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?
Begs the question why doesn’t a manufacturer just offer a proper boot?
Hi Jesse,
I agree, Blue Sea should ship that mount with a custom made proper boot. The cap they ship it with is just plain stupid.
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. https://shop.pkys.com/Blue-Sea-4018-Battery-Terminal-Insulator_p_2828.html?gad_source=1
https://www.go2marine.com/Sea-Dog-PVC-Battery-Terminal-Cover-Red-1-2-Each?gad_source=1
Thanks for never ceasing to learn yourself and then teaching the rest of us.
Hi Terrence,
Thanks for linking to that, I was not aware of that one and think it’s probably the one Jesse is referring to. Jesse?
That said, it still annoys the hell out of me that Blue Sea sell the MRBF mounts without a proper purpose designed way to cover them, and so we have to search around for some other product.
And thanks for the kind words, much appreciated.
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.
Hi Terence,
Sure lithium is worse, but even a small group 31 lead acid can deliver enough amps when shorted to melt a metal tool, so you are right to improve things, as I will by properly covering the negatives.
“ 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.
Hi John,
That must have been a nasty experience. Flooding is one of my worst boat related nightmares.
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?
Hi John,
If I remember the ABYC guide lines correctly, ignition protection is only required when installed in a compartment with a gasoline engine. The other issue to think about is that ANL and MRBF fuses would only remain safe from igniting vapours if the AIC was not exceeded. Anyway, Blue Sea specifically say that their class T fuses are ignition protected: https://www.bluesea.com/products/5502100/Class_T_Fuse_Block_with_Insulating_Cover_-_225_to_400A
Bit of motorcycle innertube cut to shape and cable tied.
Hi Abigail,
Good Option.
Coming back to this tip and the accompanying one on covering negative terminals.
As part of a DC distribution refit this winter I am using MRBF fuses on my 3 house AGM batteries with additional MRBFs being mounted on some downstream bus bars.
i purchased 2 pairs of boots to try out for coverage.
Ancor. Described as 260350 Wing Nut Battery Terminal Boots
Blue Sea. Described as Dual Post I/O CableCap
The Ancor boot is more flexible than the Blue Sea boot and seems to grip the fuse block. The cable access hole can be trimmed to accommodate larger cables easily and would certainly flex over 4/0 cable if required. My installation is using 2/0.
The Blue Sea boot covers the MRBF with cabling going through a U shaped channel. It could be trimmed for larger 4/0 cable with a bit of work. There is a groove to seat a zip tie to secure it, which would be required as the boot just sits on top of the MRBF with no grip. This boot is a more rigid material than the Ancor
Both boots would protect the complete MRBF assembly from something being dropped from above, and the majority of the sides. Both boots do not sit flush to the top surface of the battery case and sit up maybe 2mm above it. So something being pushed sideways against the boot could possibly contact the flat bar that connects to the battery screw terminal.
After handling and placing both boots in position, I am going to order more of the Ancor product to complete the install. Will most likely zip tie them in for more security.
Further on protective covers, Blue Sea makes a top cover for their 4 stud (3/8”) bus bar which I will have 3. I also have one 8 stud (3/8”) bus bar. There is no cover available for that. I’ll bend up a plexiglass top cover for that one. I considered cutting and fusing together 2 pcs of the 4 stud cover, but that will get messy and have to much flex (guess that answers the why)
Hi Bill,
Thanks for the great tips. I wondered about the Ancor covers, but good to have confirmed that they work: https://www.morganscloud.com/jhhtips/cover-the-negative-terminal-on-batteries/