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. It brings the Lynx system fully into the lithium era by enabling the integration of high-interrupt-capacity Class T fuses that protect against 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 that stick out on the left side to tie into other Lynx products. That said, there are a few key differences. First, it is part of the M10 line and is incompatible with the M8 versions of other Lynx products. Second, the Class-T Power In holds two Class T fuses, rather than the four Mega fuses you may be used to on the regular Lynx Distributor and Lynx Power In. Finally, 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 metric bolt size designations. The M indicates metric, and the number refers to the outer diameter of the threaded section. In the Lynx product ecosystem, this difference shows up in the bolts on the exposed portion of the busbar, where multiple Lynx products can attach. M10 bolts support higher torque specifications, which affects the system’s ampacity rating. For example, the Lynx BMS system uses M8 bolts for 500A and M10 bolts for 1000A.
So why move away from the tried-and-true MEGA fuse system in the classic Lynx Distributor, and lose two inputs in the process? The answer is the interrupt capacity of Class T fuses.
Fuses have two key current-related ratings: their current rating and their interrupt rating. The current rating is the familiar one: 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. MEGA fuses have a 2,000A interrupt rating. Class T fuses, by contrast, have a 20,000A interrupt rating 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 pairing lithium batteries with classic MEGA fuses were installed and commissioned. While Class T fuses now set the standard for lithium battery systems, using MEGA fuses with a Lynx Distributor is still an acceptable option. As mentioned above, lithium batteries almost always include an internal battery management system 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 up to system designers. Moving forward, we expect most new systems designed with the Class-T Power In to take center stage for battery fusing and DC distribution.
