Whats first: fuse or disconnect?

We have an installed 2kW 12 V inverter mounted close to the house battery bank. Has a 300Amp fuse about a foot away from pos terminal. But no disconnect, and the surveyor flagged it. I think it best to put the switch on the battery side of the line to kill power to the fuseholder in case a fuse change is necessary. Agree?
The existing house main switches ( POS and NEG) are wired separately.

Take a lesson from a fused disconnect. When one unit the disconnect shuts power to the fuses. You should arrange your separate disconnect and fuses the same.

Yes, i agree.

And I agree, the switch is first, then the fuses.

"And I agree, the switch is first, then the fuses."

The simplest to live with is a circuit breaker as a switch , sized to protect the feed line.

Then a class T fuse to protect the lnverter using load.

FF said:

"And I agree, the switch is first, then the fuses."

The simplest to live with is a circuit breaker as a switch , sized to protect the feed line.

Then a class T fuse to protect the lnverter using load.

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There are a lot of really cheap thermal breaker knockoffs on amazon. As in $17 for a 300 Amp model. Think i’ll stick with a ANL fuse and a 600 A blue seas disconnect.

I think it's the other way around. The fuse is to protect all the wiring downstream of it, and that includes the switch. ABYC spells it out this way too. So I think you need the fuse as close to the battery or bus bar as possible, then the disconnect switch, then the inverter. The disconnect can be places anywhere between the fuse and inverter, but the fuse needs to be as close as practicable to the battery or bus bar to minimize the length of unprotected wire. I did a quick spot check of my wiring diagrams and all the big DC loads are set up like this. The Davit, for example, has a fuse followed by disconnect switch.


I get the comparison to a fused disconnect, but I think in that case it's about removing high voltage from the fuses so they are safe to handle.

One large issue is the design of the hi AMP fuse holders. They require a wrench for replacement. So, if upstream from the disconnect, you are working with metal tools on an unfused house battery bank conductor.
Each of my Lithium batts are fused at the + term with 150A fuses. But, there are 3x in parallel.
Another example is the house main switches. Both the POS and NEG switches are headed for the bank, the POS and NEG 100A house fuses are downstream. Again, you can put the switches in a safe position for fuse replacement.

diver dave said:

One large issue is the design of the hi AMP fuse holders. They require a wrench for replacement. So, if upstream from the disconnect, you are working with metal tools on an unfused house battery bank conductor.
Each of my Lithium batts are fused at the + term with 150A fuses. But, there are 3x in parallel.
Another example is the house main switches. Both the POS and NEG switches are headed for the bank, the POS and NEG 100A house fuses are downstream. Again, you can put the switches in a safe position for fuse replacement.

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If you are worried about having the power on when changing a fuse, turn the power off. In this case, it should be turned off at the battery disconnect switch which should be on the load side of the battery fuse(s).

Will also be installing a 12V/2000Watt Inverter this season. Agree with Fuse first, then disconnect. The fuse will protect against the wire accidently disconnecting or chaffing through to an inadvertant ground (even at the Disconnect), then the Disconnect located anywhere after the Fuse and before the Inverter will protect against the Inverter inadvertantly outputing AC power. The other way around and you've got an unprotected wire segment from the battery to the Disconnect, and Disconnect to the Fuse location.

CPD said:

Will also be installing a 12V/2000Watt Inverter this season. Agree with Fuse first, then disconnect. The fuse will protect against the wire accidently disconnecting or chaffing through to an inadvertant ground (even at the Disconnect), then the Disconnect located anywhere after the Fuse and before the Inverter will protect against the Inverter inadvertantly outputing AC power. The other way around and you've got an unprotected wire segment from the battery to the Disconnect, and Disconnect to the Fuse location.

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What will be your process for replacing a blown fuse? Lets say its a class T or anl fuse

diver dave said:

What will be your process for replacing a blown fuse? Lets say its a class T or anl fuse

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I can't speak for CPD, but my procedure is to turn off the disconnect so there is no load through the fuse. Then unbolt, remove, and replace the fuse. Then turn the load back on.


With no load, you won't get any arcing at the fuse, and with 12 or 24V there is no shock hazard. You just need to be careful that you don't create a ground contact with your tools between the hot side of the fuse and some ground point.


And worst case, of you are uncomfortable with the fuse energized, You can kill the power source by turning off the battery switch.


