MRBF Fuse on Busbar

Mar 24, 2012
66
O'Day 40 BC Coast
I'm replacing some of my wiring do accommodate a 200 amp alternator and 4 Firefly G31 batteries.
Looking at the MRBF I can't figure out why they need the MRBF fuse block to connect to the positive busbar. They would need insulation at the cable connection so the connector only contacts the top of the fuse but that shouldn't be too hard.
It seems they are done in a similar fashion to this in the auto industry.

And if that is OK, why couldn't I bolt through the MRBF directly into the battery connection?
I don't have enough room to install the MRBF block on 2 of my batteries so would otherwise need ANL blocks and fuses.

Tin plated copper busbar material is available so the studs can be sized and positioned appropriately
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Jun 11, 2004
1,621
Oday 31 Redondo Beach
"...why couldn't I bolt through the MRBF directly into the battery connection"

Not sure I quite understand but if you put the fuse directly over the battery post and attach the cable terminal on top of that the fuse might blow in an over current situation but there would still be a connection between the battery post and the cable termination. I must not understand the question.
 
Mar 24, 2012
66
O'Day 40 BC Coast
I don't see how it would if it is properly insulated.
Looking at the 2 styles of mounting I see on the web, the fuse can be mounted between the hot lower bar and the cable connector with either the post isolated from the base, ala Blue Sea, or with the cable end isolated from the post, ala automotive installations.
If the fuse blows the insulation is still in place either way.
 
Jan 11, 2014
11,321
Sabre 362 113 Fair Haven, NY
And if that is OK, why couldn't I bolt through the MRBF directly into the battery connection?
I don't have enough room to install the MRBF block on 2 of my batteries so would otherwise need ANL blocks and fuses.
I'm a little confused by this. Why do you need MBRFs on all 4 of the batteries? If the batteries are wired in parallel into one big bank, then only 1 MBRF is needed. Do you intend to have 4 separate battery banks? Bigger battery banks have advantages over smaller banks.

When wiring batteries in parallel, fuses are not needed between the batteries. So if you had 2 banks of 2 batteries each, you would only need 2 MBRFs.

Yes, you can build a bus bar and use MBRFs instead of ANLs. Blue Sea makes covers for busbars (https://www.bluesea.com/products/2718/MaxiBus_Insulating_Cover_for_PN_2105_and_2126) and the MRBFs have boots to cover the connection.
 
Jan 11, 2014
11,321
Sabre 362 113 Fair Haven, NY
I want to use 1 fuse per battery because I can't put the +ve bus bar close enough to meet the 7" rule.
My batteries will be 1 bank but they are in 2 locations.
Blue Sea wants to use the MRBF fuse blocks to connect fuse to the bus bar:
https://www.bluesea.com/products/5191/MRBF_Terminal_Fuse_Block_-_30_to_300A
I could save more than 35 CAD per connection by eliminating the blocks as well as saving space and connections
Am I correct to say you want to have 2 batteries in each of 2 boxes and combine them to make one bank?

If so, you only need to have one MBRF on each bank where the + wire leaves that bank. Inside the box you will not need an MBRF.
 
May 20, 2016
3,014
Catalina 36 MK1 94 Everett, WA
I think you under estimate the difficulty in engineering a well insulated connection. They are not that expensive.
 
Jan 11, 2014
11,321
Sabre 362 113 Fair Haven, NY
I don't think it takes a lot of engineering to put an insulating nylon shoulder washer between a stud and a cable end.
I guess I'm looking for the reason Blue Sea chose to insulate the stud mounting when it is easier to insulate the other end.
This is a pretty crude piece of kit but it shows the idea.
Where did you find that fuse holder?

The nylon shoulder washer is not insulating the nut. The base of the fuse is still in contact with the clamp and the clamp is in contact with the stud.

There are issues with using a clamp on connecter on a lead post. The clamp can distort the shape of the post and lead to a loose connection.
 
Mar 24, 2012
66
O'Day 40 BC Coast
I'm not using that clamp. Firefly batteries don't have posts. But seeing it did start my thoughts.
If you look carefully you will see that the cable end is connected to the top of the fuse but insulated from the post.
Why couldn't the same technique be used to put the fuse directly onto the busbar stud?
Nylon shoulder washers are available in suitable sizes and would provide insulation. A thin stainless steel washer between the nyloc nut and the shoulder washer should protect the washer.
 
Jan 11, 2014
11,321
Sabre 362 113 Fair Haven, NY
Why couldn't the same technique be used to put the fuse directly onto the busbar stud?
Nylon shoulder washers are available in suitable sizes and would provide insulation. A thin stainless steel washer between the nyloc nut and the shoulder washer should protect the washer.
The stud would still be connected to the bus bar. And you are assuming that the SS washer would not have contact with the stud. SS is a poor conductor which is also a problem.
 
Mar 24, 2012
66
O'Day 40 BC Coast
It doesn't matter if the washer contacts the stud. After all it would be under the nut which is definitely connected to the stud. A washer added there would not carry any current. The stud would not carry any current. All the current would go from the copper busbar, through the fuse, to the copper cable end. No stainless steel involved in the circuit.
 
Feb 6, 1998
11,665
Canadian Sailcraft 36T Casco Bay, ME
I don't think it takes a lot of engineering to put an insulating nylon shoulder washer between a stud and a cable end.
I guess I'm looking for the reason Blue Sea chose to insulate the stud mounting when it is easier to insulate the other end.
This is a pretty crude piece of kit but it shows the idea.
That is a copy-cat of the MRBF style fuse. Very early on the MRBF design used an insulated washer on top, as that knock-off does, we have an entire box of them, perhaps 20 of them, rotting away, we've had them for years. I will not put them on a boat nor allow my guys to. The original design used a high temp glass reinforced plastic as the insulated-washer (1/4" studs) so it would not "cold flow" under terminal pressure and allow the lug/terminal to loosen.

They very, very quickly moved away from that design due to numerous (no fuse) type issues. Also be aware that there are more than just one type of "cube" fuse, one that blows quickly (near instantly) and one that does not and has a proper trip time delay for use as a battery fuse.
 
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Mar 24, 2012
66
O'Day 40 BC Coast
Yes, I'm not considering knock-offs but the Bussmann/Eaton ceramic bodied with an interrupt of 10,000 amps.
They're available from Arrow for 8.69 USD and I suspect Blue Sea Systems sell the same one.
Can you see any issue if used with a nylon shoulder washer and an insulating retainer nut on an M8 stud?