EG4 Batteries, breakers, fusing, etc

[BrianH]

As I'm building a system using the EG4 48v, 100AH LiFePower4 batteries, I want to make sure I'm designing a safe implementation.
In a post from March 9th, RichardfromEG4 confirmed that the built in breaker on the EG4 server rack batteries did trip providing adequate protection in the event of a dead short. Given that, inclusion of class T fuses at each battery + is not needed. Is it still desirable?
If it is desirable to fuse the batteries individually, is the Victron Lynx Distributor a viable option using MEGA fuses? It seems like a good solution except that it doesn't use class T fuses.
Finally, I'm assuming it is still a best practice to put a class T fuse between the inverter and the bus bar, correct? (I'm using a MPP LVX6048 if that's important).

 

[hclarkx]

The answer depends. A Class T fuse on the inverter isn't a bad idea, but makes only limited sense if there's significant short-circuit exposure (wire) between that fuse and the battery or batteries. A severe short-circuit in the inverter or more likely on the incoming terminals of the inverter will "see" the sum current of all parallel batteries and a Class T fuse, being "current limiting" would limit that current and the resulting damage or fire hazard. However, a Class T fuse on the battery or on each battery in the case of multiple parallel batteries would provide the same protection for the inverter AND provide fast opening for a short-circuit between the inverter and the battery terminals (where the fuse is located). This fuse being literally on the battery terminal or very close to the battery terminal provides the most short-circuit protection coverage.

We are talking potentially thousands of amps limited only by the resistance of the wire between the battery terminals and the location of the short-circuit (and by the IR of the battery/ies of course). Hence the closer the short-circuit is to the battery, the higher the current and the more hazardous the event is. Ergo a fuse close to the battery provides considerable added benefit.

Because a proper BMS will open before short-circuit current reaches it's maximum value, it too is "current limiting" and may open more quickly than the Class T fuse -- if the BMS does its job. The Class T fuse close to the battery is backup to the BMS. Not having this fuse is putting a lot of faith in the BMS. I've not seen a BMS failure under high short-circuit current documented but it can happen. In the power industry and in your home there is always backup; a second or upstream device that can interrupt short-circuit current if the protective device closes to the short-circuit fails to perform.