t class, anl, mega or mbrf

[Cpt Mark]

Originally Posted by Jammer

Hello Mark


One Fact to Consider that has not yet been touched upon in this thread is that fuses are thermal devices. In essence they are heated by current passing through them, and melt and open at a certain point.


Fuses will blow below their rating if they are in a hot environment or if they are heated by smaller cables or poor connections made to the fuse block, or by adjoining fuses in the same fuse block (if present). That may explain what happened.

Hi Jammer,

Your info is good. My fuses blew exactly as they should have. The wires were starting to melt and go brown. This was due to a brain fart on my end. I'd pulled 1 battery out of the system and had put the nut back on the post holding all the grounds and forgot to tighten it or didn't tighten it enough. It was barely on. Glad the fuses blew. BTW was smelling it before they went. Also glad that I went down the fuse rabbit hole. Learned a lot

[CarinaPDX]

Mark didn't mention that he has connected the batteries to the inverter/charger with two 1/0 (0) wires, which has the same cross section as a 4/0 (0000) cable. That is how he can use 1/0 for 400A (the recommended fuse size for the inverter/charger). According to the ABYC tables it is a bit over their recommended current but only by a small amount and the inverter will only draw that current for brief peaks, if at all.

After doing a deep dive on fuses with Mark it is pretty clear that there is a very good reason that T-fuses are recommended close to lithium batteries. I found a video that showed a MRBF fuse being shorted across a lithium battery bank; it blew up spectacularly. The good news is that it opened, and stayed open (no arcing). The bad news is that it blew out the side of the plastic case and sent molten copper flying 10 feet. So while the 10kA AIC rating is higher than any other fuse except the class-T fuses (20kA) I would not consider it safe for a lithium installation.

It would be great if some of those who have their LiFePo systems working well would publish the settings they use in their BMS for the cells used, along with the setting for the various regulators. Unfortunately there seems to not be a common vocabulary between cells and BMS's, and the purpose of each setting is rarely defined for all settings. Clearly some are critical for safe operation, while others may be just for reporting SOC.

One conclusion I have is that LiFePo is not yet plug-and-play, at least for mixed systems. It might be for all-Victron systems, at a price.

Greg

[Jammer]

I've never been myself, but a colleague of mine was once invited to visit a testing facility that Square D then used to test the interrupting capacity of its products. The way it was described to me was that they had a large rotary generator with a stator connected to large (several inches square) busbars leading into the test room. The generator had high rotational inertia and a field winding that was left unenergized as the generator was spun up to the target speed. Then field was applied leading to a tremendous yet controlled current pulse on the busbars with the amps and duration governed by the speed of the rotor and strength of the field. They could go up to well over 100,000 amps.

Anyway my friend was invited to witness a couple of tests, one at the rated AIR of the device under test, one well in excess (x10). One thing I did not know is that the tests are conducted with all covers in place and criteria for success require that no sparks or flaming debris exit the enclosure. They tested an ordinary household breaker at 10,000 amps, which passed, then tested it at 10x that which turned it into a molten mess with flaming bits of metal flying everywhere.


Another fact to consider is that the magnetic force on busbars and cables conducting over 10,000 amps is considerable. If the positive and negative cables are in contact (i.e. only separated by insulation) it can be hundreds of pounds.

So the moral of the story is that it isn't necessarily that the devices in question won't open under a fault current larger than their rating but rather that the may do so slowly and spew molten, firey debris in the course of doing so. Or they may fly apart.

Apropos this thread I doubt if many marine installations can actually deliver short circuit current in excess of 10,000 amps from any short circuit that is physically possible given the resistance of battery terminals, cables, the short itself, given the minimum length of battery cable required to achieve a short. At 12 volts just over 0.001 ohm resistance would preclude anything over 10,000 amps.