MRBF fuse on disconnect terminal

[jsuhre]

Hello all, not a frequent poster but planning to install a Victron Multiplus 12/2000/80 inverter on our O'Day 322. Space is limited and I'm trying to get the disconnect and fuse as compact as possible which got me thinking about installing a MBRF fuse on the terminal of a disconnect. Curious if I'm overlooking an obvious reason not to do this or if there's a better/alternative way to get a compact installation for the fuse/disconnect?

Thanks!

 

[jssailem]

The ABYC recommendation is to put the MBRF fuse within 7" of the power source. This would be the Battery, not the cutoff switch.

ABYC E-11.10.1.1.1 Overcurrent Protection Device Location - Ungrounded conductors shall be provided with overcurrent protection within a distance of seven inches (175mm) of the point at which the conductor is connected to the source of power measured along the conductor.

 

[colemj]

That's fine in the sense that there is nothing wrong with it. However, the fuse is supposed to be on the battery side of the wire. If this switch is coming from a bus bar or something otherwise protected by the main battery fuse, then fine.

Mark

 

[dlochner]

This is exactly the wrong place to put a fuse. The fuse is there to protect the wire, thus it needs to be at the source of power, in this case the battery. Put it on the battery + terminal and fuse it to protect the smallest wire downstream that is not otherwise protected.

 

[colemj]

The OP hasn't given a circuit diagram, so it is difficult to say whether the fuse is correct or not. There is nothing physically or electrically wrong with mounting the fuse on the switch as far as that goes.

I can think of a circuit where this would be fine. Battery-fuse-switch-fuse-inverter
|__busbar (or something else)
I don't know if my luddite formatting will survive, but basically if one terminal of the switch has the battery wire that continues on to something else like a busbar, then the switch could be wired with the fuse on the other terminal leading to the inverter. This would allow the switch to shut off the inverter from the battery correctly, with the wire protection correct.

Mark

Edit: Luddite formatting didn't work.

 

[Stu Jackson]

but basically if one terminal of the switch has the battery wire that continues on to something else like a busbar, then the switch could be wired with the fuse on the other terminal leading to the inverter. This would allow the switch to shut off the inverter from the battery correctly, with the wire protection correct.

This ^^^ is plain WRONG.

Dave already explained why:

The fuse is there to protect the wire, thus it needs to be at the source of power, in this case the battery

 

[colemj]

This ^^^ is plain WRONG.

Dave already explained why:

I think what I wrote is correct and also agrees with Dave.

For example, where are the fuses from your distribution panel? Not back at the battery, but at the "source" of the power to the wires - the fuses on the panel itself. Anything powered off a busbar has the fuse for their wires at the busbar, while the busbar is fused at the battery.

In my description, the switch is acting as a distribution post with a fuse connected to it protecting the wire to the inverter. The wire to the post (and onward) is protected by the main fuse near the battery. The only difference here is that the distribution post has an integral switch for the inverter. Everything is compact and properly protected.

But even without that, if the switch is rated for the main battery fuse and wire going to it, then there is no problem at all with the simple circuit of Battery-fuse-switch-fuse-inverter like the OP implies.

 

[jsuhre]

First thanks for all the replies

To clarify this will be a disconnect and fuse for the inverter not battery. This circuit will run: Inverter <--[short 2/0 wire]--> MRBF Fuse <--> Switch <-- [2/0 wire] --> Main Bus Bar

The Battery circuit will be configured in similar way except the MRBF will be installed on the terminal and a 2/0 wire will connect to a disconnect so: Battery <--> MRBF Fuse <--> 2/0 wire <--> Switch <--> 2/0 wire <--> Main Bus Bar

Parts pictured are actually for the battery bank and got me thinking about the possible use for the inverter.

Edit to add simple pic

Thoughts?

 

[dlochner]

First thanks for all the replies

To clarify this will be a disconnect and fuse for the inverter not battery. This circuit will run: Inverter <--[short 2/0 wire]--> MRBF Fuse <--> Switch <-- [2/0 wire] --> Main Bus Bar

The Battery circuit will be configured in similar way except the MRBF will be installed on the terminal and a 2/0 wire will connect to a disconnect so: Battery <--> MRBF Fuse <--> 2/0 wire <--> Switch <--> 2/0 wire <--> Main Bus Bar

Parts pictured are actually for the battery bank and got me thinking about the possible use for the inverter.

Edit to add simple pic
View attachment 230299


Thoughts?

If all the wire downstream from the Battery to the Inverter and to the Pane or distribution bus, are the same size, then only the battery to switch cable needs to be fused at the battery.

