Class T fuse and breaker?

[scottvanv]

Oh yeah sorry I got it the 1st time, your explanation made sense...I am glad I have them in then.

Hadn't seen Wills vid yet, here you can see some great examples of Class T used in conjunction with breakers for the batteries.

@Will Prowse Hey Will, love your videos, especially this one. But, when go look for the components, your Solar Power Systems page says, "Use these for 48V," but your Affiliate link is for 12V T Class fuses. Was this a mistake, or am I missing something?

 

[tigerwillow1]

Does a 12 volt class T fuse exist? I don't think so and it's just a typical sloppy product listing.

 

[scottvanv]

Does a 12 volt class T fuse exist? I don't think so and it's just a typical sloppy product listing.

The same product listed at Inverter Supply says it's for 12, 24, 32, or 48 volt systems, so I think you're correct. However, this listing says it's a "slow blow" fuse ... isn't T Class supposed to almost instantaneous, mainly to handle extremely high voltage system shorts?

 

[Archetype-IS]

Do you need both DC Circuit Breaker and T-Fuse? From the Mangum manual (below), it sounds like you can get away with the DC Breaker with Overcurrent protection device. Could someone confirm? T-Fuse are hard to find these days.

"
For safety and to comply with electrical code regulations, you must install a DC overcurrent protection device in the positive DC cable line to protect your DC cables. This DC overcurrent device can be a fuse or circuit breaker and must be DC rated. It must be correctly sized according to the size of DC cables being used—which means it is required to open before the cable reaches its maximum current carrying capability, thereby preventing a fi re. In a residential or commercial electrical installation, the NEC requires both overcurrent protection and a disconnect switch. If a circuit breaker is used as the overcurrent protection device, it can also be used as the required DC disconnect. For maximum protection, install the circuit breaker (or fuse/disconnect) as near as practical to the batteries. "

If a fuse is used as an overcurrent device, a Class-T type or equivalent is required. This fuse type is rated for DC operation, can handle the high short-circuit currents, and allows for momentary current surges from the inverter without opening. However, because the fuse can be energized from both directions, if it is accessible to unqualifi ed persons, the NEC requires that it be installed in a manner that the power can be disconnected on both ends of the fuse before servicing.
"

 

[Wet1]

Breaker + Fuse is an option if you need the breaker as a disconnect. I don't know which breaker specifically you're using/voltage and how fast it is; it could be fine. Personally, my battery design rule is (for 48V LiFePO4 especially) to use a Class T fuse on each pack no matter what. It's proven its use to me in the past and it's worth the money for the security it gives.

My AIO has a DC breaker in it. Do you see any issue with just installing a class T fuse/holder at the end of (each) BMS to complete the 48V DC safety system?

 

[upnorthandpersonal]

My AIO has a DC breaker in it. Do you see any issue with just installing a class T fuse/holder at the end of (each) BMS to complete the 48V DC safety system?

If the breaker on the AIO is spec'ed properly this is fine, however you should note that a fast class T will likely blow before the breaker pops. Therefor, should something go wrong (e.g. a mosfet in the inverter breaks and shorts the input) the breaker might be late responding. For this, I tend to put a smaller class T fuse at the inverter (e.g. a 100A one, I know I don't pull over 5kW) and 225A class T fuses at the batteries. This (in most cases, at least the ones I tested) prevents having all class T fuses at the batteries blow, which gets expensive with four packs in parallel.