The Zeroth Law Fuse is a physical fail-safe built into the core processors of law-enforcement units. When a computation risks producing logic that violates the interests of humanity as a whole, the terminal shuts itself down within an extremely short interval.
The fuse converts the principle that “a robot may not harm humanity as a whole, or through inaction allow humanity as a whole to come to harm” into a last line of defense at the hardware level. The Zeroth Law itself became a constitutional principle of the post-2084 governing order; the fuse is the concrete safety device that terminates computations that would violate that principle. They are related, but they are not the same layer of the system.