IEC 60127: Miniature Fuses — Why I2t and Breaking Capacity Determine Everything

The Tiny Fuse in Your Phone Charger Must Survive 35 A Surge Without Blowing

IEC 60127-10:2001 specifies miniature fuse requirements. Every USB charger and TV set has one — it must survive inrush without nuisance blowing and reliably open under fault.

I²t: the fuse soul parameter. I²t represents the energy needed to melt the fuse element. Inrush I²t (e.g., SMPS capacitor charging) must be below the fuse melting I²t — otherwise every power-on blows the fuse. But fault I²t must exceed melting I²t — a conflicting design constraint. Time-Lag (T-type) fuses use the M-effect: a low-melting-point metal alloys with a high-melting-point element. Under surge, the low-melt metal stays solid (high I²t tolerance). Under sustained fault, it melts and diffuses into the high-melt metal, forming a eutectic that opens quickly.

Breaking capacity: Miniature fuse ratings span 35 A (low-breaking, secondary protection only) to 1,500 A (high-breaking, direct mains protection). Using a low-breaking fuse on a high-fault-current circuit risks sustained arcing that never extinguishes.

TN Lab — A fuse costs pennies. It protects equipment worth thousands. Get both I²t and breaking capacity right.