AC Unit intermittent surge problem

[openrangeowners]

From one EE to another - I feel your pain when trying to figure out things like this .... you do the best to design for the worst and there is always something that pops up that you didn't expect. Since you only have 1 specific hot spot and it is the fuse leads one to believe there is too much resistance or not enough surface area passing the current along .... is the other side of the fuse getting just as hot or only the one side?

Your wiring looks very clean and good and simplified. If the ground terminal lug strip is fine (I was worried about mine with the 250Amp ones we both used), then there must be something odd with that fuse connection or path.

One thing I have a question about - that short length of 4/0 wiring between the fuses - how did you get two lugs crimped onto the 4/0 wiring ?

I am thinking now that there might be an issue with resistance in the small section based on what you stated above - maybe one of those lugs is not making good contact with the wiring .... again eliminating the 300 amp fuse and making a new longer red wire with lugs would remove that as a possible issue as well as the 300 amp fuse.

[ncitro]

JohnD You are right, the fuse should blow long before a fire. I noticed your fuse looks like a ANL 300 amp. When I switched from my Magnum to the Victron Multiplus 3000 the Victron manual called for a Class T 400 amp rather than the ANL my Magnum needed. I did not think anything of it at the time and assumed they knew what they were talking about and changed it out. Possibly the Class T could have a lower resistance than the ANL you are using?

[howson]

JohnD and ncitro

The conversation about fuse types led me to do some reading, because I had no idea of the difference between the Class T and ANL. There's a (deep) discussion on Will Prowse's forum here: https://diysolarforum.com/threads/cl...nl-fuse.13913/

A reference off of that thread led me to the marinehowto.com website. While geared to boats, I think the linked article is applicable to our trailers, too. Search for the words Over-Current Protection on this page:https://marinehowto.com/lifepo4-batteries-on-boats/ (The Over-Current Protection section is about 2/3 of the way down--it's a long page!)

If I understand all of it, a Class T fuse is what is recommended and it's there to protect the wiring from a dead short.

Doing further reading I stumbled across a "Practical Low Resistance Measurements" document (attached) at www.aeroelectric.com/articles/LowOhmsAdapter_3.pdf

It would be very interesting to build this gadget and test my wiring...maybe someday.

Please keep this thread updated with what you find on your system.

Howard

[openrangeowners]

Hi John - ran across this today .... https://diysolarforum.com/threads/25...ing-hot.17319/

[JohnD]

I have been busy with travel and have not had an opportunity to post an update. With the over heating issue, I decided to just run the Multiplus in charge-only until I got a chance to work on a solution. We got to Georgia, and plugged in to shore power with the Multiplus in charge only, and very little solar charge because of a wooded site. 36 hours later, we're sitting in the RV and the lights flicker and go out. I was very surprised, so I looked at the BVM and the batteries were indeed dead. I went out and checked the voltage from the multiplus and it was reading 13+ volts. The voltage at the fuse was 0. I came inside to make some breakfast before getting to troubleshooting the problem. I lit the stove, and the propane dwindled to a trickle of flame. Sure enough, both propane tanks and the batteries are empty. Now that takes talent!

I ran out to tractor supply to fill the tanks, came back, and removed the fuse and switch.

Melted switch with fuse attached to stud.

Melted Hole in switch casing from stud.

The fuse had melted the switch such that it no longer worked. I did some investigation and did find that the 300 amp fuse was too small for the multiplus. I know I saw that configuration in a schematic, and thought that I had confirmed that in the Victron documentation, but apparently I am mistaken. The fuse should be at least 350 amps. I have now ordered a 400 amp T class fuse as recommended by @howson.
I will replace the 400 amp ANL with the T class, and no longer have a separate fuse for the batteries.

I did some testing yesterday with a portable heater and found that I could pull 180 amps for 1/2 hour with a few surges to 320 amps (30 second tests with the microwave) and the wires stayed cool (no fuse or switch). So, I'm pretty confident that I'll be fine with the T class fuse and a new switch.

[ncitro]

JohnD That's great, glad to hear it. I'm surprised the ANL could not handle the heat, but I guess that's why they call for the class T. I'm glad I upgraded mine when I changed the inverter.