Any time the cable becomes smaller, at bus bar, switch or panel there needs to be a fuse to protect the downstream wire. The only exception would be if the battery fuse is sized to protect the smallest wire.

So, why would you mix wire sizes? A good example would be a long run where voltage drop is more of a concern than that carrying capacity. Lets say the selector switch is close to the panel and a 1/0 cable would provide both low loss and adequate capacity from the switch to the panel, but too much loss from the battery to the switch. In this case, use 2/0 to the switch fused to protect the 1/0 cable.

 

[colemj]

That's fine. You could make it a bit more compact by connecting a small bus bar between the switches and using one switch as a post to the main bus.

 

[colemj]

I missed the point that the wire from switch to inverter was the same size as used from the battery onward. In that case, you don't need the fuse on the switch at all. Just let the main battery switch handle the inverter. Unless the inverter requires a smaller fuse and you are just using the larger wire for convenience or voltage drop reasons.

Mark

 

[OS2Dude]

ABYC will allow primary fuses to be up to 42" away under certain conditions, but in general they must be within 7" of the battery. ABYC requires Class-T fuses as primary protection if the bank is Lithium or a large AGM bank. This is because the Amperage Interrupt Current (AIC) rating for other types of fuses is not high enough, as well as the fact that a Class-T fuse can not fail closed and does not explode or spark when it fails. IIRC the cutoff switch can be basically anywhere. Make sure the amperage of your cutoff switch is appropriate. All the of smaller (RED) switches I've seen have only been 200 to 250A.

MRBFs are fine for any other use, just not as the primary circuit protection. (You can't use circuit breakers either, since they can weld closed.)

I suggest you read some of the articles at MarineHowTo.com. The author is a member of the ABYC Electrical committee.

 

[dlochner]

IIRC the cutoff switch can be basically anywhere

There's an interesting distinction here between what is required in USCG commercial inspected vessels CFRs and ABYC.

ABYC says to put the switch after the fuse, presumably to protect as much of the wiring as possible.

USCG says to put the switch before the fuse to allow the power to be switched off when servicing the fuse.

Both make sense, I kind of like the USCG requirement, as it protects my fingers when installing a fuse on a live wire.

 

[Davidasailor26]

There's an interesting distinction here between what is required in USCG commercial inspected vessels CFRs and ABYC.

ABYC says to put the switch after the fuse, presumably to protect as much of the wiring as possible.

USCG says to put the switch before the fuse to allow the power to be switched off when servicing the fuse.

Both make sense, I kind of like the USCG requirement, as it protects my fingers when installing a fuse on a live wire.

If the switch is off there won’t be any load, regardless of which side the switch is on, so the fuse contacts shouldn’t arc significantly when you replace the fuse. The 12/24V systems used on our boats aren’t dangerous even if you touch from positive to neutral by hand. Just don’t use any metal tools when replacing the fuse, and be a bit more careful if you’re wearing jewelry like a metal watch band.

 

[Jan11]

While it's common knowledge to "fuse to protect the wire" it's not correct if the same size wire is being used for separate elements of the circuit like the 2/0 in this case. Fuses in series have to have lower ratings as you get farther from the source. If they are the same size you have a "race" and don't know which fuse will blow in case of a fault. They all see the same fault current.

 

[LloydB]

'The 12/24V systems used on our boats aren’t dangerous even if you touch from positive to neutral by hand.'
Similar to that statement 'don't worry about the boom it won't hurt you just keep your head down'.

 

[jsuhre]

Lots of good stuff here! Thanks for all the feed back.

Followup on the MRBF fuse - I took a skim of the ABYC standard and all I found was a footnote in table 3b saying that the values are not applicable to lithium batteries. Can you point me to the requirement for a class t? My understanding was that for 12v lithium battery banks the AIC of 10000 for a MRBF was acceptable. I like that it protects everything down stream of the battery terminal but am willing to use a class t if required. Also space is tight and 7" requirement might be tough to accommodate.

Thanks

 

[dlochner]

Lots of good stuff here! Thanks for all the feed back.

Followup on the MRBF fuse - I took a skim of the ABYC standard and all I found was a footnote in table 3b saying that the values are not applicable to lithium batteries. Can you point me to the requirement for a class t? My understanding was that for 12v lithium battery banks the AIC of 10000 for a MRBF was acceptable. I like that it protects everything down stream of the battery terminal but am willing to use a class t if required. Also space is tight and 7" requirement might be tough to accommodate.

Thanks

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.

 

[Davidasailor26]

'The 12/24V systems used on our boats aren’t dangerous even if you touch from positive to neutral by hand.'
Similar to that statement 'don't worry about the boom it won't hurt you just keep your head down'.

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.

 

[LloydB]

Minimal unless Clancy's been around. (again)