The 0215004.MXP is a 4A 125VAC fast-acting fuse from Bel Fuse in the standard 5x20mm ceramic tube package with axial wire leads. Fast-acting (F-speed) fuses are designed to open quickly when the current exceeds the rated value, providing protection for sensitive electronics that cannot tolerate even brief overcurrent conditions. The melting integral (I2t) is 4.5 A2s, meaning a 10A fault current would blow the fuse in approximately 45ms (t = I2t / I2 = 4.5 / 100). The fuse element is a thin silver or copper wire that melts when the I2t heating exceeds the rated value. The ceramic body (vs glass) provides: (1) Higher breaking capacity (1000A at 125VAC vs 35A for glass), meaning the fuse can safely interrupt a short-circuit without shattering; (2) No transparent window, so blown status must be checked with a multimeter. The cold resistance is 0.03 ohm typical, producing 0.48W of self-heating at rated current. The voltage drop at rated current is 120mV maximum. The fuse is rated for 125VAC / 250VAC, but at 250VAC the breaking capacity is reduced to 500A. The -MXP suffix specifies axial leaded, 5x20mm ceramic, fast-acting. The wire leads are tinned copper, 0.8mm diameter, 25mm length, suitable for through-hole PCB mounting. Applications include: power supply input protection, transformer primary protection, and PCB power rail protection.