Why is my Type T fuse getting hot?

[corn18]

Was load testing my RV system with a Victron MPII 12V 3000. Was drawing 318A from the battery bank for ten minutes. All wires stayed ambient. Happy about that. But the type T 400A fuse was at 106 deg F. Felt warm to the touch. Checked all the connections and they were good. Is this normal? I don't like heat in a system. It was 76 deg F ambient. 106 def F bugs me.

 

[SparkyJJO]

Fuses work by thermals. I would expect the fuse to be a bit warm when running a bit over 75% of its rating.

 

[timselectric]

Not normal.
Maybe the connection isn't clean. Or a full contact area.

 

[SparkyJJO]

Not normal.
Maybe the connection isn't clean. Or a full contact area.

I'll defer to this guy though. He knows more than me!

 

[ricardocello]

well, the resistance of the Class T fuse will generate I^2R watts of heat. It’s the law!

318 * 318 is a pretty scary number, even if the Class T is only 0.5 milliohms, that’s 50.6 watts!!
I’m sure you are conducting some of that heat down the cable.
106 fahrenheit is no biggie, but 106C sure would be!

Making sure you have perfect contact with the lugs is critical at these high currents, as @timselectric has said.

 

[corn18]

I have a spare type T I can put in and see if that gets hot. This one got abused once from a loose connection. The nut on the output side got red hot for a few minutes. Maybe that damaged the fuse.

 

[Hedges]

How about multiple photos from several angles showing connections of fuse and cables to fuse holder?
We've seen things like washers under a terminal, which adds considerable resistance.
And badly designed fuse holders with plastic in the compression path.

I have a spare type T I can put in and see if that gets hot. This one got abused once from a loose connection. The nut on the output side got red hot for a few minutes. Maybe that damaged the fuse.

You've solved the mystery.

Well, possible the fuse started to blow, but I'm guessing not.

if it is running high current now, use a DMM to check voltage from wire to lug, lug to bolt, bolt to bolt, bolt to fuse, etc.
And/or use an IR thermometer - see if fuse is the hottest, or something else.

 

[corn18]

How about multiple photos from several angles showing connections of fuse and cables to fuse holder?
We've seen things like washers under a terminal, which adds considerable resistance.
And badly designed fuse holders with plastic in the compression path.



You've solved the mystery.

I'll plop the spare in tomorrow and do another load test.

 

[timselectric]

At 80% and lower loading. There should be minimal heating of conductors and connections.
A 30° rise above ambient temperature would concern me a little.
Heating is compounded over time. Which means that it will continue to get warmer. This shouldn't happen at that load.

 

[Hedges]

I'll plop the spare in tomorrow and do another load test.

But first, see where the voltage drop is and where the heat is coming from.

if it is running high current now, use a DMM to check voltage from wire to lug, lug to bolt, bolt to bolt, bolt to fuse, etc.
And/or use an IR thermometer - see if fuse is the hottest, or something else.

 

[corn18]

Dang, 400A type T fuses have gone up 24% in 30 months. I hate inflation.

 

[corn18]

But first, see where the voltage drop is and where the heat is coming from.

if it is running high current now, use a DMM to check voltage from wire to lug, lug to bolt, bolt to bolt, bolt to fuse, etc.
And/or use an IR thermometer - see if fuse is the hottest, or something else.

I use an IR thermometer. Hard to isolate the type T from the holder, but the 4/0 wire connected to the type T holder is ambient. So is everything in the Lynx that the type T holder is attached to. I'd bet a dime on a donut when I plop the new fuse in there that everything stays cool.

 

[time2roll]

Put your volt meter across the fuse and see what it reads. Start at the wires and work your way in to direct on the fuse and see if anything jumps.
This is to verify all connections are tight.

 

[robbob2112]

Dang, 400A type T fuses have gone up 24% in 30 months. I hate inflation.

Blue Sea Class-T Fuse ~ Current Connected

The Blue Sea Systems Class T Fuse offers a high interrupt capacity for larger battery banks. Perfect for systems need a stronger fuse. Specifications: 125V DC Maximum Voltage 110A - 400A Compatible with BlueSea Class T Fuse Weight: 0.3lbs

www.currentconnected.com

$39, not bad

 

[hwy17]

I put my hand on my 150A Class T this morning while it had been running 80A for 3 minutes, and it was not palpably warm. But that's not as highly loaded or as long as yours, and mine is mounted on battery terminals so it can sink heat into the cells probably better than a fuse on warm wires.

 

[Partimewages]

I put my hand on my 150A Class T this morning while it had been running 80A for 3 minutes, and it was not palpably warm. But that's not as highly loaded or as long as yours, and mine is mounted on battery terminals so it can sink heat into the cells probably better than a fuse on warm wires.

Not to change subject but how is it mounted to the battery? Do you have a picture?

 

[hwy17]

Not to change subject but how is it mounted to the battery? Do you have a picture?

Yup, it's mounted with some aluminum bus bar. My BMS reads cell voltages in 2 banks of 8 which allows me to use a pack split fuse in the middle.

hwy17's Orion battery build

https://vevor.com/plate-shear-c_10810/hand-shear-sheet-metal-cutting-bench-mount-5--p_010353014694 Yeah I'm using quarter inch aluminum bar stock for my bus bars and apparently this can make quick work of that without me having to ruin a wood blade on my chop saw. $5 coupon (no referral) VVSOC

diysolarforum.com

 

[robbob2112]

When you assembled your system did you use any grease? I.e. no-ox-id special is made to keep the connection and the particles in it will cut through the oxides that form while the connection points are exposed to air. For aluminum it only takes a few seconds.

 

[corn18]

When you assembled your system did you use any grease? I.e. no-ox-id special is made to keep the connection and the particles in it will cut through the oxides that form while the connection points are exposed to air. For aluminum it only takes a few seconds.

I did not. It's been installed in a different location for nearly three years now. But I never checked it's temp much before and don't remember what the findings were.

 

[hwy17]

When you assembled your system did you use any grease? I.e. no-ox-id special is made to keep the connection and the particles in it will cut through the oxides that form while the connection points are exposed to air. For aluminum it only takes a few seconds.

If you mean me as well with my extensive use of aluminum bus bar. No. I have some noalox but I am not that paranoid about aluminum connections. For example, my contactors have aluminum connections and do not specify noalox.