Fuse I²t Rating

Formula

Description

The I²t (I-squared-t) rating of a fuse represents the thermal energy required to melt the fuse element, measured in ampere-squared-seconds. It is the key parameter for coordinating fuse protection with downstream components like semiconductors, capacitors, and cables. For a fuse to protect a component, the fuse I²t must be less than the component I²t withstand rating. During a short circuit, the fuse must clear the fault before the protected component is damaged by the thermal energy of the fault current.

Variables

• I²t — Thermal energy measure (A²·s)
• I — Fault current (A)
• t — Duration of the fault (s)

Practical Notes

Fuses have two I²t values: pre-arcing I²t (energy to melt the element) and total clearing I²t (including arc energy). For semiconductor protection, the total clearing I²t of the fuse must be less than the I²t withstand of the device. Fast-acting fuses have lower I²t than slow-blow fuses of the same current rating, making them better for semiconductor protection.