Replace one 500A MEGA fuse with two 250A fuses

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tschoeki asked Feb 12, '22

Replace one 500A MEGA fuse with two 250A fuses

Hi there,

I'm a little bit confused - haha :D

Preliminary information

My electronic setup basically looks like the automotive showcase from Victron with some slight differences. Key features:

400Ah Victron Smart Batteries
3000VA Multiplus
Lynx Distributor to connect the Multiplus (and 12V loads) to the batteries
2x 95mm2 cables to connect the Multiplus to the Lynx Distributor
Copper bar to connect the main switch with the fuse

Example Use Case & Problem Description

My 3000VA Multiplus is mainly used for cooking. In the following scenario, I used a 2kW electric heater in the early morning because my heating system was not ready at this time. (Not a use case anymore, and just for a better understanding of what was going on).

As the electric heater was warming up the inside of our Sprinter van at half power (1kW) for 90mins, suddenly the Cerbo GX showed an over-temperature warning. I got up and took a look at the system. And right at the mounting point of the main system fuse (see the mark in the image below) the copper bar I used to connect the main switch with the fuse was extraordinarily hot (60°C).

I indeed did a test of the whole system months before and did know that this point was going somewhat of an issue as it will heat up first if the system is under high load. So I placed the Multiplus temperature sensor there, just in case. And, well, the alarm struck.

Thoughts & Question

Now I was thinking about how to solve this. (I also asked a friend how is an electrician, but for households and not for 12V/24V systems.)

I'm not quite sure if the problem exists because of the huge load that needs to "get through" this single fuse? Or can it be the different materials the fuse and the copper bar are made of that are causing friction and therefore this point heats up? Because all other parts of the system aren't even hot - sure - they are warm, but not hot :)

So, how about replacing the one 500A MEGA fuse with two 250A fuses to split the load like it is done with the Multiplus connection to the Lynx Distributor? I can't find any answers on this via Google or here.

So, what do you think? Any suggestions? Alternatives?

Thanks in advance

Fabian :)

P.S. Here's a picture of my setup :)

MultiPlus Quattro Inverter Charger wiring fuses temperature Temperature Alarm

screenshot-2022-02-12-at-120529.png (191.9 KiB)

elektro-sprinter-large.jpeg (259.6 KiB)

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Answer 1 · 2022-02-12T12:26:02Z

Al answered · Feb 12, 2022 at 12:26 PM

Just a few checks first:

Are your cable lugs properly crimped?

Are the cable lugs in direct contact with the Mega fuse? (No washers in-between)

Are the connections torqued up and tight?

Nice compact setup you have but, is there any cooling for it? (fan or passive)

Why do you need the 500a Mega fuse if you already have 2 x 250a fuses from the Lynx Distributor?

*edit. Sorry depending on if you have dc loads separate to the Multi, I guess the Lynx does need its own fuse from the battery, which in principle 2 x 250 would probably be ok, a wiring diagram would be useful.

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tschoeki commented · Feb 12, 2022 at 03:45 PM

Thanks, @Al!

To answer your questions:

Are your cable lugs properly crimped? Yes! plus additionally soldered within the cable lugs
Are the cable lugs in direct contact with the Mega fuse? (No washers in-between) Yes! Had stainless steel washers there before, and it was worse :D
Are the connections torqued up and tight? Also yes! double-checked that
Nice compact setup you have but, is there any cooling for it? (fan or passive) Passive I guess - All components are mounted on perforated plates
Why do you need the 500a Mega fuse if you already have 2 x 250a fuses from the Lynx Distributor? Well, I guess for safety reasons? Otherwise, that's a good idea. I could get rid of the fuse there.

Here's the wiring diagram. It should be pretty correct - especially regarding the topic:

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sprinter-wiring-diagram-80.jpg (588.2 KiB)

Answer 2 · 2022-02-12T17:32:59Z

Alexandra answered · Feb 12, 2022 at 05:32 PM

@tschoeki

Sometimes de-burring the lugs and copper bar is needed. While a terminal can be torqued the contact is not flat / full surface contacting.

Saw a setup with a similar issue and that is all it was.

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tschoeki commented · Feb 13, 2022 at 08:28 AM

Hey @Alexandra, thanks for your suggestion. The hole I drilled into the copper bar is already de-burred and the surface is flat. But, and that brings me to an idea, I'll check the hole diameter again - maybe it is too big and therefore the fuse has not an ideal amount of contact surface.

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Answer 3 · 2022-02-13T08:50:18Z

Matthias Lange - DE answered · Feb 13, 2022 at 08:50 AM

You have to check where the heat is coming from.
You wrote that the copper bar between the fuse and the switch is getting hot.
But if the heat is coming from the fuse the cable on the other side should also getting hot.
Do you have access to a thermal camera?

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tschoeki commented · Feb 13, 2022 at 09:00 AM

Hey @Matthias Lange - DE, thanks for your suggestions and yes I do have access to a thermal camera. Good point - will check this :)

From what I know from checking all the components with my hand the source of the heat is the spot where the fuse is mounted to the copper bar. The other side of the fuse is not hot.

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nickdb ♦♦ commented · Feb 13, 2022 at 09:33 AM

Recently I had a similar issue where I could smell hot plastic. On tracing it out the entire mini db was very hot, opening the case and I could see insulation starting to melt near the fuse holder.

Turns out a manufacturing defect caused the fuse to be loose in its holder, so it was arcing. The more it arced the hotter it got, the more the holder deformed the more it arced.

flir have a very nice usb camera for mobile phones, it really helps find hot spots before they cascade into bigger problems.

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Answer 4 · 2022-02-13T11:58:05Z

gn-ro answered · Feb 13, 2022 at 11:58 AM

If only one end of the fuse heats, then there is your problem: a bad contact or solder. You must check the fuse and the contact to the bus bar.

You can split the problem if you can turn the fuse around see if the other end is heating too. If this is the case, then the problem is not with the fuse but with the bus bar contacts.

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tschoeki commented · Feb 13, 2022 at 02:04 PM

Thanks, @gn_ro! This sounds absolutely reasonable and I will check this soonish :)

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Replace one 500A MEGA fuse with two 250A fuses

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