Fuse Types? Why is it a big deal?

[mitchk1303]

Hello all. I am trying to wrap my head around the reasoning for using different fuse types. I understand the interrupting amperage rating. I guess my question is about the necessity of T-series fuses.

I will start with my potential setup:
2x 206Ah lithium batteries with high current protection
1x 100A 12v fuse block
1 x 3kw victron inverter/charger

I am planning on using 2 225A T-series (or maybe MRBF?) fuses on each battery with 2/0 cable to the bus bar.
I think I will use one 4/0 cable and 400A fuse from the bus bar to the inverter.

My question is why is T-series fuses recommended for the bus bar to the inverter? Isn't the system already protected by both the battery short circuit protection as well as the fuses in between the battery and bus bar? It seems like the T-series fuse would be redundant. I get having a fuse to protect the cable but it seems as though it wouldn't have to have a super high AIC rating? Any thoughts would be appreciated!

 

[John Frum]

Hello all. I am trying to wrap my head around the reasoning for using different fuse types. I understand the interrupting amperage rating. I guess my question is about the necessity of T-series fuses.

I will start with my potential setup:
2x 206Ah lithium batteries with high current protection

Depends how they are configured
lithium covers a lot of chemistries best to be specific.
In you case its LFP.
For the topology that I suggested in your other thread which has necessary context, I suggest you use MRBF fuses on the positive terminal because they are cheaper than class-t have have enough breaking capacity for the usage model.

My question is why is T-series fuses recommended for the bus bar to the inverter?

Its not.

Isn't the system already protected by both the battery short circuit protection as well as the fuses in between the battery and bus bar? It seems like the T-series fuse would be redundant. I get having a fuse to protect the cable but it seems as though it wouldn't have to have a super high AIC rating? Any thoughts would be appreciated!

Again the branch circuits are protected by a fuse to hopefully isolate a fault to the branch thus leaving the rest of the system functional.
The class-t fuses are used to protect the batteries.

 

[DerpsyDoodler]

The bottom line is if your battery has adequate, reliable over-curent protection built-in, class-T isn't wholly necessary. I'm using a DIY pack and BMS. I want the extra protection of the class T so that in event the BMS protection doesn't do its job, the fuse will.

 

[Zil]

Often, mostly?, internal battery protection means the BMS.
The BMS is the last stand protection of the LFP battery. The system must be designed and parameters set so the BMS never trips during normal operation. There should be an adequate fuse at the battery positive post, or very close to the post. Li batteries can output tremendous amounts of energy during a short circuit. Some fuses will not interrupt this current. MRBF will interrupt 10,000 amperes up to about 14 volts dc.
Class-T fuses will interrupt 20,000 amperes at 125 volts dc.
If multiple batteries are connected in parallel, each battery positive must be wired and fused to the maximum current expected. If the inverter needs a 4/0, then each battery should not be wired with 2/0. They need 4/0 as well.

 

[DerpsyDoodler]

If the inverter needs a 4/0, then each battery should not be wired with 2/0. They need 4/0 as well.

They can be wired however you want to wire them, as long as the wire is properly protected by overcurrent protection rated to interrupt the possible ISC at whatever your operating voltage is. whether that be a class-t fuse or a suitable breaker. It doesn't _have_ to be a fuse. It _has_ to be over-current protection that meets spec/code.

 

[Zil]

That is true! Fuses Protect Wires! But i would never recommend a dc breaker used as the catastrophic fuse at a LiFePo battery, or on any high amperage dc circuit. Pay attention to the interrupt capacity of any circuit protection.
What happens to the fuses sized for 2/0 if some of the parallel battery fails, but the load exceeds the fuse capacity of the bank. Everything goes down all at once. Or the circuit breaker fails closed and the wire gets hot.

 

[Roswell Bob]

Fast fuses can also keep components from rupturing during a supercurrent event.