Fuse holder nut was glowing RED!

corn18

Village Idiot
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Sep 9, 2021
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I installed my Lynx distributor today and decided to replace my Blue Sea 250A circuit breaker with a 400A class T fuse. The class T fuse holder is from Blue Sea Systems and the fuse is a LittleFuse 400A class T. Got it all hooked up and tested DC only, Solar and low power AC. All worked great. So I started loading up the inverter. Turned on the fireplace on Lo (800W) and all was good. So I turned on the microwave. 2000W total and all good. So I put the fireplace on Hi. Inverter is outputting 2527W and drawing 286A. Got a low batt alert so I turned everything off. Went to check my wires on the Lynx and HOLY CRAP! The upper nut holding the class T fuse in the fuse holder is glowing RED HOT! Oh crap!

I let it cool off and checked the torque. It was not torqued enough. I must have just hand tightened it when I first installed it. I must have missed it when I checked the torque on every other connection. I tightened it up and redid the test and everything stayed cool.

I'm pretty sure if I had let this keep going, something would have melted or caught fire. And the thing that bugs me is there is no protection for this. This was a piss poor connection heating up but not blowing any fuses or circuit breakers. Scary stuff.

The nut circled in red at the top of the class T fuse is the one that was glowing red hot.

DC wiring 290RL lynx t400.jpg
 

Brett V

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When I worked for a living, I used to kill the AC and run the radio sites on battery for an hour while I did a PM on the microwave. One time about 5 minutes in to the job I started getting low voltage alarms and discovered a battery bolt on a busbar smoking. It gets your adrenaline going!
After removing the busbar and cleaning it, a proper torque fixed the problem.
 

rmaddy

Full-time Solar-powered Trailer Life
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No fuses or breakers should have tripped. There was no overcurrent. Just a loose connection causing a lot of extra resistance (heat). Glad you caught it in time.

Good reminder to double check all connections.
 

Steve_S

Offgrid Cabineer, N.E. Ontario, Canada
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Yep, that sounds like a loose connection. make sure surfaces are clean (wipe down with rubbing alcohol) and then reconnect everything and torque to spec. Recheck torque after a day or two again to make sure.
 

DJSmiley

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Thats why a flir is still on my wish list.

Run at max load for 30-60min, and check with the thermal camera. It will also spot bad crimps and so on.
 

sunshine

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I'm pretty sure if I had let this keep going, something would have melted or caught fire. And the thing that bugs me is there is no protection for this.
That is why a fire resistant backing board and enclosure away from flammable material is recommended!
 

Zil

Just another ass on the web
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May 27, 2020
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Just check the mv drop across the connections with your multimeter. Will catch the problem long before it gets warm.

I would think the inverter drawing that much power would shut down on low voltage as the bad connection got that hot.
 

Pappion

Retired Engineer Tech
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Nov 26, 2020
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271
Actually fuses blow due heat. They are self heating. That hot terminal would take out the fuse, eventually.
My roof top AC had bad connection taking out the fuses.
I no longer own this home, I buy all my EXs a house. Now when I date the first thing I ask: Do you want a house?
 

Short_Shot

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Actually fuses blow due heat. They are self heating. That hot terminal would take out the fuse, eventually.
My roof top AC had bad connection taking out the fuses.
I no longer own this home, I buy all my EXs a house. Now when I date the first thing I ask: Do you want a house?
Not necessarily. And the hot joint itself won't contribute any extra heating to the fuse unless the fuse is bolted directly to it.

What normally causes "extra heating" to blow a fuse in this situation is increased resistance at the connection driving up the current needed for a device to operate by pulling down the voltage.


I've seen many glowing red nuts just like this that ran that way for who knows how long before anyone noticed.

Once it was over a week between a "repair" and finding the loose connection. Never blew a breaker or fuse.
 

eXodus

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Blue Sea 250A circuit breaker with a 400A class T fuse. The class T fuse holder is from Blue Sea Systems and the fuse is a LittleFuse 400A class T. G
glad you are fine.

That is one of the reasons I go up in Voltage as soon as an application hits 100A. Tiny assembly errors and mechanical issues can have huge impacts.

The average welder has 140-200A and I know how much damage a welder can to even the thickest metal.

I don't like venturing into that territory. I know it can be done safely, but this thread is reinforcing my preconception not to build a welder.
 

Short_Shot

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glad you are fine.

That is one of the reasons I go up in Voltage as soon as an application hits 100A. Tiny assembly errors and mechanical issues can have huge impacts.

The average welder has 140-200A and I know how much damage a welder can to even the thickest metal.

I don't like venturing into that territory. I know it can be done safely, but this thread is reinforcing my preconception not to build a welder.
*casually welds at 30 amps*
 

chrisski

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The nut circled in red at the top of the class T fuse is the one that was glowing red hot.
Can you tell us how this connection has been over the last couple of weeks And also what you did like cleaning terminals with wire brushes and acetone, etc..?
 

eXodus

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*casually welds at 30 amps*
DC amps not AC amps.

"A transformer-style welding power supply converts the moderate voltage and moderate current electricity from the utility mains (typically 230 or 115 VAC) into a high current and low voltage supply, typically between 17V and 45V (open-circuit) volts and 55 to 590 amperes."

 

Short_Shot

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DC amps not AC amps.

"A transformer-style welding power supply converts the moderate voltage and moderate current electricity from the utility mains (typically 230 or 115 VAC) into a high current and low voltage supply, typically between 17V and 45V (open-circuit) volts and 55 to 590 amperes."

I have a Primeweld 225 AC/DC tig welder in the garage my man. I'm aware of the difference and I know what I said.

I've welded some pretty thin stuff before lol

That reference of yours is a joke, fyi. 55 is by no means a lower limit.
 

eXodus

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I have a Primeweld 225 AC/DC tig welder in the garage my man. I'm aware of the difference and I know what I said.

I've welded some pretty thin stuff before lol

That reference of yours is a joke, fyi. 55 is by no means a lower limit.
Sorry was not my intend to offend anyone. My apologies.

was the first reference google populated - I didn't do a reference check.

I got a sensor welding station which goes down even further.
 

Short_Shot

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Sorry was not my intend to offend anyone. My apologies.

was the first reference google populated - I didn't do a reference check.

I got a sensor welding station which goes down even further.
Not offended or anything, just was making a joke about low welding current vs your reference to welding as the bar you set.

As you stated you can be safe at high currents, but I wanted to illustrate that you can also be unsafe at low currents.

100a is a pretty good point for most people in my opinion.
 

corn18

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Can you tell us how this connection has been over the last couple of weeks And also what you did like cleaning terminals with wire brushes and acetone, etc..?
It was brand new out of the box so no cleaning was done. Now that the nut is torqued properly, everything stays cool.
 

Pappion

Retired Engineer Tech
Joined
Nov 26, 2020
Messages
271
Actually fuses blow due heat. They are self heating. That hot terminal would take out the fuse, eventually.
My roof top AC had bad connection taking out the fuses.
I no longer own this home, I buy all my EXs a house. Now when I date the first thing I ask: Do you want a house?
What is this so hard to understand?
Power dissipated = Current * Current * Ohms. The current moving thru the resistance heats up the fuse element and it melts down.
Little Fuse document on temperature derating
 
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