Bussmann 3A Fast Acting Melamine Fuse with 600VAC Voltage Rating; NOS Series

Fast-acting, nonindicating UL Class H and K5 fuses are for general purpose use in existing installations that require a high-sensitivity, fast-blow fuse. These fuses are highly sensitive to changes in current flow and don't tolerate temporary overload currents or power surges, so they open within seconds of being exposed to overloads and short circuits. They have no indicator for identifying a blown fuse, so they're sometimes used with an indicating fuse block or cover. Class H fuses are rated from 65A to 600A with an interrupting rating (IR) of 10kA for low-interrupt applications. Class K5 fuses are rated up to 60A with an interrupting rating (IR) of 50kA for higher interrupt applications and are current-limiting to prevent electrical damage by disconnecting circuits before they're exposed to the full magnitude and duration of an overcurrent. They're not labeled as current-limiting since they don't have a rejection feature.

UL Class H and K5 fuses are used as replacement fuses for overcurrent protection in older HVAC installations. These fuses are dimensionally the same, so they can be interchanged to change from a current-limiting system using Class K fuses to a noncurrent-limiting system using Class H fuses. Class H fuses are used in low-interrupt applications while Class K fuses have a higher interrupting rating.