The real purpose of the disconnect is to enable service of the load device, and to turn off the load through the fuse for fuse service. With something like an inverter, even if the inverter itself is turned off, you can get significant arcing when you connect/disconnect the DC cables because of the large input capacitors in the inverter. That creates a challenge if you are trying to change a fuse without a disconnect. If you don't get an arc disconnecting the fuse, you will definitely get one reconnecting.

You are now describing a layout with a disconnect on both sides of the fuse. While certainly possible, not so on my boat.

twistedtree said:

...With something like an inverter, even if the inverter itself is turned off, you can get significant arcing when you connect/disconnect the DC cables ....

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And I just did experience this exact effect with the Magnum; 30 minutes ago.

Just did a Lead extraction, and Li insertion ceremony.

diver dave said:

You are now describing a layout with a disconnect on both sides of the fuse. While certainly possible, not so on my boat.

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I'm describing what I believe is the prescribed ABYC wiring, which is:


- Battery,
- Fuse within 7" or I think a bit longer if the cable is sheathed,
- Battery disconnect switch.


Now you can start connecting loads, noting that there are a few exceptions for things you can fuse and directly connect to the battery.


Your inverter is a load and probably needs to be fused. There are two things that would trigger it. One would be if the wire to the inverter requires a fuse smaller than the battery fuse. The second would be if the inverter manufacturer call for it, which I think all do.



The disconnect part is optional, but is very convenient for service, especially if you don't want to shut down the whole DC system via the battery disconnect switch. You can use a breaker of some sort and get both in one package. Or you can use a fuse followed by a switch.


The reason for the fuse/switch order is that the wiring has a limit to how much current it can carry before you risk insulation breakdown and fire. The fuse is the limiting device, and needs to be at the beginning of the circuit, not midway through it.



Now I'm making an assumption here that may not be true, and that's that after the battery bank fuse and disconnect switch, you have some sort of DC power distribution point. It may be a simple terminal post, or a bus bar, or something more complex. Where power is drawn from that distribution point, it needs to be fused based on the circuit wire size. Now it's very possible that your boat doesn't have a battery disconnect switch, or battery fuse(s). I don't know when that became part of ABYC, or the degree to which various manufacturers follow those guidelines.

diver dave said:

And I just did experience this exact effect with the Magnum; 30 minutes ago.

Just did a Lead extraction, and Li insertion ceremony.

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It gets your attention, doesn't it . I've got a couple of tools with attempted spot weld points on them.

Boat is French designed and built; and carries a CE mark. Batts go right to main switches, POS and NEG. Then a pair of 100 A fuses, POS and NEG, and feeds the dc distrib panel.
Inverter was added later, and wires direct to battery bank, no switches, and only a single ANL 300 A fuse in POS.
Windlass uses its own feed with a breaker near the bank.
Both engines have both POS and NEG disconnects in their start batts.

I’m planning to add a disconnect in the inverter feed.

Absolutely right!

twistedtree said:

It gets your attention, doesn't it . I've got a couple of tools with attempted spot weld points on them.

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Okay... this makes me feel better. I thought I was the only one....

The proper configuration is fuse within 7” of battery, or as close as possible. If more than 7” sheath the cable. In a typical large battery bank it should be a Class T fuse. Then you can put the switch in a convenient location so you can get to it quickly in an emergency.
If you have the switch off you won’t get that big connection spark!
French built boats typically have terrible electrical systems when it comes to US and ABYC standards. I’ve seen a lot of French build boats that came out of the factory with inverter charger systems that didn’t have any fuse or switch between the battery bank and the inverter. And that whole negative battery switch thing makes no sense to me! ?*

Brent Hodges said:

French built boats typically have terrible electrical systems when it comes to US and ABYC standards. I’ve seen a lot of French build boats that came out of the factory with inverter charger systems that didn’t have any fuse or switch between the battery bank and the inverter. And that whole negative battery switch thing makes no sense to me! ?*

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U forgot about tying 20 conductors on one Wago terminal block. Yes, I have that now [emoji848] To their credit, all those black wires are uniquely and permanently numbered. Something Americans will only do on aircraft and spaceships.
As a kid, my father owned a couple Peugeot cars. I still recall some design consternation’s with those. Luckily, balanced out with a couple Mercedes thereafter.