I’m not an electrician, sparky or similar, just an old guy having a pure off grid system installed centred around:
Multiplus II 48/10000/140
Pylontech US5000 x 4
Lynx Power In, Shunt & Distributor
RS450/100.
I’m trying to understand some of the basics so that with installation complete I’m not left totally in the dark as to what is what. The installer has presented an option of Lynx Class T power in or Lynx Power in (I suspect due cost). I’ve been reading lots of other posts but it’s oh so confusing.
My understanding is that battery cables should be fuse protected as close to the battery as possible. In reading about Class T devices they appear to protect against a dead short. Victron’s class T power in manual only shows class T fuses (mounted inside the Lynx power in). No specific cable protection that I can see so obviously I’m missing something.
Questions.
Should a Class T fuse should be specified as standard for my intended system?
If so, should a standard fuse also be added closer to the battery?
That’s not even getting into CNL, CNN, Mega, EF3, AB1 and the rest.
I went with the EF3 fuses. I swapped out the internal Mega fuses in my batteries with them.
The issue with standard mega fuses is that large lithium batteries have so much instantaneous current capacity that a mega fuse can just arc over in a fault event and the current will keep flowing.
Thanks for the reply - you are correct and here you have to forgive my layman’s terms as they may not technically be right. I see the Class T as your disaster situation safety device, what I’m not understanding is how they protect a cable experiencing an overcurrent, for whatever reason, that is below the Class T trigger point but would trip a fuse. Does that make sense?
They do not if the short is made between the battery and the fuse. A lot of batteries have some form of protection built in so the fuse is for when the internal battery protection fails.
In simple terms using a regular vs class T fuse depends on how much current the battery can deliver. If there is a short some batteries can deliver so much current regular fuses basically just melt and don’t interrupt the flow of electricity. Your installer should be able to tell you which one you need.
If you want to dive into the numbers, look for the maximum current the fuse can handle.
Then consider the short-circuit current a LFP cell can deliver is for some cell types north of 30 kA.
This value you can compute yourself given 3.2V cells and internal resistance ~0.1 mOhm for e.g. an MB56 cell (link) → I=V/R = 3.2/(0.1e-3) = 32 kA.
These are crazy currents, even for a class T fuse. EF3 generally has a higher breaking capacity, but it’s slower. In my own installations I have an EF3 inside the battery pack at the + side and a class T in the class T power in before the + battery cable.
I have to step back at bit and clarify something. I’m an installers nightmare as I like to understand what they are doing but in effect no next to nothing about what they are doing.
Take a Class T or EF3 or similar, with some made up rating numbers of rated current at 150A, and breaking capacity of 50kA@200Vdc. I’m with you on the 50kA, that I understand. The 150A current rating - are we saying that this unit will act in the same way as a conventional fuse when loaded beyond 150A?