Reading time-current curves, I²t and current limitation
A time-current curve plots how long a fuse takes to melt against the current flowing through it. I²t expresses the energy a fuse lets through before it clears. Together they tell you whether a fuse will protect a device without nuisance operation under normal load.
Who this is for: Engineers selecting fuses for semiconductors, drives and power conversion equipment, and anyone comparing a replacement fuse against an original.
What a time-current curve shows
The horizontal axis is prospective current and the vertical axis is time, both logarithmic. The plotted band shows the range within which the fuse element melts. Higher current means faster melting, which is why the curve slopes down to the right.
Two figures are usually distinguished: pre-arcing time, which is how long the element takes to melt, and total clearing time, which adds the arcing period until current actually stops.
What I²t means
I²t is the integral of current squared over time, and it is proportional to the energy the fuse passes. It is normally quoted as two values — pre-arcing I²t and total let-through I²t.
It matters most when protecting semiconductors. A thyristor or diode has its own I²t withstand figure, and the fuse protecting it must let through less energy than the device can survive. Comparing those two numbers is the core of semiconductor fuse selection.
Current limitation
A current-limiting fuse clears so quickly that it interrupts before the prospective fault current reaches its first peak. The peak the downstream equipment actually experiences is therefore lower than the prospective peak.
Peak let-through curves plot that relationship. They are what you use to show that a fuse protects equipment from mechanical and thermal stress under fault.
Comparing a replacement against an original
When replacing a fuse that is no longer made, matching voltage and ampere rating is necessary but not sufficient. The replacement should have a comparable melting characteristic and a let-through energy no greater than the original, and it must physically fit and mount the same way.
This is why DENCO reviews the application rather than answering a substitution from a table.
Limits of this guide
- This guide explains how to read published fuse data. It is not a substitute for a protection coordination study.
- Device withstand figures must come from the semiconductor or equipment manufacturer, not from the fuse data.
- Derating for ambient temperature, enclosure, cycling and altitude applies and is not covered here.
- Curves are published per fuse family; ask DENCO for the current revision rather than working from a general curve.
Related products
Sources
- DENCO product catalog, January 2025 — curves published per family
- NOTE: the catalog’s own introductory section (pp. 9–20) is a damaged translation containing at least one corrupted numeric value. It has deliberately NOT been reproduced here and must be rewritten from source data before any figure from it is published.
Technical review
This guide is educational and is not a protection coordination study. Confirm any selection with DENCO engineering.