. I had a shipping delay in getting my fuse holders, and so I ordered dual MRBF holders on Amazon for next-day delivery (as the safety improvement project was held up on just those two pieces).

The result is what you can see here:
It's quite a bit simpler than most of my fanciful diagrams. Just Option 1, except that it is a "dually".
These fit nicely into the battery terminal clamps which have nice M10 bolts on them.
The only thing I would ask for improvement is one right-handed and one left-handed terminal clamp. I found neither of the threaded bolts were reversible, so I was left with the askew routing that you see in the photo.

The fuller context of the system is here:

Ironically, the single cube fuse holders I got did not have long enough posts to mount two (with decent lugs). But, the dual fuse holders (where it's not necessary) have extremely long posts that would easily accept two cube fuses per post. I found this amusing.

Most importantly, I'm not going to burn down my garage now.
Looks like a nice tidy setup you have. I was tight for space in my caravan, so much preferred the post mounted fuse; no hot spots either!
Now we just need a battery terminal designed for the fuse to eliminate the holder ;)