What to Know About Victron’s Class-T Fuse Distributor
The Lynx Class-T Power In is Victron’s latest addition to the Lynx DC distribution ecosystem. This new product ushers the Lynx system fully into the lithium era by allowing the integration of high interrupt capacity Class T fuses which fully protect from the high short circuit current potential of modern lithium batteries.
The Class-T Power In has a familiar form factor: two bus bars under the blue plastic cover with ends sticking out the left-hand side to tie into other Lynx products. Beyond this, there are some key differences. One thing to note is that this is a member of the M10 line and is incompatible with the M8 versions of other Lynx products. Another thing to be aware of is that the Class-T Power In has a capacity for two Class T fuses rather than the four Mega fuses you may be used to on the regular Lynx Distributor and Power In. Also, the Class-T Power In sits 4 inches off its mounted surface (vs. 2.5 inches on the Power In).
What’s the difference between M8 and M10?
M8 and M10 are measurement classifications for bolts. The M stands for metric, and the number stands for the outer diameter of the threaded section. In the Lynx product ecosystem, the difference is found in the bolts on the extruding portion of the busbar, where multiple Lynx products can attach together. The M10 bolts allow for higher torque specs, affecting the system's ampacity rating. We can see this in the Lynx BMS system, which utilizes M8 bolts for the 500A and M10 bolts for 1000A.
So why move from the tried and true MEGA fuse system of the classic Lynx Distributor and lose two inputs while we’re at it? The answer is in the interrupt capacity of Class T fuses.
Fuses have two main numbers associated with amperage: their current rating and their interrupt rating. The current rating is what we are all familiar with; a 200A MEGA fuse will allow that current indefinitely and then break when subjected to a higher current, such as 250A, for a period of time. However, what if that same fuse was hit with 4,000A? This is where the interrupt rating comes into play. When high amounts of amperage are present, the fuse must be constructed to complete the disconnect without self-destructing or creating an arc. The MEGA fuses have a 2,000A interrupt rating. On the other hand, Class T fuses have an interrupt rating of 20,000A and can safely handle the 4,000A to disconnect the short circuit as intended.
The next logical question is: why are we now worried about 4,000A plus short circuit currents? This is due to the incredibly low internal resistance of modern lithium batteries. The low internal resistance translates to the potential for very high short-circuit currents to the tune of thousands of amps. For example, a single 48V LiFePo4 battery can potentially unload 2,500A in a dead short. While most batteries have an internal BMS that acts as the first line of defense against short circuits, the introduction of the Class T fuse and the compatible Class-T Power In raises the bar on safety and redundancy for Victron power systems.
So what about all the lithium battery systems already installed with Lynx Distributors and MEGA fuses? Are they ticking time bombs? No. Most lithium batteries come equipped with a BMS that serves as the first line of defense in the event of a short circuit. Power systems using Pylontech LiFePO4 batteries go a step further, with two layers of protection built into the BMS to isolate the cells if necessary. Paired with MEGA fuses to protect your DC wire from regular high amperage scenarios, a lithium system with quality batteries provides a good bit of protection to begin with. The new Class T system offers an added level of security that works across the board regardless of what kind of system it is paired with. This is something that many of us in the Victron world have been waiting for, and we expect the new Class-T Power In to become a standard component in state-of-the-art lithium power systems.
On a final note: long before the introduction of the Class-T Power In, many Victron systems paired lithium batteries with the classic MEGA fuses have been installed and commissioned. While the Class T fuses now set the standard for lithium battery systems, the older style of using MEGA fuses with a Lynx Distributor is still an acceptable option. As previously mentioned, lithium batteries come nearly universally with an internal battery management system that is designed to disconnect the battery in the event of a short circuit. A MEGA fuse is still more than capable of protecting your wire from the moderately high amperage it is designed for, and coupled with the BMS, creates an effective protection system. Ultimately, the choice is in the hands of system designers but, moving forward, we expect most new systems designed with the Class-T Power In to take center stage for battery fusing and DC distribution.
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